Bulk vs. Surface Recombination in Polymer-Fullerene Solar Cells

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-C-61 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.