Non-Geminate Recombination as the Primary Determinant of Open-Circuit Voltage in Polythiophene:Fullerene Blend Solar Cells: an Analysis of the Influence of Device Processing Conditions

Abstract

[EN] The physical origin of the open-circuit voltage in bulk heterojunction solar cells is still not well understood. While signi¿ cant evidence exists to indicate that the open-circuit voltage is limited by the molecular orbital energies of the heterojunction components, it is clear that this picture is not suf¿ cient to explain the signi¿ cant variations which often occur between cells fabricated from the same heterojunction components. We present here an analysis of the variation in open-circuit voltage between 0.4¿0.65 V observed for a range of P3HT/PCBM solar cells where device deposition conditions, electrode structure, active-layer thickness and device polarity are varied. The analysis quanti¿ es non-geminate recombination losses of dissociated carriers in these cells, measured under device operating conditions. It is found that at opencircuit, losses due to non-geminate recombination are suf¿ ciently large that other loss pathways may effectively be neglected. Variations in open-circuit voltage between different devices are shown to arise from differences in the rate coef¿ cient for non-geminate recombination, and from differences in the charge densities in the photoactive layer of the device. The origin of these differences is discussed, particularly with regard to variations in ¿ lm microstructure. By separately quantifying these differences in rate coef¿ cient and charge density, and by application of a simple physical model based upon the assumption that open-circuit is reached when the ¿ ux of charge photogeneration is matched by the ¿ ux of non-geminate recombination, we are able to calculate correctly the open-circuit voltage for all the cells studied to within an accuracy of ± 5 mV.