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Single-step sulfo-selenization method for achieving low open circuit voltage deficit with band gap front-graded Cu2ZnSn(S,Se)4 thin films

Single-step sulfo-selenization method for achieving low open circuit voltage deficit with band gap front-graded Cu2ZnSn(S,Se)4 thin films
Hwang, Dae KueKo, Byoung SooJeon, Dong HwanKang, Jin KyuSung, Shi JoonYang, Kee JeongNam, Da HyunCho, So YeonCheong, Hyeon SikKim, Dae Hwan
DGIST Authors
Hwang, Dae KueKang, Jin Kyu; Sung, Shi Joon; Yang, Kee JeongKim, Dae Hwan
Issue Date
Solar Energy Materials and Solar Cells, 161, 162-169
Article Type
Band Gap Front-GradingCompositional DistributionCopperCZTSSe Thin FilmsElectron MicroscopyEnergy Dispersive SpectroscopyEnergy Dispersive X Ray SpectroscopyEnergy GapGradingHigh Resolution Transmission Electron MicroscopyLow Open Circuit Voltage DeficitOpen Circuit VoltageOptical PropertiesPower Conversion EfficienciesScanning Electron MicroscopyScanning Transmission Electron MicroscopySelenizationSelenization MethodsSelenization ProcessSemiconducting Selenium CompoundsSolar CellsSulfo-SelenizationThin-Film Solar CellsThin-FilmsThin Film CircuitsTiming CircuitsTransmission Electron MicroscopyX Ray Spectroscopy
In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu2ZnSn(S,Se)4 (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161 eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029 eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (Voc) of 0.505 V, short-circuit current density (Jsc) of 31.61 mA/cm2, fill factor (FF) of 64.6%, and Voc deficit of 525 mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, Voc of 0.424 V, Jsc of 32.83 mA cm−2, FF of 51.9%, and Voc deficit of 576 mV, the Voc and Voc deficit of the CZTSSe cell improved considerably. The high Voc, low Voc deficit, and less loss of Jsc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film. © 2016 Elsevier B.V.
Elsevier B.V.
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Convergence Research Center for Solar Energy1. Journal Articles

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