@article{schubert_collins_mangold_howard_schindler_vandewal_roland_behrends_kraffert_steyrleuthner_et al._2014, title={Correlated Donor/Acceptor Crystal Orientation Controls Photocurrent Generation in All-Polymer Solar Cells}, volume={24}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201304216}, DOI={10.1002/adfm.201304216}, abstractNote={New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all‐polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer:fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi‐crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all‐polymer solar cells. These issues are examined by combining structural and photo‐physical characterization on a series of poly(3‐hexylthiophene) (donor) and P(NDI2OD‐T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo‐excited charge carriers. Advanced X‐ray and electron‐based studies reveal the effect of chloronaphthalene co‐solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, efficient split‐up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors.}, number={26}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Schubert, Marcel and Collins, Brian A. and Mangold, Hannah and Howard, Ian A. and Schindler, Wolfram and Vandewal, Koen and Roland, Steffen and Behrends, Jan and Kraffert, Felix and Steyrleuthner, Robert and et al.}, year={2014}, month={Apr}, pages={4068–4081} }
 @article{roland_schubert_collins_kurpiers_chen_facchetti_ade_neher_2014, title={Fullerene-Free Polymer Solar Cells with Highly Reduced Bimolecular Recombination and Field-Independent Charge Carrier Generation}, volume={5}, ISSN={1948-7185}, url={http://dx.doi.org/10.1021/JZ501506Z}, DOI={10.1021/jz501506z}, abstractNote={Photogeneration, recombination, and transport of free charge carriers in all-polymer bulk heterojunction solar cells incorporating poly(3-hexylthiophene) (P3HT) as donor and poly([N,N'-bis(2-octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD-T2)) as acceptor polymer have been investigated by the use of time delayed collection field (TDCF) and time-of-flight (TOF) measurements. Depending on the preparation procedure used to dry the active layers, these solar cells comprise high fill factors (FFs) of up to 67%. A strongly reduced bimolecular recombination (BMR), as well as a field-independent free charge carrier generation are observed, features that are common to high performance fullerene-based solar cells. Resonant soft X-ray measurements (R-SoXS) and photoluminescence quenching experiments (PQE) reveal that the BMR is related to domain purity. Our results elucidate the similarities of this polymeric acceptor with the superior recombination properties of fullerene acceptors.}, number={16}, journal={The Journal of Physical Chemistry Letters}, publisher={American Chemical Society (ACS)}, author={Roland, Steffen and Schubert, Marcel and Collins, Brian A. and Kurpiers, Jona and Chen, Zhihua and Facchetti, Antonio and Ade, Harald and Neher, Dieter}, year={2014}, month={Aug}, pages={2815–2822} }
 @article{tumbleston_collins_yang_stuart_gann_ma_you_ade_2014, title={The influence of molecular orientation on organic bulk heterojunction solar cells}, volume={8}, ISSN={["1749-4893"]}, DOI={10.1038/nphoton.2014.55}, number={5}, journal={NATURE PHOTONICS}, author={Tumbleston, John R. and Collins, Brian A. and Yang, Liqiang and Stuart, Andrew C. and Gann, Eliot and Ma, Wei and You, Wei and Ade, Harald}, year={2014}, month={May}, pages={385–391} }
 @article{tumbleston_gadisa_liu_collins_samulski_lopez_ade_2013, title={Modifications in Morphology Resulting from Nanoimprinting Bulk Heterojunction Blends for Light Trapping Organic Solar Cell Designs}, volume={5}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/AM402363R}, DOI={10.1021/am402363r}, abstractNote={Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption.}, number={16}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Tumbleston, John R. and Gadisa, Abay and Liu, Yingchi and Collins, Brian A. and Samulski, Edward T. and Lopez, Rene and Ade, Harald}, year={2013}, month={Aug}, pages={8225–8230} }
 @article{cochran_amir_sivanandan_ku_seo_collins_tumbleston_toney_ade_hawker_et al._2013, title={Synthesis, solid-state, and charge-transport properties of conjugated polythiophene-S,S-dioxides}, volume={51}, ISSN={["1099-0488"]}, DOI={10.1002/polb.23167}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Cochran, Justin E. and Amir, Elizabeth and Sivanandan, Kulandaivelu and Ku, Sung-Yu and Seo, Jung Hwa and Collins, Brian A. and Tumbleston, John R. and Toney, Michael F. and Ade, Harald and Hawker, Craig J. and et al.}, year={2013}, month={Jan}, pages={48–56} }
 @article{bartelt_beiley_hoke_mateker_douglas_collins_tumbleston_graham_amassian_ade_et al._2013, title={The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells}, volume={3}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201200637}, abstractNote={Abstract}, number={3}, journal={ADVANCED ENERGY MATERIALS}, author={Bartelt, Jonathan A. and Beiley, Zach M. and Hoke, Eric T. and Mateker, William R. and Douglas, Jessica D. and Collins, Brian A. and Tumbleston, John R. and Graham, Kenneth R. and Amassian, Aram and Ade, Harald and et al.}, year={2013}, month={Mar}, pages={364–374} }
