Effect of Plasmonic Au Nanoparticles on Inverted Organic Solar Cell Performance

We studied the competition between energy loss due to quenching and enhanced absorption arising from the near-field plasmonic effect of Au nanoparticles (NPs) on the performance of organic solar cells (OSCs), by fabricating inverted architecture OSCs with electron transport layers consisting of an Au NPs monolayer covered by a ZnO overlayer. The distance between Au NPs and a poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer was controlled by systematically varying the ZnO overlayer thickness, and its effect on the absorption spectra, photoluminescence (PL) dynamics, and OSC performance of these films is examined. We found that when P3HT:PCBM is in direct contact with the Au NPs, PL intensity was reduced and the OSC device was shunted with lower values in photocurrent, open circuit voltage, and fill factor. With a thin (8 nm) ZnO overlayer, higher absorption and PL signal were observed, and some OSC device parameters improved, but the short circuit current density remained low. ZnO overlayer thickness greater than 30 nm negated any impact of the AuNPs on the absorption of the active layer. A small (∼5%) efficiency enhancement was achieved in the 16 nm ZnO overlayer devices; however, this improvement resulted from increased fill factor rather than photocurrent. Optical field simulation using the finite-difference time-domain (FDTD) method showed that redistribution of the optical field by Au NPs resulted in a 0.6% absorption reduction over the entire P3HT:PCBM active layer.