Impurity effects in a-Si:H solar cells due to air, oxygen, nitrogen, phosphine, or monochlorosilane in the plasma

Abstract

It is demonstrated that solar cells fabricated using plasma‐deposited a‐Si:H alloys can be seriously degraded by the incorporation of certain impurities during deposition of the a‐Si:H materials. Nominally intrinsic layers are adversely affected by the addition to the plasma of air, N₂+O₂ mixtures (although by neither N₂ nor O₂ separately), PH₃, or SiH₃Cl (monochlorosilane). For example, the conversion efficiency of Pd/p‐i‐n solar cells is lowered from 3.1% to 1.5% by the presence in the plasma of 3000 ppm air during deposition of the i‐layer. In this case modification of the a‐Si:H gap‐state density owing to synergistic effects of oxygen and nitrogen in the plasma leads to a collapse of the space‐charge region and a reduction of the μτ product for holes. The deleterious effects on device performance of phosphine and monochlorosilane are also discussed.