Transport resistance dominates the fill factor losses in record organic solar cells

TL;DR

This paper identifies transport resistance as the main factor limiting the fill factor in high-efficiency organic solar cells, and provides methods to measure, understand, and reduce it to improve device performance.

Contribution

It offers a comprehensive analysis of transport resistance in organic photovoltaics and introduces practical approaches to characterize and mitigate its effects.

Findings

Transport resistance causes significant fill factor losses in organic solar cells.

Even the best devices are affected by transport resistance-related performance losses.

Strategies to reduce transport resistance can lead to higher efficiency organic solar cells.

Abstract

Organic photovoltaics are a promising solar cell technology well-suited to mass production using roll-to-roll processes. The efficiency of lab-scale solar cells has exceeded 20% and considerable attention is currently being given to understanding and minimising the remaining loss mechanisms preventing higher efficiencies. While recent efficiency improvements are partly owed to reducing non-radiative recombination losses at open-circuit, the low fill factor due to a significant transport resistance is becoming the Achilles heel of organic photovoltaics. The term transport resistance refers to a voltage and light intensity dependent charge collection loss in low-mobility materials. In this Perspective, we demonstrate that even the highest efficiency organic solar cells reported to-date have significant performance losses that can be attributed to transport resistance and that lead to high fill factor losses. We provide a closer look at the transport resistance and the material properties influencing it. We describe how to experimentally characterise and quantify the transport resistance by providing easy to follow instructions. Furthermore, the causes and theory behind transport resistance are detailed. In particular, we integrate the relevant figures of merit and different viewpoints on the transport resistance. Finally, we outline strategies that can be followed to minimise these charge collection losses in future solar cells.