Influences of extended selenization on Cu₂ZnSnSe₄ solar cells prepared from quaternary nanocrystal ink

Kesterite Cu₂ZnSnSe₄ (CZTSe) thin films prepared by the selenization of mechanochemically synthesized Cu₂ZnSnS₄ (CZTS) nanocrystal films are systematically investigated as a function of the annealing time in terms of the phase purity, microstructure, composition, and device characteristics. It is shown that selenization for an extended time does not cause a noticeable amount of Sn loss or segregation of Zn-rich layers. Thus, the prolonged annealing leads to improvements (reduction) in the shunt conductance, reverse saturation current, and diode ideality factor. However, it also leads to a deterioration of the series resistance, of which influence turned out to overwhelm all of the aforementioned positive effects on the device performance. As a consequence, the CZTSe solar cell exhibits its highest efficiency (5.43%) at the shortest annealing time (10 min). Impedance spectroscopy is demonstrated to be of good use in detecting the change in the back contact of CZTSe solar cells during annealing. The impedance spectra of the CZTSe solar cells are analyzed in association with the microstructures of the back-contact electrodes, demonstrating that the increase in the series resistance is attributed to the formation of the resistive MoSe₂ layer.