Origin of Reduced Polaron Recombination in Organic Semiconductor Devices

TL;DR

This paper introduces a model explaining the significantly reduced bimolecular recombination rates in organic solar cells by considering spatial carrier concentration gradients, aligning well with experimental observations across temperatures.

Contribution

The model accounts for spatial carrier gradients in organic solar cells, providing a new explanation for reduced recombination rates and their temperature dependence.

Findings

Recombination rate is 1-4 orders of magnitude lower than Langevin predictions.

Recombination rate approaches Langevin value at low temperatures.

Model successfully describes temperature dependence of recombination.

Abstract

We propose a model to explain the reduced bimolecular recombination rate found in state-of-the-art bulk heterojunction solar cells. When compared to the Langevin recombination, the experimentally observed rate is one to four orders of magnitude lower, but gets closer to the Langevin case for low temperatures. Our model considers the organic solar cell as device with carrier concentration gradients, which form due to the electrode/blend/electrode device configuration. The resulting electron concentration under working conditions of a solar cell is higher at the cathode than at the anode, and vice versa for holes. Therefore, the spatially dependent bimolecular recombination rate, proportional to the local product of electron and hole concentration, is much lower as compared to the calculation of the recombination rate based on the extracted and thus averaged charge carrier concentrations. We consider also the temperature dependence of the recombination rate, which can for the first time be described with our model.