On electrode pinning and charge blocking layers in organic solar cells

We use device modelling for studying the losses introduced by metallic electrodes in organic solar cells' device structure. We first discuss the inclusion of pinning at the integer charge transfer state in device models, with and without using the image charge potential. In the presence of disorder, the space charge introduced due to the image potential enhances the pinning by more than 0.2 eV. The explicit introduction of the image potential creates band-gap narrowing at the contact, thus affecting both dark leakage current and photo conversion efficiency. We find that there are two regimes in which the contacts may limit the performance. For low (moderate) barriers, the contacts introduce minority carrier recombination at the contacts that adds to the bulk recombination channels. Only for high barriers, the contacts directly limit the open circuit voltage and impose a value that is equal to the contact's energy difference. Examining the device structures with blocking layers, we find that these are mainly useful for the low to moderate contacts' barriers and that for the high barrier case, the enhancement of open circuit voltage may be accompanied by the introduction of serial resistance or S shape.