Hydrogenated amorphous silicon solar cells

A simple description of the operation of the hydrogenated amorphous silicon (a-SiH_x) pin solar cell is given and general guidelines for increasing the efficiency are established. The use of heterostructures in which the n and p layers have larger band gaps than the intrinsic (i) layer helps to reduce losses in efficiency due to optical absorption in the doped layers and back diffusion of carriers across the n-i and p-i interfaces. The density of gap states of the intrinsic layer is inferred from measurements of optical absorption, photoconductivity, drift mobility, collection efficiency and capacitance. Based on the density of gap states of the best intrinsic material available, an upper theoretical limit of 17% is estimated for the solar cell efficiency. Materials' improvements required to achieve experimentally a 10% efficient a-SiH_x solar cell are discussed.