Competition between recombination and extraction of free charges determines the fill factor of organic solar cells.

Davide Bartesaghi,Steffen Roland,Dieter Neher,L Jan Anton Koster,Juliane Kniepert,Irene Del Carmen Pérez,Mathieu Turbiez

doi:10.1038/ncomms8083

Abstract

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.

Highlights

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult
We show that to achieve fill factor (FF) of over 0.8, further improvements in charge transport are needed while reducing recombination
The charges photogenerated inside the active layer on absorption of photons move driven by the external electric field and by their concentration gradient

Summary

Introduction

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Researchers have put considerable effort in the development of this technology, seeking to increase both the efficiency and the stability of OPV devices1–4 The achievement of these goals is strongly dependent on the understanding of the physics of the devices. If the generation of charges changes significantly between open-circuit and short-circuit conditions, this will influence the FF Such field-dependent generation of charge carriers has been shown to be the main determinant in some donor/acceptor combinations. In most of the high-efficiency OPV systems, geminate recombination is greatly reduced and bimolecular recombination is the main mechanism for charge recombination12 It is the competition between recombination and extraction of charges that principally determines the dependence of the photocurrent on bias, and the FF13. Some authors combined transport and recombination by treating the problem in terms of the mobility-lifetime product

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