Understanding the fill-factor limit of organic solar cells

TL;DR

This paper investigates the fundamental factors limiting the fill factor in organic solar cells, developing an analytical model to understand how electric field effects influence efficiency, and suggests strategies to improve performance.

Contribution

It provides a comprehensive analysis of the FF in OSCs, introduces a new model linking electric field and excited state energetics, and proposes methods to enhance FF by managing exciton dynamics.

Findings

Stark effect and field-dependent charge transfer impact FF significantly.

Increasing exciton lifetime can suppress geminate decay and boost FF.

Analytical model accurately correlates electric field with excited state energetics.

Abstract

Although the power conversion efficiencies of organic solar cells (OSCs) have surpassed 20%, they still lag behind commercial inorganic solar cells and emerging perovskite solar cells. To bridge this efficiency gap, improving the fill factor (FF) is critical, provided other photovoltaic parameters are not compromised. However, the fundamental understanding of the FF in OSCs remains incomplete. In this work, we systematically investigate a wide range of OSCs with the FF values spanning 0.27 to 0.80, and analyse the effect of free charge generation and recombination on the FF in OSCs. To explain our observations, we developed an analytical model that quantitatively correlates the applied electric field with the energetics of excited states in donor-acceptor blends. By combining device characterisation, spectroscopy, and theoretical modelling, we reveal that the Stark effect and the field-dependent charge transfer significantly impact the FF in state-of-the-art OSCs with low voltage losses. Our findings highlight that suppressing geminate decay by increasing exciton lifetime is a promising strategy for boosting the FF and achieving future efficiency gains in OSCs.