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Organic photovoltaic (OPV) cells have recently attracted attention due to their low cost and easy fabrication. However, their efficiency remains low compared to silicon-based photovoltaics. OPV cells generate power through the creation of an exciton by light. The diffusion length for the electron-hole pair in most organic photovoltaic material is on the order of 100 nm, which limits the overall thickness of the device. The maximum 100 nm thickness of the cell reduces the absorption of the device, which ultimately lowers its efficiency. The absorption may be boosted at certain wavelengths by the excitation of surface plasmons. Surface plasmons are collective oscillations of conduction electrons in the metal along a metal-dielectric interface, and create enhancements of the electromagnetic fields up to 20 times near the metal surface. To test the possible enhancement from surface plasmons, OPV cells are made using a conjugated polymer and fullerene-based active layer with either an aluminum or silver bottom electrode. Periodic grating structures were formed between the active layer and the metal electrode to facilitate surface plasmon excitation from visible light. Surface plasmons are verified to exist on Al cathode cells by reflection and transmission measurements. Although there is no statistically significant change in efficiency, fill factor, open circuit voltage or short circuit current observed for our devices on patterned versus unpatterned OPV cells, this could be due to the large error that results from having small numbers of samples. While there is no overall gain in incident photon conversion efficiency (IPCE) for the samples with gratings to couple surface plasmons, OPV cells with silver cathodes demonstrated gains in IPCE potentially due to surface plasmons around 400 to 450 nm for both 555 nm and 833 nm period gratings. Further experiments are necessary to fully investigate the role of the surface plasmon in this increase in efficiency.

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