Impurity Photovoltaic Effect in Cadmium Sulfide

Abstract

Photovoltaic cells were fabricated by evaporating a thin copper layer on low‐resistivity single‐crystal n‐type cadmium sulfide. It was found that the spectral response of the cells in the infrared was enhanced by as much as two hundred fold when the cell was illuminated by radiation with energy corresponding to the energy gap (green light). The enhancement of the infrared photovoltaic response is attributed to creation of additional minority carriers by transitions of electrons from the valence band to impurity levels. These impurity levels were emptied by the green excitation making infrared transitions to the impurity states more favorable. The net effect is the broadening of the spectral response of the cell. Transient measurements of the emptying and filling of the impurity states were made by illuminating the sample from two monochromatic light sources. The recombination statistics of holes was analyzed. A single recombination level model was adequate to explain the results. Recombination center concentration and hole and electron capture cross sections were calculated with the results: N_r=1.3×10¹⁷/cm³, S_n=1.2×10⁻²³ cm², and S_p=2.08=10⁻²⁰ cm².