@article{schrickx_kashani_buck_ding_rech_flagg_li_kudenov_you_richter_et al._2024, title={Exceptional Alignment in a Donor-Acceptor Conjugated Polymer via a Previously Unobserved Liquid Crystal Mesophase}, volume={4}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202315183}, abstractNote={Abstract}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Schrickx, Harry M. and Kashani, Somayeh and Buck, Lauren and Ding, Kan and Rech, Jeromy J. and Flagg, Lucas Q. and Li, Ruipeng and Kudenov, Michael W. and You, Wei and Richter, Lee J. and et al.}, year={2024}, month={Apr} }
 @article{schrickx_gyurek_moore_hernandez-pagan_doherty_kudenov_brendan t. o'connor_2024, title={Flexible Self-Powered Organic Photodetector with High Detectivity for Continuous On-Plant Sensing}, volume={2}, ISSN={["2195-1071"]}, DOI={10.1002/adom.202400005}, abstractNote={Abstract}, journal={ADVANCED OPTICAL MATERIALS}, author={Schrickx, Harry M. and Gyurek, Sydney and Moore, Caleb and Hernandez-Pagan, Edmaritz and Doherty, Colleen J. and Kudenov, Michael W. and Brendan T. O'Connor}, year={2024}, month={Feb} }
 @article{wang_xiao_siddika_shamsi_frey_qian_bai_brendan t. o'connor_dickey_2024, title={Glassy gels toughened by solvent}, volume={6}, ISSN={["1476-4687"]}, url={https://doi.org/10.1038/s41586-024-07564-0}, DOI={10.1038/s41586-024-07564-0}, journal={NATURE}, author={Wang, Meixiang and Xiao, Xun and Siddika, Salma and Shamsi, Mohammad and Frey, Ethan and Qian, Wen and Bai, Wubin and Brendan T. O'Connor and Dickey, Michael D.}, year={2024}, month={Jun} }
 @article{ren_song_zhu_o'connor_dong_2023, title={All Electrohydrodynamic Printed Flexible Organic Thin Film Transistors}, volume={6}, ISSN={["2365-709X"]}, url={https://doi.org/10.1002/admt.202300410}, DOI={10.1002/admt.202300410}, abstractNote={Abstract}, journal={ADVANCED MATERIALS TECHNOLOGIES}, author={Ren, Ping and Song, Runqiao and Zhu, Yong and O'Connor, Brendan and Dong, Jingyan}, year={2023}, month={Jun} }
 @article{charles_edwards_ravishankar_calero_henry_rech_saravitz_you_ade_o'connor_et al._2023, title={Emergent molecular traits of lettuce and tomato grown under wavelength-selective solar cells}, volume={14}, ISSN={["1664-462X"]}, DOI={10.3389/fpls.2023.1087707}, abstractNote={The integration of semi-transparent organic solar cells (ST-OSCs) in greenhouses offers new agrivoltaic opportunities to meet the growing demands for sustainable food production. The tailored absorption/transmission spectra of ST-OSCs impacts the power generated as well as crop growth, development and responses to the biotic and abiotic environments. To characterize crop responses to ST-OSCs, we grew lettuce and tomato, traditional greenhouse crops, under three ST-OSC filters that create different light spectra. Lettuce yield and early tomato development are not negatively affected by the modified light environment. Our genomic analysis reveals that lettuce production exhibits beneficial traits involving nutrient content and nitrogen utilization while select ST-OSCs impact regulation of flowering initiation in tomato. These results suggest that ST-OSCs integrated into greenhouses are not only a promising technology for energy-neutral, sustainable and climate-change protected crop production, but can deliver benefits beyond energy considerations.}, journal={FRONTIERS IN PLANT SCIENCE}, author={Charles, Melodi and Edwards, Brianne and Ravishankar, Eshwar and Calero, John and Henry, Reece and Rech, Jeromy and Saravitz, Carole and You, Wei and Ade, Harald and O'Connor, Brendan and et al.}, year={2023}, month={Feb} }
 @article{siddika_peng_balar_dong_zhong_you_ade_oconnor_2023, title={Molecular interactions that drive morphological and mechanical stabilities in organic solar cells}, volume={7}, ISSN={["2542-4351"]}, DOI={10.1016/j.joule.2023.06.002}, abstractNote={<h2>Summary</h2> Morphological and mechanical stabilities of organic solar cells (OSCs) are of paramount importance to ensure long-lived devices. However, the fundamental drivers of these stability metrics and their competing relationship have yet to be well defined. Here, several high-performance polymers and small molecule acceptors (SMAs) are considered to assist in the development of a comprehensive view of the molecular drivers of, and interrelationships between, morphological and mechanical stabilities. We find that the SMAs drive much of the embrittlement and diffusion characteristics in the blend films. However, the heterointeraction of the SMA and polymer, probed through dynamic mechanical analysis, is a key contributing factor to the film toughness. The heterointeraction energy is ideally maximally negative (i.e., repulsive), deviating from the geometric mean of the homointeraction energy. These findings assist in introducing a framework to understand the active layer stability and highlight material properties that lead to morphologically stable and physically robust OSCs.}, number={7}, journal={JOULE}, author={Siddika, Salma and Peng, Zhengxing and Balar, Nrup and Dong, Xinyun and Zhong, Xiaowei and You, Wei and Ade, Harald and OConnor, Brendan T.}, year={2023}, month={Jul}, pages={1593–1608} }
 @article{al shafe_schrickx_ding_ade_brendan t. o'connor_2023, title={Rubber-Toughened Organic Solar Cells: Miscibility-Morphology-Performance Relations}, volume={8}, ISSN={["2380-8195"]}, DOI={10.1021/acsenergylett.3c01124}, abstractNote={Tough organic solar cell (OSC) active layers are necessary to achieve robust, flexible, and stretchable devices. A major challenge is that the brittle small molecule acceptor (SMA) in polymer/SMA bulk heterojunctions results in films prone to mechanical failure. To improve mechanical toughness, we investigate the use of a thermoplastic elastomer (styrene-b-ethylene-butylene-styrene) (SEBS) as an additive in high-performance photoactive layers. We find a consistent transition of all measured parameters [e.g., fracture energy (Gc) and power conversion efficiency (PCE)] at a SEBS concentration of 5–10 wt %, which is driven by the miscibility of the SEBS. We use this insight to optimize the SEBS loading for PCE and toughness. Optimized OSCs are found to increase Gc significantly with a marginal decrease in PCE, resulting in a record Gc·PCE metric, considering all OSC photoactive layers. The pronounced miscibility–function relationship provides a framework to optimize elastomer addition in OSCs for performance and toughness.}, journal={ACS ENERGY LETTERS}, author={Al Shafe, Abdullah and Schrickx, Harry M. and Ding, Kan and Ade, Harald and Brendan T. O'Connor}, year={2023}, month={Aug} }
 @article{shanahan_oh_son_siddika_pendleton_brendan t. o'connor_you_2023, title={Strategic Incorporation of Cleavable Side Chains Improves Thermal Stability of PffBT-T4-Based Polymer Solar Cells}, volume={35}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.3c02181}, abstractNote={Thermal degradation by intrinsic morphological change and extrinsic oxidation remain outstanding challenges for bulk heterojunction (BHJ)-based polymer solar cells (PSCs). Postprocessing thermocleavage of side chains on the donor conjugated polymers using ester pyrolysis is a proven method to kinetically trap the morphology via increased glass transition temperature (Tg) and improved thermal oxidation resistance. We previously showed that having a certain fraction of thermocleavable side chains (TCS) incorporated into the archetypical polymer P3HT, achieved through the copolymerization of a TCS-functionalized thiophene monomer, can offer high thermal stability to its BHJ devices without compromising significant performance metrics. This work expands the concept of using the copolymerized TCS monomer unit to balance the stability with efficiency into the state-of-the-art PSC system PffBT-T4-OD:PCBM, where the original octyl–decyl (OD) branched side chains are partially replaced with TCS in 50–70 mol % in the copolymers. Structural differences of the fully cleavable PffBT-T4-TCS polymer and P3ET polymers disclose that increasing backbone rigidity and alkyl chain length can increase the temperature of eliminating alkyl chains by up to 20 °C in the solid state. Dynamic mechanical analysis shows the cleaved PffBT-T4-TCS polymer has significantly increased thermal relaxation temperatures and high storage modulus over a large temperature range. Thermal stability testing at 100 °C in air reveals that increasing the TCS content drastically increases the polymer resistance to oxidation. PSCs made with the fully cleavable PffBT-T4-TCS polymer offer only a meager efficiency of 0.2%, while the copolymer with 60 mol % TCS can deliver a PCE of 3.2% with its BHJ device, double the previous highest reported efficiency for TCS-containing polymer-based PSCs. Importantly, the copolymer with the 60 mol % TCS-based device is stable, retaining 80% of the initial performance after accelerated aging tests (100 °C, 2 weeks). Together with our previous works, these new findings demonstrate that using partial cleavage of side chains could be a general strategy to gain both efficiency and stability for conjugated polymer-based PSCs.}, number={23}, journal={CHEMISTRY OF MATERIALS}, author={Shanahan, Jordan and Oh, Jiyeon and Son, Sung Yun and Siddika, Salma and Pendleton, David and Brendan T. O'Connor and You, Wei}, year={2023}, month={Nov}, pages={10139–10149} }
 @article{song_ren_liu_zhu_dong_brendan t. o'connor_2023, title={Stretchable Organic Transistor Based Pressure Sensor Employing a Porous Elastomer Gate Dielectric}, volume={4}, ISSN={["2365-709X"]}, url={http://dx.doi.org/10.1002/admt.202202140}, DOI={10.1002/admt.202202140}, abstractNote={Abstract}, journal={ADVANCED MATERIALS TECHNOLOGIES}, publisher={Wiley}, author={Song, Runqiao and Ren, Ping and Liu, Yuxuan and Zhu, Yong and Dong, Jingyan and Brendan T. O'Connor}, year={2023}, month={Apr} }
 @article{altaqui_schrickx_gyurek_sen_escuti_brendan t. o'connor_kudenov_2022, title={Cephalopod-inspired snapshot multispectral sensor based on geometric phase lens and stacked organic photodetectors}, volume={61}, ISSN={["1560-2303"]}, DOI={10.1117/1.OE.61.7.077104}, abstractNote={Abstract. Multispectral imaging (MSI) is a valuable sensing modality for applications that require detecting a scene’s chemical characteristics. Existing MSI techniques utilize a filter wheel or color filter arrays, which are subject to reduced temporal or spatial resolution. In this work, we present a cephalopod-inspired multispectral organic sensor (CiMOS) based on geometric phase lenses (GPLs) and organic photovoltaics (OPVs) to enable aberration-based color sensing. We mimic the approach by which animals with single-type photoreceptors perceive colors via chromatic aberration. The intrinsic chromatic aberration of GPLs allows for multispectral sensing by stacking precisely patterned OPVs within specific spectrally dependent focal lengths. We provide simulations and a proof of concept of the CiMOS and highlight its advantages, including its simple design and snapshot multi-color detection using only a single axial position. Experimental results demonstrate the sensor’s ability to detect four colors with full width at half maximum spectral resolution as low as 35 nm.}, number={7}, journal={OPTICAL ENGINEERING}, author={Altaqui, Ali and Schrickx, Harry and Gyurek, Sydney and Sen, Pratik and Escuti, Michael and Brendan T. O'Connor and Kudenov, Michael}, year={2022}, month={Jul} }
 @article{liu_zheng_o'connor_dong_zhu_2022, title={Curvilinear soft electronics by micromolding of metal nanowires in capillaries}, volume={8}, ISSN={["2375-2548"]}, url={https://doi.org/10.1126/sciadv.add6996}, DOI={10.1126/sciadv.add6996}, abstractNote={
            Soft electronics using metal nanowires have attracted notable attention attributed to their high electrical conductivity and mechanical flexibility. However, high-resolution complex patterning of metal nanowires on curvilinear substrates remains a challenge. Here, a micromolding-based method is reported for scalable printing of metal nanowires, which enables complex and highly conductive patterns on soft curvilinear and uneven substrates with high resolution and uniformity. Printing resolution of 20 μm and conductivity of the printed patterns of ~6.3 × 10
            6
            S/m are achieved. Printing of grid structures with uniform thickness for transparent conductive electrodes (TCEs) and direct printing of pressure sensors on curved surfaces such as glove and contact lens are also realized. The printed hybrid soft TCEs and smart contact lens show promising applications in optoelectronic devices and personal health monitoring, respectively. This printing method can be extended to other nanomaterials for large-scale printing of high-performance soft electronics.
