S-shaped current-voltage characteristics of organic solar devices

Measuring the current-voltage characteristic of organic bulk heterojunction solar devices sometimes reveals an S-shaped deformation. We qualitatively produce this behavior by a numerical device simulation assuming a reduced surface recombination. Furthermore we show how to experimentally create these double diodes by applying an oxygen plasma etch on the indium-tin-oxide anode. Restricted charge transport over material interfaces accumulates space charges and therefore creates S-shaped deformations. Finally we discuss the consequences of our findings for the open-circuit voltage V-oc.