Gold Nanorods on the Cathode Electrode for Enhancing the Efficiency of Polymer Solar Cells

Different densities of gold nanorods (GNRs) were incorporated on the back electrode of bulk heterojunction organic solar cell (OSC). GNRs layers (1, 3, and 5) were deposited on top of the poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) layer using spin-casting technique. According to the optical and structural characterizations, the solar cell devices incorporated with one layer of gold nanorods showed an enhancement in both power conversion efficiency and short-circuit current by up to 14% and 22% respectively as compared to the devices without gold nanorods. This result suggests that GNRs in the back electrode of polymer solar cells act as backscattering elements. They not only increase the optical path length in the active layer but also store energy in localized surface plasmon resonance mode. Both mechanisms lead to enhancement of light absorption and in turn contribute to photocurrent generation and the overall power conversion efficiency. On the other hand, the solar cells with high density GNRs on the back electrode showed inferior performance compared to that of low density integrated ones. The decrease in PCE would stem from enhanced charge recombination induced by high density GNRs. Furthermore, generation of intense local electric fields named hotspots, would reduce the charge transportation and exciton dissociation probability. In such cases, the power conversion efficiency of the device is observed to be less than that for one layer GNRs or even the control device.