          }, number={46}, journal={SCIENCE ADVANCES}, author={Liu, Yuxuan and Zheng, Michael and O'Connor, Brendan and Dong, Jingyan and Zhu, Yong}, year={2022}, month={Nov} }
 @article{booth_khanna_schrickx_siddika_al shafe_brendan t. o'connor_2022, title={Electrothermally Actuated Semitransparent Shape Memory Polymer Composite with Application as a Wearable Touch Sensor}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.2c10290}, abstractNote={A semitransparent shape memory polymer (SMP):silver nanowire (AgNW) composite is demonstrated to be capable of low-temperature actuation, thus making it attractive for wearable electronics applications that require intimate contact with the human body. We demonstrate that the SMP:AgNW composite has tunable electrical and optical transparency through variation of the AgNW loading and that the AgNW loading did not significantly change the mechanical behavior of the SMP. The SMP composite is also capable of electrical actuation through Joule heating, where applying a 4 V bias across the AgNWs resulted in full shape recovery. The SMP was found to have high strain sensitivity at both small (<1%) and large (over 10%) applied strain. The SMP could sense strains as low as 0.6% with a gauge factor of 8.2. The SMP composite was then utilized as a touch sensor, able to sense and differentiate tapping and pressing. Finally, the composite was applied as a wearable ring that was thermally actuated to conformably fit onto a finger as a touch sensor. The ring sensor was able to sense finger tapping, pressing, and bending with high signal-to-noise ratios. These results demonstrate that SMP:AgNW composites are a promising design approach for application in wearable electronics.}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Booth, Ronald E. and Khanna, Chetna and Schrickx, Harry M. and Siddika, Salma and Al Shafe, Abdullah and Brendan T. O'Connor}, year={2022}, month={Nov} }
 @article{ravishankar_booth_hollingsworth_ade_sederoff_decarolis_brendan t. o'connor_2022, title={Organic solar powered greenhouse performance optimization and global economic opportunity}, volume={15}, ISSN={["1754-5706"]}, url={https://doi.org/10.1039/D1EE03474J}, DOI={10.1039/d1ee03474j}, abstractNote={This work integrates greenhouse energy demand, solar power production, and plant growth modeling to assess the economic opportunity of organic solar powered greenhouses. Results show these systems have positive economic outlook across broad climates.}, number={4}, journal={ENERGY & ENVIRONMENTAL SCIENCE}, publisher={Royal Society of Chemistry (RSC)}, author={Ravishankar, Eshwar and Booth, Ronald E. and Hollingsworth, Joseph A. and Ade, Harald and Sederoff, Heike and DeCarolis, Joseph F. and Brendan T. O'Connor}, year={2022}, month={Mar} }
 @article{booth_schrickx_hanby_liu_qin_ade_zhu_brendan t. o'connor_2022, title={Silver Nanowire Composite Electrode Enabling Highly Flexible, Robust Organic Photovoltaics}, volume={6}, ISSN={["2367-198X"]}, DOI={10.1002/solr.202200264}, abstractNote={Using Ag nanowires (NWs) is a promising approach to make flexible and transparent conducting electrodes for organic photovoltaics (OPVs). However, the roughness of the NWs can decrease device performance. Herein, a Ag NW electrode embedded in a UV‐curable epoxy that uses a simple mechanical lift‐off process resulting in highly planar electrodes is demonstrated. A bimodal blend of Ag NWs with varying aspect ratios is used to optimize the transparency and conductivity of the electrode. In addition, a ZnO layer is coated on the Ag NWs prior to the embedding process to ensure low contact resistance in the OPV cells. The resulting resin‐embedded ZnO‐encapsulated silver nanowire (REZEN) electrode is found to have excellent mechanical stability. REZEN electrode‐based OPV cells exhibit comparable performance with reference devices, achieving maximum power conversion efficiency (PCE) of 13.5% and 13.6% respectively. The REZEN‐based OPV cells are also mechanically robust, retaining 97% of their PCE after 5000 cycles at R = 1.2 mm and 94% PCE after 1000 cycles at R = 0.55 mm. This flexibility is among the highest reported for freestanding devices. Thus, the REZEN electrode is a promising and simple strategy to achieve mechanically robust ITO‐free flexible OPV cells.}, journal={SOLAR RRL}, author={Booth, Ronald E. and Schrickx, Harry M. and Hanby, Georgia and Liu, Yuxuan and Qin, Yunpeng and Ade, Harald and Zhu, Yong and Brendan T. O'Connor}, year={2022}, month={Jun} }
 @article{ghasemi_balar_peng_hu_qin_kim_rech_bidwell_mask_mcculloch_et al._2021, title={A molecular interaction-diffusion framework for predicting organic solar cell stability}, volume={20}, ISSN={["1476-4660"]}, DOI={10.1038/s41563-020-00872-6}, abstractNote={Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property-function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy E a  scales linearly with the enthalpic interaction parameters χ H  between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in E a  to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property-function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability.}, number={4}, journal={NATURE MATERIALS}, author={Ghasemi, Masoud and Balar, Nrup and Peng, Zhengxing and Hu, Huawei and Qin, Yunpeng and Kim, Taesoo and Rech, Jeromy J. and Bidwell, Matthew and Mask, Walker and McCulloch, Iain and et al.}, year={2021}, month={Apr}, pages={525-+} }
 @article{ren_liu_song_o'connor_dong_zhu_2021, title={Achieving High-Resolution Electrohydrodynamic Printing of Nanowires on Elastomeric Substrates through Surface Modification}, volume={3}, ISSN={["2637-6113"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85099220488&partnerID=MN8TOARS}, DOI={10.1021/acsaelm.0c00747}, abstractNote={Stretchable electronics based on nanomaterials has received much interest recently. However, it is challenging to print 1D nanomaterials (e.g., nanowires) with high resolution on stretchable elasto...}, number={1}, journal={ACS APPLIED ELECTRONIC MATERIALS}, publisher={American Chemical Society (ACS)}, author={Ren, Ping and Liu, Yuxuan and Song, Runqiao and O'Connor, Brendan and Dong, Jingyan and Zhu, Yong}, year={2021}, month={Jan}, pages={192–202} }
 @article{ravishankar_charles_xiong_henry_swift_rech_calero_cho_booth_kim_et al._2021, title={Balancing crop production and energy harvesting in organic solar-powered greenhouses}, volume={2}, ISSN={["2666-3864"]}, DOI={10.1016/j.xcrp.2021.100381}, abstractNote={Adding semitransparent organic solar cells (ST-OSCs) to a greenhouse structure enables simultaneous plant cultivation and electricity generation, thereby reducing the greenhouse energy demand. However, there is a need to establish the impact of such systems on plant growth and indoor climate and to optimize system tradeoffs. In this work, we consider plant growth under OSCs and system-relevant design. We evaluate the growth of red leaf lettuce under ST-OSC filters and compare the impact of three different OSC active layers that have unique transmittance. We find no significant differences in the fresh weight and chlorophyll content of the lettuce grown under these OSC filters. In addition, OSCs provide an opportunity for further light and thermal management of the greenhouse through device design and optical coatings. The OSCs can thus affect plant growth, power generation, and thermal load of the greenhouse, and this design trade space is reviewed and exemplified.}, number={3}, journal={CELL REPORTS PHYSICAL SCIENCE}, publisher={Elsevier BV}, author={Ravishankar, Eshwar and Charles, Melodi and Xiong, Yuan and Henry, Reece and Swift, Jennifer and Rech, Jeromy and Calero, John and Cho, Sam and Booth, Ronald E. and Kim, Taesoo and et al.}, year={2021}, month={Mar} }
 @article{altaqui_schrickx_sen_li_rech_lee_balar_you_kim_escuti_et al._2021, title={Bio-inspired spectropolarimetric sensor based on tandem organic photodetectors and multi-twist liquid crystals}, volume={29}, ISSN={["1094-4087"]}, url={https://doi.org/10.1364/OE.431858}, DOI={10.1364/OE.431858}, abstractNote={Simultaneous spectral and polarimetric imaging enables versatile detection and multimodal characterization of targets of interest. Current architectures incorporate a 2×2 pixel arrangement to acquire the full linear polarimetric information causing spatial sampling artifacts. Additionally, they suffer from limited spectral selectivity and high color crosstalk. Here, we demonstrate a bio-inspired spectral and polarization sensor structure based on integrating semitransparent polarization-sensitive organic photovoltaics (P-OPVs) and liquid crystal polymer (LCP) retarders in a tandem configuration. Color tuning is realized by leveraging the dynamic chromatic retardation control of LCP films, while polarization sensitivity is realized by exploiting the flexible anisotropic properties of P-OPVs. The structure is marked by its ultra-thin design and its ability to detect spectral and polarimetric contents along the same optical axis, thereby overcoming the inherent limitations associated with conventional division-of-focal plane sensors.}, number={26}, journal={OPTICS EXPRESS}, publisher={The Optical Society}, author={Altaqui, Ali and Schrickx, Harry and Sen, Pratik and Li, Lingshan and Rech, Jeromy and Lee, Jin-Woo and Balar, Nrup and You, Wei and Kim, Bumjoon J. and Escuti, Michael and et al.}, year={2021}, month={Dec}, pages={43953–43969} }
 @article{altaqui_sen_schrickx_rech_lee_escuti_you_kim_kolbas_brendan t. o'connor_et al._2021, title={Mantis shrimp-inspired organic photodetector for simultaneous hyperspectral and polarimetric imaging}, volume={7}, ISSN={["2375-2548"]}, url={https://doi.org/10.1126/sciadv.abe3196}, DOI={10.1126/sciadv.abe3196}, abstractNote={Semitransparent polarization-sensitive organic detectors reveal unprecedented degrees of freedom for multidimensional imaging.}, number={10}, journal={SCIENCE ADVANCES}, publisher={American Association for the Advancement of Science (AAAS)}, author={Altaqui, Ali and Sen, Pratik and Schrickx, Harry and Rech, Jeromy and Lee, Jin-Woo and Escuti, Michael and You, Wei and Kim, Bumjoon J. and Kolbas, Robert and Brendan T. O'Connor and et al.}, year={2021}, month={Mar} }
 @article{peng_jiang_qin_li_balar_brendan t. o'connor_ade_ye_geng_2021, title={Modulation of Morphological, Mechanical, and Photovoltaic Properties of Ternary Organic Photovoltaic Blends for Optimum Operation}, volume={11}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.202003506}, DOI={10.1002/aenm.202003506}, abstractNote={Abstract}, number={8}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Peng, Zhongxiang and Jiang, Kui and Qin, Yunpeng and Li, Miaomiao and Balar, Nrup and Brendan T. O'Connor and Ade, Harald and Ye, Long and Geng, Yanhou}, year={2021}, month={Feb} }
 @article{altaqui_kolbas_escuti_brendan t. o'connor_kudenov_2021, title={Organic-based photodetectors for multiband spectral imaging}, volume={60}, ISSN={["2155-3165"]}, DOI={10.1364/AO.417069}, abstractNote={Using organic photodetectors for multispectral sensing is attractive due to their unique capabilities to tune spectral response, transmittance, and polarization sensitivity. Existing methods lack tandem multicolor detection and exhibit high spectral cross talk. We exploit the polarization sensitivity of organic photodetectors, together with birefringent optical filters to design single-pixel multispectral detectors that achieve high spectral selectivity and good radiometric performance. Two different architectures are explored and optimized, including the Solc-based and multitwist-retarder-based organic photodetectors. Although the former demonstrated a higher spectral resolution, the latter enables a more compact sensor as well as greater flexibility in device fabrication.}, number={8}, journal={APPLIED OPTICS}, author={Altaqui, Ali and Kolbas, Robert M. and Escuti, Michael J. and Brendan T. O'Connor and Kudenov, Michael W.}, year={2021}, month={Mar}, pages={2314–2323} }
 @article{balar_rech_siddika_song_schrickx_sheikh_ye_bonilla_awartani_ade_et al._2021, title={Resolving the Molecular Origin of Mechanical Relaxations in Donor-Acceptor Polymer Semiconductors}, volume={32}, ISSN={["1616-3028"]}, url={https://doi.org/10.1002/adfm.202105597}, DOI={10.1002/adfm.202105597}, abstractNote={Abstract}, number={4}, journal={ADVANCED FUNCTIONAL MATERIALS}, publisher={Wiley}, author={Balar, Nrup and Rech, Jeromy James and Siddika, Salma and Song, Runqiao and Schrickx, Harry M. and Sheikh, Nadeem and Ye, Long and Bonilla, Anthony Megret and Awartani, Omar and Ade, Harald and et al.}, year={2021}, month={Oct} }
