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

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.