Singlet Exciton Decay is The Main Competing Pathway to Free Charge Generation in Organic Solar Cells with Low Energetic Offset

Since the advent of low bandgap non-fullerene acceptors (NFAs), the performance of organic solar cells (OSCs) has improved significantly. In combination with donor materials with complementary absorption, such NFA-based bulk heterojunction blends show efficient conversion of photons into electrons over a broad spectral range. Therefore, an important topic of current OSC research is to reduce the voltage loss by non-radiative recombination while maintaining the short-circuit current (Jsc) and the fill factor (FF) at a high level. The most popular approach to optimize the open circuit voltage of NFA-based blends is to reduce the HOMO-HOMO energy offset. Unfortunately, this is generally accompanied by a significant reduction of Jsc and FF - the reason for this being controversially discussed in the literature.^[1–4]<br/><br/>Here we combine a wide range of methods, from spectroelectrochemistry to determine the blend energetics, to femtosecond transient absorption to study the early excitation dynamics, to bias-dependent photoluminescence spectroscopy in the steady state, to study the mechanisms and efficiency of free charge generation in relation to the HOMO-HOMO offset in selected Y-based blends. We find no evidence of the hybridisation of the interfacial charge transfer (CT) state with the local singlet exciton (LE), nor do we observe significant losses due to non-radiative CT recombination competing with charge separation. Instead, singlet exciton decay is identified as the main competing pathway for free charge generation in low energy offset OSCs. This interpretation is consistent with the observed strong decrease in Jsc when the HOMO-HOMO offset decreases from 0.32 eV to 0.17 eV. Our results show that the optimal range of the HOMO-HOMO offset for achieving optimal performance is quite narrow and lies at 0.3 eV - the exciton binding energy of Y6 aggregates.^[5]<br/><br/>[1] B. Sun, N. Tokmoldin, O. Alqahtani, A. Patterson, C. S. P. De Castro, D. B. Riley, M. Pranav, A. Armin, F. Laquai, B. A. Collins, D. Neher, S. Shoaee, <i>Adv. Energy Mater.</i> <b>2023</b>, <i>2300980</i>, DOI 10.1002/aenm.202300980.<br/>[2] M. Pranav, T. Hultzsch, A. Musiienko, B. Sun, A. Shukla, F. Jaiser, S. Shoaee, D. Neher, <i>APL Mater.</i> <b>2023</b>, <i>11</i>, DOI 10.1063/5.0151580.<br/>[3] D. Qian, S. M. Pratik, Q. Liu, Y. Dong, R. Zhang, J. Yu, N. Gasparini, J. Wu, T. Zhang, V. Coropceanu, X. Guo, M. Zhang, J. L. Bredas, F. Gao, J. R. Durrant, <i>Adv. Energy Mater.</i> <b>2023</b>, <i>2301026</i>, 1.<br/>[4] J. S. Müller, M. Comí, F. Eisner, M. Azzouzi, D. Herrera Ruiz, J. Yan, S. S. Attar, M. Al-Hashimi, J. Nelson, <i>ACS Energy Lett.</i> <b>2023</b>, <i>8</i>, 3387.<br/>[5] S. Shoaee, H. M. Luong, J. Song, Y. Zou, T. Nguyen, D. Neher, <i>Adv. Mater.</i> <b>2023</b>, DOI 10.1002/adma.202302005.