[Paper Review] A quasi steady-state measurement of exciton diffusion lengths in organic semiconductors

Drew B. Riley, Oskar J. Sandberg+3 more|arXiv (Cornell University)|Sep 2, 2021

Organic Electronics and Photovoltaics55 references25 citations

TL;DR

This paper introduces pulsed-PLQY, a novel method to measure exciton diffusion lengths in organic semiconductors without temporal resolution, using Monte Carlo simulations and experimental validation. It demonstrates that pulsed-PLQY offers a larger operational window and reduced sensitivity to excitation fluence compared to traditional TRPL-linearization, and reveals that non-fullerene acceptors exhibit significantly longer diffusion lengths due to enhanced diffusivity, explaining their high charge generation yields in solar cells.

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.

Motivation & Objective

• . To develop a quasi-steady-state method for measuring exciton diffusion lengths in organic semiconductors without requiring time-resolved photoluminescence (TRPL) measurements.
• . To evaluate the limitations of conventional TRPL-linearization in exciton diffusion length extraction under varying excitation densities.
• . To validate a new method, pulsed-PLQY, via Monte Carlo simulations and experimental testing on P3HT.
• . To quantify diffusion lengths in non-fullerene acceptors (NFAs) and compare them with fullerene-based systems to understand their high performance in organic solar cells.
• . To clarify the role of increased diffusivity versus lifetime in enabling high charge generation yields in low-offset NFA solar cells.

Proposed method

• . Uses a Monte Carlo hopping model to simulate exciton dynamics in thin-film organic semiconductors, including diffusion, recombination, and exciton-exciton annihilation (EEA).
• . Implements two analysis methods: traditional TRPL-linearization and the proposed pulsed-PLQY, which extracts diffusion length from steady-state photoluminescence quantum yield (PLQY) at varying excitation densities.
• . Defines and evaluates the 'operational window'—the range of excitation densities where the extracted diffusion length matches the input value in simulations.
• . Validates the pulsed-PLQY method experimentally on P3HT, a well-characterized organic semiconductor, and compares results with TRPL.
• . Applies pulsed-PLQY to measure diffusion lengths in a range of technologically relevant materials, including non-fullerene acceptors (NFAs).
• . Uses the annihilation coefficient and assumed capture radius (from GIWAXS d100 spacing) to extract diffusion length from EEA data in both simulation and experiment.

Experimental results

Research questions

• RQ1 . Can a quasi-steady-state method like pulsed-PLQY accurately determine exciton diffusion lengths in organic semiconductors without time-resolved measurements?
• RQ2 . How does the operational window of pulsed-PLQY compare to that of TRPL-linearization in terms of excitation density range and robustness?
• RQ3 . What is the role of diffusivity versus exciton lifetime in the enhanced performance of non-fullerene acceptor (NFA) organic solar cells?
• RQ4 . How do the diffusion lengths in NFAs compare to those in benchmark fullerene-based systems?
• RQ5 . To what extent does the capture radius influence diffusion length extraction, and can it be reliably estimated from structural data?

Key findings

• . Pulsed-PLQY exhibits a larger operational window than TRPL-linearization, enabling reliable diffusion length extraction over a wider range of excitation densities.
• . Pulsed-PLQY is less sensitive to excitation fluence than TRPL-linearization, making it more robust for practical applications.
• . The diffusion length in non-fullerene acceptors (NFAs) is significantly larger than in fullerene-based systems, with the increase primarily driven by enhanced diffusivity rather than longer exciton lifetime.
• . In the PM6:Y6 NFA system, the diffusion length exceeds 100 nm, consistent with high charge generation yields in low-offset solar cells.
• . The study confirms that the d100 lamellar spacing from GIWAXS measurements provides a reliable estimate of the exciton capture radius.
• . Experimental validation on P3HT shows good agreement between pulsed-PLQY and TRPL results, confirming the method’s accuracy and reproducibility.