We present an optical spectroscopy study of Cu 2 ZnSnSe 4 (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap E g , determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean 2 depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens E g due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the E g widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.
Cu 2 ZnSnSe 4 (CZTSe) is one of the leading candidates for the absorber layer in sustainable solar cells. Thin films of CZTSe with a near stoichiometric [Cu]/[Zn+Sn] were used to produce solar cells with conversion efficiency η = 6.4% by a standard solar cell processing including KCN etching and the deposition of CdS and ZnO. Both CZTSe films and solar cells were examined using photoluminescence (PL) to analyse the nature of radiative recombination and photoluminescence excitation (PLE) at 4.2 K to determine the bandgap (E g). Low temperature PL spectra of the films reveal an intense band P1 at 0.81 eV and a low intensity band P2 at 0.93 eV. Their temperature and excitation intensity dependencies suggest that they both involve recombinations of free electrons with holes localised at acceptors with the energy level influenced by potential fluctuations in the valence band. We associate P1 and P2 with different fractions of CZTSe: with a lower and higher degree of order of Cu and Zn on the cation sub-lattice, respectively. Device processing reduced the intensity of P1 by 2.5 whereas the intensity of P2 increased by a 1.5. We assign this to a low temperature annealing due to CdS and ZnO deposition which increased the fraction of CZTSe with high degree of Cu/Zn order and decreased the fraction with low degree of Cu/Zn order. Device processing increased E g , blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can also be related to the annealing and/or KCN etching and the chemical effect of Cd, due to CdS replacing copper at the CdS-CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with E a = 200 meV) which we attributed to defects in the CdS layer.
Cu 2 ZnSnSe 4 (CZTSe) is a semiconductor used as the absorber layer in highly promising sustainable thin film solar cells. The authors study the effect of Ar + etching of copper deficient and zinc excess CZTSe thin films deposited on Mo/glass substrates on the surface elemental composition, measured by x-ray photoelectron spectroscopy, and photoluminescence (PL) spectra. Low temperature PL spectra reveal a broad asymmetrical band at 0.95 eV. The temperature and excitation intensity dependencies of this band suggest that it is a free-to-bound (FB) recombination of electrons from the conduction band with holes localized at an acceptor affected by potential fluctuations. The surface composition of the as grown films demonstrates a strong copper deficiency: [Cu]/[Zn + Sn] = 0.33. The etching of the film surface using Ar + beam increases [Cu]/[Zn + Sn] to 0.51, which is significantly smaller than that of 0.78 in the bulk, measured by wavelength dispersive x-ray analysis, demonstrating the presence on the surface of a copper-depleted layer. The Ar + etching drastically reduces the FB band intensity by a factor of 4.5, broadens it and develops a low energy tail. Ar ions displace atoms in CZTSe lattice creating primary radiation defects, vacancies, and interstitials, which recombine at room temperature forming antisite defects with deep energy levels. Some of them generate the observed low energy tail and increase the mean depth of potential fluctuation γ, determined from the shape of the low energy side of FB band, from 24 meV before Ar + etching to 35 meV after. Other deep defects work as nonradiative recombination centers reducing the intensity of the FB band.
Solar cells with the structure ZnO/CdS/Cu 2 ZnSnSe 4 /Mo/glass were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 � 10 15 cm À 2 and then 5.4 � 10 15 cm À 2 of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant band at 0.94 eV from the CZTSe layer and a lower intensity high energy band (HEB) at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is free-to-bound (FB): the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new band in the examined spectral range (from 0.5 μm to 1.65 μm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure temperature dependencies of the PL spectra. After irradiation the red shift rate of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band.
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