New molecular precursor for kesterite solar cells

Scientists from India’s SRM Institute of Science and Technology (SRMIST) have developed a new molecular precursor that can be used in kesterite films for applications in thin-film solar cells based on the same compound.

They presented their findings in “Superstrate type CZTS solar cell with all solution processed functional layers at low temperature,” which was recently published in Solar Energy. The precursor, which is claimed to be highly stable, was used to build a superstrate type solar cell based on kesterite/cadmium sulfide heterojunction with titanium oxide one-dimensional nanostructures.

The kesterite film was manufactured at a mild annealing temperature of 250 C, without sulfurization. It was then used in a heterojunction cell featuring layers of fluorine-doped tin oxide, titanium dioxide, cadmium sulfide, kesterite and aluminum.

“The superstrate configuration is able to overcome the re-combination loss and also can offer additional advantages by allowing the easier implementation of light trapping, improved back contact design, or the realization of tandem device structures,” the scientists explained.

The low-temperature manufacturing process can prevent cadmium sulfide diffusion and the degradation of the fluorine-doped tin oxide layer, while enabling low-cost fabrication. The cell achieved a conversion efficiency of 1.04%, which the scientists say is among the highest levels seen for such devices in the existing literature.

The kesterite layer is based on a precursor solution that the researchers had already developed. This solution was obtained by dissolving copper chloride, zinc acetate dihydrate, tin chloride dihydrate and thioacetamide in 10 ml of 2-methoxyethanol under stirring for 10 minutes to get a yellow transparent solution. The solution was then stored in a closed glass bottle under ambient conditions to observe its stability. The UV visible spectra showed no significant change in its absorbance properties and the solution showed an exceptional shelf life of more than 20 months, the scientists claimed, adding that, in a previous research, they had used a molybdenum-coated soda lime glass as a substrate for the kesterite film.

“The CZTS film was prepared by dip coating route using the precursor solution and then dried at 130 C followed by annealing at 500 C for 30 minutes in the presence of argon gas,” the scientists said.

The UV visible spectra showed no significant change in its absorbance properties. Images taken taken through Field Emission Scanning Electron Microscopy (FESEM) also showed that the obtained film was uniform and smooth, and that particles of kesterite covered its entire substrate. The film was tested in a simple heterojunction device made of glass, molybdenum, kesterite, cadmium sulfide and aluminum, with a vacuum-free laminated conductive tape being used as the top electrode.

“All the results, including the hall effect measurement as well as the heterojunction device characteristics, confirmed the formation of device quality CZTS absorber layer, which can lead the future work to fabricate the optimized thin film solar cells with high efficiency,” the researchers said.

They took hall effect measurements of the device and claimed that the p-type semiconducting nature of the film showed superior values of mobility. The researchers said that the cells show promising values for short circuit current (ISC) and open-circuit voltage (VOC).