Doped hydrogenated amorphous silicon a-Si:H films of only a few nanometer thin find application in a-Si:H/crystalline silicon heterojunction solar cells. Although such films may yield a field effect at the interface, their electronic passivation properties are often found to be inferior, compared to those of their intrinsic counterparts. In this article, based on H2 effusion experiments, the authors argue that this phenomenon is caused by Fermi energy dependent Si–H bond rupture in the a-Si:H films, for either type of doping. This results in the creation of Si dangling bonds, counteracting intentional doping of the a-Si:H matrix, and lowering the passivation quality. © 2009 American Institute of Physics.