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.