Nanoporous TiO2/Cu1.8S heterojunctions for solar energy conversion

Thin films of p-type Cu1.8S have been deposited onto smooth and nanoporous n-type TiO2 with Atomic Layer Chemical Vapor Deposition (AL-CVD). As precursors, Cu(thd)2 and H2S have been used and self-limited deposition takes place between 125 and 240 °C. The crystalline phase and growth rate strongly depend on the process conditions; below 175 °C CuS and above this temperature, Cu1.8S is formed. Photospectroscopy shows that Cu1.8S has a bandgap of 1.75 eV and an absorption coefficient of 2.3×104 cm−1 at 500 nm. A 35-nm-thick Cu1.8S film on flat TiO2 substrates shows a short circuit photocurrent of 30 μA/cm2 and an open circuit photovoltage of 200 mV at broad-band irradiation of 2.8 kW/m2. The internal quantum efficiency is 6% at 500 nm.