Studies of the optimization of recombination layers for inverted tandem polymer solar cells

We report on an optimization study of recombination layers in inverted tandem bulk heterojunction polymeric solar cells. Modification of bilayers of MoOx/Ag with a polymeric surface modifier, polyethylenimine ethoxylated (PEIE), is found to significantly improve the performance of these recombination layers. The improvement is assigned to a significant reduction by 1.5 eV of the work function of the Ag with PEIE. An analysis of the electrical, optical, and surface properties of the recombination trilayer is presented. Inverted tandem polymer solar cells, fabricated with two identical active layers comprising a donor polymer poly(3-hexylthiophene) (P3HT), and a fullerene derivative acceptor, [6,6]-phenyl C61 butyric acid methyl ester (PC60BM), provide a proof-of-principle demonstration of the effectiveness of the MoOx/Ag/PEIE trilayer as recombination layer. Such tandem cells yield an open-circuit voltage of 1042 mV and fill factors of 0.62, comparable to those of single cells (0.63) under simulated AM 1.5G, 100 mW/cm2 illumination. Preliminary shelf lifetime studies suggest that these trilayer recombination layers show good air stability.