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Title: The impact of sodium on the sub-bandgap states in CZTSe and CZTS

We compare the optically active sub-bandgap states in polycrystalline Cu{sub 2}ZnSnSe{sub 4} (CZTSe) and Cu{sub 2}ZnSnS{sub 4} (CZTS) thin films as a function of sodium content. In all samples studied, we find that CZTSe has a lower concentration of radiative defect-derived states compared to CZTS and that the states are also shallower in CZTSe compared to CZTS. Further, we find that sodium impacts the relative ratios of two sub-bandgap peaks in the 4 K photoluminescence (PL) spectra of CZTSe (one at ∼0.85 eV and another at ∼0.92 eV). We propose that both of these sub-bandgap peaks stem from intrinsic point defects in CZTSe rather than from electronic states introduced by sodium; this is supported by a measurement on a sodium-free single-crystal of CZTSe. We also show that films with stronger emission through the shallower sub-bandgap states at 4 K display room-temperature PL closer to the bandgap energy. For all sodium quantities studied, one broad PL peak is observed in the 4 K PL spectrum of CZTS which also shifts towards the band edge with increasing sodium. A reduced overall defect density and the fact that the states that are present are shallower together may help account for the lower V{sub OC} deficits in CZTSe andmore » the empirical observations that sodium improves device performance.« less
Authors:
; ; ; ; ;  [1] ; ;  [2]
  1. Physical Sciences Department, IBM T.J. Watson Research Center, 1101 Kitchawan Rd, Yorktown Heights, New York 10598 (United States)
  2. Institute of Energy Conversion, University of Delaware, Newark, Delaware 19716 (United States)
Publication Date:
OSTI Identifier:
22398794
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 106; Journal Issue: 12; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; COMPARATIVE EVALUATIONS; CONCENTRATION RATIO; COPPER COMPOUNDS; ENERGY GAP; EV RANGE; MONOCRYSTALS; PHOTOLUMINESCENCE; POINT DEFECTS; POLYCRYSTALS; SODIUM; TEMPERATURE RANGE 0273-0400 K; THIN FILMS; TIN SELENIDES; TIN SULFIDES; ZINC COMPOUNDS