Journal
CHEMISTRY OF MATERIALS
Volume 27, Issue 15, Pages 5279-5287Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.chemmater.5b01473
Keywords
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Funding
- Framework 7 program under the project KESTCELLS [FP7-PEOPLE-2012-ITN-316488]
- European Regional Development Founds (ERDF, FEDER Programa Competitivitat de Catalunya)
- MINECO (Ministerio de Economia y Competitividad de Espana) [PCIN-2013-128-C02-01, ENE2013-49136-C4-1-R]
- Government of Spain [RYC-2011-09212]
- PTA [PTA2012-7852-A]
- FPI-MINECO [RES-2011-045774]
- China Scholarship Council (CSC) [201206340113]
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A detailed study explaining the beneficial effects of low temperature postdeposition annealing combined with selective surface etchings for Cu2ZnSnSe4 (CZTSe) based solar cells is presented. After performing a selective oxidizing surface etching to remove ZnSe secondary phases typically formed during the synthesis processes an additional 200 degrees C annealing step is necessary to increase device performance from below 3% power conversion efficiency up to 8.3% for the best case. This significant increase in efficiency can be explained by changes in the surface chemistry which results in strong improvement of the CdS/CZTSe heterojunction commonly used in this kind of absorber/buffer/window heterojunction solar cells. XPS measurements reveal that the 200 degrees C annealing promotes a Cu depletion and Zn enrichment of the etched CZTSe absorber surface relative to the CZTSe bulk. Raman measurements confirm a change in Cu/Zn ordering and an increase in defect density. Furthermore, TEM microstructural investigations indicate a change of grain boundaries composition by a reduction of their Cu content after the 200 degrees C annealing treatment. Additionally, insights in the CdS/CZTSe interface are gained showing a significant amount of Cu in the CdS buffer layer which further helps the formation of a Cu-depleted surface and seems to play an important role in the formation of the pn-heterojunction.
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