Solar cells showing improved efficiency, amounting to about 14 percent for overall solar power conversion, are obtained by annealing InP/CdS solar cells in a slightly reducing atmosphere for about 15 minutes in a temperature range preferably from about 550° centrigrade to about 600° centigrade. In an annealing temperature range from 400° centigrade to 625° centigrade, an inversely dependent adjustment of annealing time is found desirable. The atmosphere preferably comprises mainly a substantially inert component and typically comprises an H.sub.2 + N.sub.2 mixture, such as forming gas (15% H.sub.2 + 85% N.sub.2).
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