Band-gap-graded Cu2ZnSn(S1-x,Sex)4 Solar Cells Fabricated by an Ethanol-based, Particulate Precursor Ink Route

Abstract

Solution processing of earth-abundant Cu2ZnSn(S1-x,Sex)4 (CZTSSe) absorber materials is an attractive research area in the economical and large-scale deployment of photovoltaics. Here, a band-gap-graded CZTSSe thin-film solar cell with 7.1% efficiency was developed using non-toxic solvent-based ink without the involvement of complex particle synthesis, highly toxic solvents, or organic additives. Despite the high series resistance due to the presence of a thick Mo(S,Se)x layer and Zn(S,Se) aggregates, a high short-circuit current density (JSC) was generated. In addition, there was no significant difference in open circuit voltages (VOC) between CZTS (0.517 V) and CZTSSe (0.505–0.479 V) cells, despite a significant band gap change from 1.51 eV to 1.24 eV. The high JSC and less loss of VOC are attributed to the effect of band gap grading induced by Se grading in the CZTSSe absorber layer. Our environmentally benign ink approach will enable the realization of low-cost, large-area, high-efficiency thin-film solar cells.

Effect of selenization conditions on the growth and properties of Cu2ZnSn(S,Se)4 thin films

“…To achieve this goal, tuning optical and electrical properties of the material are required since the best experimental conversion efficiency reported so far are below 10% for pure CZTS based solar cells and there is still a huge gap between theoretical and practical results [4]. Recent studies indicate an improvement by incorporation of Se in CZTS so as to form Cu 2 ZnSn(S x ,Se 1-x ) 4 alloy, known as CZTSSe [5,6,7]. The variation of sulphur to selenium ratio in this compound changes the structural, electrical and optical properties of the film [8].…”

Optimization of Cu2ZnSnS4 Absorber Layers via Sulfurization of Electrodeposited Cu-Zn-Sn Precursors for Photovoltaic Applications

“…Poor carrier diffusion in the bulk of the CZTS layers is also observed with our TRPL results. [190,193,194], the band gap of absorbers in the solar cells are in reasonable agreement with reported band gap values (1.45-1.6 eV) [56,57,196] . …”

“…The relatively high EQE result of CZTS_Citrate is consistent with the higher Jsc of this device compared to CZTS_Pyro. However, the highest value in the EQE curves are much lower than the reported values [187,190].…”

“…The EQE spectra show significant absorption at short wavelengths around 400 nm, which originates from the ZnO phase, and the absorption around 500 nm due to the CdS layer [132,190]. The absorption of CZTS_Citrate cell declines faster in the blue region of the spectrum (~450-500 nm) compared to CZTS_Pyro.…”

Effect of the stacked structure on performance in CZTSSe thin film solar cells

“…We speculate that the multilayer stack of Cu and Sn accelerated the reaction rate, which reduced the loss of Sn, thereby enabling it to react to form single phase CZTSSe. [25,26]. It is generally believed that band gap grading can be enhanced to mitigate recombination in order to increase the open circuit voltage and photo-generated carrier collection [26].…”