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 (ΔE_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 ΔE_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 via 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.

References.
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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. Adv. Energy Mater. 2021, 11, 2003570.
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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. Nat Commun. 2020, 11, 833.