Linking charge carrier recombination to photovoltaic responses in organic solar cells with Y-shaped non-fullerene acceptors

Lorena Perdigon-Toro^a^,^b, Le Quang Phuong^b, Yingping Zou^c, Safa Shoaee^b, Dieter Neher^a

^aUniversity of Potsdam, Soft Mater Physics, Institute of Physics and Astronomy, Germany

^bUniversity of Potsdam, Optoelectronics of Disordered Semiconductors, Institute of Physics and Astronomy, Germany

^cCentral South University, College of Chemistry and Chemical Engineering, China, Yuelu District, Changsha, China, 410083, Changsha, China

Materials for Sustainable Development Conference (MATSUS)
Proceedings of Online nanoGe Fall Meeting 20 (OnlineNFM20)

#NewOPV20. Non-fullerene Electron acceptors Within Organic Photovoltaics

Online, Spain, 2020 October 20th - 23rd

Organizers: Vida Engmann and Morten Madsen

Contributed talk, Lorena Perdigon-Toro, presentation 161
Publication date: 4th October 2020

Organic solar cells based on Y-shaped non-fullerene acceptors, such as Y6, have gained considerable attention because of their high performance in single layer devices [1]. We recently demonstrated that free charge generation in the blend of Y6 with the donor polymer PM6 is barrierless [2]. It was proposed that the efficient charge separation in this blend is related to the high morphological order in this blend. In this work, we compare the charge carrier dynamics of PM6:Y6 with the blend of PM6 with a less-aggregating Y6-derivative, denoted as N4 [3]. Interestingly, the later blend performs fairly well but displays a significantly lower open-circuit voltage (V_OC). This hints at different recombination properties. By performing time delayed collection field (TDCF) and bias assisted charge extraction (BACE) experiments, we found that in both systems, bimolecular recombination is the dominant loss under illuminations equivalent to 1 sun. Surprisingly, the bimolecular recombination rate in PM6:N4 is about five times lower than that in PM6:Y6, in apparent contradiction to the lower V_OC of the blend. To study the reasons behind this discrepancy, quasi-steady-state photoinduced absorption (PIA) was applied to the blend film and to the full device to determine the steady state carrier density without and with the presence of electrodes. These measurements consistently unravel a higher concentration of charge carriers in the PM6:N4 blend under comparable illuminations while at the same time ruling out a significant contribution from recombination losses at the interfaces and across the transporting layers in these devices. Thus, considering the data for bimolecular recombination rates, the carrier concentrations and the negligible surface recombination, we propose that the smaller V_OC of the PM6:N4 blend is due to larger energetic disorder and a lower energy of the charge transfer states – highlighting the need to establish high structural and energetic order to realize efficient devices.

References:

[1] Yuan, J.; Zhang, Y.; Zhou, L.; Zhang, G.; Yip, H.-L.; Lau, T.-K.; Lu, X.; Zhu, C.; Peng, H.; Johnson, P. A.; et al. Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with Electron-Deficient Core. Joule 2019, 3, 1140–1151.

[2] Perdigón-Toro, L.; Zhang, H.; Markina, A.; Yuan, J.; Hosseini, S. M.; Wolff, C. M.; Zuo, G.; Stolterfoht, M.; Zou, Y.; Gao, F.; et al. Barrierless Free Charge Generation in the High-Performance PM6:Y6 Bulk Heterojunction Non-Fullerene Solar Cell. Adv. Mater. 2020, 1906763.

[3] Jiang, K.; Wei, Q.; Lai, J. Y. L.; Peng, Z.; Kim, H. K.; Yuan, J.; Ye, L.; Ade, H.; Zou, Y.; Yan, H. Alkyl Chain Tuning of Small Molecule Acceptors for Efficient Organic Solar Cells. Joule 2019, 3, 3020–3033.

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