Elucidating The Performance Impeding Role of Exciton Dissociation in Low Offset Nonfullerene Acceptor-Based Solar Cells

The performance of organic solar cells has made large strides with power conversion efficiencies exceeding 19%, and the milestone of 20% well within sight¹. The emergence of non-fullerene acceptors (NFAs) has played a vital role in these advancements². Particularly, organic blends comprising of NFAs with polymeric donors (D) having low energetic offset between the ionization energies (Δ<i>E</i>_IE) of the two components have demonstrated excellent photovoltaic performance with superior charge generation yields in conjunction with reduced voltage losses^3,4. However, the process of free charge generation and the origin of performance limiting loss pathways has been a subject of debate. More specifically, the critical role of low Δ<i>E</i>_IE on the voltage losses and charge-generation efficiencies ask for a more detailed analysis. In this work, we systematically explore the role of energetic offset through methodical assessment of free charge generation process in a sample set of Y-series acceptors (Y6 and Y5) with well know polymeric donor, PM6. Herein, the PM6:Y5 material system is found to have a relatively lower energetic offset as compared to high performing system PM6:Y6. Our sample set uses the NFAs Y5 and Y6 blended with different molecular weights of the polymer donor PM6, spanning a large PCE range from 15% to 1%. This poor photovoltaic performance is further accompanied with pronounced field-dependence of free charge generation as demonstrate <i>via</i> time delayed collection field (TDCF) measurements. Using transient absorption spectroscopy (TAS), we find that the poor performing PM6:Y5 material system suffers from inefficient charge transfer at the interface, ultimately limiting the overall photovoltaic performance of Y5 based blends. We highlight the significance of driving force through field-dependent TAS measurements by demonstrating concomitant increment in free charge generation and exciton dissociation yields under the application of external electric field. These results supported with bias-dependent steady-state and transient photoluminescence studies provides a holistic view of the overall process and propounds that poor exciton dissociation is one of the main performance limiting channel in materials systems with diminishing energetic offset.<br/><br/><b>References.</b><br/>1. Liu, F., Zhou, L., Liu, W., Zhou, Z., Yue, Q., Zheng, W., Sun, R., Liu, W. Y., Xu, S., Fan, H., Feng, L., Yi, Y., Zhang, W., Zhu, X., Organic Solar Cells with 18% Efficiency Enabled by an Alloy Acceptor: A Two-in-One Strategy<i>. Adv. Mater. </i>2021, <i>33</i>, 2100830<br/>2. Armin, A., Li, W., Sandberg, O. J., Xiao, Z., Ding, L., Nelson, J., Neher, D., Vandewal, K., Shoaee, S., Wang, T., Ade, H., Heumüller, T., Brabec, C., Meredith, P., A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells. <i>Adv. Energy Mater. </i>2021, <i>11</i>, 2003570.<br/>3. Bertrandie, J., Han, J., De, C. S. P., Yengel, E., Gorenflot, J., Anthopoulos, T., Laquai, F., Sharma, A., Baran, D., The Energy Level Conundrum of Organic Semiconductors in Solar Cells. <i>Adv. Mater.</i> 2022, <i>34</i>, 2202575<br/>4. Zhong, Y., Causa’, M., Moore, G.J. et al. Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers. <i>Nat Commun. </i>2020<i>,</i> <i>11</i>, 833.