Recombination dynamics in polythiophene : fullerene solar cells

Abstract

Organic devices based on polymer:fullerene blend films are attracting extensiveinterest as low cost solar cells, with power conversion efficiencies over 5%.Improvements in performance are dependent on developing a better understanding ofthe pertinent loss processes. This in turn requires the ability to reliably determinecharge densities (n) and carrier lifetimes (?n) in real devices under standard operatingconditions. In this thesis, we address the recombination dynamics in organic solarcells based on blends of poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), P3HT:PCBM devices, one of the bestdevices to date, using both experimental and modelling studies. Initially, a drift-diffusion model was used to study the basic principles of solar celloperation, with particular focus on investigating the ?corrected photocurrent?, wherethe effects of dark injection are removed. We then have employed a series ofexperimental techniques ? including transient photovoltage and photocurrent,transient absorption spectroscopy and charge extraction ? to determine the carrierlifetimes and charge densities in standard annealed P3HT:PCBM devices underoperation. The results of our studies for a device under open-circuit conditions showthat the open-circuit voltage (Voc) is primarily governed by a trap dependentbimolecular recombination process. By applying charge extraction studies on devicesunder forward bias in the dark, we show that the dark current is also governed by thesame trap dependent bimolecular recombination mechanism which determines Voc. Based on the understanding of charge carrier dynamics at Voc and the forward biasdark current, a simple model has been developed to simulate ?light? current-voltage(J-V) curves. Despite the simplicity of this model, remarkably good agreement wasobserved with experimental J-V data.EThOS - Electronic Theses Online ServiceGBUnited Kingdo