Effect of double substitutions of Cd and Cu on optical band gap and electrical properties of non-colloidal PbS thin films

Controllable band gap has been pursued to absorb a proper range of light by p-type absorber semiconductors for better performance photovoltaic devices. Here we introduce double substitutions with Cd and Cu for non-colloidal p-type PbS thin films to cover a broader range of optical band gap from 1.22 to 1.78 eV. Thin films of (Pb_1-xCd_x)_1-yCu_yS (x = 0-0.3 and y = 0-0.3) were grown by a single step chemical bath deposition process at a low temperature of 70 °C. The incorporation of Cd resulted in a wider band gap but changed the type of semiconductor into n-type above x = 0.2. Only the proper substitutions with both Cd and Cu induced an optimal band gap of 1.63 eV, which means a substantial improvement compared to 1.22 eV for pure PbS thin film, while maintaining p-type conductivity. Interestingly, excessive Cu substitutions beyond y = 0.2 inhibited crystallization significantly and generated an undesirably high carrier concentration.