 @article{son_samson_siddika_brendan t. o'connor_you_2021, title={Thermocleavage of Partial Side Chains in Polythiophenes Offers Appreciable Photovoltaic Efficiency and Significant Morphological Stability}, volume={33}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.1c01305}, abstractNote={The intrinsic degradation of conjugated polymer (CP) based solar cells (PSCs) due to morphological change by heat is an outstanding challenge. Increasing the glass transition temperature (Tg) of the materials used in PSCs can largely mitigate the thermal instability, yet most CPs used in high-efficiency PSCs only show low Tg values, mainly due to the long and bulky side chains needed for solution processing of such polymers. Thermally removing cleavable side chains is an effective approach to regain the high Tg of CPs after the film formation, thereby achieving higher stability; however, previous results using polythiophenes only achieved moderate efficiency (0.8% with PC61BM) after a high temperature (300 °C) treatment to remove all side chains. To better understand and utilize thermocleavable side chains (TCSs), we explore a series of regioregular polythiophenes having TCSs and hexyl side chains by varying the ratio of different side chains, from 0 mol % TCSs to 100 mol % TCSs at an increment of 20 mol %. Through a systematic investigation, we find that the polymers with more TCSs than hexyl side chains exhibit sufficient stability under a rather harsh condition (100 °C, in air and under continuous ambient light). While a complete removal of alkyl chains might offer a higher stability, the device efficiency was very low (∼0.14%); by contrast, the polymer having ∼70 mol % of TCSs achieved the highest efficiency (∼1.5%) after alkyl chain cleavage at 200 °C and significant morphological stability. Under our stability test (150 °C, 24 h and ambient light), these specific polymer:PC61BM based solar cells were able to retain 90% of the original efficiency. These key findings, together with mechanistic understanding of the thermocleavage process, provide valuable insight into the impact of TCS and present a new design rationale to achieve PSCs with both high efficiency and improved stability.}, number={12}, journal={CHEMISTRY OF MATERIALS}, author={Son, Sung Yun and Samson, Stephanie and Siddika, Salma and Brendan T. O'Connor and You, Wei}, year={2021}, month={Jun}, pages={4745–4756} }
 @article{schrickx_sen_booth_altaqui_burleson_rech_lee_biliroglu_gundogdu_kim_et al._2021, title={Ultra-High Alignment of Polymer Semiconductor Blends Enabling Photodetectors with Exceptional Polarization Sensitivity}, volume={10}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202105820}, abstractNote={Abstract}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Schrickx, Harry M. and Sen, Pratik and Booth, Ronald E. and Altaqui, Ali and Burleson, Jacob and Rech, Jeromy J. and Lee, Jin-Woo and Biliroglu, Melike and Gundogdu, Kenan and Kim, Bumjoon J. and et al.}, year={2021}, month={Oct} }
 @article{ravishankar_booth_saravitz_sederoff_ade_brendan t. o'connor_2020, title={Achieving Net Zero Energy Greenhouses by Integrating Semitransparent Organic Solar Cells}, volume={4}, ISSN={["2542-4351"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85079138223&partnerID=MN8TOARS}, DOI={10.1016/j.joule.2019.12.018}, abstractNote={Greenhouses vastly increase agricultural land-use efficiency. However, they also consume significantly more energy than conventional farming due in part to conditioning the greenhouse space. One way to mitigate the increase in energy consumption is to integrate solar modules onto the greenhouse structure. Semitransparent organic solar cells (OSCs) are particularly attractive given that their spectral absorption can be tuned to minimize the attenuation of sunlight over the plants photosynthetically active spectrum. Here, the benefits of integrating OSCs on the net energy demand of greenhouses within the U.S. are determined through a detailed energy balance model. We find that these systems can have an annual surplus of energy in warm and moderate climates. Furthermore, we show that sunlight reduction entering the greenhouse can be minimized with appropriate design. These results demonstrate that OSCs are an excellent candidate for implementing in greenhouses and provide an opportunity to diversify sustainable energy generation technology.}, number={2}, journal={JOULE}, author={Ravishankar, Eshwar and Booth, Ronald E. and Saravitz, Carole and Sederoff, Heike and Ade, Harald W. and Brendan T. O'Connor}, year={2020}, month={Feb}, pages={490–506} }
 @article{hollingsworth_ravishankar_o'connor_johnson_decarolis_2020, title={Environmental and economic impacts of solar-powered integrated greenhouses}, volume={24}, ISSN={["1530-9290"]}, DOI={10.1111/jiec.12934}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF INDUSTRIAL ECOLOGY}, author={Hollingsworth, Joseph A. and Ravishankar, Eshwar and O'Connor, Brendan and Johnson, Jeremiah X. and DeCarolis, Joseph F.}, year={2020}, month={Feb} }
 @article{song_yao_liu_wang_dong_zhu_brendan t. o'connor_2020, title={Facile Approach to Fabricating Stretchable Organic Transistors with Laser-Patterned Ag Nanowire Electrodes}, volume={12}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.0c15339}, DOI={10.1021/acsami.0c15339}, abstractNote={Stretchable electronics are poised to revolutionize personal healthcare and robotics, where they enable distributed and conformal sensors. Transistors are fundamental building blocks of electronics, and there is a need to produce stretchable transistors using low-cost and scalable fabrication techniques. Here, we introduce a facile fabrication approach using laser patterning and transfer printing to achieve high-performance, solution-processed intrinsically stretchable organic thin-film transistors (OTFTs). The device consists of Ag nanowire (NW) electrodes, where the source and drain electrodes are patterned using laser ablation. The Ag NWs are then partially embedded in a poly(dimethylsiloxane) (PDMS) matrix. The electrodes are combined with a PDMS dielectric and polymer semiconductor, where the layers are individually transfer printed to complete the OTFT. Two polymer semiconductors, DPP-DTT and DPP-4T, are considered and show stable operation under the cyclic strain of 20 and 40%, respectively. The OTFTs maintain electrical performance by adopting a buckled structure after the first stretch-release cycle. The conformability and stretchability of the OTFT is also demonstrated by operating the transistor while adhered to a finger being flexed. The ability to pattern highly conductive Ag NW networks using laser ablation to pattern electrodes as well as interconnects provides a simple strategy to produce complex stretchable OTFT-based circuits.}, number={45}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Song, Runqiao and Yao, Shanshan and Liu, Yuxuan and Wang, Hongyu and Dong, Jingyan and Zhu, Yong and Brendan T. O'Connor}, year={2020}, month={Nov}, pages={50675–50683} }
 @article{yang_sen_brendan t. o'connor_kudenov_2020, title={Optical crosstalk and off-axis modeling of an intrinsic coincident polarimeter}, volume={59}, ISSN={["2155-3165"]}, DOI={10.1364/AO.59.000156}, abstractNote={Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure the emitted, reflected, or transmitted state of polarization. An intrinsic coincident (IC) full-Stokes polarimeter was previously demonstrated by our group, in a free space configuration, by using stain-aligned polymer-based organic photovoltaics. To minimize the model’s complexity, these were tilted to avoid crosstalk from back-reflections. We present a theoretical model of a monolithic IC polarimeter that considers the back-reflection’s influence for on-axis light. The model was validated using a monolithic four-detector polarimeter, which achieved an error of less than 3%. Additionally, an off-axis model was produced and validated for a simpler two detector polarimeter, demonstrating an error between the TM and TE polarized components of less than 3% for angles spanning an 18° incidence cone.}, number={1}, journal={APPLIED OPTICS}, author={Yang, Ruonan and Sen, Pratik and Brendan T. O'Connor and Kudenov, Michael W.}, year={2020}, month={Jan}, pages={156–164} }
 @article{yang_sen_o'connor_kudenov_2020, title={Optimization of an intrinsic coincident polarimeter and quantitative architectural comparison of different polarimeter techniques}, volume={59}, ISSN={["1560-2303"]}, DOI={10.1117/1.OE.59.2.024111}, abstractNote={Abstract. Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure a scene’s polarization state. An intrinsic coincident (IC) full-Stokes polarimeter was previously demonstrated and optimized to achieve high temporal and spatial resolution. We optimized the IC polarimeter by introducing additional waveplates or measurement channels and compared it with existing polarimeter architectures under signal-independent Gaussian noise and signal-dependent Poisson noise. The quantitative comparison of noise variances showed that the IC and division-of-amplitude polarimeters have the lowest noise variances due to their higher signal collection ability. Both polarimeters have a factor of 2 and 2 improved signal-to-noise ratio, in the S0 component, for Gaussian and Poisson noises, respectively, as compared to division of time, division of focal plane, and division of aperture polarimeters. While the division of amplitude and IC polarimeters outperforms other approaches, the IC polarimeter has a significantly simpler design, potentially allowing for cost-effective, high-performance polarimetric imaging.}, number={2}, journal={OPTICAL ENGINEERING}, author={Yang, Ruonan and Sen, Pratik and O'Connor, Brendan and Kudenov, Michael}, year={2020}, month={Feb} }
 @article{carpenter_ghasemi_gann_angunawela_stuard_rech_ritchie_brendan t. o'connor_atkin_you_et al._2019, title={Competition between Exceptionally Long-Range Alkyl Sidechain Ordering and Backbone Ordering in Semiconducting Polymers and Its Impact on Electronic and Optoelectronic Properties}, volume={29}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201806977}, abstractNote={Abstract}, number={5}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Carpenter, Joshua H. and Ghasemi, Masoud and Gann, Eliot and Angunawela, Indunil and Stuard, Samuel J. and Rech, Jeromy James and Ritchie, Earl and Brendan T. O'Connor and Atkin, Joanna and You, Wei and et al.}, year={2019}, month={Feb} }
 @article{hu_ye_ghasemi_balar_rech_stuard_you_brendan t. o'connor_ade_2019, title={Highly Efficient, Stable, and Ductile Ternary Nonfullerene Organic Solar Cells from a Two-Donor Polymer Blend}, volume={31}, ISSN={["1521-4095"]}, url={https://publons.com/wos-op/publon/18518240/}, DOI={10.1002/adma.201808279}, abstractNote={Abstract}, number={17}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Hu, Huawei and Ye, Long and Ghasemi, Masoud and Balar, Nrup and Rech, Jeromy James and Stuard, Samuel J. and You, Wei and Brendan T. O'Connor and Ade, Harald}, year={2019}, month={Apr} }
 @article{sun_song_balar_sen_kline_brendan t. o'connor_2019, title={Impact of Substrate Characteristics on Stretchable Polymer Semiconductor Behavior}, volume={11}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.8b16457}, abstractNote={Stretchable conductive polymer films are required to survive not only large tensile strain but also stay functional after the reduction in applied strain. In the deformation process, the elastomer substrate that is typically employed plays a critical role in response to the polymer film. In this study, we examine the role of a polydimethylsiloxane (PDMS) elastomer substrate on the ability to achieve stretchable PDPP-4T films. In particular, we consider the adhesion and near-surface modulus of the PDMS tuned through UV/ozone (UVO) treatment on the competition between film wrinkling and plastic deformation. We also consider the role of PDMS tension on the stability of films under cyclic strain. We find that increasing the near-surface modulus of the PDMS and maintaining the PDMS in tension throughout the cyclic strain process promote plastic deformation over film wrinkling. In addition, the UVO treatment increases film adhesion to the PDMS resulting in a significantly reduced film folding and delamination. For a 20 min UVO-treated PDMS, we show that a PDPP-4T film root-mean-square roughness is consistently below 3 nm for up to 100 strain cycles with a strain range of 40%. In addition, although the film is plastically deforming, the microstructural order is largely stable as probed by grazing incidence X-ray scattering and UV-visible spectroscopy. These results highlight the importance of neighboring elastomer characteristics on the ability to achieve stretchable polymer semiconductors.}, number={3}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Sun, Tianlei and Song, Runqiao and Balar, Nrup and Sen, Pratik and Kline, R. Joseph and Brendan T. O'Connor}, year={2019}, month={Jan}, pages={3280–3289} }
 @article{yao_ren_song_liu_huang_dong_o'connor_zhu_2019, title={Nanomaterial‐Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications}, volume={32}, ISSN={0935-9648 1521-4095}, url={http://dx.doi.org/10.1002/adma.201902343}, DOI={10.1002/adma.201902343}, abstractNote={Abstract}, number={15}, journal={Advanced Materials}, publisher={Wiley}, author={Yao, Shanshan and Ren, Ping and Song, Runqiao and Liu, Yuxuan and Huang, Qijin and Dong, Jingyan and O'Connor, Brendan T. and Zhu, Yong}, year={2019}, month={Aug}, pages={1902343} }
 @article{sen_yang_rech_feng_ho_huang_so_kline_you_kudenov_et al._2019, title={Panchromatic All-Polymer Photodetector with Tunable Polarization Sensitivity}, volume={7}, ISSN={["2195-1071"]}, DOI={10.1002/adom.201801346}, abstractNote={Abstract}, number={4}, journal={ADVANCED OPTICAL MATERIALS}, author={Sen, Pratik and Yang, Ruonan and Rech, Jeromy J. and Feng, Yuanxiang and Ho, Carr Hoi Yi and Huang, Jinsong and So, Franky and Kline, R. Joseph and You, Wei and Kudenov, Michael W. and et al.}, year={2019}, month={Feb} }
