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Title: Multiple phases of Cu{sub 2}ZnSnSe{sub 4} detected by room temperature photoluminescence

Abstract

Cu{sub 2}ZnSnSe{sub 4} based solar cells are promising but suffer from low open circuit voltage relative to their band gap. Additionally, the bandgap as extrapolated from quantum efficiency (QE) measurements varies without clear correlation to the growth conditions. Using room temperature photoluminescence, we show that different materials with different bandgaps coexist within micrometer sized areas of the absorbers. Simulations of the effect of multiple bandgaps on both the absorption and the Shockley-Queisser radiative recombination limit, explain the variations of the bandgap extrapolated from QE and the deficiencies of the solar cell parameters.

Authors:
; ; ; ;  [1]
  1. Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, 4422 Belvaux (Luxembourg)
Publication Date:
OSTI Identifier:
22314293
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 116; Journal Issue: 7; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ABSORPTION; ELECTRIC POTENTIAL; PHOTOLUMINESCENCE; QUANTUM EFFICIENCY; RECOMBINATION; SIMULATION; SOLAR CELLS; TEMPERATURE RANGE 0273-0400 K

Citation Formats

Djemour, Rabie, Redinger, Alex, Mousel, Marina, Gütay, Levent, and Siebentritt, Susanne, E-mail: susanne.siebentritt@uni.lu. Multiple phases of Cu{sub 2}ZnSnSe{sub 4} detected by room temperature photoluminescence. United States: N. p., 2014. Web. doi:10.1063/1.4892101.
Djemour, Rabie, Redinger, Alex, Mousel, Marina, Gütay, Levent, & Siebentritt, Susanne, E-mail: susanne.siebentritt@uni.lu. Multiple phases of Cu{sub 2}ZnSnSe{sub 4} detected by room temperature photoluminescence. United States. doi:10.1063/1.4892101.
Djemour, Rabie, Redinger, Alex, Mousel, Marina, Gütay, Levent, and Siebentritt, Susanne, E-mail: susanne.siebentritt@uni.lu. Thu . "Multiple phases of Cu{sub 2}ZnSnSe{sub 4} detected by room temperature photoluminescence". United States. doi:10.1063/1.4892101.
@article{osti_22314293,
title = {Multiple phases of Cu{sub 2}ZnSnSe{sub 4} detected by room temperature photoluminescence},
author = {Djemour, Rabie and Redinger, Alex and Mousel, Marina and Gütay, Levent and Siebentritt, Susanne, E-mail: susanne.siebentritt@uni.lu},
abstractNote = {Cu{sub 2}ZnSnSe{sub 4} based solar cells are promising but suffer from low open circuit voltage relative to their band gap. Additionally, the bandgap as extrapolated from quantum efficiency (QE) measurements varies without clear correlation to the growth conditions. Using room temperature photoluminescence, we show that different materials with different bandgaps coexist within micrometer sized areas of the absorbers. Simulations of the effect of multiple bandgaps on both the absorption and the Shockley-Queisser radiative recombination limit, explain the variations of the bandgap extrapolated from QE and the deficiencies of the solar cell parameters.},
doi = {10.1063/1.4892101},
journal = {Journal of Applied Physics},
number = 7,
volume = 116,
place = {United States},
year = {Thu Aug 21 00:00:00 EDT 2014},
month = {Thu Aug 21 00:00:00 EDT 2014}
}
  • The interest in Cu2ZnSn(S,Se)4 (CZTS) for photovoltaic applications is motivated by similarities to Cu(In,Ga)Se2 while being comprised of non-toxic and earth abundant elements. However, CZTS suffers from a Voc deficit, where the Voc is much lower than expected based on the band gap, which may be the result of a high concentration of point-defects in the CZTS lattice. Recently, reports have observed a low-temperature order/disorder transition by Raman and optical spectroscopies in CZTS films and is reported to describe the ordering of Cu and Zn atoms in the CZTS crystal structure. To directly determine the level of Cu/Zn ordering, wemore » have used resonant-XRD, a site, and element specific probe of long range order. We used CZTSe films annealed just below and quenched from just above the transition temperature; based on previous work, the Cu and Zn should be ordered and highly disordered, respectively. Our data show that there is some Cu/Zn ordering near the low temperature transition but significantly less than high chemical order expected from Raman. To understand both our resonant-XRD results and the Raman results, we present a structural model that involves antiphase domain boundaries and accommodates the excess Zn within the CZTS lattice.« less
  • Cu{sub 2}ZnSnSe{sub 4} thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu{sub 2}Zn{sub 5}SnSe{sub 8} and Cu{sub 2}Zn{sub 6}SnSe{sub 9} composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu{sub 2}ZnSnSe{sub 4} and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meVmore » below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu{sub 2}Zn{sub 5}SnSe{sub 8} and Cu{sub 2}Zn{sub 6}SnSe{sub 9} phases. Possible effects of these phases on solar cell performance are discussed.« less
  • Cu{sub 2}ZnSnS{sub 4} is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu{sub 2}ZnSnS{sub 4}-Cu{sub 2}SnS{sub 3} tie line in the quasi-ternary Cu{sub 2}S-ZnS-SnS{sub 2} phase system, and this effect is attributed tomore » structural similarity between the Cu{sub 2}ZnSnS{sub 4}, Cu{sub 2}SnS{sub 3}, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu{sub 2}ZnSnS{sub 4}-Cu{sub 2}SnS{sub 3} tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu{sub 2}ZnSnS{sub 4} on non-radiative recombination processes. Implications for processing and performance of Cu{sub 2}ZnSnS{sub 4} solar cells are discussed.« less
  • We report that the interest in Cu 2ZnSn(S,Se) 4 (CZTS) for photovoltaic applications is motivated by similarities to Cu(In,Ga)Se 2 while being comprised of non-toxic and earth abundant elements. However, CZTS suffers from a V oc deficit, where the V oc is much lower than expected based on the band gap, which may be the result of a high concentration of point-defects in the CZTS lattice. Recently, reports have observed a low-temperature order/disorder transition by Raman and optical spectroscopies in CZTS films and is reported to describe the ordering of Cu and Zn atoms in the CZTS crystal structure. Tomore » directly determine the level of Cu/Zn ordering, we have used resonant-XRD, a site, and element specific probe of long range order. We used CZTSe films annealed just below and quenched from just above the transition temperature; based on previous work, the Cu and Zn should be ordered and highly disordered, respectively. Our data show that there is some Cu/Zn ordering near the low temperature transition but significantly less than high chemical order expected from Raman. Finally, to understand both our resonant-XRD results and the Raman results, we present a structural model that involves antiphase domain boundaries and accommodates the excess Zn within the CZTS lattice.« less