Improving Efficiencies of Kesterite-Based CZTSSe Solar Cell by Introducing CdS Nanolayers for Forming Passivating p+-CTSSe Point Contact

• Main Authors: Huang, Chih-Yang, 黃至揚
• Other Authors: Hwang, Jih-Shang
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/f3f83k

碩士 === 國立臺灣海洋大學 === 光電科學研究所 === 106 === In the thesis, solar cell conversion efficiencies of CZTSSe solar cells were proved to be greatly improved by a simple but effective method, wherein a CdS nano-layer was deposited between the Mo substrate and the metal precursors prior to CZTSSe sulfo-selenization, producing p+-CTSSe (or p+-CTS(e)) and ZnSSe (or ZnS(e)), which help reduce interface recombination, on the bottom of the CZTSSe absorber layer. Experimental results evidenced that p+-CTS(e) would help forming back surface field to enhance carrier collection, whereas the wide band-gap ZnS(e) regions could block part of conducting path way through Mo/CZTSSe interface to reduce interface recombination. During sulfo-selenization, cadmium would also diffuse into the CZTSSe layer, which would help suppress Cu-Zn disorder to prevent formation of deep level defects. As a result, CZTSSe solar cells performances can be greatly improved. The formation of p+-CTS(e) and ZnS(e) structure at the bottom of the CZTSSe absorber layer, as well as the homogenous cadmium diffusion into CZTSSe, was evidenced by TEM EDS mapping. According to semi-quantitative analysis, the band alignment of CZTSSe:Cd/p+-CTS(e)/Mo was revealed, which would form back surface field to help reduce electron-hole recombination and enhance conductivity. Likewise, CZTSSe:Cd/p+-CTS(e)/Mo could block part of conducting path way through Mo/CZTSSe interface to reduce interface recombination. Experimental results also show that introducing a CdS nanolayer would suppress the reaction between CZTSSe and Mo substrate, which would reduce voids of Mo(S1-xSex)2 and Sn(S1-xSex) and resulted in carriers transport improvement. Light current and dark current analysis were conducted to realize the interface recombination. By introducing CdS nanolayers between the metal precursor and Mo substrate, the reverse saturation current reduced greatly from 2.19 to 0.13 mA/cm2 and ideal factor decreased from 4.39 to 2.39, which mean the p+-CTS(e) point contact and the ZnS(e) passivation layer did help reducing interface recombination. The main improvement of EQE was observed from 500 to 1200nm, which further proved that the enhancement was due to the reduction of back contact recombination. According to a log scale plot of EQE, samples with CdS nanolayer had lower defect density and shallower defects with improved band tails. From time resloved photoluminese, CdS modified samples also show a longer carrier life time due to less Cu-Zn disoreder. Finally, high efficiency CZTSSe solar cell were attained through the aforementioned CdS nanolayer method. With CdS nanolyer used, the fill factor of CZTSSe solar cells was enhanced from 44% (no CdS nanolyer) to 64%; the short circuit current density was increased from 30.92 to 32.24 mA/cm2; and eventually the conversion efficiency reached 9.61%.