 @article{ye_xiong_chen_zhang_fei_henry_heeney_o’connor_you_ade_et al._2019, title={Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells}, volume={31}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.201808153}, DOI={10.1002/adma.201808153}, abstractNote={Abstract}, number={17}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Ye, Long and Xiong, Yuan and Chen, Zheng and Zhang, Qianqian and Fei, Zhuping and Henry, Reece and Heeney, Martin and O’Connor, Brendan T. and You, Wei and Ade, Harald and et al.}, year={2019}, month={Apr} }
 @article{balar_rech_henry_ye_ade_you_o’connor_2019, title={The Importance of Entanglements in Optimizing the Mechanical and Electrical Performance of All-Polymer Solar Cells}, volume={31}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.9b01011}, DOI={10.1021/acs.chemmater.9b01011}, abstractNote={Organic solar cells that have all-polymer active layers may have several advantages compared with polymer–small molecule systems including improved mechanical and thermodynamic stability; however, ...}, number={14}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Balar, Nrup and Rech, Jeromy James and Henry, Reece and Ye, Long and Ade, Harald and You, Wei and O’Connor, Brendan T.}, year={2019}, month={Jun}, pages={5124–5132} }
 @article{min_chen_chen_sun_lee_li_zhu_o'connor_parsons_chang_2018, title={Atomic Layer Deposition: Conformal Physical Vapor Deposition Assisted by Atomic Layer Deposition and Its Application for Stretchable Conductors (Adv. Mater. Interfaces 22/2018)}, volume={5}, ISSN={2196-7350 2196-7350}, url={http://dx.doi.org/10.1002/ADMI.201870109}, DOI={10.1002/ADMI.201870109}, abstractNote={Physical vapor deposition of thin films on nanostructure topography is typically limited by line-of-sight transfer. This study demonstrates that by introducing an oxide interlayer using atomic layer deposition, the film uniformity and electrical properties can be improved. Enabling conformal coating at low cost, this method can find applications in manufacturing of integrated circuit and stretchable electronics. More details can be found in article number 1801379 by Joong-Hee Min, Chih-Hao Chang, and co-workers.}, number={22}, journal={Advanced Materials Interfaces}, publisher={Wiley}, author={Min, Joong‐Hee and Chen, Yi‐An and Chen, I‐Te and Sun, Tianlei and Lee, Dennis T. and Li, Chengjun and Zhu, Yong and O'Connor, Brendan T. and Parsons, Gregory N. and Chang, Chih‐Hao}, year={2018}, month={Nov}, pages={1870109} }
 @article{min_chen_chen_sun_lee_li_zhu_brendan t. o'connor_parsons_chang_2018, title={Conformal Physical Vapor Deposition Assisted by Atomic Layer Deposition and Its Application for Stretchable Conductors}, volume={5}, ISSN={["2196-7350"]}, DOI={10.1002/admi.201801379}, abstractNote={Abstract}, number={22}, journal={ADVANCED MATERIALS INTERFACES}, author={Min, Joong-Hee and Chen, Yi-An and Chen, I-Te and Sun, Tianlei and Lee, Dennis T. and Li, Chengjun and Zhu, Yong and Brendan T. O'Connor and Parsons, Gregory N. and Chang, Chih-Hao}, year={2018}, month={Nov} }
 @article{yang_sen_brendan t. o'connor_kudenov_2018, title={Monolithic Intrinsic Coincident Polarimeter using Organic Photovoltaics}, volume={10655}, ISSN={["1996-756X"]}, DOI={10.1117/12.2500476}, abstractNote={Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure the emitted, reflected, or transmitted state of polarization (SOP). An Intrinsic Coincident full-Stokes Polarimeter (ICP) was previously demonstrated by our group, in a free space configuration, by using stain-aligned polymer-based organic photovoltaics (OPVs). These were tilted to avoid back-reflection cross-talk. In this paper, we present a theoretical model of a monolithic ICP which considers the back-reflection’s influence. This includes a comparison between the free space model to the new monolithic model. Experimental demonstrations yield less than 3% error between our model and the experiment data.}, journal={POLARIZATION: MEASUREMENT, ANALYSIS, AND REMOTE SENSING XIII}, author={Yang, Ruonan and Sen, Pratik and Brendan T. O'Connor and Kudenov, Michael W.}, year={2018} }
 @article{sen_xiong_zhang_park_you_ade_kudenov_brendan t. o'connor_2018, title={Shear-Enhanced Transfer Printing of Conducting Polymer Thin Films}, volume={10}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.8b09968}, abstractNote={Polymer conductors that are solution-processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune the adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp, enabling large-area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high-performance semitransparent organic solar cells.}, number={37}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Sen, Pratik and Xiong, Yuan and Zhang, Qanqian and Park, Sungjune and You, Wei and Ade, Harald and Kudenov, Michael W. and Brendan T. O'Connor}, year={2018}, month={Sep}, pages={31560–31567} }
 @article{balar_brendan t. o'connor_2017, title={Correlating Crack Onset Strain and Cohesive Fracture Energy in Polymer Semiconductor Films}, volume={50}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.7b01282}, abstractNote={Polymer semiconductors are an attractive material system for flexible and stretchable electronic devices owing to their potentially favorable mechanical attributes. Establishing the thermomechanical behavior of polymer semiconductors is thus an important consideration to ensure successful operation in these applications. One of the most common mechanical characterization methods for these materials is to manipulate the thin films while on an elastomer substrate. A primary measurement with this approach is the film’s crack onset strain (COS), a measure of ductility. It is simple and effective; however, it is a highly qualitative view of film mechanical stability, particularly in flexible device applications. Alternatively, cohesive fracture energy (Gc) provides a direct quantitative measure of the mechanical integrity of the film. While fracture energy provides important insight into mechanical stability, it typically requires a more complex measurement method than the film on elastomer tests. Here, we com...}, number={21}, journal={MACROMOLECULES}, author={Balar, Nrup and Brendan T. O'Connor}, year={2017}, month={Nov}, pages={8611–8618} }
 @article{yu_chung_shewmon_ho_carpenter_larrabee_sun_jones_ade_o'connor_et al._2017, title={Flexible Inorganic Ferroelectric Thin Films for Nonvolatile Memory Devices}, volume={27}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201700461}, DOI={10.1002/ADFM.201700461}, abstractNote={Next‐generation wearable electronics call for flexible nonvolatile devices for ubiquitous data storage. Thus far, only organic ferroelectric materials have shown intrinsic flexibility and processability on plastic substrates. Here, it is shown that by controlling the heating rate, ferroelectric hafnia films can be grown on plastic substrates. The resulting highly flexible capacitor with a film thickness of 30 nm yields a remnant polarization of 10 µC cm−2. Bending tests show that the film ferroelectricity can be retained under a bending radius below 8 mm with up to 1000 bending cycles. The excellent flexibility is due to the extremely thin hafnia film thickness. Using the ferroelectric film as a gate insulator, a low voltage nonvolatile vertical organic transistor is demonstrated on a plastic substrate with an extrapolated date retention time of up to 10 years.}, number={21}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Yu, Hyeonggeun and Chung, Ching-Chang and Shewmon, Nate and Ho, Szuheng and Carpenter, Joshua H. and Larrabee, Ryan and Sun, Tianlei and Jones, Jacob L. and Ade, Harald and O'Connor, Brendan T. and et al.}, year={2017}, month={Apr}, pages={1700461} }
 @article{yang_sen_o'connor_kudenov_2017, title={Intrinsic coincident full-Stokes polarimeter using stacked organic photovoltaics}, volume={56}, ISSN={["2155-3165"]}, DOI={10.1364/ao.56.001768}, abstractNote={An intrinsic coincident full-Stokes polarimeter is demonstrated by using strain-aligned polymer-based organic photovoltaics (OPVs) that can preferentially absorb certain polarized states of incident light. The photovoltaic-based polarimeter is capable of measuring four Stokes parameters by cascading four semitransparent OPVs in series along the same optical axis. This in-line polarimeter concept potentially ensures high temporal and spatial resolution with higher radiometric efficiency as compared to the existing polarimeter architecture. Two wave plates were incorporated into the system to modulate the S3 Stokes parameter so as to reduce the condition number of the measurement matrix and maximize the measured signal-to-noise ratio. Radiometric calibration was carried out to determine the measurement matrix. The polarimeter presented in this paper demonstrated an average RMS error of 0.84% for reconstructed Stokes vectors after normalized to S0. A theoretical analysis of the minimum condition number of the four-cell OPV design showed that for individually optimized OPV cells, a condition number of 2.4 is possible.}, number={6}, journal={APPLIED OPTICS}, author={Yang, Ruonan and Sen, Pratik and O'Connor, B. T. and Kudenov, M. W.}, year={2017}, month={Feb}, pages={1768–1774} }
 @article{yang_sen_o'connor_kudenov_2017, title={Intrinsic coincident full-Stokes polarimeter using stacked organic photovoltaics and architectural comparison of polarimeter techniques}, volume={10407}, ISBN={["978-1-5106-1271-6"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2273885}, abstractNote={An intrinsic coincident full-Stokes polarimeter is demonstrated by using stain-aligned polymer-based organic photovoltaics (OPVs) which can preferentially absorb certain polarized states of incident light. The photovoltaic-based polarimeter is capable of measuring four stokes parameters by cascading four semitransparent OPVs in series along the same optical axis. Two wave plates were incorporated into the system to modulate the S3 stokes parameter so as to reduce the condition number of the measurement matrix. The model for the full-Stokes polarimeter was established and validated, demonstrating an average RMS error of 0.84%. The optimization, based on minimizing the condition number of the 4-cell OPV design, showed that a condition number of 2.4 is possible. Performance of this in-line polarimeter concept was compared to other polarimeter architectures, including Division of Time (DoT), Division of Amplitude (DoAm), Division of Focal Plane (DoFP), and Division of Aperture (DoA) from signal-to-noise ratio (SNR) perspective. This in-line polarimeter concept has the potential to enable both high temporal (as compared with a DoT polarimeter) and high spatial resolution (as compared with DoFP and DoA polarimeters). We conclude that the intrinsic design has the same ~√2 SNR advantage as the DoAm polarimeter, but with greater compactness.}, journal={POLARIZATION SCIENCE AND REMOTE SENSING VIII}, author={Yang, Ruonan and Sen, Pratik and O'Connor, B. T. and Kudenov, M. W.}, year={2017} }
 @article{brendan t. o'connor_awartani_balar_2017, title={Morphological considerations of organic electronic films for flexible and stretchable devices}, volume={42}, ISSN={["1938-1425"]}, DOI={10.1557/mrs.2017.6}, abstractNote={Abstract}, number={2}, journal={MRS BULLETIN}, author={Brendan T. O'Connor and Awartani, Omar M. and Balar, Nrup}, year={2017}, month={Feb}, pages={108–114} }
 @article{moshonas_stathopoulos_o'connor_bedeloglu_savaidis_vasiliadis_2017, title={Optical modeling of fiber organic photovoltaic structures using a transmission line method}, volume={56}, ISSN={["2155-3165"]}, DOI={10.1364/ao.56.009351}, abstractNote={An optical model has been developed and evaluated for the calculation of the external quantum efficiency of cylindrical fiber photovoltaic structures. The model is based on the transmission line theory and has been applied on single and bulk heterojunction fiber-photovoltaic cells. Using this model, optimum design characteristics have been proposed for both configurations, and comparison with experimental results has been assessed.}, number={34}, journal={APPLIED OPTICS}, author={Moshonas, N. and Stathopoulos, N. A. and O'Connor, B. T. and Bedeloglu, A. Celik and Savaidis, S. P. and Vasiliadis, S.}, year={2017}, month={Dec}, pages={9351–9358} }
