Impact of the selenisation temperature on the structural and optical properties of CZTSe absorbers

We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 °C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4% for selenisation at 500 °C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 °C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 °C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed

A luminescence study of Cu2ZnSnSe4/Mo/glass films and solar cells with near stoichiometric copper content

Cu2ZnSnSe4 (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 (Eg). 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 cati on 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 d egree of Cu/Zn order and decreased the fraction with low degree of Cu/Zn order. Device processing increased Eg, 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 Ea = 200 meV) which we attributed to defects in the CdS layer

Stimulated emission and optical properties of solid solutions of Cu(In,Ga)Se2 direct band gap semiconductors

Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se2 thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se2 thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm2. It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se2 thin films

Photoluminescence, Stimulated and Laser Emission in CuInSe2crystals

Excitonic quality CuInSe2 crystals were studied using low-temperature (10 K) photoluminescence (PL) excited by continuous wave and nanosecond pulsed lasers at power densities from 0.01 to 76 kW/cm2. Increasing the excitation power density level to 26 kW/cm2 resulted in the appearance of a stimulated emission SE-band in the PL spectra at 1.035 eV. Further increase in the excitation level to 39 kW/cm2 generated on the top of the SE band, a structure of equidistant sharp lines attributed to laser emission. © 2021 Author(s).This work was supported by State Program of Scientific Research of the Republic of Belarus “Physical Material Science, New Materials and Technologies” (Project No. 1.4.4) and Belarusian Republican Foundation of Basic Research (Grant No. F20M-058). The research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (“Spin” No. AAAA-A18-118020290104-2)

Influence of the copper content on the optical properties of CZTSe thin films

We present an optical spectroscopy study of Cu2ZnSnSe4 (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 Eg, 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 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 Eg 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 Eg 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

Photoluminescence, stimulated and laser emission in CuInSe2 crystals

Excitonic quality CuInSe2 crystals were studied using low-temperature (10 K) photoluminescence (PL) excited by continuous wave and nanosecond pulsed lasers at power densities from 0.01 kW/cm2 to 76 kW/cm2 . Increasing the excitation power density level to 26 kW/cm2 resulted in the appearance of a stimulated emission SE-band in the PL spectra at 1.035 eV. Further increase in the excitation level to 39 kW/cm2 generated on the top of the SE band a structure of equidistant sharp lines attributed to laser emission. at 1.035 eV. Further increase in the excitation level to 39 kW/cm2 generated a structure of equidistant sharp lines attributed to laser emission on the top of the SE band. The lasing regime suggests the presence of volumes with parallel faces (microcracks or grain boundaries), which act as laser mirrors within the CuInSe2 crystals

EFFECT OF CHEMICAL TREATMENT OF THIN FILMS Cu2ZnSnSe4 ON PHOTOLUMINESCENCE OF SOLAR CELLS CREATED ON THEIR BASIS

The effect of etching regimes of thin films of Cu2ZnSnSe4 (prior to the deposition of CdS) on the bandgap (Eg) and photoluminescence (PL) spectra were studied by examining solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo/glass using PL and PL-excitation tech-niques

INFLUENCE OF IRRADIATION OF Cu(In, Ga)Se2 THIN FILMS BY 10 MeV ELECTRONS ON PHOTOLUMINESCENCE SPECTRA

Thin films of Cu(In,Ga)Se2 on Mo coated glass substrates were studied by photolumines-cence (PL) technique before and after irradiation with a dose of 1.8·1015 cm–2 of 10 MeV electrons to determine the nature of radiation defects in Cu(In,Ga)Se2.Работа выполнена при финансовой поддержке РНФ (проект № 17-12-01500)