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Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both Ag,Cu 2ZnSnSe4 and Cu2Zn Sn,Ge Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X ray diffraction and low temperature X ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn Sn,Ge Se4, the thin film absorbers show a very similar behavior as Cu2Zn Sn,Ge Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn S bonds instead of Ge S bonds. In Ag,Cu 2ZnSnSe4, the dependence of the Ag Se and Cu Se bond lengths on Ag Ag Cu might indicate an energetic advantage in the formation of certain local configurations

The performance of Cu2ZnSnSe4 solar cells is presently limited by low values of open circuit voltage which are a consequence of strong band tailing and high level of nonradiative recombination. Recently, the partial substitution of Cu, Zn, and Sn by other elements has shown the potential to overcome this limitation. We explored the structural changes and the effect on the optoelectronic properties of the partial substitution of Cu with Ag in Cu2ZnSnSe4. This paper clari amp; 64257;es the crystal structure of Ag1 amp; 8722;xCux 2ZnSnSe4 solid solution series, deducing possible cationic point defects and paying special attention to the presence of Cu Zn disorder with a combination of neutron and x ray diffraction. The optoelectronic properties of the solid solution series are assessed using re amp; 64258;ection and quantitative photoluminescence spectroscopy, which allows us to estimate the fraction of nonradiative recombination, which would contribute to the open circuit voltage loss in devices. The results strongly suggest Ag incorporation as a promising route to eliminate Cu Zn disorder and to reduce nonradiative recombination losses in Cu2ZnSnSe4.

As the open circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu Zn disorder, replacing of Cu with Ag this density is predicted to drop. In this study, neutron diffraction and diffuse reflectance were applied to evaluate the crystal structure and optoelectronic properties of the Ag 1 x Cu x 2 ZnSnSe 4 solid solution ACZTSe . The results allow us to suggest that in spite of both end members of the solid solution being reported to crystallize in kesterite type structure, at least Ag 1 x Cu x 2 ZnSnSe4 with x 0.17 and 0.46 crystallize in the stannite type structure and thus block Cu Zn disorder

33rd European Photovoltaic Solar Energy Conference and Exhibition; 1074-1076, One of the most detrimental problems in the further development of thin film solar cells based on kesterite type compound semiconductors is the limitation in open circuit voltage (VOC). The latter, according to many theoretical and experimental studies, is mostly related to the high concentration of intrinsic defects, mainly with ZnCu antisites. Recently, a way for overcoming this problem by partial substitution of Cu cations by Ag was proposed. This may lead to a strong decrease of intrinsic defects due to higher formation energy of ZnAg defects and as result to increase of VOC. Taking this into account, we performed an investigation of (AgxCu1-x)2ZnSnS4 thin films deposited by the spray pyrolysis method. The as-deposited thin films with 10, 15 and 20 % of Ag were annealed at 450 °C for 60 min in presence of elemental sulfur. Structural investigations by XRD, as well as Raman spectroscopy studies confirmed the formation of solid solutions. Photoluminescence investigations showed one broad band, which exhibits the blue shift with the increase of Ag concentration. This could be explained by changes in the activation energy of the defect levels involved in radiative transition rather than with the band gap change.

One of the most detrimental problems in the further development of thin film solar cells based on kesterite type compound semiconductors is the limitation in open circuit voltage VOC . The latter, according to many theoretical and experimental studies, is mostly related to the high concentration of intrinsic defects, mainly with ZnCu antisites. Recently, a way for overcoming this problem by partial substitution of Cu cations by Ag was proposed. This may lead to a strong decrease of intrinsic defects due to higher formation energy of ZnAg defects and as result to increase of VOC. Taking this into account, we performed an investigation of AgxCu1 x 2ZnSnS4 thin films deposited by the spray pyrolysis method. The as deposited thin films with 10, 15 and 20 of Ag were annealed at 450 C for 60 min in presence of elemental sulfur. Structural investigations by XRD, as well as Raman spectroscopy studies confirmed the formation of solid solutions. Photoluminescence investigations showed one broad band, which exhibits the blue shift with the increase of Ag concentration. This could be explained by changes in the activation energy of the defect levels involved in radiative transition rather than with the band gap change. Keywords Kesterite, Thin Film, Spray Pyrolysis, XRD, Raman Spectroscopy

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1186-1189, Thin Cu2ZnSn(SxSe1-x)4 films with x = 1.0 and 0.85 were prepared by the spray pyrolysis method with subsequent annealing in the presence of elemental sulfur or selenium. The surface sensitive Raman measurements indicated a high crystalline quality of all samples and absence of secondary phases. Small amount of Cu2-xS secondary phase was detected from grazing incidence X-ray diffraction patterns (information depth of about 500 nm). The resistivity, (T), is governed by the Mott variable-range hopping conduction at high temperatures, when the mean hopping length Rh (T) exceeds the mean distance between the acceptors, RA, but is still smaller than the typical inter-grain distance, aig. However, with lowering the temperature the resistivity becomes dominated by inter-grain tunneling, provided that Rh (T) > aig. The analysis of the (T) function has yielded the values of the acceptor concentration and the localization radius, with the evidence that all the samples are characterized by a state far different from the metal-insulator transition. The radius of the mean size grain as well as values of aig were in addition determined.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1147-1151, Modulated surface photovoltage (SPV) spectra, frequency dependent SPV signals and SPV transients were measured on a Cu1.95Zn1.1Sn0.96Se4 (CZTSe) single phase powder at different modulation frequencies, temperatures and excitation wavelengths. The sign and the onset energies of the modulated SPV signals depended on temperature and modulation frequency. A band of acceptor states with an energy of 65 meV above the valence band edge was observed and the disorder in the bulk of Cu1.95Zn1.1Sn0.96Se4 crystallites is described by band tails with a tail parameter of 30 meV. It was shown that the band gap of Cu1.95Zn1.1Sn0.96Se4 crystallites can be obtained from the spectra of the parameters of the fits of SPV transients by stretched exponentials (1.00 eV at -80°C).

31st European Photovoltaic Solar Energy Conference and Exhibition; 1358-1361, Spectral and time dependent surface photovoltage (SPV) measurements were performed on Cu2ZnSn(SxSe1-x)4 (CZTSSe) thin films prepared on ITO/glass substrates by spray pyrolysis at ambient atmosphere from aqueous solutions and subsequent selenization of Cu2ZnSnS4 (CZTS) layers. The morphology, stoichiometry and phases of the crystalline thin films were studied by electron microscopy, energy dispersive x-ray analysis and xray diffraction, respectively. The CZTSSe thin films show mostly the kesterite type CZTSSe phases and are slightly copper deficient. The Se/(S+Se) ratio amounted to about 0.27. The onset energies of SPV signals were observed at about 1.0 and 1.5 eV. The transition at 1.0 eV can be attributed to the SnSe secondary phase with an indirect band gap of 0.90 eV. The energy of 1.5 eV corresponds to the band gap of the CZTSSe with about 25 % of Se content. A slow relaxation process and a change of the sign set on at photon energies above 1.5 eV. Results are discussed in relation to the phase analysis.