 @article{sun_scott_wang_kline_bazan_brendan t. o'connor_2017, title={Plastic Deformation of Polymer Blends as a Means to Achieve Stretchable Organic Transistors}, volume={3}, ISSN={["2199-160X"]}, DOI={10.1002/aelm.201600388}, abstractNote={Intrinsically stretchable semiconductors will facilitate the realization of seamlessly integrated stretchable electronics. In this study, a new approach to achieve intrinsically stretchable semiconductors is introduced by blending a rigid high‐performance donor–acceptor polymer semiconductor poly[4‐(4,4‐dihexadecyl‐4H‐cyclopenta[1,2‐b:5,4‐b′]dithiopen‐2‐yl)‐alt[1,2,5]thiadiazolo[3,4‐c]pyridine] with a ductile polymer semiconductor poly(3‐hexylthiophene). Under large tensile strains of up to 75%, the polymers are shown to orient in the direction of strain, and when the strain is reduced, the polymers reversibly deform. During cyclic strain, the local packing order of the polymers is shown to be remarkably stable. The saturated field effect charge mobility is shown to be consistently above 0.04 cm2 V−1 s−1 for up to 100 strain cycles with strain ranging from 10% to 75% when the film is printed onto a rigid test bed. At the 75% strain state, the charge mobility is consistently above 0.15 cm2 V−1 s−1. Ultimately, the polymer blend process introduced here results in an excellent combination of device performance and stretchability providing an effective approach to achieve intrinsically stretchable semiconductors.}, number={1}, journal={ADVANCED ELECTRONIC MATERIALS}, author={Sun, Tianlei and Scott, Joshua I. and Wang, Ming and Kline, Regis Joseph and Bazan, Guillermo C. and Brendan T. O'Connor}, year={2017}, month={Jan} }
 @article{ye_xiong_li_ghasemi_balar_turner_gadisa_hou_o’connor_ade_et al._2017, title={Precise Manipulation of Multilength Scale Morphology and Its Influence on Eco-Friendly Printed All-Polymer Solar Cells}, volume={27}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201702016}, DOI={10.1002/adfm.201702016}, abstractNote={Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all‐polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all‐polymer bulk‐heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade‐coated films. Resonant soft X‐ray scattering is used to elucidate the multiphasic morphology of these printed all‐polymeric films and complements with the use of grazing incidence wide‐angle X‐ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive‐free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.}, number={33}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Ye, Long and Xiong, Yuan and Li, Sunsun and Ghasemi, Masoud and Balar, Nrup and Turner, Johnathan and Gadisa, Abay and Hou, Jianhui and O’Connor, Brendan T. and Ade, Harald and et al.}, year={2017}, month={Jul}, pages={1702016} }
 @article{balar_xiong_ye_li_nevola_dougherty_hou_ade_o’connor_2017, title={Role of Polymer Segregation on the Mechanical Behavior of All-Polymer Solar Cell Active Layers}, volume={9}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/ACSAMI.7B13719}, DOI={10.1021/acsami.7b13719}, abstractNote={An all-polymer bulk heterojunction (BHJ) active layer that removes the use of commonly used small molecule electron acceptors is a promising approach to improve the thermomechanical behavior of organic solar cells. However, there has been limited research on their mechanical properties. Here, we report on the mechanical behavior of high-performance blade-coated all-polymer BHJ films cast using eco-friendly solvents. The mechanical properties considered include the elastic modulus, crack onset strain, and cohesive fracture energy. We show that the mechanical behavior of the blend is largely unaffected by significant changes in the segregation characteristics of the polymers, which was varied systematically through solvent formulation. In comparison to a polymer:fullerene BHJ counterpart, the all-polymer films were found to have lower stiffness and increased ductility. Yet, the fracture energy of the all-polymer films is not significantly improved compared to that of the polymer:fullerene films. This study highlights that improved mechanical behavior of all-polymer systems cannot be assumed, and that details of the molecular structure, molecular weight, and film morphology play an important role in both the optoelectronic and mechanical properties. Furthermore, we show that simple composite modeling provides a predictive tool for the mechanical properties of the polymer blend films, providing a framework to guide future optimization of the mechanical behavior.}, number={50}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Balar, Nrup and Xiong, Yuan and Ye, Long and Li, Sunsun and Nevola, Daniel and Dougherty, Daniel B. and Hou, Jianhui and Ade, Harald and O’Connor, Brendan T.}, year={2017}, month={Dec}, pages={43886–43892} }
 @article{kim_gadisa_schaefer_yao_gautam_balar_ghasemi_constantinou_so_o'connor_et al._2017, title={Strong polymer molecular weight-dependent material interactions: impact on the formation of the polymer/fullerene bulk heterojunction morphology}, volume={5}, ISSN={2050-7488 2050-7496}, url={http://dx.doi.org/10.1039/C7TA03052E}, DOI={10.1039/c7ta03052e}, abstractNote={The morphological evolution is initiated by L–L or L–S phase separation (left) and further developed by molecular mobility, governed by polymer–solvent interactions which determine the final domain size of the BHJ layer (right).}, number={25}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={Kim, Joo-Hyun and Gadisa, Abay and Schaefer, Charley and Yao, Huifeng and Gautam, Bhoj R. and Balar, Nrup and Ghasemi, Masoud and Constantinou, Iordania and So, Franky and O'Connor, Brendan T. and et al.}, year={2017}, pages={13176–13188} }
 @article{hoffman_mcafee_pazoki_apperson_brendan t. o'connor_dougherty_2017, title={Temperature controlled interlayer disorder in ultrathin films of alpha-sexithiophene}, volume={642}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2017.09.011}, abstractNote={The surface potential difference between the first and second layer of α-sexithiophene (6T) films on silane passivated SiO2 is observed using Kelvin Probe Force Microscopy. The relative surface potential between the first two layers changes sign depending on whether the films are grown at 70 °C or 120 °C substrate temperature. Complementary wide angle X-ray scattering observations show that this difference can be interpreted as the result of a higher concentration of interlayer hole traps due to poor out-of-plane ordering in films deposited at lower temperature. Molecular sliding and shifts in tilt angle are proposed as the microscopic origin of out-of-plane disorder leading to trapped charges.}, journal={THIN SOLID FILMS}, author={Hoffman, Benjamin C. and Mcafee, Terry and Pazoki, Sara and Apperson, Aubrey and Brendan T. O'Connor and Dougherty, Daniel B.}, year={2017}, month={Nov}, pages={182–187} }
 @article{zhao_awartani_o'connor_zikry_2016, title={A direct correlation of x-ray diffraction orientation distributions to the in-plane stiffness of semi-crystalline organic semiconducting films}, volume={108}, ISSN={["1077-3118"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84968866659&partnerID=MN8TOARS}, DOI={10.1063/1.4948533}, abstractNote={Large charge mobilities of semi-crystalline organic semiconducting films could be obtained by mechanically aligning the material phases of the film with the loading axis. A key element is to utilize the inherent stiffness of the material for optimal or desired alignment. However, experimentally determining the moduli of semi-crystalline organic thin films for different loading directions is difficult, if not impossible, due to film thickness and material anisotropy. In this paper, we address these challenges by presenting an approach based on combining a composite mechanics stiffness orientation formulation with a Gaussian statistical distribution to directly estimate the in-plane stiffness (transverse isotropy) of aligned semi-crystalline polymer films based on crystalline orientation distributions obtained by X-ray diffraction experimentally at different applied strains. Our predicted results indicate that the in-plane stiffness of an annealing film was initially isotropic, and then it evolved to transverse isotropy with increasing mechanical strains. This study underscores the significance of accounting for the crystalline orientation distributions of the film to obtain an accurate understanding and prediction of the elastic anisotropy of semi-crystalline polymer films.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Zhao, Bingxiao and Awartani, Omar and O'Connor, Brendan and Zikry, Mohammed A.}, year={2016}, month={May} }
 @article{awartani_zhao_currie_kline_zikry_brendan t. o'connor_2016, title={Anisotropic Elastic Modulus of Oriented Regioregular Poly(3-hexylthiophene) Films}, volume={49}, ISSN={["1520-5835"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954288748&partnerID=MN8TOARS}, DOI={10.1021/acs.macromol.5b02680}, abstractNote={Specific morphological features of polymer semiconductors are often promoted in devices to optimize optoelectronic behavior. Less studied is the role of morphology on the mechanical properties of the film, such as elastic modulus, which is an important property for the development of flexible and stretchable devices. To gain insight into the morphological origin of elasticity in polymer semiconductors and its relationship to charge transport, we study the anisotropic in-plane elastic modulus of strain-aligned regioregular poly(3-hexylthiophene) (P3HT) films and compare the results to previously measured field effect charge mobility. The film morphology is varied through the amount of applied strain and post strain thermal annealing. Morphological characterization includes UV–vis optical spectroscopy and X-ray diffraction. The elastic modulus is measured using a buckling-based measurement technique. The elastic modulus of the film is found to decrease as the film is plastically strained. Thermally annealin...}, number={1}, journal={MACROMOLECULES}, author={Awartani, Omar M. and Zhao, Bingxiao and Currie, Tyler and Kline, R. Joseph and Zikry, Mohammed A. and Brendan T. O'Connor}, year={2016}, month={Jan}, pages={327–333} }
 @article{ye_xiong_yao_dinku_zhang_li_ghasemi_balar_hunt_o'connor_et al._2016, title={High Performance Organic Solar Cells Processed by Blade Coating in Air from a Benign Food Additive Solution}, volume={28}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/ACS.CHEMMATER.6B03083}, DOI={10.1021/acs.chemmater.6b03083}, abstractNote={Solution processable conjugated organic materials have gained tremendous interest motivated by their potential of low cost, lightweight and especially easy manufacturing of large-area and flexible electronics. Toxic halogen-containing solvents have been widely used in the processing of organic electronics, particularly organic photovoltaics (OPVs). To transition this technology to more commercially attractive manufacturing approaches, removing these halogenated solvents remains one of the key challenges. Our morphological (hard/soft X-ray scattering) and calorimetric characterizations reveal that using o-methylanisole, a certified food additive, as processing solvent can achieve similar crystalline properties and domain spacing/purity with that achieved by widely used binary halogenated solvents (chlorobenzene and 1,8-diiodooctane), thus yielding comparable photovoltaic performance in spin-casted films. To move a step forward, we further present the potential of o-methylanisole as processing solvent in th...}, number={20}, journal={Chemistry of Materials}, publisher={Link}, author={Ye, L. and Xiong, Y. and Yao, H. and Dinku, A.G. and Zhang, H. and Li, S. and Ghasemi, M. and Balar, N. and Hunt, A. and O'Connor, B.T. and et al.}, year={2016}, pages={7451–7458} }
 @article{roy_awartani_sen_o'connor_kudenov_2016, title={Intrinsic coincident linear polarimetry using stacked organic photovoltaics}, volume={24}, ISSN={["1094-4087"]}, DOI={10.1364/oe.24.014737}, abstractNote={Polarimetry has widespread applications within atmospheric sensing, telecommunications, biomedical imaging, and target detection. Several existing methods of imaging polarimetry trade off the sensor's spatial resolution for polarimetric resolution, and often have some form of spatial registration error. To mitigate these issues, we have developed a system using oriented polymer-based organic photovoltaics (OPVs) that can preferentially absorb linearly polarized light. Additionally, the OPV cells can be made semitransparent, enabling multiple detectors to be cascaded along the same optical axis. Since each device performs a partial polarization measurement of the same incident beam, high temporal resolution is maintained with the potential for inherent spatial registration. In this paper, a Mueller matrix model of the stacked OPV design is provided. Based on this model, a calibration technique is developed and presented. This calibration technique and model are validated with experimental data, taken with a cascaded three cell OPV Stokes polarimeter, capable of measuring incident linear polarization states. Our results indicate polarization measurement error of 1.2% RMS and an average absolute radiometric accuracy of 2.2% for the demonstrated polarimeter.}, number={13}, journal={OPTICS EXPRESS}, author={Roy, S. Gupta and Awartani, O. M. and Sen, P. and O'Connor, B. T. and Kudenov, M. W.}, year={2016}, month={Jun}, pages={14737–14747} }
