Hole-mobility limit of amorphous silicon solar cells

Solar cells based on hydrogenated amorphous silicon a-Si:H and related materials are now manufactured in large quantities, so it is remarkable that there is no consensus re- garding the optoelectronic processes that determine their photovoltaic conversion efficiency. There are at least two broad issues that have led to ambiguities regarding the de- vice physics. First, the solar cells are nip or pin structures, and the cell efficiency is potentially affected by the proper- ties of the intrinsic and doped layers and also by the inter- faces between them. Second, at least for the intrinsic layer, both disorder limitation of carrier mobilities, and also carrier capture by deep levels such as dangling bonds, are each known to be important, but it is difficult to distinguish the two effects on the cell's efficiency. In this letter, we present temperature-dependent mea- surements and analyses for a series of a-Si:H nip solar cells prepared at United Solar Ovonic Corporation that indicate that the efficiencies of these cells at solar intensities are mostly determined by the hole mobility of the intrinsic layer. Essentially the same conclusion of mobility or space-change limited photocurrent was reached recently for polymer-