Abstract
The carrier recombination in organic solar cells is investigated by numerical modeling to explore the weak dependence of the open circuit voltage on the workfunction of the electrodes. Two recombination mechanisms are proposed. In ohmic contact with low carrier injection barrier, the photo carriers recombine pre-dominantly with dark carriers diffused from the electrode into the semiconductor and the recombination reduces open circuit voltage as mobility increases. The dark carrier induced recombination is the main limit of the power conversion efficiency for BHJ structure. As the barrier is increased such as Schottky barrier structure, the dark-carrier induced recombination is excluded so that photo-carrier induced recombination can be inhibited by increasing mobility and gives a better performance. As a given semiconductor, with decreased workfunction difference of the electrodes, reduces simultaneously the dark carrier recombination and the flat band voltage.The balance between these two opposite factors give a nearly constant open circuit voltage. Instead of using a Schottky contact, this study demonstrate an ideal bi-layer structure to reduce the dark-carrier induced recombination by separating carrier generation region from electrodes. Hence an improvement of power conversion efficiency over 60% is obtained compared with blending structures.