Spontaneous formation of light-trapping nano-structures for top-illumination organic solar cells

By introduction of nano-structured crystallite capping layers, the power conversion efficiency of top-illumination organic solar cells is improved from 4.2 +/- 0.1% to 6.0 +/- 0.2%, representing a 44% enhancement. This is caused by the increase in J(SC) and led by the enhancement in the local E distribution inside the active layers. Comprehensive finite-difference time domain simulation reveals two main reasons for this enhancement: (1) the nano-structured capping layers can be treated as gradient-index films that effectively increase the light entering the devices. (2) The nano-structured capping layers can also diffract the light from original normal-incident paths, hence increasing the absorption length inside the active layers.