Quasi-Steady-State Measurement of Exciton Diffusion Lengths in Organic Semiconductors

Abstract

This paper is a contribution to the Physical Review Applied collection titled Photovoltaic Energy Conversion. Exciton diffusion plays a decisive role in various organic optoelectronic applications, including lasing, photodiodes, light-emitting diodes, and solar cells. Understanding the role that exciton diffusion plays in organic solar cells is crucial to understanding the recent rise in power conversion efficiencies brought about by nonfullerene acceptor (NFA) molecules. Established methods for quantifying exciton diffusion lengths in organic semiconductors require specialized equipment designed for measuring high-resolution time-resolved photoluminescence (TRPL). In this paper we introduce an approach, named pulsed-photoluminescence quantum yield (PLQY), to determine the diffusion length of excitons in organic semiconductors without any temporal measurements. Using a Monte Carlo model, the dynamics within a thin-film semiconductor are simulated and the results are analyzed using both pulsed-PLQY and TRPL methods. It is found that pulsed-PLQY has a larger operational window and depends less on the excitation fluence than the TRPL approach. The simulated results are validated experimentally on a well-understood organic semiconductor, after which pulsed-PLQY is used to evaluate the diffusion length in a variety of technologically relevant materials. It is found that the diffusion lengths in NFAs are much larger than in the benchmark fullerene and that this increase is driven by an increase in diffusivity. This result helps explain the high charge generation yield in low-offset state-of-the-art NFA solar cells.