Journal
SCIENCE CHINA-MATERIALS
Volume 64, Issue 1, Pages 52-60Publisher
SCIENCE PRESS
DOI: 10.1007/s40843-020-1408-x
Keywords
kesterite solar cell; V-oc deficit; SnCl4; reaction pathway; heterojunction heat treatment
Categories
Funding
- National Natural Science Foundation of China [21571106, U1902218]
- Postgraduate Research and Practice Innovation Program of Jiangsu Province
- Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington - National Science Foundation [NNCI-1542101]
- University of Washington
- Molecular Engineering & Sciences Institute
- Clean Energy Institute
- National Institutes of Health
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The study investigates the influence of different tin coordination complexes on CZTSSe thin film solar cells, revealing that using Sn2+ as precursor results in poor film quality and device performance, while Sn4+ precursor leads to higher performance absorber materials.
The limiting factor preventing further performance improvement of the kesterite (sulfide Cu2ZnSnS4(CZTS), selenide Cu2ZnSnSe4(CZTSe), and their alloying Cu2ZnSn(S,Se)(4)(CZTSSe)) thin film solar cells is the large open-circuit voltage deficit (V-oc,V-def) issue, which is 0.345 V for the current world record device with an efficiency of 12.6%. In this study, SnCl4 and SnCl2 center dot 2H(2)O were respectively used as tin precursor to investigate the V-oc,V-def issue of dimethyl sulfoxide (DMSO) solution processed CZTSSe solar cells. Different complexations of tin compounds with thiourea (Tu) and DMSO resulted in different reaction pathways from the solution to the absorber material and thus dramatic differences in photovoltaic performance. The coordination of Sn2+ with Tu led to the formation of SnS, ZnS and Cu2S in the precursor film, which converted to selenides first and then fused to CZTSSe, resulting in poor film quality and device performance. The highest efficiency obtained from this film was 8.84% with a V-oc,V-def of 0.391 V. The coordination of Sn(4+)with DMSO facilitated direct formation of CZTS phase in the precursor film which directly converted to CZTSSe during selenization, resulting in compositional uniform absorber and high device performance. A device with an active area efficiency of 12.2% and a V-oc,V-def of 0.344 V was achieved from the Sn4+ solution processed absorber. Furthermore, CZTSSe/CdS heterojunction heat treatment (JHT) significantly improved the performance of the Sn4+ device but had slightly negative effect on the Sn2+ device. A champion CZTSSe solar cell with a total area efficiency of 12.4% (active area efficiency of 13.6%) and a low V(oc,def)of 0.297 V was achieved from the Sn4+ solution. Our results demonstrate the preformed uniform CZTSSe phase enabled by Sn4+ precursor is the key for the highly efficient CZTSSe absorber. The lowest V-oc,V-def and high efficiency achieved here shines new light on the future of CZTSSe solar cell.
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