Investigation on carrier recombination and energetics of kesterite materials for photovoltaic applications
Date of Issue2016
School of Materials Science and Engineering
Are still in their infancy, with the solution based techniques requiring either a highly toxic and dangerous solvent such as hydrazine, or an organic solvent such as methanol. Thus, an environmentally green and cost effective method is in current high demand. In this work, an aqueous-based chemical spray pyrolysis technique was used to deposit a uniform CZTS layer on Mo coated glass. The film was further processed in a high temperature selenization furnace to obtain a highly crystalline CZTSSe absorber. Using X-ray diffraction (XRD) and Raman spectroscopy, no detectable secondary phase was observed from the CZTSSe absorbers. A uniform elemental distribution was suggested from SIMS depth profiling, except for slightly increased concentrations of Zn and S close to the Mo contact layer. The S/Se ratio can be tuned by applying different amounts of Se in the selenization process, which resulted in similar solar cell conversion efficiencies above 5%. The highest efficiency obtained after process optimization was 7.5%, which to the best of our knowledge, is the highest efficiency obtained for a kesterite containing solar cell fabricated by an aqueous-based solution method. The carrier recombination behavior in CZTSSe absorber was investigated by temperature and power dependent photoluminescence (PL) spectroscopy. The donor-acceptor pairs (DAPs) transition was concluded as the dominant recombination mechanism. Non-radiative recombination was responsible for the large temperature quenching effect observed for the PL intensity. The thermal activation energies of the non-radiative recombination channels have been calculated and present a higher value for the post-optimized CZTSSe absorber, indicating a less severe non-radiative recombination for post-optimized CZTSSe. The energetics alignment of the CZTSSe solar cells was investigated by impedance spectroscopy. Three solar cell devices with a similar short circuit current density (Jsc) were investigated and revealed a similar recombination resistance from the analysis of their impedance spectra. The calculated results from the Mott-Schottky relationship indicated a similar carrier concentration but a variation in the energetics alignment, which was considered as one of the critical factors affecting the Voc. The schematic band alignment was presented for a better understanding Voc origin. Alternative CMTSSe (M=Ca, Ni, Mg, Mn) absorbers were also fabricated via a spray pyrolysis method followed by selenization. XRD and Raman spectroscopy showed that the CMTSSe (M=Ca, Ni, Mg) samples all coexisted with additional binary and tertiary phases. Similar to the CZTSSe, the samples of CMnTSSe presented a single kesterite phase. Furthermore, the solar cell devices based on the CMnTSSe absorber generated a 0.07% conversion efficiency, indicating it as a potential alternative for CZTSSe-based absorbers.