Single crystal gallium phosphide solar cells

Abstract

Solar cells have been fabricated from epitaxially deposited single crystal gallium phosphide. Two types of cells have been found: (1) Those having their major spectral response at about 0·75 μ and referred. to as extrinsic, and (2) those which have their main response at 0·45 μ and are referred to as intrinsic cells.

For the extrinsic cells, at 23°C in sunlight, an open circuit voltage (V_oc) of about 0·6 V with a short circuit current density, J_sc, of 4 mA/cm² has been found. Conversion efficiencies somewhat above 1 per cent have been noted. Correspondingly, for the 0·45 μ cells a V_oc at 23°C of 1·35 V and a J_sc of 1·4 mA/cm² have been obtained with a conversion efficiency, measured in sunlight, up to 1 per cent. Cell areas have ranged from 0·2 cm² to 0·5 cm².

With increasing temperature, the V_oc of both types of cells decreases with a temperature coefficient, β of about 3 mV/deg. The J_sc of the 0·45 μ cell increases with temperature and the temperature variation can be expressed as

with ΔE≈ ·05 eV. The extrinsic cell, on the other hand, shows no change in J_sc to 200°C, after which it decreases rapidly.

The p-n junctions of these cells appear to be graded as evidenced by the capacity-voltage (c-v) relation which suggests a 1/c³ variation with voltage. From measurement of the minority carrier diffusion length using the technique of Logan and Chynoweth [1] the minority carrier lifetime in these cells is found to vary between 10⁻¹⁰ and 10⁻¹² sec.