Understanding the limits of plasmonic enhancement in organic photovoltaics

Plasmonic enhancement in organic photovoltaics has been extensively studied in the past decade. However, the reported improvements in power conversion efficiency (PCE) are highly inconsistent due to a poor understanding of the limitations of how plasmonics affect charge generation and transport in solar cells. In this work, we address these long-standing uncharted questions as to when plasmonic enhancements are useful and when they are not. We do this with a model system consisting of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]:[6,6]-phenyl C61 butyric acid methyl ester PCDTBT polymer active layer with silver nanostructures embedded in the poly(3,4-ethylenedioxythiophene):polystyrene PEDOT:PSS sulfonate hole-transport layer. We demonstrate that: (a) plasmonic enhancements are most pronounced when the charge carrier mobilities of the donor and acceptor materials are unbalanced; (b) the introduction of plasmonic nanostructures in devices with balanced charge transport usually results in a decrease in efficiency; (c) plasmonic enhancement is highly shape-dependent; (d) for devices with asymmetric mobilities, as long as the species with low mobility is extracted at the contact where light is incident, device efficiency will be boosted; and (e) increase in light absorption in the active layer has minimal impact on PCE; the efficiency is primarily driven by exciton generation and charge collection efficiency. The findings of our work provide a generalized framework to guide researchers as to when plasmonic effects could be helpful to a device and when they could degrade performance