The effect of annealing temperature on Cu2ZnGeSe4 thin films and solar cells grown on transparent substrates

Semi-transparent solar cells are the next step for photovoltaics into our daily life. Over the last years, kesterite-type material has attracted a special attention to be used as an absorber in thin-film solar cells because of its low toxicity and earth abundant constituents. Here, Cu2ZnGeSe4 (CZGSe) thin films are grown by co-evaporation and subsequent annealing at a maximum temperature of 480 degrees C or 525 degrees C onto Mo/V2O5/FTO/glass stacks. The goal of this work is to investigate the influence of the annealing temperature on the composition, morphology, vibrational properties, and transmittance of CZGSe layers, the formation of secondary phases, and distribution of elements within the absorber layer as well as on the optoelectronic properties of the corresponding solar cell devices. Raising the annealing temperature to 525 degrees C leads to a more uniform distribution of Cu, Zn, Ge and Se throughout the absorber layer, a reduction of the presence of the GeSe2 secondary phase, which is mainly detected at 480 degrees C, a larger grain size and the formation of a thicker MoSe2 layer at the CZGSe/back contact interface. The strategy of increasing the annealing temperature allows for improved J-V characteristics and higher spectral response resulting in an enhanced device performance of 5.3% compared to 4.2% when using 525 degrees C and 480 degrees C, respectively. Both absorber layers present an optical band gap energy of 1.47 eV. Furthermore, higher annealing temperature has beneficial effect to the CZGSe-based devices without losses in total transmitted light because of the higher diffuse transmittance. This work shows first promising semi-transparent CZGSe-based solar cells possibly open up new routes of applications.

Automated summary

Increasing the annealing temperature of CZGSe thin films to 525 degrees Celsius improves the performance of solar cell devices, leading to more uniform element distribution, larger grain size, and reduced presence of secondary phases. This strategy enhances the J-V characteristics and spectral response of the devices, resulting in enhanced overall device performance.