 @article{zhao_awartani_o'connor_zikry_2016, title={Microstructural Behavior and Failure Mechanisms of Organic Semicrystalline Thin Film Blends}, volume={54}, ISSN={["1099-0488"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954289843&partnerID=MN8TOARS}, DOI={10.1002/polb.23991}, abstractNote={Organic thin film blends of P3HT semiconducting polymers and PCBM fullerenes have enabled large-scale semiconductor fabrication pertaining to flexible and stretchable electronics. However, molecular packing and film morphologies can significantly alter mechanical stability and failure behavior. To further understand and identify the fundamental mechanisms affecting failure, a multiphase microstructurally based formulation and nonlinear finite-element fracture methodology were used to investigate the heterogeneous deformation and failure modes of organic semicrystalline thin film blends. The multiphase formulation accounts for the crystalline and amorphous behavior, polymer tie-chains, and the PCBM aggregates. Face-on packing orientations resulted in extensive inelastic deformation and crystalline rotation, and this was characterized by ductile failure modes and interfacial delamination. For edge-on packing orientations, brittle failure modes and film cracking were due to lower inelastic deformation and higher film stress in comparison with the face-on orientations. The higher crystallinity of P3HT and larger PCBM aggregates associated with larger domain sizes, strengthened the film and resulted in extensive film cracking. These predictions of ductile and brittle failure are consistent with experimental observations for P3HT:PCBM films. The proposed predictive framework can be used to improve organic film ductility and strength through the control of molecular packing orientations and microstructural mechanisms. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 896–907}, number={9}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Zhao, Bingxiao and Awartani, Omar and O'Connor, Brendan and Zikry, Mohammed A.}, year={2016}, month={May}, pages={896–907} }
 @article{antonik_o'connor_ferguson_2016, title={Performance and Design Comparison of a Bulk Thermoelectric Cooler With a Hybrid Architecture}, volume={8}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4032637}, DOI={10.1115/1.4032637}, abstractNote={This paper compares the economic viability and performance outcomes of two different thermoelectric device architectures to determine the advantages and appropriate use of each configuration. Hybrid thermoelectric coolers (TECs) employ thin-film thermoelectric materials sandwiched between a plastic substrate and form a corrugated structure. Roll-to-roll (R2R) manufacturing and low-cost polymer materials offer a cost advantage to the hybrid architecture at the sacrifice of performance capabilities while conventional bulk devices offer increased performance at a higher cost. Performance characteristics and cost information are developed for both hybrid and conventional bulk single-stage thermoelectric modules. The design variables include device geometry, electrical current input, and thermoelectric material type. The tradeoffs between cooling performance and cost will be explored, and the thermoelectric system configuration is analyzed for both hybrid and conventional bulk TECs.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Antonik, Margaret and O'Connor, Brendan T. and Ferguson, Scott}, year={2016}, month={Mar} }
 @inproceedings{antonik_ferguson_o'connor_2016, title={Performance comparison of a bulk thermoelectric cooler with a hybrid device architecture}, DOI={10.1115/detc2015-47610}, abstractNote={This paper compares the economic viability and performance outcomes of two different thermoelectric device architectures to determine the advantages and appropriate use of each configuration. Hybrid thermoelectric coolers employ thin-film thermoelectric materials sandwiched between a plastic substrate and formed into a corrugated structure. Roll-to-roll manufacturing and low-cost polymer materials offer a cost advantage to the hybrid architecture at the sacrifice of performance capabilities while conventional bulk devices offer increased performance at a higher cost. Performance characteristics and cost information are developed for both hybrid and conventional bulk single-stage thermoelectric modules. The design variables include device geometry, electrical current input, and thermoelectric material type. The trade-offs between cooling performance and cost will be explored and the thermoelectric system configuration analyzed for both hybrid and conventional bulk thermoelectric coolers.}, booktitle={International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, 2015, vol 2b}, author={Antonik, M. and Ferguson, S. M. and O'Connor, B. T.}, year={2016} }
 @article{scott_xue_wang_kline_hoffman_dougherty_zhou_bazan_brendan t. o'connor_2016, title={Significantly Increasing the Ductility of High Performance Polymer Semiconductors through Polymer Blending}, volume={8}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.6b01852}, abstractNote={Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, the development of methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor-acceptor polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT), with ductile poly(3-hexylthiophene). We found that the ductility of the blend films is significantly improved compared to that of neat PCDTPT films, and when the blend film is employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is due to intermixing of the polymers throughout the film thickness. Importantly, the application of large strains to the ductile films is shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.}, number={22}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Scott, Joshua I. and Xue, Xiao and Wang, Ming and Kline, R. Joseph and Hoffman, Benjamin C. and Dougherty, Daniel and Zhou, Chuanzhen and Bazan, Guillermo and Brendan T. O'Connor}, year={2016}, month={Jun}, pages={14037–14045} }
 @article{o'connor_xue_sun_2015, title={Charge transport in highly aligned conjugated polymers}, volume={9568}, ISSN={["0277-786X"]}, DOI={10.1117/12.2187646}, abstractNote={Charge transport in conjugated polymers has a complex dependence on film morphology. Aligning the polymer chains in the plane of the film simplifies the morphology of the system allowing for insight into the morphological dependence of charge transport. Highly aligned conjugated polymers have also been shown to lead to among the highest reported field effect mobilities in these materials to date. In this talk, a comparison will be made between aligned polymer films processed using two primary methods, nanostructured substrate assisted growth and mechanical strain. A number of polymer systems including P3HT, pBTTT, N2200, and PCDTPT are considered, and the processed films are analyzed in detail with optical spectroscopy, AFM, TEM, and X-ray scattering providing insight into the molecular features that allow for effective alignment. By contrasting the morphology of these films, several insights into underlying charge transport limitations can be made. A number of key morphological features that lead to high field effect mobility and charge transport anisotropy in these films will be discussed. In addition, several unique features of organic thin film transistor device behavior in these systems will be examined including the commonly observed gate voltage dependence of saturated field effect mobility.}, journal={ORGANIC FIELD-EFFECT TRANSISTORS XIV; AND ORGANIC SENSORS AND BIOELECTRONICS VIII}, author={O'Connor, Brendan and Xue, Xiao and Sun, Tianlei}, year={2015} }
 @article{roy_awartani_sen_o'connor_kudenov_2015, title={Complete Intrinsic Coincident Polarimetry using Stacked Organic Photovoltaics}, volume={9613}, ISSN={["0277-786X"]}, DOI={10.1117/12.2188972}, abstractNote={Measuring the 2 dimensional Stokes vector, to determine the polarization state of light, finds application in multiple areas, including the characterization of aerosol size distributions, target identification, quality control by evaluating the distribution of stress birefringence, resolving data channels in telecommunications, and for evaluating biological tissues in medical imaging. Conventional methods, such as channeled and division of focal plane polarimeters, usually limit spatial resolution, while others, like division of aperture or division of amplitude polarimeters, have higher complexity and less compactness. To help solve these issues, we have developed a system that uses semitransparent organic photovoltaics (OPVs) as photodetectors. The active area of the devices consist of biaxially oriented polymer films, which enables the device to preferentially absorb certain polarized states of incident light, depending on the orientation of the polymer chains. Taking advantage of the cells’ transparency and ease of processing, compared to inorganic materials, enables multiple devices to be “stacked” along the optical axis. Presently, experiments have been conducted to detect linear polarization states of light. We use three stacked OPVs, where each device can measure one of the first three Stokes parameters simultaneously, thereby ensuring high spatial and temporal resolution with inherent spatial registration. In this paper, the fabrication of the OPVs and the design and calibration technique is documented, along with experimental data, supporting the hypothesis.}, journal={POLARIZATION SCIENCE AND REMOTE SENSING VII}, author={Roy, S. Gupta and Awartani, O. M. and Sen, P. and O'Connor, B. T. and Kudenov, M. W.}, year={2015} }
 @article{sun_peavey_shelby_ferguson_brendan t. o'connor_2015, title={Heat shrink formation of a corrugated thin film thermoelectric generator}, volume={103}, ISSN={["1879-2227"]}, DOI={10.1016/j.enconman.2015.07.016}, abstractNote={A thin film thermoelectric (TE) generator with a corrugated architecture is demonstrated formed using a heat-shrink fabrication approach. Fabrication of the corrugated TE structure consists of depositing thin film thermoelectric elements onto a planar non-shrink polyimide substrate that is then sandwiched between two uniaxial stretch-oriented co-polyester (PET) films. The heat shrink PET films are adhered to the polyimide in select locations, such that when the structure is placed in a high temperature environment, the outer films shrink resulting in a corrugated core film and thermoelectric elements spanning between the outer PET films. The module has a cross-plane heat transfer architecture similar to a conventional bulk TE module, but with heat transfer in the plane of the thin film thermoelectric elements, which assists in maintaining a significant temperature difference across the thermoelectric junctions. In this demonstration, Ag and Ni films are used as the thermoelectric elements and a Seebeck coefficient of 14 μV K−1 is measured with a maximum power output of 0.22 nW per couple at a temperature difference of 7.0 K. We then theoretically consider the performance of this device architecture with high performance thermoelectric materials in the heat sink limited regime. The results show that the heat-shrink approach is a simple fabrication method that may be advantageous in large-area, low power density applications. The fabrication method is also compatible with simple geometric modification to achieve various form factors and power densities to customize the TE generator for a range of applications.}, journal={ENERGY CONVERSION AND MANAGEMENT}, author={Sun, Tianlei and Peavey, Jennifer L. and Shelby, M. David and Ferguson, Scott and Brendan T. O'Connor}, year={2015}, month={Oct}, pages={674–680} }
 @article{awartani_kudenov_kline_brendan t. o'connor_2015, title={In-Plane Alignment in Organic Solar Cells to Probe the Morphological Dependence of Charge Recombination}, volume={25}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201403377}, abstractNote={Bulk heterojunction (BHJ) organic solar cells are fabricated with the polymer semiconductor aligned in the plane of the film to probe charge recombination losses associated with aggregates characterized by varying degrees of local order. 100% uniaxial strain is applied on ductile poly(3‐hexylthiophene):phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) BHJ films and characterize the resulting morphology with ultraviolet‐visible absorption spectroscopy and grazing incidence X‐ray diffraction. It is found that the strained films result in strong alignment of the highly ordered polymer aggregates. Polymer aggregates with lower order and amorphous regions also align but with a much broader orientation distribution. The solar cells are then tested under linearly polarized light where the light is selectively absorbed by the appropriately oriented polymer, while maintaining a common local environment for the sweep out of photogenerated charge carriers. Results show that charge collection losses associated with a disordered BHJ film are circumvented, and the internal quantum efficiency is independent of P3HT local aggregate order near the heterojunction interface. Uniquely, this experimental approach allows for selective excitation of distinct morphological features of a conjugated polymer within a single BHJ film, providing insight into the morphological origin of recombination losses.}, number={8}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Awartani, Omar and Kudenov, Michael W. and Kline, R. Joseph and Brendan T. O'Connor}, year={2015}, month={Feb}, pages={1296–1303} }
