A quasi steady-state measurement of exciton diffusion lengths in organic semiconductors

TL;DR

This paper introduces pulsed-PLQY, a new method for measuring exciton diffusion lengths in organic semiconductors that does not require time-resolved measurements, and demonstrates its advantages over traditional TRPL techniques.

Contribution

The paper presents pulsed-PLQY as a novel, less equipment-intensive method for measuring exciton diffusion lengths, validated through simulations and experiments on organic semiconductors.

Findings

Pulsed-PLQY has a larger operational region than TRPL.

Diffusion lengths in NFA semiconductors are significantly larger than in fullerenes.

Increased diffusion lengths are driven by higher diffusivity in NFA materials.

Abstract

Understanding the role that exciton diffusion plays in organic solar cells is a crucial to understanding the recent rise in power conversion effciencies brought about by non-fullerene acceptors (NFA). Established methods for measuring exciton diffusion lengths in organic solar cells require specialized equipment designed for measuring high-resolution time-resolved photoluminescence (TRPL). Here we introduce a technique, coined pulsed-PLQY, to measure the diffusion length of organic solar cells without any temporal measurements. Using a Monte-Carlo model we simulate the dynamics within a thin film semiconductor and analyse the results using both pulsed-PLQY and TRPL methods. We find that pulsed-PLQY has a larger operational region and depends less on the excitation fuence than the TRPL approach. We validate these simulated results by preforming both measurements on organic thin films and reproduce the predicted trends. Pulsed-PLQY is then used to evaluate the diffusion length in a variety of technologically relevant organic semiconductors. It is found that the diffusion lengths in NFA's are much larger than in the benchmark fullerene and that this increase is driven by an increase in diffusivity.