Recombination dynamics in polythiophene:fullerene solar cells

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Title: Recombination dynamics in polythiophene:fullerene solar cells
Authors: Shuttle, Christopher G
Item Type: Thesis or dissertation
Abstract: Organic devices based on polymer:fullerene blend films are attracting extensive interest as low cost solar cells, with power conversion efficiencies over 5%. Improvements in performance are dependent on developing a better understanding of the pertinent loss processes. This in turn requires the ability to reliably determine charge densities (n) and carrier lifetimes (τn) in real devices under standard operating conditions. In this thesis, we address the recombination dynamics in organic solar cells 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 best devices to date, using both experimental and modelling studies. Initially, a drift-diffusion model was used to study the basic principles of solar cell operation, with particular focus on investigating the ‘corrected photocurrent’, where the effects of dark injection are removed. We then have employed a series of experimental techniques – including transient photovoltage and photocurrent, transient absorption spectroscopy and charge extraction – to determine the carrier lifetimes and charge densities in standard annealed P3HT:PCBM devices under operation. The results of our studies for a device under open-circuit conditions show that the open-circuit voltage (Voc) is primarily governed by a trap dependent bimolecular recombination process. By applying charge extraction studies on devices under forward bias in the dark, we show that the dark current is also governed by the same trap dependent bimolecular recombination mechanism which determines Voc. Based on the understanding of charge carrier dynamics at Voc and the forward bias dark current, a simple model has been developed to simulate ‘light’ current-voltage (J-V) curves. Despite the simplicity of this model, remarkably good agreement was observed with experimental J-V data.
Issue Date: Jul-2008
Date Awarded: Feb-2009
Supervisor: Durrant, James
Author: Shuttle, Christopher G
Department: Chemistry
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Chemistry PhD theses

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