Impurity effects in a-Si:H solar cells

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, N/sub 2/ + O/sub 2/ mixtures (although by neither N/sub 2/ nor O/sub 2/ separately), PH/sub 3/ or SiH/sub 3/Cl (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 the reduction of the ..mu.. tau product for holes. The deleterious effects on device performance of phosphine and monochlorosilane are also discussed.