The Potential of Hydrogenated Amorphous Silicon-Sulfur Alloys for Absorber Materials in Photovoltaic Devices

The group VI element, sulfur, is an inefficient donor in hydrogenated amorphous silicon (a-Si:H). The compensation by sulfur donors of the p-type conductivity obtained with diborane in a-Si:H provides additional evidence of the role of sulfur as a donor. By adding equal amounts of diborane and hydrogen sulfide in the plasma the dark conductivity at room temperature can be reduced by one to two orders of magnitude compared to the corresponding p-type a-Si:H with the same boron concentration. Unlike phosphorus doping, a portion of the sulfur-related donors is passivated by hydrogen in the annealed state. This passivated portion can be rendered electrically active by optical excitation. This effect is similar to that which has been called persistent photoconductivity (PPC) and occurs in some compensated samples of a-Si:H and in some multilayer structures. The PPC effect has the opposite effect on both the photo- and dark conductivities from the Staebler-Wronski effect. For this reason it is possible to find an appropriate S/Si ratio where the two effects cancel as far as the conductivity is concerned. For an appropriate concentration of S in “intrinsic” a-Si:H one can obtain samples with high photoconductivity and essentially no degradation in either the dark or the photo-conductivities upon prolonged optical excitation (light soaking). These results suggest that at least the majority carriers are unaffected; however, it remains unclear what effect this second metastability will have on the minority carriers, and hence on PV device applications. The general idea that the addition of a second metastability to hydrogenated amorphous silicon (a-Si:H) might counteract the deleterious consequences of the Staebler-Wronski effect is presented.