  Measuring the current-voltage characteristic of organic bulk heterojunction
solar devices sometimes reveals an s-shaped deformation. We qualitatively
produce this behaviour 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 (ITO)
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 $V_{oc}$
