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

• Double substitutions of noncolloidal PbS thin films with Cd and Cu were studied.
• Single step chemical bath deposition was conducted for (Pb1−xCdx)1−yCuyS films.
• Band gap tunability from 1.22 to 1.78 eV was achieved with the Cd/Cu substitutions.
• The optimal film of (Pb0.8Cd0.2)0.9Cu0.1S showed an ideal band gap of 1.63 eV.
• Excessive Cu was not desirable due to unstable growth and high carrier concentration.

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 (Pb1−xCdx)1−yCuyS (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.