@article{ho_pei_qin_zhang_peng_angunawela_jones_yin_iqbal_reynolds_et al._2022, title={Importance of Electric-Field-Independent Mobilities in Thick-Film Organic Solar Cells}, volume={10}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.2c11265}, DOI={10.1021/acsami.2c11265}, abstractNote={In organic solar cells (OSCs), a thick active layer usually yields a higher photocurrent with broader optical absorption than a thin active layer. In fact, a ∼300 nm thick active layer is more compatible with large-area processing methods and theoretically should be a better spot for efficiency optimization. However, the bottleneck of developing high-efficiency thick-film OSCs is the loss in fill factor (FF). The origin of the FF loss is not clearly understood, and there a direct method to identify photoactive materials for high-efficiency thick-film OSCs is lacking. Here, we demonstrate that the mobility field-dependent coefficient is an important parameter directly determining the FF loss in thick-film OSCs. Simulation results based on the drift-diffusion model reveal that a mobility field-dependent coefficient smaller than 10-3 (V/cm)-1/2 is required to maintain a good FF in thick-film devices. To confirm our simulation results, we studied the performance of two ternary bulk heterojunction (BHJ) blends, PTQ10:N3:PC71BM and PM6:N3:PC71BM. We found that the PTQ10 blend film has weaker field-dependent mobilities, giving rise to a more balanced electron-hole transport at low fields. While both the PM6 blend and PTQ10 blend yield good performance in thin-film devices (∼100 nm), only the PTQ10 blend can retain a FF = 74% with an active layer thickness of up to 300 nm. Combining the benefits of a higher JSC in thick-film devices, we achieved a PCE of 16.8% in a 300 nm thick PTQ10:N3:PC71BM OSC. Such a high FF in the thick-film PTQ10 blend is also consistent with the observation of lower charge recombination from light-intensity-dependent measurements and lower energetic disorder observed in photothermal deflection spectroscopy.}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Ho, Carr Hoi Yi and Pei, Yusen and Qin, Yunpeng and Zhang, Chujun and Peng, Zhengxing and Angunawela, Indunil and Jones, Austin L. and Yin, Hang and Iqbal, Hamna F. and Reynolds, John R. and et al.}, year={2022}, month={Oct} }
 @article{jones_ho_schneider_zhang_pei_wang_zhan_marder_toney_so_et al._2022, title={Insights into the Local Bulk-Heterojunction Packing Interactions and Donor-Acceptor Energy Level Offsets in Scalable Photovoltaic Polymers}, volume={7}, ISSN={["1520-5002"]}, url={http://dx.doi.org/10.1021/acs.chemmater.2c01121}, DOI={10.1021/acs.chemmater.2c01121}, abstractNote={An energy level offset in organic solar cells (OSCs) is necessary for efficient charge generation and separation. To date, there are several polymer donor–non-fullerene acceptor (NFA) bulk-heterojunction (BHJ) systems with a negligible ionization energy (IE) level offset achieving high power conversion efficiencies (PCEs) over 15%. Although these donor–acceptor pairs perform well in solar cells, there is little understanding on why some systems can achieve this phenomenon, and therefore, many of these BHJs are discovered through a trial-and-error process. Here, we investigate how OSC efficiencies can be modulated by adjusting the IE level offset in a series of PTQ10 n:m random terpolymer donors by means of solar cell performance (open circuit voltage (VOC) and short-circuit current (JSC)) when paired with Y6 and IDIC acceptors. PTQ10’s IE level was adjusted through a copolymerization of thiophene (n), bithiophene (m), and quinoxaline monomer units in different ratios, whereby 10% bithiophene leads to a 0.05 eV decrease in the polymer’s IE. The incorporation of 10% bithiophene (PTQ10 90:10) led to a 1.3 ± 0.5 mA/cm2 increase in JSC when paired with Y6 (PCE = 13.8 ± 0.4%) in conjunction with an incremental decrease in VOC and fill factor (FF) when compared to PTQ10 (PCE = 14.7 ± 0.1%). Increasing the bithiophene content to 20% (PTQ10 80:20) exacerbated the decrease in VOC and FF further without the benefit of increased JSC. The drop in FF with increasing bithiophene incorporation correlated with increasing edge-on orientation in the neat polymer and polymer:Y6 BHJ blend films, shown by grazing-incidence wide-angle X-ray scattering measurements. High-field solid-state (ss)NMR spectroscopy analysis of single component PTQ10, Y6, and PTQ10:Y6 BHJ blends provides a complementary insight into how a low IE level offset system (PTQ10:Y6) imparts high performance. By resolving inter- and intramolecular packing interactions at sub-nanometer distances, ssNMR results offer key insights into the changes in local structures and conformations in the vicinity of the alkoxy PTQ10 side chains and in the Y6 end group in BHJ blends when compared to the neat compounds. Despite the changes in local structures, the BHJ morphology maintains pure D-A domains and preserves the microstructure, which correlates with the high-performing solar cells. A synergic combination of chemical design, multiscale morphology characterization, and device physics shown in this study provides an excellent strategy to investigate the BHJ and its role in organic solar cell performance.}, journal={CHEMISTRY OF MATERIALS}, author={Jones, Austin L. and Ho, Carr Hoi Yi and Schneider, Sebastian A. and Zhang, Junxiang and Pei, Yusen and Wang, Jiayu and Zhan, Xiaowei and Marder, Seth R. and Toney, Michael F. and So, Franky and et al.}, year={2022}, month={Jul} }