Organic Solar Cells based on Y-Series Non-Fullerene Acceptors: From Charge Separation to Device Performance

Dieter Neher^a, Lorena Perdigón-Toro^a, Nurlan Tokmoldin^a, Le Phuong^a, Guangzheng Zuo^a, Safa Shoai^a

^aInstitute of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Spring Meeting 2022 (NSM22)

#OrgCD22. Charge Dynamics of State-of-the-art Organic Solar Cells

Online, Spain, 2022 March 7th - 11th

Organizers: Ardalan Armin and Safa Shoaee

Invited Speaker, Dieter Neher, presentation 178
DOI: https://doi.org/10.29363/nanoge.nsm.2022.178
Publication date: 7th February 2022

Organic solar cells (OSCs) are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs) [1]. One particularly successful type of NFAs are Y-type moleucles such as Y6 [2]. In blends with PM6 and other donor polymers, high efficiency (>18 %) have been recently achieved in single junction devices, with the perspective to approach the commercially relevant 20 %. Here, we summarizes our recent understanding of the processes governing the performance of OSCS based on Y6 and related compounds. For PM6:Y6, we find that free charge generation is essentially barrierless[3] and that the fill factor of the device is essentially limited by the diffusion length of the charges, which is smaller than the active layer thickness [4]. This puts the understanding of the recombination processes at the focus of the further understanding. By studying the emission properties of such cells, we conclude that the radiative recombination of free charges in Y6-based cells proceeds almost entirely through the re-occupation and decay of the Y6 singlet, but that this pathway contributes to less than 1 % of the total recombination current [5]. As such, the open-circuit voltage of the PM6:Y6 blend is almost entirely determined by the energetics and kinetics of the charge transfer (CT) state, irrespective of the position and emission properties of the singlet state. Most recently, we addressed the role of energetic disorder in the competition between charge extraction and recombination [6] By performing temperature dependent charge transport and recombination studies, we come to a consistent picture of the the density of state distributions for free charges, which allows us to analytically describe the dependence of the open-circuit voltage on temperature and illumination intensity. We conclude that energetic disorder of charge separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.

References:

[1] (1) Green, M. A.; Dunlop, E. D.; Hohl-Ebinger, J.; Yoshita, M.; Kopidakis, N.; Hao, X. Solar Cell Efficiency Tables (Version 58). Prog. Photovoltaics Res. Appl. 2021, 29 (7), 657–667.

[2] (1) Yuan, J.; Zhang, Y.; Zhou, L.; Zhang, G.; Yip, H.-L.; Lau, T.-K.; Lu, X.; Zhu, C.; Peng, H.; Johnson, P. A.; Leclerc, M.; Cao, Y.; Ulanski, J.; Li, Y.; Zou, Y. Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with Electron-Deficient Core. Joule 2019, 3 (4), 1140–1151.

[3] (1) Perdigón‐Toro, L.; Zhang, H.; Markina, A.; Yuan, J.; Hosseini, S. M.; Wolff, C. M.; Zuo, G.; Stolterfoht, M.; Zou, Y.; Gao, F.; Andrienko, D.; Shoaee, S.; Neher, D. Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell. Adv. Mater. 2020, 32 (9), 1906763.

[4] (1) Tokmoldin, N.; Vollbrecht, J.; Hosseini, S. M.; Sun, B.; Perdigón‐Toro, L.; Woo, H. Y.; Zou, Y.; Neher, D.; Shoaee, S. Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths. Adv. Energy Mater. 2021, 2100804.

[5] (1) Perdigón-Toro, L.; Phuong, L. Q.; Zeiske, S.; Vandewal, K.; Armin, A.; Shoaee, S.; Neher, D. Excitons Dominate the Emission from PM6:Y6 Solar Cells, but This Does Not Help the Open-Circuit Voltage of the Device. ACS Energy Lett. 2021, 6 (2), 557–564.

[6] (1) Zuo, G.; Shoaee, S.; Kemerink, M.; Neher, D. General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells. Phys. Rev. Appl. 2021, 16 (3), 034027.

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