 @article{xue_chandler_zhang_kline_fei_heeney_diemer_jurchescu_brendan t. o'connor_2015, title={Oriented Liquid Crystalline Polymer Semiconductor Films with Large Ordered Domains}, volume={7}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.5b08710}, abstractNote={Large strains are applied to liquid crystalline poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) (pBTTT) films when held at elevated temperatures resulting in in-plane polymer alignment. We find that the polymer backbone aligns significantly in the direction of strain, and that the films maintain large quasi-domains similar to that found in spun-cast films on hydrophobic surfaces, highlighted by dark-field transmission electron microscopy imaging. The highly strained films also have nanoscale holes consistent with dewetting. Charge transport in the films is then characterized in a transistor configuration, where the field effect mobility is shown to increase in the direction of polymer backbone alignment, and decrease in the transverse direction. The highest saturated field-effect mobility was found to be 1.67 cm(2) V(-1) s(-1), representing one of the highest reported mobilities for this material system. The morphology of the oriented films demonstrated here contrast significantly with previous demonstrations of oriented pBTTT films that form a ribbon-like morphology, opening up opportunities to explore how differences in molecular packing features of oriented films impact charge transport. Results highlight the role of grain boundaries, differences in charge transport along the polymer backbone and π-stacking direction, and structural features that impact the field dependence of charge transport.}, number={48}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Xue, Xiao and Chandler, George and Zhang, Xinran and Kline, R. Joseph and Fei, Zhuping and Heeney, Martin and Diemer, Peter J. and Jurchescu, Oana D. and Brendan T. O'Connor}, year={2015}, month={Dec}, pages={26726–26734} }
 @article{owoyele_ferguson_brendan t. o'connor_2015, title={Performance analysis of a thermoelectric cooler with a corrugated architecture}, volume={147}, ISSN={["1872-9118"]}, DOI={10.1016/j.apenergy.2015.01.132}, abstractNote={A thermoelectric (TE) cooler architecture is presented that employs thin film thermoelectric elements on a plastic substrate in a corrugated structure sandwiched between planar thermal interface plates. This design represents a hybrid of a conventional bulk TE device and an in-plane thin film TE design. This design is attractive as it may benefit from low cost thin-film processing in a roll-to-roll fashion onto low-cost plastics substrates while maintaining a cross-plane heat flux for large area applications and a geometry that assists in maintaining a significant temperature difference across the thermoelectric elements. First, the performance of a single thermocouple is analyzed and the effect of the parasitic heat loss through the plastic substrate is examined. The performance of an array of thermocouples is then considered and the effects of various geometric parameters are analyzed with particular focus on the packing density of thermoelectric legs. The results show that while the coefficient of performance (COP) is comparable to a conventional bulk element TE cooler, the cooling power density drops off dramatically with a decrease in stacking angle of the legs. A comparison is then made between the heat sink demands of the hybrid TE design and a conventional bulk TE device where it is found that the lower cooling power density of the hybrid TE results in a reduction of heat sink demands as compared to bulk TE modules. The modeled performance suggest that the hybrid TE device may be advantageous in low cooling power density applications over relatively large areas where the low-cost nature of the device is maximized and less elaborate heat sink designs work effectively, cumulatively improving cost competitiveness.}, journal={APPLIED ENERGY}, author={Owoyele, Opeoluwa and Ferguson, Scott and Brendan T. O'Connor}, year={2015}, month={Jun}, pages={184–191} }
 @article{kitchen_awartani_kline_mcafee_ade_brendan t. o'connor_2015, title={Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation}, volume={7}, ISSN={["1944-8244"]}, DOI={10.1021/am508855s}, abstractNote={The role of molecular orientation of a polar conjugated polymer in polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open-circuit voltage (V(OC)), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a V(OC) comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that, in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in part by the orientation distribution of the P3HT at the interface with PCBM. Finally, the photoresponses of the devices are also shown to have a dependence on P3HT stacking orientation.}, number={24}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Kitchen, Brent and Awartani, Omar and Kline, R. Joseph and McAfee, Terry and Ade, Harald and Brendan T. O'Connor}, year={2015}, month={Jun}, pages={13208–13216} }
 @article{brendan t. o'connor_reid_zhang_kline_richter_gundlach_delongchamp_toney_kopidakis_rumbles_2014, title={Morphological Origin of Charge Transport Anisotropy in Aligned Polythiophene Thin Films}, volume={24}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201303351}, abstractNote={The morphological origin of anisotropic charge transport in uniaxially strain aligned poly(3‐hexylthiophene) (P3HT) films is investigated. The macroscale field effect mobility anisotropy is measured in an organic thin film transistor (OTFT) configuration and compared to the local aggregate P3HT mobility anisotropy determined using time‐resolved microwave conductivity (TRMC) measurements. The field effect mobility anisotropy in highly aligned P3HT films is substantially higher than the local mobility anisotropy in the aggregate P3HT. This difference is attributed to preferentially aligned polymer tie‐chains at grain boundaries that contribute to macroscale charge transport anisotropy but not the local anisotropy. The formation of sharp grains between oriented crystalline P3HT, through tie chain removal by thermal annealing the strained aligned films, results in an order of magnitude drop in the measured field effect mobility for charge transport parallel to the strain direction. The field effect mobility anisotropy is cut in half while the local mobility anisotropy remains relatively constant. The local mobility anisotropy is found to be surprisingly low in the aligned films, suggesting that the π−π stacking direction supports charge carrier mobility on the same order of magnitude as that in the intrachain direction, possibly due to poor intrachain mobility through chain torsion.}, number={22}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Brendan T. O'Connor and Reid, Obadiah G. and Zhang, Xinran and Kline, R. Joseph and Richter, Lee J. and Gundlach, David J. and DeLongchamp, Dean M. and Toney, Michael F. and Kopidakis, Nikos and Rumbles, Garry}, year={2014}, month={Jun}, pages={3422–3431} }
 @article{awartani_kudenov_brendan t. o'connor_2014, title={Organic photovoltaic cells with controlled polarization sensitivity}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4868041}, abstractNote={In this study, we demonstrate linearly polarized organic photovoltaic cells with a well-controlled level of polarization sensitivity. The polarized devices were created through the application of a large uniaxial strain to the bulk heterojunction poly(3-hexylthiophene):Phenyl-C61-butyric acid methyl ester (P3HT:PCBM) film and printing the plastically deformed active layer onto a PEDOT:PSS and indium tin oxide coated glass substrate. The P3HT:PCBM layer is processed such that it is able to accommodate high strains (over 100%) without fracture. After printing the strained films, thermal annealing is used to optimize solar cell performance while maintaining polarization sensitivity. A dichroic ratio and short circuit current ratio of ≈6.1 and ≈1.6 were achieved, respectively.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Awartani, Omar and Kudenov, Michael W. and Brendan T. O'Connor}, year={2014}, month={Mar} }
 @article{gargi_kline_delongchamp_fischer_toney_brendan t. o'connor_2013, title={Charge Transport in Highly Face-On Poly(3-hexylthiophene) Films}, volume={117}, ISSN={["1932-7447"]}, DOI={10.1021/jp4050644}, abstractNote={We report that the π-stacking direction in poly(3-hexylthiophene) (P3HT) films can be made to orient strongly out-of plane by uniaxially straining films in orthogonal directions, providing a valuable opportunity to evaluate charge transport in a very unusual microstructure for this material. The structure of the films was characterized using UV–visible spectroscopy, X-ray diffraction, and near-edge X-ray absorption fine structure spectroscopy, showing that unstrained films have a weakly edge-on stacking character with a large orientation distribution, whereas films strained biaxially by 100% in orthogonal directions have highly face-on stacking. In the biaxially strained films the face-on packing occurs while the P3HT long axis orientation is found to be only weakly anisotropic in-plane. Charge transport is characterized in an organic thin-film transistor (OTFT) configuration, showing that the saturated field effect mobility in the biaxially strained films is greater than that for unstrained films for cha...}, number={34}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Gargi, Deepak and Kline, R. Joseph and DeLongchamp, Dean M. and Fischer, Daniel A. and Toney, Michael F. and Brendan T. O'Connor}, year={2013}, month={Aug}, pages={17421–17428} }
 @article{awartani_lemanski_ro_richter_delongchamp_brendan t. o'connor_2013, title={Correlating Stiffness, Ductility, and Morphology of Polymer:Fullerene Films for Solar Cell Applications}, volume={3}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201200595}, abstractNote={Abstract}, number={3}, journal={ADVANCED ENERGY MATERIALS}, author={Awartani, Omar and Lemanski, Bethany I. and Ro, Hyun Wook and Richter, Lee J. and DeLongchamp, Dean M. and Brendan T. O'Connor}, year={2013}, month={Mar}, pages={399–406} }
 @article{ro_akgun_brendan t. o'connor_hammond_kline_snyder_satija_ayzner_toney_soles_et al._2012, title={Poly(3-hexylthiophene) and [6,6]-Phenyl-C-61-butyric Acid Methyl Ester Mixing in Organic Solar Cells}, volume={45}, ISSN={["1520-5835"]}, DOI={10.1021/ma3008527}, abstractNote={The mixing behavior of the hole- and electron-transporting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) blends plays a key role in determining the nanoscale morphology, which is believed to be a decisive factor in determining device performance. We present a systematic investigation of the mixing behavior between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in model multilayer structures. The bilayer structures are composed of amorphous PCBM that is mechanically laminated to different P3HT layers with varying degrees of crystallinity. We find that mixing is significantly decreased as the crystallinity of P3HT is increased. The mixing behavior can be explained as resulting from (1) nearly complete miscibility of PCBM with amorphous P3HT (based on our results from regiorandom P3HT) and (2) the existence of tie chains between crystalline P3HT domains that restrain the swelling of the P3HT layer by PCBM. We also introduce a unique PCBM–P3HT–PCBM tril...}, number={16}, journal={MACROMOLECULES}, author={Ro, Hyun Wook and Akgun, Bulent and Brendan T. O'Connor and Hammond, Matthew and Kline, R. Joseph and Snyder, Chad R. and Satija, Sushil K. and Ayzner, Alexander L. and Toney, Michael F. and Soles, Christopher L. and et al.}, year={2012}, month={Aug}, pages={6587–6599} }
 @article{o'connor_kline_conrad_richter_gundlach_toney_delongchamp_2011, title={Anisotropic Structure and Charge Transport in Highly Strain-Aligned Regioregular Poly(3-hexylthiophene)}, volume={21}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201100904}, abstractNote={Abstract}, number={19}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={O'Connor, Brendan and Kline, R. Joseph and Conrad, Brad R. and Richter, Lee J. and Gundlach, David and Toney, Michael F. and DeLongchamp, Dean M.}, year={2011}, month={Oct}, pages={3697–3705} }
 @article{o’connor_chan_chan_conrad_richter_kline_heeney_mcculloch_soles_delongchamp_2010, title={Correlations between Mechanical and Electrical Properties of Polythiophenes}, volume={4}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/nn1018768}, DOI={10.1021/nn1018768}, abstractNote={The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.}, number={12}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={O’Connor, Brendan and Chan, Edwin P. and Chan, Calvin and Conrad, Brad R. and Richter, Lee J. and Kline, R. Joseph and Heeney, Martin and McCulloch, Iain and Soles, Christopher L. and DeLongchamp, Dean M.}, year={2010}, month={Nov}, pages={7538–7544} }
