Electrical and Optical Properties of (n)ZnO/(p)CdTe Heterojunction and Its Performance as a Photovoltaic Converter

Thin film heterojuctions of the type (n)ZnO/(p)CdTe with different doping concentration were prepared by vacuum evaporation, and their electrical and optical properties, both in dark and under illumination at room temperature as well as elevated temperatures, were studied. Different junction parameters such as ideality factors, barrier heights, Richardson constant, short-circuit current, etc. were determined from I–V characteristics. These parameters were found to change significantly on hydrogenation and annealing of the junctions and also on variation of temperature. The structures showed the change of the photovoltaic (PV) effect, giving a fill factor of 0.57 for hydrogen (H)-treated with an open-circuit voltage of 345 mV and a short-circuit current density of 75.72× 10−4 mA·cm−2 and 0.42 for untreated with an open-circuit voltage of 244 mV and a short-circuit current density of 52.00× 10−4 mA·cm−2 for doping concentrations of Na=2.43× 1016/cm3(2.53% Sb doped CdTe) and Nd=3.88× 1015/cm3(2.74% Al doped ZnO). The junctions were found to possess a series resistance as high as 437 Ω for an untreated sample and 95 Ω for H-treated samples even under illumination. Proper doping, annealing, and hydrogenation are necessary to reduce the series resistance so as to achieve an ideal solar cell.