Effect of Gold Nanorods on the Performance of Polymer:Fullerene Organic Solar Cells

This research is an experimental investigation on the effect of inserting gold nanorods in various locations of conjugated polymer solar cells that comprise poly(3-hexylthiophene-2,5-diyl) as the electron donor, and [6,6]-phenyl-C61-butyric-acid-methyl-ester as the electron acceptor, on the cells performance. Since gold nanorods support at least two major plasmonic modes associated with metallic nanoparticles, incorporating such nanoparticles into thin films of polymer solar cells is supposed to trap light inside the cells in a broad wavelength range, leading to increasing absorptivity as well as power conversion efficiency. First, several experiments were performed to manufacture devices with a good and reproducible efficiency by optimizing the fabrication conditions, particularly the lithium fluoride thickness as well as the annealing process. This optimization succeeded in producing reproducible devices with an enhanced power conversion efficiency from 0.36% to 1.67%. Secondly, various approaches were used to introduce gold nanorods in our devices. Rods were deposited in contact with either the cells’ front electrode, or the rear one. They were also blended with the solution of the anodic buffer layer, or the one of the photoactive layer. We compared the photovoltaic parameters extracted from completed devices made with/without gold nanorods, as well as their spectroscopic and structure properties. We found that for each location of rods in our devices there was an optimal concentration of the rods to produce enhancement in the devices’ performance. Based on theoretical considerations, devices enhancement was related to either the far field or near field effect induced by the presence of rods. It was found that increasing or decreasing the rods density from the optimal one reduced the overall efficiency of resulting devices. We experimentally verified that there was a relationship between the enhancement in the devices efficiency and the multi-mode excitations associated with gold nanorods. We also found that the influence of plasmonics on absorption of the devices depended on the thickness of the devices’ photoactive layers. Using the rod shape of gold nanoparticles to increase the device performance is indeed a promising approach since a fairly low density of the rods in the layer succeeded in increasing remarkably the devices efficiency by up to 21.3 %