 @article{yan_collins_gann_wang_ade_mcneill_2012, title={Correlating the Efficiency and Nanomorphology of Polymer Blend Solar Cells Utilizing Resonant Soft X-ray Scattering}, volume={6}, ISSN={["1936-086X"]}, DOI={10.1021/nn204150f}, abstractNote={Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (F8TBT) or poly([N,N'-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-12-bithiophene)) (P(NDI2OD-T2)). Both P3HT:F8TBT and P3HT:P(NDI2OD-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of size ~5-10 nm that evolve with annealing and larger domains of size ~100 nm that are insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length scale. For both P3HT:P(NDI2OD-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5-10 nm length scale. Grazing-incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI2OD-T2) crystallites 25-40 nm in thickness that are embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is also observed in blends with P(NDI2OD-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness.}, number={1}, journal={ACS NANO}, author={Yan, Hongping and Collins, Brian A. and Gann, Eliot and Wang, Cheng and Ade, Harald and McNeill, Christopher R.}, year={2012}, month={Jan}, pages={677–688} }
 @article{collins_cochran_yan_gann_hub_fink_wang_schuettfort_mcneill_chabinyc_et al._2012, title={Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films}, volume={11}, ISSN={1476-1122 1476-4660}, url={http://dx.doi.org/10.1038/nmat3310}, DOI={10.1038/nmat3310}, abstractNote={Molecular orientation critically influences the mechanical, chemical, optical and electronic properties of organic materials. So far, molecular-scale ordering in soft matter could be characterized with X-ray or electron microscopy techniques only if the sample exhibited sufficient crystallinity. Here, we show that the resonant scattering of polarized soft X-rays (P-SoXS) by molecular orbitals is not limited by crystallinity and that it can be used to probe molecular orientation down to size scales of 10 nm. We first apply the technique on highly crystalline small-molecule thin films and subsequently use its high sensitivity to probe the impact of liquid-crystalline ordering on charge mobility in polymeric transistors. P-SoXS also reveals scattering anisotropy in amorphous domains of all-polymer organic solar cells where interfacial interactions pattern orientational alignment in the matrix phase, which probably plays an important role in the photophysics. The energy and q-dependence of the scattering anisotropy allows the identification of the composition and the degree of orientational order in the domains.}, number={6}, journal={Nature Materials}, publisher={Springer Nature}, author={Collins, B. A. and Cochran, J. E. and Yan, H. and Gann, E. and Hub, C. and Fink, R. and Wang, C. and Schuettfort, T. and McNeill, C. R. and Chabinyc, M. L. and et al.}, year={2012}, month={Apr}, pages={536–543} }
 @article{collins_ade_2012, title={Quantitative compositional analysis of organic thin films using transmission NEXAFS spectroscopy in an X-ray microscope}, volume={185}, ISSN={["1873-2526"]}, DOI={10.1016/j.elspec.2012.05.002}, abstractNote={Near edge X-ray absorption fine structure (NEXAFS) spectroscopy is well suited for the quantitative determination of the composition of soft matter thin films. Combined with the high spatial resolution of a scanning transmission X-ray microscope, compositional maps of submicron morphologies can be derived and have been used successfully to characterize a number of materials systems. However, multiple sources of known systematic errors limit the accuracy and are frequently not taken into account. We show that these errors can be significant (more than 10%) and demonstrate simple methods to eliminate them. With suitable precautions, a compositional measurement can be made on a thin film sample in a matter of minutes with sub-micron spatial resolution and sub-percent compositional precision. NEXAFS measurements are furthermore known to be sensitive to anisotropic molecular orientation and a strategy to account for that and extract preferential molecular orientation relative to a reference is presented. The spatial resolution of the measurement can be increased to below 100 nm at the expense of compositional precision, depending on the point spread function of the zone plate focusing optics of the microscope.}, number={5-7}, journal={JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA}, author={Collins, Brian A. and Ade, Harald}, year={2012}, month={Aug}, pages={119–128} }
