Study of alkali (Na, K)-doped Cu$_2$ZnSnS$_4$ thin films prepared by sol–gel method

The non-doped and alkali (Na, K)-doped Cu$_2$ZnSnS$_4$ (CZTS) thin films were prepared using the sol–gel spin coating method on the glass substrate, and the changes in the structural, optical, and electrical characteristics of the films were examined and compared. The structural study results obtained showed that all the prepared samples had a kesterite structure. The scanning electron microscopy and Raman analysis showed that the samples' surface and crystalline quality were significantly changed by doping, and an appropriate amount of dopants can improve them. The optical study showed that the energy gap values for the CZTS layers were in the range of 1.40–1.61 eV, which is desirable for solar cells. Moreover, good optical conductivity values (10$^{12}$ to 10$^{15}$ S$^{-1}$) and high absorption coefficients (up to 1.8 $\cdot$ 10$^5$ cm$^{-1}$ in the visible region) were obtained for the CZTS thin films. Investigation of the sample’s electrical properties indicated that non-doped and doped CZTS was $p$-type, and therefore doping did not change the type of the charter of thin films. Moreover, the carrier concentration of the samples significantly increased up to 8.00 $\cdot$ 10$^{19}$ cm$^{-3}$ (one order of magnitude increase) with the doping. The samples' photovoltaic properties showed that the fabricated ZnS|CZTS hetero-junction exhibited good rectifying behavior and the doped layers had better diode parameters. Overall, the results showed that the CZTS thin films doped with an appropriate amount of dopants (1% Na and 1.5% K) had better structural, optical, and electrical properties. The photo-electrical study of the samples showed that for all samples, photocurrent under light illumination significantly increased (especially for Na-doped CZTS thin films), indicating that the CZTS thin films are suitable for solar energy conversion.