# Effect of Imbalanced Charge Transport on the Interplay of Surface and Bulk Recombination in Organic Solar Cells

## TL;DR

This study investigates how imbalanced charge transport affects surface recombination and open-circuit voltage in organic solar cells, revealing the significant influence of contact work function variations and charge mobility imbalance.

## Contribution

It provides a detailed analysis of the impact of charge mobility imbalance on surface recombination and $V_{oc}$, including new analytical expressions and generalized simulation results.

## Key findings

- Small anode work function changes significantly affect $V_{oc}$ dependence on light intensity.
- Imbalanced charge transport shifts the dominant recombination mechanism from bulk to surface.
- Analytical models describe $V_{oc}$ behavior under space charge pile-up conditions.

## Abstract

Surface recombination has a major impact on the open-circuit voltage ($V_\text{oc}$) of organic photovoltaics. Here, we study how this loss mechanism is influenced by imbalanced charge transport in the photoactive layer. As a model system, we use organic solar cells with a two orders of magnitude higher electron than hole mobility. We find that small variations in the work function of the anode have a strong effect on the light intensity dependence of $V_\text{oc}$. Transient measurements and drift-diffusion simulations reveal that this is due to a change in the surface recombination rather than the bulk recombination. We use our numerical model to generalize these findings and determine under which circumstances the effect of contacts is stronger or weaker compared to the idealized case of balanced charge transport. Finally, we derive analytical expressions for $V_\text{oc}$ in the case that a pile-up of space charge is present due to highly imbalanced mobilities.