Energy Gap Dependence of Singlet Exciton Lifetimes in Non-Fullerene Acceptors
Tomoki Ohmura a, Shun Yamaguchi a, Kota Tsujioka a, Yasunari Tamai a b, Hideo Ohkita a
a Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Japan
b PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
Poster, Tomoki Ohmura, 127
Publication date: 23rd October 2018

Non-fullerene acceptors have great contribution to improving a power conversion efficiency of organic photovoltaics these days because of their superior light-harvesting properties in the near-IR region and their highly tunable molecular energy levels compared with fullerene counterparts. However, little is known about the reason why non-fullerene acceptors can enhance the photocurrent generation effectively even in the near-IR region. In order to address the origin of the efficient photocurrent generation in the near-IR region, we focused on the exciton diffusion, which is limited by lifetime in the excited state. We measured singlet exciton lifetimes of various non-fullerene acceptor neat films (IEICO-4F, 3TT-FIC, IEICO, IT-4F, ITIC, IT-M, O-IDTBR, O-IDFBR, and FBR) by transient absorption spectroscopy. For comparison, we also measured singlet exciton lifetimes of various conjugated polymer films. By using PL quantum efficiency, we evaluated non-radiative decay rates of various non-fullerene acceptors and conjugated donor polymers with different bandgaps. As a result, we found that non-radiative decay rates of non-fullerene acceptors have weaker energy gap dependence than that of conjugated polymers. Interestingly, non-radiative decay rates are as slow as ~2 × 1010 s-1 for non-fullerene acceptors while they are as fast as ~10 × 11 s-1 for conjugated polymers at around 1000 nm. We therefore conclude that the effective photocurrent generation is partially due to longer exciton lifetimes of non-fullerene acceptors in the near-IR region.

We would like to thank Professor Osaka at Hiroshima University for providing non-fullerene acceptors 3TT-FIC, IEICO, and IT-4F. This study was partly supported by JST ALCA Grant Number JPMJAL1404 and JSPS KAKENHI Grant Number JP26248033, Japan.

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