Highly conductive Ga-doped polycrystalline (Zn,Cd)S films

Abstract

Highly conducting Ga-doped (Zn,Cd)S thin films with various Zn contents are used as collector/window layers in chalcopyritebased heterojunction solar cells. The films were prepared by means of thermal evaporation. In order to elucidate the effects of Ga doping on carrier transport and optical properties, Hall effect, thermoelectric power and optical transmission measurements have been carried out. Both Hall data and thermoelectric power data show a decrease of mobility and of carrier concentration with increasing Zn content. The comparison of n Hall and n thermo versus conductivity of the films indicates polar optical phonon scattering for lower concentration levels and scattering determined by charged impurities at carrier concentrations exceeding 3 X 10 19 cm 3. With increasing Ga doping, carrier concentration n Hall and mobility μ Hall show different maxima depending on Zn content. For Zn 0.2 Cd 0.8 S films, a maximum carrier concentration of about 10 20 cm -3 at 0.5 at % Ga and a maximum mobility of about 40 cm 2/ V· s at 1 at% Ga can be achieved. Above a specific Ga doping level, both carrier concentration and Hall mobility diminish. Optical data show a significant Moss-Burstein shift (up to 300 meV) for carrier concentrations exceeding 3 X 10 19 cm -3. The electrical and optical data are in agreement.