Radiative recombination in Cu2ZnSnSe4 thin films with Cu deficiency and Zn excess

Thin films of Cu2ZnSnSe4 (CZTSe) with copper de�ficiency and zinc excess were fabricated at Northumbria University by the selenisation of metallic precursors deposited on Mo/glass and bare glass substrates. Absorption and photoluminescence (PL) measurements were used to examine the �film on glass whereas fi�lms on Mo/glass were used to produce a solar cell with ef�ficiency of 8.1%. Detailed temperature and excitation intensity analysis of PL spectra allows identifi�cation of the main recombination mechanisms as band-to-tail and band-to-band transitions. The latter transition was observed in the spectra from 6 to 300 K

Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu2ZnSnSe4/Mo/glass

The effect of solar cell processing (including etching in KCN along with deposition of CdS and ZnO) on photoluminescence (PL) spectra and bandgap Eg (measured at 4.2 K by photoluminescence excitation) of Cu2ZnSnSe4 films, produced by selenising metallic precursors at 450 °C, 500 °C and 550 °C, was studied. Temperature and excitation intensity analysis of the P1 dominant band in the PL spectra of solar cells suggests that after processing this band still can be assigned to the free-to-bound recombination of free electrons with holes bound at deep acceptor levels influenced by valence band-tails. However processing increased the intensity of P1 and blue shifted it. The strongest effect was observed for the film selenised at 500 °C. For the film selenised at 450 °C the blue shift and increase in the intensity were smaller and only a slight intensity rise was found for the film selenised at 550 °C. The intensity increase we assign to a reduction in the concentration of non-radiative recombination centers on the surface because of the etching and changes in doping due to inter-diffusion of Cd, S, Se and Zn after the deposition of CdS. Such an inter-diffusion depends on the elemental composition of the films defining the chemistry of defects and influencing Eg which increased in the film selenised at 500 °C but decreased in the other films. Processing increased the P1 shift rate (j-shift) with excitation power change in all the films demonstrating a higher compensation degree in the solar cells which is consistent with the formation of an interface layer containing new donors CdCu

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 (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

Landau levels of the C-exciton in CuInSe2 studied by magneto-transmission

The electronic structure of the solar cell absorber CuInSe2 is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass mso = (0.31 ± 0.12)m0 for the C valence sub-band

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

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

ДЕГРАДАЦИЯ ПАРАМЕТРОВ ФОТОПРЕОБРАЗОВАТЕЛЕЙ СОЛНЕЧНОЙ ЭНЕРГИИ НА ОСНОВЕ ПОЛУПРОВОДНИКОВЫХ ТВЕРДЫХ РАСТВОРОВ Cu(In,Ga)Se2 ПРИ ЭЛЕКТРОННОМ ОБЛУЧЕНИИ

Polycrystalline Cu(In,Ga)Se2 (CIGS) thin films were grown on molybdenum-coated soda-lime glass substrates by co-evaporation of the elements Cu, In, Ga and Se from independent sources. The effect of electron irradiation on the electrical and optical properties of CIGS thin films and solar cells with the structure ZnO:Al/i-ZnO/CdS/CIGS/Mo/glass was studied. It was found that the degradation of the electrical parameters of solar cells (open-circuit voltage, short-circuit current density and efficiency) took place due to the formation of radiation defects (recombination centers) with deep energy levels in the bandgap of CIGS. It was revealed that after electron irradiation intensity of near band-edge luminescence band at about 1,1 eV decreased considerably and bands of luminescence with maxima at 0,93 and 0,75 eV appeared.Методом испарения элементов Cu, In, Ga и Se из независимых источников на натрийсодержащих стеклянных подложках, покрытых слоем молибдена, выращены тонкие поликристаллические пленки Cu(In,Ga)Se2 (CIGS). Изучено влияние электронного облучения на электрические и оптические свойства тонких пленок CIGS и солнечных элементов со структурой ZnO:Al/i-ZnO/CdS/CIGS/Mo/стекло. Установлено, что деградация электрических параметров солнечных элементов (напряжения холостого хода, плотности тока короткого замыкания и коэффициента полезного действия) обусловлена образованием радиационных дефектов (рекомбинационных центров) с глубокими энергетическими уровнями в запрещенной зоне базовых слоев CIGS. Обнаружено, что после электронного облучения значительно уменьшается интенсивность близкраевой люминесценции в области 1,1 эВ и появляются полосы люминесценции с максимумами 0,93 и 0,75 эВ

Erbium upconversion luminescence from sol-gel derived multilayer porous inorganic perovskite film

Erbium-doped barium titanate (BaTiO3:Er) xerogel film with a thickness of about 500 nm was formed on the porous strontium titanate (SrTiO3) xerogel film on Si substrate after annealing at 800 °C or 900 °C. The elaborated structures show room temperature upconversion luminescence under 980 nm excitation with the photoluminescence (PL) bands at 523, 546, 658, 800 and 830 nm corresponding to 2H11/2→4I15/2, 4S3/2→4I15/2, 4F9/2→4I15/2 and 4I9/2→4I15/2 transitions of trivalent erbium. Raman and X-ray diffraction (XRD) analysis of BaTiO3:Er\porous SrTiO3\Si structure showed the presence of perovskite phases. Its excellent up-conversion optical performance will greatly broaden its applications in perovskite solar cells and high-end anti-counterfeiting technologies