Origin of the light intensity dependence of the short-circuit current of polymer/fullerene solar cells

TLDR

In polymer/fullerene solar cells the short‑circuit current follows a sub‑linear power law Jsc∝I^α (α≈0.85–1) rather than a linear dependence, a deviation often attributed to bimolecular recombination. This work aims to show that the light‑intensity dependence of the photocurrent is controlled by space‑charge buildup caused by unequal electron and hole mobilities. The authors test this by tuning the mobility mismatch over one to three orders of magnitude through annealing in a bulk heterojunction model system. They confirm that the sub‑linear Jsc–I relationship arises from space‑charge effects linked to the mobility difference, validating the proposed mechanism.

Abstract

A typical feature of polymer/fullerene based solar cells is that the current density under short-circuit conditions (Jsc) does not scale exactly linearly with light intensity (I). Instead, a power law relationship is found given by Jsc∝Iα, where α ranges from 0.85 to 1. In a number of reports this deviation from unity is speculated to arise from the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in bulk heterojunction solar cells is governed by the build-up of space-charge in the device as a consequence of a difference in electron- and hole mobility. We have verified this for an experimental model system in which the mobility difference can be tuned from one to three orders of magnitude by changing the annealing treatment.