Transport Resistance Dominates the Fill Factor Losses in Record Organic Solar Cells

Organic photovoltaics (OPV) 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 minimizing 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 (FF) due to a significant transport resistance is becoming the Achilles heel of OPV. The term transport resistance refers to a voltage and light intensity-dependent charge collection loss in low-mobility materials. In this perspective, it is demonstrated that even the highest efficiency organic solar cells (OSCs) reported to-date have significant performance losses that can be attributed to transport resistance and that lead to high FF losses. A closer look at the transport resistance and the material properties influencing it is provided. How to experimentally characterize and quantify the transport resistance is described by providing easy to follow instructions. Furthermore, the causes and theory behind transport resistance are detailed. In particular, the relevant figures of merit (FoMs) and different viewpoints on the transport resistance are integrated. Finally, we outline strategies that can be followed to minimize these charge collection losses in future solar cells.