Effect of potassium and germanium dopants on optical, structural and electrical properties of copper zinc tin sulphoselenide solar absorber layer

Abstract

In present research, the role of Potassium (K) and Germanium (Ge) dopants in Copper Zinc Tin Sulphoselenide (CZTSSe) absorber thin solar film prepared using non-vacuum spray pyrolysis deposition technique has been investigated. K-doped CZTS precursor solution was prepared using one-pot approach with different concentrations of K (0.0, 0.5, 1.0, 1.5, 2.0 and 2.5) mol %, dissolved using dimethyl sulfoxide solvent. The solution was then sprayed on soda lime glass (SLG) substrate using ultrasonic spray coater at 300 °C. The deposited thin films were selenized in tube furnace using three-step temperature approach (300 °C, 500 °C and 550 °C) with 30 minutes ramping time in nitrogen environment. Deposited K-doped CZTSSe thin films were characterized by ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy and 3D microscope to determine the optical properties and thickness of K-doped CZTSSe structure. X-ray diffractometer was employed for the structural and crystallinity analyses, whereas field emission scanning electron microscope was used to study surface morphologies. Energy dispersive X-ray spectrometer was used to study the elemental composition of the film while Hall effect measurement system was used for measuring the charge carrier density. Based on results, 1.5 mol % of Kdoped was selected for fabrication process of (K,Ge)-doped CZTSSe solar absorber layer. Same processes were performed to synthesize thin films except with different molar concentrations of Ge (10, 15, 20, 25 and 30)% in dimethylformamide as dissolving agent. The effects of different Ge concentrations were studied. UV-Vis- NIR spectra have shown high absorption coefficient which was more than 10000 cm-1 for each sample. The bandgap increased as the concentration of Ge was increased, which inferred the capability of Ge to tune the CZTSSe bandgap to increase the open circuit voltage. X-ray spectra showed better crystallinity at 25% and 30% of Ge dopant, while micrographs from field emission scanning electron microscope revealed that 25% Ge has better crystal growth. The charge carrier density in the absorber layer also increased with increase in dopant concentration. Based on the findings, (K,Ge)-doped CZTSSe thin film with 1.5 mol % K and 25% Ge has the best properties.