Balance between light trapping and charge carrier collection: Electro-photonic optimization of organic photovoltaics with ridge-patterned back electrodes

Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell efficiencies. However, one cannot overlook the side-effects to charge carrier collection which are introduced when seeking optical enhancements. A comprehensive electro-photonic model is utilized to study the optical and electrical effects of patterning poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] and poly(3-hexylthiophene) based solar cells with simple optical grating structures. It is found that for the most part, optical absorption improvements are attenuated by enhanced electrical losses. Optimized device structures that overcome this tradeoff are proposed and their detailed electro-optical characteristics are discussed. When the hole mobility is smaller than the electron mobility, the results suggest that in general, an inverted structure has a better chance to outperform a flat active layer than a conventional architecture in an OPV cell with the ridge-patterned back electrode. (C) 2013 AIP Publishing LLC.