• M. A. Sulimov, M. N. Sarychev, M. V. Yakushev, J. Márquez-Prieto, I. Forbes, Y. V. Ivanov, P. R. Edwards, A. V. Mudryi, J. Krustok, and R. W. Martin, “Effects of irradiation of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass solar cells by 10 MeV electrons on photoluminescence spectra,,” Materials Science in Semiconductor Processing, vol. 121, p. 105301, 2021.

Solar cells with the structure ZnO/CdS/Cu₂ZnSnSe₄/Mo were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 10¹⁵ cm⁻² and then 5.4 × 10¹⁵ cm⁻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 FB band at 0.94 eV from the CZTSe layer and a lower intensity 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 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 bands 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 excitation intensity and temperature dependencies of the PL spectra. After irradiation the rate of red shift 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

@Article{strathprints73346,
author = {M. A. Sulimov and M. N. Sarychev and M. V. Yakushev and J. M{\'a}rquez-Prieto and I. Forbes and V. Yu. Ivanov and P. R. Edwards and A. V. Mudryi and J. Krustok and R. W. Martin},
journal = {Materials Science in Semiconductor Processing},
title = {Effects of irradiation of {ZnO/CdS/Cu₂ZnSnSe₄/Mo}/glass solar cells by 10 {MeV} electrons on photoluminescence spectra,},
year = {2021},
month = {July},
pages = {105301},
volume = {121},
abstract = {Solar cells with the structure ZnO/CdS/Cu₂ZnSnSe₄/Mo were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 10¹⁵ cm⁻² and then 5.4 × 10¹⁵ cm⁻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 FB band at 0.94 eV from the CZTSe layer and a lower intensity 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 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 bands 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 excitation intensity and temperature dependencies of the PL spectra. After irradiation the rate of red shift 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},
url = {https://strathprints.strath.ac.uk/73346/},
}

• M. V. Yakushev, M. A. Sulimov, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, “A luminescence study of Cu₂ZnSnSe₄/Mo/glass films and solar cells with near stoichiometric copper content,” Journal of Physics D: Applied Physics, vol. 36, p. 61208, 2019.

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 {\ensuremath{\eta}} = 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.

@Article{strathprints66083,
author = {M. V. Yakushev and M.A. Sulimov and J. M{\'a}rquez-Prieto and I. Forbes and P.R. Edwards and V.D. Zhivulko and O.M. Borodavchenko and A. V. Mudryi and J. Krustok and R. W. Martin},
title = {A luminescence study of Cu₂ZnSnSe₄/Mo/glass films and solar cells with near stoichiometric copper content},
journal = {Journal of Physics D: Applied Physics},
year = {2019},
volume = {36},
pages = {061208},
month = {November},
note = {This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://iopscience.iop.org/journal/0022-3727.},
abstract = {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 {\ensuremath{\eta}} = 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.},
keywords = {solar cells, photoluminescence, CZTSe, Physics, Physics and Astronomy(all)},
url = {https://strathprints.strath.ac.uk/66083/}
}

• M. A. Sulimov, M. V. Yakushev, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, “Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass,” Thin Solid Films, vol. 672, pp. 146-151, 2019.

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.

@Article{strathprints66586,
author = {M.A. Sulimov and M.V. Yakushev and J. M{\'a}rquez-Prieto and I. Forbes and P.R. Edwards and V.D. Zhivulko and O.M. Borodavchenko and A. V. Mudryi and J. Krustok and R.W. Martin},
title = {Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass},
journal = {Thin Solid Films},
year = {2019},
volume = {672},
pages = {146-151},
month = {January},
abstract = {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.},
keywords = {copper zinc tin selenide, solar cells, photoluminescence, selenisation, optical spectroscopy, Physics, Materials Chemistry, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Metals and Alloys},
url = {https://strathprints.strath.ac.uk/66586/}
}

• M. A. Sulimov, M. V. Yakushev, I. Forbes, J. M. Prieto, A. V. Mudryi, J. Krustok, P. R. Edwards, and R. W. Martin, “A PL and PLE study of high Cu content Cu₂ZnSnSe₄ films on Mo/glass and solar cells,” Physics of the Solid State, vol. 61, p. 908–917, 2019.

Cu₂ZnSnSe₄(CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap (Eg) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j-shift) j(P1) = 14 meV and j(P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from 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). We attributed P3 to defects in the CdS layer.

