Charge Carrier Dynamics in Low-offset Organic Solar Cells
Yifan Dong a, Hyojung Cha b, James R Durrant a
a Department of Chemistry, MSRH, White City Campus, Imperial College London, Imperial College Road, London, United Kingdom
b Kyungpook National University
Proceedings of SUNRISE September Symposium 2021 ‘Powering Green Recovery’ (SUNRISEIII)
Online, Spain, 2021 September 20th - 22nd
Organizers: Hari Upadhyaya, Adrian Walters, James Durrant, Sara Walters and Georgia Bevan
Invited Speaker, Yifan Dong, presentation 037
Publication date: 14th September 2021

Organic solar cells (OSCs) typically employ electron donor and acceptor materials to faciliate efficient charge generation from excitons. Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss (Eg/q – VOC) in polymer:non-fullerene acceptor (NFA) OSCs. While a lot of studies have investigated the CT state energetics and its correlation with device photovoltage as well as voltage losses, the CT state dynamics still remains relatively unexplored. In my talk, I will discuss how transient absorption spectroscopy (TAS) is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. TAS measurements show that the CT state lifetime follows an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset is reduced. These results highlight how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

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