@article{henry_balar_ade_2023, title={In-Situ Ellipsometry for the Determination of Thermal Transitions and Relaxations in Organic Photovoltaic Materials}, volume={35}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.3c00714}, abstractNote={Characterization of thermodynamic transitions and kinetic processes in optoelectronic materials is critical for understanding the optimized processing conditions and final device structural stability. Differential scanning calorimetry (DSC) is traditionally used to determine melting, crystallization, and glass transition temperatures as well as additional transitions, such as polymorphic solid–solid transitions. These transition temperatures are utilized to understand the general structure–property relations of materials and can be used to inform processing protocols in device fabrication in order to facilitate the formation of preferable microstructures for optimized electronic properties. However, traditional DSC experiments are limited to bulk-like samples and cannot characterize device relevant, controlled thin film geometries. Here we demonstrate that ellipsometry is at least as capable as traditional DSC experiments to determine relevant thermal transitions through a direct comparison using a range of optoelectronic and benchmarking materials. In addition, ellipsometry measurement protocols can uncover kinetic characteristics and possible additional transitions that are not observed in traditional DSC. Furthermore, ellipsometry observes density changes associated with free volume and molecular packing and associated hysteresis during temperature sweeps directly, an avenue of inquiry underutilized to date. We anticipate that ellipsometry protocols will allow for a more widely used, powerful complement to DSC characterization.}, number={18}, journal={CHEMISTRY OF MATERIALS}, author={Henry, Reece and Balar, Nrup and Ade, Harald}, year={2023}, month={Sep}, pages={7406–7421} }
 @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{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{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{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{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{peng_balar_ghasemi_ade_2021, title={Upper and Apparent Lower Critical Solution Temperature Branches in the Phase Diagram of Polymer:Small Molecule Semiconducting Systems}, volume={12}, ISSN={["1948-7185"]}, DOI={10.1021/acs.jpclett.1c02848}, abstractNote={Solution-processable semiconducting materials are complex materials with a wide range of applications. Despite their extensive study and utility, their molecular interactions as manifested, for example, in phase behavior are poorly understood. Here, we aim to understand the phase behavior of conjugated systems by determining phase diagrams spanning extensive temperature ranges for various combinations of the highly disordered semiconducting polymer (PTB7-Th) with crystallizable (IT-M and PC61BM) and noncrystallizable (di-PDI) small molecule acceptors (SMAs), with polystyrene as an amorphous control, a nonsemiconducting commodity polymer. We discover that the apparent binodal of the studied blends frequently consists of an upper critical solution temperature (UCST) and lower critical solution temperature (LCST) branch, exhibiting a sharp kink where the branches join. Our work suggests that phase diagrams might be a probe in combination with sophisticated models to understand the complexity of semiconducting materials, including microstructure and molecular interactions.}, number={44}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Peng, Zhengxing and Balar, Nrup and Ghasemi, Masoud and Ade, Harald}, year={2021}, month={Nov}, pages={10845–10853} }
 @article{balar_siddika_kashani_peng_rech_ye_you_ade_brendan t. o'conner_2020, title={Role of Secondary Thermal Relaxations in Conjugated Polymer Film Toughness}, volume={32}, ISSN={["1520-5002"]}, url={https://publons.com/wos-op/publon/35208553/}, DOI={10.1021/acs.chemmater.0c01910}, abstractNote={Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long term thermal and mechanical stability of associated devices, there is ...}, number={15}, journal={CHEMISTRY OF MATERIALS}, author={Balar, Nrup and Siddika, Salma and Kashani, Somayeh and Peng, Zhengxing and Rech, Jeromy James and Ye, Long and You, Wei and Ade, Harald and Brendan T. O'Conner}, year={2020}, month={Aug}, pages={6540–6549} }
 @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{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{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{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{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} }