 @article{gann_young_collins_yan_nasiatka_padmore_ade_hexemer_wang_2012, title={Soft x-ray scattering facility at the Advanced Light Source with real-time data processing and analysis}, volume={83}, ISSN={["1089-7623"]}, DOI={10.1063/1.3701831}, abstractNote={We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.}, number={4}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Gann, E. and Young, A. T. and Collins, B. A. and Yan, H. and Nasiatka, J. and Padmore, H. A. and Ade, H. and Hexemer, A. and Wang, C.}, year={2012}, month={Apr} }
 @article{he_collins_watts_ade_mcneill_2012, title={Studying Polymer/Fullerene Intermixing and Miscibility in Laterally Patterned Films with X-Ray Spectromicroscopy}, volume={8}, ISSN={["1613-6810"]}, DOI={10.1002/smll.201102382}, abstractNote={Abstract}, number={12}, journal={SMALL}, author={He, Xiaoxi and Collins, Brian A. and Watts, Benjamin and Ade, Harald and McNeill, Christopher R.}, year={2012}, month={Jun}, pages={1920–1927} }
 @article{collins_li_mcneill_ade_2011, title={Fullerene-Dependent Miscibility in the Silole-Containing Copolymer PSBTBT-08}, volume={44}, ISSN={["1520-5835"]}, DOI={10.1021/ma201883a}, abstractNote={A high fullerene molecular miscibility of over 40 wt % is found in the copolymer poly((4,4-octyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT-08)—a member of the PSBTBT low-bandgap polymer family that have produced power conversion efficiencies as high as 5.9% in solar cells. This observation suggests molecular miscibility plays a key role in the photovoltaic effect in this system. The level of miscibility is additionally measured to be highly dependent on the fullerene species with significant differences between C60- and C70-based fullerenes, highlighting a new parameter to be monitored and controlled when considering different fullerene moieties and species in organic solar cells. Surprisingly, a wide-angle X-ray scattering study reveals no significant crystallinity in the PSBTBT with octyl side chains, potentially the cause of low mobilities and in stark contrast to dodecyl and ethylhexyl PSBTBTs, which demonstrates the importance of the side chain in device mo...}, number={24}, journal={MACROMOLECULES}, author={Collins, Brian A. and Li, Zhe and McNeill, Christopher R. and Ade, Harald}, year={2011}, month={Dec}, pages={9747–9751} }
 @article{collins_tumbleston_ade_2011, title={Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells: Toward an Enlightened Understanding of Device Morphology and Stability}, volume={2}, ISSN={["1948-7185"]}, DOI={10.1021/jz2014902}, abstractNote={Rapid improvements in organic solar cell efficiency have brought these devices into the spotlight as a potential source of abundant electricity. Despite much empirical progress, fundamental understanding is still lacking. Due to the required three-dimentional nanoscale morphology, determining structure–performance relationships has been a major challenge, and a convergent understanding has yet to emerge. We discuss recent major advances in delineating the characteristics of the most common organic solar cells. The large variations in device performance reported, the contradictory morphologies observed, and the determination of the underlying driving forces need to be resolved. Deeper understanding that can provide a roadmap to improved devices will only occur through refined consideration of material characteristics and fabrication procedures in conjunction with increased use of advanced characterization. We additionally highlight the recently discovered partial miscibility of the component materials, its...}, number={24}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Collins, Brian A. and Tumbleston, John R. and Ade, Harald}, year={2011}, month={Dec}, pages={3135–3145} }
 @article{collins_gann_guignard_he_mcneill_ade_2010, title={Molecular Miscibility of Polymer-Fullerene Blends}, volume={1}, ISSN={["1948-7185"]}, DOI={10.1021/jz101276h}, abstractNote={The device function of polymer bulk heterojunction (BHJ) solar cells has been commonly interpreted to arise from charge separation at discrete interfaces between phase-separated materials and subsequent charge transport through these phases without consideration of phase purity. To probe composition, the miscibility of poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) with phenyl-C61-butyric acid methyl ester (PCBM) has been determined, while the effects of polymer crystallinity on miscibility are probed using P3HT grades of varying regioregularity. It is found that, while no intercalation occurs in P3HT crystals, amorphous portions of P3HT and MDMO-PPV contain significant concentrations of PCBM, calling into question models based on pure phases and discrete interfaces. Furthermore, depth profiles of P3HT/PCBM bilayers reveal that even short annealing causes significant interdiffusion of both materials, showing that under no conditions do pure am...}, number={21}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Collins, Brian A. and Gann, Eliot and Guignard, Lewis and He, Xiaoxi and McNeill, Christopher R. and Ade, Harald}, year={2010}, month={Nov}, pages={3160–3166} }