Importance of optical confinement in organic thin film solar cells

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO|glass substrate after the post-metallization annealing. We prepared an ITO surface with an average roughness, R_a, of 1 - 9 nm by chemical etching. When solar cell structures were made on these ITO surfaces, the diffuse reflection intensity increases with the increase in ITO surface roughness. Also, the solar cell performance, J_sc increases drastically with ITO roughness after annealing. This feature suggests that the diffuse reflection of Al|P3HT/C₆₀ interface is quite important for optical confinement of the cells, after post-metallization annealing. The effect is quite effective to improve cell performances.