Transport resistance strikes back: unveiling its impact on fill factor losses in organic solar cells.

The fill factor (FF) is a critical parameter for solar cell efficiency, but its analytical description is challenging due to the interplay between recombination and charge extraction processes. A significant factor contributing to FF losses, beyond recombination, that has not received much attention is the influence of charge transport. In most state-of-the-art organic solar cells, the primary limitations of the FF do not just arise from non-radiative recombination, but also from low conductivity of the organic semiconductors. A closer look reveals that even in the highest efficiency cells, performance losses due to transport resistance are significant. This finding highlights the need for refined models to predict the FF accurately. Here, we extend the analytical model for transport resistance to a more general case by systematically incorporating energetic disorder. We introduce a straightforward set of equations to predict the FF of a solar cell, enabling the differentiation of losses attributed to recombination and transport resistance. Our analytical model is validated with a large set of experimental current-voltage and light intensity-dependent open-circuit voltage data for a wide range of temperatures. Based on our findings, we provide valuable insights into strategies for mitigating FF losses, guiding the development of more efficient solar cell designs and optimisation strategies.