The kesterite–stannite structural transition as a way to avoid Cu/Zn disorder in kesterites: the exemplary case of the Cu2(Zn,Mn)SnSe4

The solid solution series between Cu2ZnSnSe4, crystallizing in the kesterite type structure, and Cu2MnSnSe4, adopting the stannite type structure, i.e. Cu2(Zn1−xMnx)SnSe4, was studied by a combination of neutron and X-ray powder diffraction. Powder samples with 0 ≤ x ≤ 1 were synthesized by the solid state reaction of the pure elements and it was confirmed by wavelength-dispersive X-ray spectroscopy that each contained a homogeneous, off-stoichiometric quaternary phase. The lattice parameters and cation site occupancy factors were determined simultaneously by the Rietveld analysis of the neutron and X-ray powder diffraction data. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in the crystal structure of the Cu2(Zn1−xMnx)SnSe4 mixed crystals, from which a cation distribution model was derived. For the end member Cu2ZnSnSe4, the disordered kesterite structure was confirmed and for Cu2MnSnSe4, the stannite structure was confirmed. The cross-over from the kesterite to stannite type structure by Zn2+ ↔ Mn2+ substitution in the Cu2Zn1−xMnxSnSe4 solid solution can be seen as a cation re-distribution process among the positions (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu+, Zn2+ and Mn2+. The Sn4+ cation does not take part in this process and remains on the 2b site. Moreover, the cross-over is also visible in the ratio of the lattice parameters c/(2a), showing a characteristic dependence on the chemical composition. The order parameter Q, the quantitative measure of Cu/BII disorder (BII = Zn and Mn), shows a distinct dependence on the Mn/(Mn + Zn) ratio. In Zn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals, the order parameter Q ∼ 0.7 and drops to Q ∼ 0 (complete Cu/BII disorder) in the compositional region 0.3 ≥ x ≥ 0.7. In Mn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals, adopting the stannite type structure, the order parameter reaches almost Q ∼ 1 (order). Thus, it can be concluded that only Mn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals do not show Cu/BII disorder. A similar trend of the dependence on the chemical composition of both Cu/BII-disorder and the band gap energy Eg in Cu2(Zn1−xMnx)SnSe4 mixed crystals was observed

A pitfall for classification

High-efficiency kesterite-based thin film solar cells typically feature Cu- poor, Zn-rich absorbers although secondary phases occur easily in non- stoichiometric Cu2ZnSnSe4. We therefore applied high-resolution X-ray fluorescence analysis using a synchrotron nanobeam to study the local composition of a CZTSe cross section lamella cut from a sample with an integral composition of Zn/Sn = 1.37 and Cu/(Zn+Sn) = 0.55. We find submicrometer-sized ZnSe-, SnSe/SnSe2-, and even CuSe/Cu2Se-like secondary phases, while the local compositions of the kesterite are highly Zn-rich yet barely Cu-poor with 1.5 ≤ Zn/Sn ≤ 2.2 and Cu/(Zn+Sn) ∼ 1.0. Consequently, great care must be taken when relating the integral composition to other material properties including the device performance

Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu–Se and Zn–Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells

Vibrational properties of sulfoselenide solid solutions

In this work, Raman spectroscopy and X-ray diffraction were applied together to evaluate the crystal structure and the phonon modes of photovoltaic grade Cu 2ZnSn(SxSe1−x)4 thin films, leading to a complete characterization of their structural and vibrational properties. Vibrational characterization has been based on Raman scattering measurements performed with different excitation wavelengths and polarization configurations. Analysis of the experimental spectra has permitted identification of 19 peaks, which positions are in good accord with theoretical predictions. Besides, the observation of Cu 2ZnSnS4-like A symmetry peaks related to S vibrations and Cu 2ZnSnSe4-like A symmetry peaks related to Se vibrations, additional Raman peaks, characteristic of the solid solution and previously not reported, are observed, and are attributed to vibrations involving both S and Se anions

Point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterites

The efficiency of kesterite-based solar cells is limited by various non-ideal recombination paths, amongst others by a high density of defect states and by the presence of binary or ternary secondary phases within the absorber layer. Pronounced compositional variations and secondary phase segregation are indeed typical features of non-stoichiometric kesterite materials. Certainly kesterite-based thin film solar cells with an off-stoichiometric absorber layer composition, especially Cu-poor/Zn-rich, achieved the highest efficiencies, but deviations from the stoichiometric composition lead to the formation of intrinsic point defects (vacancies, anti-sites, and interstitials) in the kesterite-type material. In addition, a non-stoichiometric composition is usually associated with the formation of an undesirable side phase (secondary phases). Thus the correlation between off-stoichiometry and intrinsic point defects as well as the identification and quantification of secondary phases and compositional fluctuations in non-stoichiometric kesterite materials is of great importance for the understanding and rational design of solar cell devices. This paper summarizes the latest achievements in the investigation of identification and quantification of intrinsic point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterite-type materials