 @article{o’connor_nothern_pipe_shtein_2010, title={High-efficiency broadband solar cell architectures based on arrays of volumetrically distributed narrowband photovoltaic fibers}, volume={18}, ISSN={1094-4087}, url={http://dx.doi.org/10.1364/oe.18.00a432}, DOI={10.1364/oe.18.00a432}, abstractNote={We propose a novel solar cell architecture consisting of multiple fiber-based photovoltaic (PV) cells. Each PV fiber element is designed to maximize the power conversion efficiency within a narrow band of the incident solar spectrum, while reflecting other spectral components through the use of optical microcavity effects and distributed Bragg reflector (DBR) coatings. Combining PV fibers with complementary absorption and reflection characteristics into volume-filling arrays enables spectrally tuned modules having an effective dispersion element intrinsic to the architecture, resulting in high external quantum efficiency over the incident spectrum. While this new reflective tandem architecture is not limited to one particular material system, here we apply the concept to organic PV (OPV) cells that use a metal-organic-metal-dielectric layer structure, and calculate the expected performance of such arrays. Using realistic material properties for organic absorbers, transport layers, metallic electrodes, and DBR coatings, 17% power conversion efficiency can be reached.}, number={S3}, journal={Optics Express}, publisher={The Optical Society}, author={O’Connor, Brendan and Nothern, Denis and Pipe, Kevin P. and Shtein, Max}, year={2010}, month={Aug}, pages={A432} }
 @article{an_o’connor_pipe_shtein_2009, title={Organic photodetector with spectral response tunable across the visible spectrum by means of internal optical microcavity}, volume={10}, ISSN={1566-1199}, url={http://dx.doi.org/10.1016/j.orgel.2009.06.003}, DOI={10.1016/j.orgel.2009.06.003}, abstractNote={We demonstrate an organic photodetector (OPD) structure in which the active layers and a thick optical spacer are sandwiched between two metallic electrodes, forming a Fabry–Perot resonant cavity. The second resonant mode of this cavity can be positioned by means of an optical spacer so that its maximum intensity overlaps with the donor–acceptor interface, leading to a peak in the external quantum efficiency (EQE) of the OPD for this resonant wavelength. The photoresponse can thus be tuned across the visible spectrum by adjusting the spacer thickness, while the full width half maximum remains approximately 50 nm. Because the active layers can be thin in this approach, the EQE is not sacrificed, and the device can achieve a relatively high response frequency that does not suffer from the inclusion of the optical spacer. We simulate the photoresponse of OPD structure using transfer matrix optical calculations and an exciton diffusion model; our simulation also explicitly accounts for interface roughness measured by atomic force microscopy. Angular dependence of the OPD's response is also measured and discussed.}, number={6}, journal={Organic Electronics}, publisher={Elsevier BV}, author={An, Kwang Hyup and O’Connor, Brendan and Pipe, Kevin P. and Shtein, Max}, year={2009}, month={Sep}, pages={1152–1157} }
 @article{o’connor_pipe_shtein_2008, title={Fiber based organic photovoltaic devices}, volume={92}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2927533}, DOI={10.1063/1.2927533}, abstractNote={A fiber-shaped organic photovoltaic cell is demonstrated, utilizing concentric thin films of small molecular organic compounds. Illuminated at normal incidence to the fiber axis through a thin metal electrode, the cell exhibits 0.5% power conversion efficiency, compared to 0.76% for a planar control device. The fiber device efficiency is nearly independent of illumination angle, increasing its power generation over the planar counterpart for diffuse illumination. Losses due to partial shading of the fiber surface are minimal, while the coated fiber length is limited only by the experimental deposition chamber geometry—factors favoring scale-up to woven energy harvesting textiles.}, number={19}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={O’Connor, Brendan and Pipe, Kevin P. and Shtein, Max}, year={2008}, month={May}, pages={193306} }
 @article{an_o’connor_pipe_zhao_shtein_2008, title={Scanning optical probe microscopy with submicrometer resolution using an organic photodetector}, volume={93}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2963033}, DOI={10.1063/1.2963033}, abstractNote={A high-resolution scanning optical microscopy technique is demonstrated, in which an organic photodetector on a silicon-based scanning probe cantilever scans a sample, simultaneously recording optical and topographic data with submicrometer resolution, while showing no measurable degradation during the scan. Potential applications of the probe include characterization of optoelectronic materials and devices, as well as simultaneous topographic and fluorescence microscopy of biological samples. Extension to these applications is aided by the fact that the probe is compatible with conventional atomic force microscopy systems and does not suffer some of the practical difficulties of existing near-field scanning optical microscopy systems.}, number={3}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={An, Kwang H. and O’Connor, Brendan and Pipe, Kevin P. and Zhao, Yiying and Shtein, Max}, year={2008}, month={Jul}, pages={033311} }
 @article{o’connor_haughn_an_pipe_shtein_2008, title={Transparent and conductive electrodes based on unpatterned, thin metal films}, volume={93}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3028046}, DOI={10.1063/1.3028046}, abstractNote={Transparent electrodes composed of ultrathin, unpatterned metal films are investigated in planar heterojunction (PHJ) and bulk heterojunction organic photovoltaic (OPV) cells. Optimal electrode composition and thickness are deduced from electrical and optical models and experiments, enabling a PHJ-OPV cell to be realized using a silver anode, achieving power conversion efficiency parity with an analogous cell that uses an indium tin oxide anode. Beneficial aspects of smooth, unpatterned metal films as transparent electrodes in OPV cells are also discussed in the text.}, number={22}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={O’Connor, Brendan and Haughn, Chelsea and An, Kwang-Hyup and Pipe, Kevin P. and Shtein, Max}, year={2008}, month={Dec}, pages={223304} }
 @article{o'connor_an_zhao_pipe_shtein_2007, title={Fiber Shaped Light Emitting Device}, volume={19}, ISSN={0935-9648 1521-4095}, url={http://dx.doi.org/10.1002/adma.200700627}, DOI={10.1002/adma.200700627}, abstractNote={Advances in fiber and fabric technology have been applied in a wide variety of contexts, ranging from structural composites to fiber-optic communications, and more recently, electronic textiles. There is growing interest in optoelectronic devices having a fiber form factor, including pulled fiber photodetectors, optically pumped surface emitting fiber lasers, photoluminescent polymer-coated fibers, fiberbased Grätzel and thin film semiconductor solar cells, as well as organic light emitting devices (OLEDs) on a fiber. Fiber-based OLEDs in particular have wide ranging applications from fabric integrated light sources to low cost solidstate lighting. However, there is a relative dearth of scientific literature providing a sound physical understanding of the impact that the fiber geometry has on device performance, or the challenges facing the realization of fiber-based and fabricintegrated optoelectronics. In this work we focus on molecular organic compounds as the active device layers due to their inherent mechanical flexibility and compatibility with low cost device fabrication techniques. Using these materials, we demonstrate a fiberbased OLED and analyze the physical effects that arise due to the non-planar device geometry. The thickness of the active layers in these devices is approximately 100 nm, orders of magnitude thinner than the typical fiber diameter (e.g. 50– 1,000 lm), suggesting that energy conversion functionality can be incorporated into an individual fiber without affecting its mechanical characteristics. Additionally, as will be analyzed below, the electroluminescence spectrum is measured to be independent of the observation angle, which is in contrast to planar OLED geometries. An archetypal OLED structure is considered, as illustrated in Figure 1a, and applied to the fiber geometry, Figure 1b. The OLED consists of organic charge transport and emission layers sandwiched between a metallic anode and cathode. For the fiber-based device the layers were deposited conformally onto a 480 lm thick polyimide-coated silica fiber using vacuum thermal evaporation at 10 Torr; the cathode was deposited through a shadow mask. (Please refer to the Experimental Sec. for details on device growths.) Figure 1c shows a photograph of a 1 mm segment of the OLED formed concentrically around the fiber, emitting characteristic green light under forward electrical bias. Light emission in this device occurs through the semitransparent outer electrode. For a direct comparison of optoelectronic performance, a similar OLED structure was deposited on planar silicon and polyimide substrates, as will be discussed below. Figure 2 compares the current density-voltage (J–V) characteristics of fiber OLEDs to those of the analogous planar devices. The similarity in the current-voltage relationship between the planar and fiber cells suggests comparable layer thickness of the hole and electron transport layers on each type of substrate. The slightly larger leakage current at low bias (< 2.5 V) in one of the fiber-based devices is attributable to the greater surface roughness of the fiber substrate. While the electrical characteristics do not differ materially between the fiberand the planar-shaped OLEDs, their emission characteristics are expected to differ substantially due to the microcavity effects typical of OLEDs. Specifically, the microcavity effects present in the planar device structure lead to a strong variation in the peak wavelength with emission anC O M M U N IC A IO N}, number={22}, journal={Advanced Materials}, publisher={Wiley}, author={O'Connor, B. and An, K. H. and Zhao, Y. and Pipe, K. P. and Shtein, M.}, year={2007}, month={Nov}, pages={3897–3900} }
 @article{zhao_an_chen_o'connor_pipe_shtein_2007, title={Localized Current Injection and Submicron Organic Light-Emitting Device on a Pyramidal Atomic Force Microscopy Tip}, volume={7}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl071883w}, DOI={10.1021/nl071883w}, abstractNote={An organic light-emitting device was fabricated on a commercial atomic force microscopy (AFM) probe having a pyramidal tip by a lithography-free vacuum thermal evaporation (VTE) process. The line-of-sight molecular transport characteristic of VTE results in controlled thickness variation across the nonplanar substrate, such that localized current injection occurs at the tip region. Furthermore, the high curvature of the AFM tip vertex concentrates the electric field, causing highly localized bipolar charge injection, accompanied by photon emission from a region less than a micrometer across. This light source exhibits a range of features potentially attractive for applications such as probe-based optical microscopy, nanoscale light sensing, and chemical detection.}, number={12}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Zhao, Yiying and An, Kwang H. and Chen, Shuo and O'Connor, Brendan and Pipe, Kevin P. and Shtein, Max}, year={2007}, month={Dec}, pages={3645–3649} }
 @article{o’connor_an_pipe_zhao_shtein_2006, title={Enhanced optical field intensity distribution in organic photovoltaic devices using external coatings}, volume={89}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2399937}, DOI={10.1063/1.2399937}, abstractNote={An external dielectric coating is shown to enhance energy conversion in an organic photovoltaic cell with metal anode and cathode by increasing the optical field intensity in the organic layers. Improved light incoupling in the device is modeled using transfer matrix simulations and is confirmed by in situ measurement of the photocurrent during growth of the coating. The optical field intensity in optimized cell geometries is predicted to exceed that in analogous devices using indium tin oxide, both cell types having equivalent anode sheet resistance, suggesting a broader range of compatible substrates (e.g., metal foils) and device processing techniques.}, number={23}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={O’Connor, Brendan and An, Kwang H. and Pipe, Kevin P. and Zhao, Yiying and Shtein, Max}, year={2006}, month={Dec}, pages={233502} }
 @article{an_o’connor_pipe_zhao_shtein_2006, title={Organic light-emitting device on a scanning probe cantilever}, volume={89}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2353816}, DOI={10.1063/1.2353816}, abstractNote={Organic light-emitting devices (OLEDs) were fabricated on scanning probe cantilevers using a combination of thermally evaporated molecular organic compounds and metallic electrodes. Ion beam milling was used to define the emissive region in the shape of a ring having a diameter of less than 5μm and a narrow width. Stable light emission was observed from the device at forward bias, with a current-voltage response similar to that of archetypal OLEDs. Based on this device, a novel electrically pumped scanning optical microscopy tool is suggested.}, number={11}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={An, Kwang H. and O’Connor, Brendan and Pipe, Kevin P. and Zhao, Yiying and Shtein, Max}, year={2006}, month={Sep}, pages={111117} }