  The fill factor ($FF$) is a critical parameter for solar cell efficiency, yet
its analytical description is challenging due to the interplay between
recombination and charge extraction processes. An often overlooked yet
significant factor contributing to $FF$ losses, beyond recombination, is the
influence of charge transport. In most state-of-the-art organic solar cells,
the primary limitation of the $FF$ arises not from recombination but rather
from low conductivity, highlighting the need for refined models to predict the
$FF$ accurately. Here, we extend the analytical model for transport resistance
to a more general case. Drawing from a large set of experimental
current-voltage and light intensity-dependent open-circuit voltage data, we
systematically incorporate crucial details previously omitted in the model.
Consequently, 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 study provides valuable insights
into strategies for mitigating $FF$ losses based on the experimentally
validated analytical model, guiding the development of more efficient solar
cell designs and optimization strategies.
