State-of-the art polymer-fullerene solar cells reach power conversion efficiencies of up to 6%, featuring low polaron
recombination rates. In order to identify limiting factors, we investigated the photocurrent of poly(3-hexyl thiophene)
(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) solar cells experimentally. From our investigations, we find
the photocurrent to be determined mainly by polaron pair dissociation and charge extraction. Focussing on the polaron
pair dissociation, we apply Monte Carlo simulations in order to understand the unexpectedly high internal yield of this
separation process. We find that a long effective conjugation length of the polymer chains leads to delocalisation of the
positively charged constituent of polaron pairs, a hole, making it easier to escape the Coulomb attraction to the electron.
However, we identify an additional loss mechanisms, which our Monte Carlo simulations show to be significant: losses of
polaron pairs at the semiconductor/electrode interfaces due to diffusion of the pairs. We discuss how the different processes
influencing the photocurrent can be accounted for analytically.
Studying polymer:fullerene solar cells, we find that the bimolecular recombination is much lower than the anticipated
Langevin recombination. In our study, we investigate the performance of annealed P3HT:PCBM bulk
heterojunction solar cells by current-voltage characterisation, and complement these measurements with
photo-CELIV experiments. We find that at room temperature the bimolecular recombination rate is reduced by a
factor of about 50 as compared to the Langevin rate. We discuss the implications of this reduction on the solar
cell performance, using a one-dimensional numeric simulator to analyse the experimental results, and compare
the reduced recombination rates with the Langevin rates.