Growth Mechanism and Properties of Nanostructure Cu₂ZnSnSe₄ Thin Films and Solar Cells

Abstract

The quality of the absorber layer is considerably important in thin-film solar cells. The precursor stacking with uniform distribution of metal elements is usually considered to be critical while preparing high-quality kesterite-structured thin-film solar cells. However, the Mo/Zn/Cu/Sn/Cu stacking order may be more reasonable after considering the growth mechanism of films. Herein, the growth mechanism and thus the solar cells prepared with these two stacking sequences Mo/Zn/Cu/Sn/Cu and Mo/Sn/Cu/Zn/Sn/Cu are studied in-depth. The crosssectional images reveal that the Cu 2 ZnSnSe 4 (CZTSe) film prepared with the precursor stacking order of Mo/Zn/Cu/Sn/ Cu shows large grains without nanoscale small grains at the bottom of the film, which is attributed to a top-to-bottom growth mechanism. However, the CZTSe film prepared with the precursor stacking order Mo/Sn/Cu/Zn/Sn/Cu has a bilayer structure. The crystallization of CZTSe at the top of the film is good, but its crystallization at the bottom of the film is poor because the SnSe 2 liquid phase that assists the growth of CZTSe grains is absent in the bottom layer. The difference in nanoscale morphology and the growth mechanism of CZTSe films is mainly due to the difference in the locations of Zn layers in the precursor films. Therefore, the best CZTSe solar cells have been fabricated with the selenized Mo/Zn/Cu/Sn/Cu films and exhibit an efficiency of 10.28%. Further development of a high-quality kesterite-structured thin film is expected based on the growth mechanism studied herein.

Solvothermal Synthesis of Cu2ZnSnSe4 Nanoparticles and Their Visible-Light-Driven Photocatalytic Activity

“…As can be appreciated, the multiple peak fitting revealed two peaks centered at 173 and 190 cm −1 . All these peaks are attributed to and are characteristics of the kesterite Cu2ZnSnSe4 phase and well matched to literature reports [4,8,35,45,52,53]. The most intense peak in the Raman spectrum of the tetragonal kesterite Cu2ZnSnSe4 phase is observed at 190 cm −1 , which corresponds to the purely anionic vibrations (A1 mode) of selenium surrounded with motionless neighboring atoms [3,35,42].…”

“…The average crystallite size evaluated from the (112) CZTSe diffraction peak was ~35 nm, inferring its nanocrystalline character. However, it is worth mentioning that due to the overlapping between characteristic XRD peaks of CZTSe and the diffraction peaks of Cu x Se, Cu 2 SnSe 3 , and ZnSe XRD patterns [4,8,42,51], the coexistence of these phases cannot be completely excluded based solely on XRD analysis [4,8,42,51]. Therefore, to confirm the structural properties of the CZTSe nanocrystalline samples and provide a more comprehensive understanding, Raman spectroscopy has been used as a complementary tool to distinguish and ensure the formation of the pure Cu 2 ZnSnSe 4 phase.…”

“…The kesterite family includes quaternary and quinary compounds such as Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and their alloys (Cu 2 ZnSn(S x ,Se 1−x ) 4 (CZTSSe), where 0 ≤ x ≤ 1) [2][3][4][5]. The kesterite constituent elements are less toxic and are the more usual elements of copper, zinc, and sulfur, which are the 26th, 25th, and 17th most abundant elements in the Earth's crust, respectively [2,[6][7][8][9][10]. All these kesterite semiconductor materials exhibited outstanding optoelectronic properties such as broad optical absorbance over the entire visible region with high absorption coefficient (>10 4 cm −1 ) [6,7,9,11] and optimal optical bandgap (0.9-1.6 eV), allowing the material to harvest maximum photons [12], good photo-stability [13], and multi-dimensional symmetric carrier transport [11,14].…”