pubs.acs.org/NanoLett
Plasmon-Enhanced Charge Carrier Generation in Organic Photovoltaic Films Using Silver Nanoprisms Abhishek P. Kulkarni, Kevin M. Noone, Keiko Munechika, Samuel R. Guyer, and David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195 ABSTRACT We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenylC61-butyric acid methyl ester on top of films of silver nanoprisms (∼40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells. KEYWORDS Localized surface plasmon resonance, metal nanoparticles, light trapping, plasmon-enhanced absorption, far-field scattering, plasmon-enhanced solar cells, organic bulk heterojunction
O
rganic photovoltaic (OPV) cells based on solutionprocessable conjugated polymers are an attractive, low-cost alternative to conventional silicon-based thin film solar cells due in part to the promise of highthroughput manufacturing.1,2 Light absorption in an OPV creates strongly bound electron-hole pairs (excitons) that need to diffuse to a donor/acceptor (D/A) interface to be dissociated into free charges. OPV device design and optimization is complicated by the fact that the light absorption depth is ∼10× larger than the exciton diffusion length in most organic polymers (the “excitonic bottleneck”), which often forces a trade off between light absorption and exciton harvesting efficiency.1,2 While the bulk heterojunction blended architecture partially alleviates the problem of exciton dissociation in optically thick films, the internal quantum efficiency (IQE) in bulk heterojunction cells often decreases rapidly with increasing film thickness.1 This drop in IQE is particularly significant for many new low band gap, red, and near-IR absorbing materials because they typically have lower absorption coefficients compared to blue and green absorbers.3 Many light-trapping strategies are being explored to improve light-harvesting efficiency in optically thin OPV devices4 including folded device architectures,3,4 aperiodic dielectric stacks,5 diffraction gratings,4 and plasmon resonant metallic nanostructures.6-11 Enhanced sensitivities and photocurrents have been theoretically predicted12-16 and experimentally reported17-23 for thin-film inorganic photo-
diodes and solar cells (where there is also a mismatch between minority carrier diffusion length and absorption depth) by exploiting light scattering from metal nanoparticles (∼50-100 nm diameter spheres). Similar approaches in OPVs have had more limited success, partly because of the use of smaller (