@Article{Sulimov2019PSS61,
author = {M. A. Sulimov and M. V. Yakushev and I. Forbes and J. M. Prieto and A. V. Mudryi and Ju. Krustok and P. R. Edwards and R. W. Martin},
title = {A {PL} and {PLE} study of high {Cu} content {Cu₂ZnSnSe₄} films on {Mo}/glass and solar cells},
journal = {Physics of the Solid State},
year = {2019},
volume = {61},
pages = {908--917},
abstract = {Cu₂ZnSnSe₄(CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap (Eg) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j-shift) j(P1) = 14 meV and j(P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from 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). We attributed P3 to defects in the CdS layer.},
url = {https://dx.doi.org/10.1134/S1063783419050214}
}

• M. V. Yakushev, M. A. Sulimov, E. Skidchenko, J. Márquez-Prieto, I. Forbes, P. R. Edwards, M. V. Kuznetsov, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, “Effects of Ar⁺ etching of Cu₂ZnSnSe₄ thin films : an x-ray photoelectron spectroscopy and photoluminescence study,” Journal of Vacuum Science and Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, vol. 36, p. 61208, 2018.

Cu2ZnSnSe4 (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 {\ensuremath{\gamma}}, 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.

@Article{strathprints66330,
author = {Michael V. Yakushev and Mikhail A. Sulimov and Ekaterina Skidchenko and Jose M{\'a}rquez-Prieto and Ian Forbes and Paul R. Edwards and Mikhail V. Kuznetsov and Vadim D. Zhivulko and Olga M. Borodavchenko and Alexander V. Mudryi and Juri Krustok and Robert W. Martin},
title = {Effects of Ar⁺ etching of Cu₂ZnSnSe₄ thin films : an x-ray photoelectron spectroscopy and photoluminescence study},
journal = {Journal of Vacuum Science and Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena},
year = {2018},
volume = {36},
pages = {061208},
month = {November},
abstract = {Cu2ZnSnSe4 (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 {\ensuremath{\gamma}}, 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.},
keywords = {Cu2ZnSnSe4, argon ion etching, photoluminescence, Physics, Condensed Matter Physics, Renewable Energy, Sustainability and the Environment, Electronic, Optical and Magnetic Materials},
url = {https://strathprints.strath.ac.uk/66330/}
}

• M. V. Yakushev, M. A. Sulimov, J. Márquez-Prieto, I. Forbes, J. Krustok, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, and R. W. Martin, “Influence of the copper content on the optical properties of CZTSe thin films,” Solar Energy Materials and Solar Cells, vol. 168, pp. 69-77, 2017.

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.

@Article{strathprints60524,
author = {M. V. Yakushev and M. A. Sulimov and J. M{\'a}rquez-Prieto and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and R. W. Martin},
title = {Influence of the copper content on the optical properties of {CZTSe} thin films},
journal = {Solar Energy Materials and Solar Cells},
year = {2017},
volume = {168},
pages = {69-77},
month = {April},
abstract = {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.},
keywords = {copper, thin films, optical spectroscopy, photoluminescence excitation, stoichiometric film, Cu2ZnSnSe4, defects, zinc, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
url = {http://strathprints.strath.ac.uk/60524/},
}

• J. Márquez-Prieto, M. V. Yakushev, I. Forbes, J. Krustok, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, M. Dimitrievska, V. Izquierdo-Roca, N. Pearsall, and R. W. Martin, “Impact of the selenisation temperature on the structural and optical properties of CZTSe absorbers,” Solar Energy Materials and Solar Cells, vol. 152, p. 42–50, 2016.

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.

@Article{strathprints55956,
author = {J. M{\'a}rquez-Prieto and M. V. Yakushev and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and M. Dimitrievska and V. Izquierdo-Roca and N. Pearsall and R. W. Martin},
title = {Impact of the selenisation temperature on the structural and optical properties of {CZTSe} absorbers},
journal = {Solar Energy Materials and Solar Cells},
year = {2016},
volume = {152},
pages = {42--50},
month = {April},
abstract = {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.},
keywords = {Cu2ZnSnSe4, selenisation, optical spectroscopy, structure, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
url = {http://strathprints.strath.ac.uk/55956/}
}

• M. V. Yakushev, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, A. V. Mudryi, J. Krustok, and R. W. Martin, “Radiative recombination in Cu₂ZnSnSe₄ thin films with Cu deficiency and Zn excess,” Journal of Physics D: Applied Physics, vol. 48, iss. 47, p. 475109, 2015.

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.

@Article{strathprints54657,
author = {M V Yakushev and J M{\'a}rquez-Prieto and I Forbes and P R Edwards and V D Zhivulko and A V Mudryi and J Krustok and R W Martin},
title = {Radiative recombination in {Cu₂ZnSnSe₄} thin films with {Cu} deficiency and {Zn} excess},
journal = {Journal of Physics D: Applied Physics},
year = {2015},
volume = {48},
number = {47},
pages = {475109},
month = {November},
abstract = {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.},
keywords = {Cu2ZnSnSe4, solar cells, photoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54657/}
}

• M. V. Yakushev, I. Forbes, A. V. Mudryi, M. Grossberg, J. Krustok, N. S. Beattie, M. Moynihan, A. Rockett, and R. W. Martin, “Optical spectroscopy studies of Cu₂ZnSnSe₄ thin films,” Thin Solid Films, vol. 582, p. 154–157, 2015.

Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.

@Article{strathprints55983,
author = {M.V. Yakushev and I. Forbes and A.V. Mudryi and M. Grossberg and J. Krustok and N.S. Beattie and M. Moynihan and A. Rockett and R.W. Martin},
title = {Optical spectroscopy studies of {Cu₂ZnSnSe₄} thin films},
journal = {Thin Solid Films},
year = {2015},
volume = {582},
pages = {154--157},
month = {May},
abstract = {Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.},
keywords = {Cu2ZnSnSe4, thin films, photoluminescence, defects, absorption, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/55983/}
}

• M. V. Yakushev, P. Maiello, T. Raadik, M. J. Shaw, P. R. Edwards, J. Krustok, A. V. Mudryi, I. Forbes, and R. W. Martin, “Investigation of the structural, optical and electrical properties of Cu3BiS3 semiconducting thin films,” Energy Procedia, vol. 60, p. 166–172, 2014.

The elemental composition, structural, optical and electronic properties of p-type Cu3BiS3 thin films are investigated. The films are shown to be single phase orthorhombic, with a measured composition of Cu3.00Bi0.92S3.02. A surface oxidation layer is also clarified using energy dependent X-ray microanalysis. Photoreflectance spectra demonstrate two band gaps (EgX =1.24 eV and EgY =1.53 eV at 4 K) associated with the X and Y valence sub-bands. The photocurrent excitation measurements suggest a direct allowed nature of EgX. Photoluminescence spectra at 5 K reveal two broad emission bands at 0.84 and 0.99 eV quenching with an activation energy of 40 meV.

@article{strathprints51127,
volume = {60},
title = {Investigation of the structural, optical and electrical properties of Cu3BiS3 semiconducting thin films},
author = {M.V. Yakushev and P. Maiello and T. Raadik and M.J. Shaw and P.R. Edwards and J. Krustok and A.V. Mudryi and I. Forbes and R.W. Martin},
year = {2014},
pages = {166--172},
journal = {Energy Procedia},
keywords = {Cu3BiS3, thin films, solar cells, Raman spectroscopy, photoluminescence, photoreflectance, Physics, Energy(all)},
url = {http://strathprints.strath.ac.uk/51127/},
abstract = {The elemental composition, structural, optical and electronic properties of p-type Cu3BiS3 thin films are investigated. The films are shown to be single phase orthorhombic, with a measured composition of Cu3.00Bi0.92S3.02. A surface oxidation layer is also clarified using energy dependent X-ray microanalysis. Photoreflectance spectra demonstrate two band gaps (EgX =1.24 eV and EgY =1.53 eV at 4 K) associated with the X and Y valence sub-bands. The photocurrent excitation measurements suggest a direct allowed nature of EgX. Photoluminescence spectra at 5 K reveal two broad emission bands at 0.84 and 0.99 eV quenching with an activation energy of 40 meV.}
}

• M. V. Yakushev, P. Maiello, T. Raadik, M. J. Shaw, P. R. Edwards, J. Krustok, A. V. Mudryi, I. Forbes, and R. W. Martin, “Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics,” Thin Solid Films, vol. 562, p. 195–199, 2014.

Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.

@article{strathprints48646,
volume = {562},
title = {Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics},
author = {M.V. Yakushev and P. Maiello and T. Raadik and M.J. Shaw and P.R. Edwards and J. Krustok and A.V. Mudryi and I. Forbes and R.W. Martin},
year = {2014},
pages = {195--199},
journal = {Thin Solid Films},
keywords = {photoluminescence, thin films, solar cells, semiconductors, electronic structure, raman spectroscopy, photoreflectance, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/48646/},
abstract = {Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.}
}

• F. Luckert, D. I. Hamilton, M. V. Yakushev, N. S. Beattie, G. Zoppi, M. Moynihan, I. Forbes, A. V. Karotki, A. V. Mudryi, M. Grossberg, J. Krustok, and R. W. Martin, “Optical properties of high quality Cu₂ZnSnSe₄ thin films,” Applied Physics Letters, vol. 99, iss. 6, p. 62104, 2011.

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

@Article{strathprints34031,
author = {F. Luckert and D. I. Hamilton and M. V. Yakushev and N. S. Beattie and G. Zoppi and M. Moynihan and I. Forbes and A. V. Karotki and A. V. Mudryi and M. Grossberg and J. Krustok and R. W. Martin},
title = {Optical properties of high quality Cu₂ZnSnSe₄ thin films},
journal = {Applied Physics Letters},
year = {2011},
volume = {99},
number = {6},
pages = {062104},
month = {August},
abstract = {Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.},
keywords = {photoluminescence, selenisation, dependence, energy, copper compounds, excitons, phonons, photoionisation, Raman spectra, semiconductor thin films, sputter deposition, tin compounds, zinc compounds, TIC - Bionanotechnology, Optics. Light, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/34031/}
}