Crystal Growth and the Structure of a New Quaternary Adamantine Cu☐GaGeS₄

Single crystals of quaternary adamantine-type Cu☐GaGeS4 were grown using the chemical vapor transport technique, with iodine as the transport agent. Dark red transparent crystals were grown in a temperature gradient of ΔT = 900–750 °C. Chemical characterization by X-ray fluorescence showed the off-stoichiometric composition of Cu☐GaGeS4 crystals—in particular, a slight Ge deficiency was observed. By X-ray diffraction, Cu☐GaGeS4 was found to adopt the chalcopyrite-type structure with the space group I4¯2d. Cation distribution in this structure was analyzed by multiple energy anomalous synchrotron X-ray diffraction, and it was found that Cu and vacancies occupied the 4a site, whereas Ga and Ge occupied the 4b site. The band gap energies of several off-stoichiometric Cu☐GaGeS4 crystals were determined by UV-Vis spectroscopy and ranged from 2.1 to 2.4 eV. A non-linear correlation of the band gap energy with the Ge content of the compound was shown to follow the usual bowing behavior of semiconductor alloys, with a bowing parameter of b = −1.45 (0.08)

Cation distribution in Cu2ZnSnSe4, Cu2FeSnS4 and Cu2ZnSiSe4 by multiple‐edge anomalous diffraction

Multiple‐Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Semiconductors from this group of materials are promising candidates for photovoltaic applications. Their properties strongly depend on point defects, in particular related to cation order–disorder. However, Cu+, Zn2+ and Fe2+ have very similar scattering factors and are all but indistinguishable in usual X‐ray diffraction experiments. Anomalous diffraction utilizes the dependency of the atomic scattering factors f′ and f′′ of the energy of the radiation, especially close to the element‐specific absorption edges. In the MEAD technique, individual Bragg peaks are tracked over an absorption edge. The intensity changes depending on the structure factor can be highly characteristic for Miller indices selected for a specific structural problem, but require very exact measurements. Beamline KMC‐2 at synchrotron BESSY II, Berlin, has been recently upgraded for this technique. Anomalous X‐ray powder diffraction and XAFS compliment the data. Application of this technique confirmed established cation distribution in Cu2ZnSnSe4 (CZTSe) and Cu2FeSnS4 (CFTS). In contrast to the literature, cation distribution in Cu2ZnSiSe4 (CZSiSe) is shown to adopt a highly ordered wurtz‐kesterite structure type.Multiple‐Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Application of this technique confirms established cation distribution in Cu2ZnSnSe4 (CZTSe) and Cu2FeSnS4 (CFTS), but in Cu2ZnSiSe4 (CZSiSe) the cation distribution is shown to adopt a highly ordered wurtz‐kesterite structure type in contrast to the literature. imag

Conductivity mechanisms and influence of the Cu/Zn disorder on electronic properties of the powder Cu2ZnSn(S1-xSex)4 solid solutions

One of the major reasons for a recent stuck of the development of kesterite based photovoltaic devices is related to the problems in their open circuit voltage. Several limitations can be pointed out as a possible origin. In particular, for a case of Cu and Zn containing kesterite compounds, a lattice disorder connected to these cations is considered to bring in an important contribution to the limitations above. Extensive studies showed a significant influence of this disorder being an intrinsic property of kesterites to structural, optical and vibrational properties of the related materials. However, detailed investigations focused to the role of disorder in formation of the electrical properties of the kesterite materials are still lacking. In order to cover this gap, here is investigated resistivity of Cu2ZnSn(S1-xSex)4 (CZTSSe) powder samples with x = 0.48–1.00 at temperatures between ~ 10 and 300 K. A detailed analysis of the measured data permits to establish various conductivity mechanisms within different temperature ranges, and to obtain a set of important macroscopic and microscopic electronic parameters. By itself, their dependence on x does not reveal any univocal behavior. In contrast, all the electronic parameters above exhibit a clear correlation with the order parameter Q. This is explained completely by a strong sensitivity of the electronic properties of CZTSSe to the Cu/Zn disorder

Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu$_2$ZnSnS$_4$ (CZTS), Cu$_2$ZnSnSe$_4$ (CZTSe) and Cu$_2$ZnGeSe$_4$ (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu–Se and Zn–Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells

Discrepancy between integral and local composition in off-stoichiometric Cu2ZnSnSe4 kesterites: a pitfall for classification

High-efficiency kesterite-based thin film solar cells typically feature Cu-poor, Zn-rich absorbers although secondary phases occur easily in non-stoichiometric CuZnSnSe. We therefore applied high-resolution X-ray fluorescence analysis using a synchrotron nanobeam to study the local composition of a CZTSe cross section lamella cut from a sample with an integral composition of Zn/Sn = 1.37 and Cu/(Zn+Sn) = 0.55. We find submicrometer-sized ZnSe-, SnSe/SnSe-, and even CuSe/CuSe-like secondary phases, while the local compositions of the kesterite are highly Zn-rich yet barely Cu-poor with 1.5 ≤ Zn/Sn ≤ 2.2 and Cu/(Zn+Sn) ∼ 1.0. Consequently, great care must be taken when relating the integral composition to other material properties including the device performance.This work was funded by ESRF, DFG (Deutsche Forschungsgemeinschaft) under Grant No. SCHN 1283/2-1, BMBF (Bundesministerium für Bildung und Forschung) under the “nano@work” project (Grant No. 05 K16SJ1), KESTCELLS 316488, FP7-PEOPLE-2012 ITN, Multi-ITN project, and MINECO (Ministerio de Economía y Competitividad de España) under the NASCENT Project (ENE2014-56237-C4-1-R). S.G. thanks the Government of Spain for the FPI fellowship (BES-2014-068533) and E.S. for the “Ramón y Cajal” fellowship (RYC-2011-09212)