G. Gurieva

Multiwavelength excitation Raman scattering of Cu2ZnSn(SxSe1−x)4 (0 ≤ x ≤ 1) polycrystalline thin films: 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 Cu2ZnSn(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 Cu2ZnSnS4-like A symmetry peaks related to S vibrations and Cu2ZnSnSe4-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.

Single Crystal X-ray Structure Investigation of Cu2ZnSnSe4

The crystal structure of Cu2ZnSnSe4 (CZTSe) has been investigated using two types of structure models known as stannite and kesterite. The single crystal X-ray diffraction study has been performed for two CZTSe single crystals samples obtained by the Bridgman (I) and chemical transport reaction (II) methods. The best refinement was obtained for the model in the space group I-42m, which suggests that cooper and zinc atoms alternate in the d Wickoff position of the space group and statistically occupy it with equal probability.

Spectroscopic ellipsometry study of Cu2ZnSnSe4 bulk crystals

Using spectroscopic ellipsometry we investigated and analyzed the pseudo-optical constants of Cu2ZnSnSe4 bulk crystals, grown by the Bridgman method, over 0.8–4.5 eV photon energy range. The structures found in the spectra of the complex pseudodielectric functions were associated to E0, E1A, and E1B interband transitions and were analyzed in frame of the Adachis model. The interband transition parameters such as strength, threshold energy, and broadening were evaluated by using the simulated annealing algorithm. In addition, the pseudo-complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity were derived over 0.8–4.5 eV photon energy range.

Intrinsic point defects in off-stoichiometric Cu2ZnSnSe4: A neutron diffraction study

This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite type CZTSe by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation anti site defects (CuZn, ZnCu, ZnSn, SnZn, and CuZn), as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline powder samples synthesized by the solid state reaction. The results show that the point defects present in off-stoichiometric CZTSe agree with the off-stoichiometry type model, assuming certain cation substitutions accounting for charge balance. In addition to the known off-stoichiometry types A–H, new types (I–L) have been introduced. For the very first time, a correlation between the chemical composition of the CZTSe kesterite type phase and the occurring intrinsic point defects is presented. In addition to the off-stoichiometry type specific defects, the Cu/Zn disorder is always present in the CZTSe phase. In Cu-poor/Zn-rich CZTSe, a composition considered as the one that delivers the best photovoltaic performance, mainly copper vacancies, ZnCu and ZnSn anti sites are present. Also, this compositional region shows the lowest degree of Cu/Zn disorder.This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite type CZTSe by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation anti site defects (CuZn, ZnCu, ZnSn, SnZn, and CuZn), as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline powder samples synthesized by the solid state reaction. The results show that the point defects present in off-stoichiometric CZTSe agree with the off-stoichiometry type model, assuming certain cation substitutions accounting for charge balance. In addition to the known off-stoichiometry types A–H, new types (I–L) have been introduced. For the very first time, a correlation between the chemical composition of the CZTSe kesterite type phase and the occurring intrinsic point defects is presented. In addition to the off-stoichiometry type specific defects, the Cu/Zn disorder is always present in the CZTSe phase. In Cu-poor/Zn-rich CZTSe, a composition consid...

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 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.

Quantitative anomalous powder diffraction analysis of cation disorder in kesterite semiconductors

Kesterite type compound semiconductors, containing copper and zinc, have photovoltaic properties depending on cation distribution in the crystal structure. Anomalous diffraction allows discrimination of isoelectronic cations, in principle allowing a straightforward determination of site occupation factors from data collected at multiple energies close to the X-ray absorption edges of copper and zinc. However, extremely strong correlation between structural parameters precludes this. We present a recipe based on the direct dependency between refined occupation factors and atomic scattering power, which allows to lift the correlations and to detect issues of individual diffraction patterns or assumptions in the model, thereby allowing for reliable quantitative analysis of the Cu/Zn distribution.

X-ray diffraction investigation on Cu2ZnSiSe4 single and polycrystalline crystals

Abstract Cu2ZnSiSe4 belong to the adamantine family of quaternary chalcogenides crystallizing in the wurtzstannite structure. Recent ab-initio calculations show, that the lowest energy structure of Cu2ZnSiSe4 is the wurtzkesterite type structure in contrast to wurtzstannite type, usually obtained in experiments. To clarify this issue a structural study on single crystals of Cu2ZnSiSe4 was performed for the first time. The structural characterization of the single crystals was carried out by X-ray diffraction at two different temperatures – room temperature and 150 K. The XRD data analysis shows, that Cu2ZnSiSe4 single crystals adopt the orthorhombic wurtzstannite type structure (space group Pmn21) and lattice parameters a = 7.809 Å, b = 6.778 Å, c = 6.447 Å at 150 K, and lattice parameters a = 7.821 Å, b = 6.734 Å, c = 6.453 Å at room temperature were derived. The structural parameters were confirmed for the polycrystalline Cu2ZnSiSe4 bulk sample.

Temperature dependent transient surface photovoltage spectroscopy of a Cu1.95Zn1.1Sn0.96Se4 kesterite single phase powder

An off-stoichiometric but single phase Cu1.95Zn1.1Sn0.96Se4 kesterite powder was investigated by temperature dependent transient surface photovoltage (SPV) spectroscopy. SPV signals excited at different wavelengths were transformed into SPV spectra that depended on the response time of measurement. Shallow electronic states and states with transition energies at 0.83 eV or 0.78… 0.9 eV were distinguished. The temperature dependence of the band gap of Cu1.95Zn1.1Sn0.96Se4 was obtained. Results were discussed on the basis of defects in Cu-poor and Zn-rich kesterite.

Structural characterization of off-stoichiometric kesterite-type Cu2ZnGeSe4 compound semiconductors: from cation distribution to intrinsic point defect density

The substitution of Ge4+ for Sn4+ in Cu2ZnSn(S,Se)4 (CZTSSe) kesterite-type absorber layers for thin film solar cells has been proven to enhance the opto-electronic properties of the material. By cationic substitution, in general, the optical bandgap can be more readily designed for the purpose of bandgap engineering, and the substitution of Sn4+ by Ge4+, in particular, widens the optical bandgap such that it can be employed for both photovoltaics and solar fuel quarrying by photocatalytic water splitting. This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite-type Cu2ZnGeSe4 (CZGSe) by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation antisite defects (CuZn, ZnCu, ZnGe, CuGe) as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline materials synthesized by solid state reaction. In addition to the off-stoichiometry-type specific defects, Cu/Zn disorder is always present in the kesterite-type CZGSe phase. While compositional changes are clearly reflected by the tetragonal deformation c/2a, the lattice parameters a and c seem differently responding to point defect types and concentration variations, respectively. The CuGe antisite defect which is known to greatly deteriorate the opto-electronic properties exists only in Cu-rich CZGSe, but appears already in CZGSe with Cu/(Zn + Ge) ≈ 1. Furthermore, we showed by diffuse reflectance hyperspectral imaging a widening of the energy bandgap in off-stoichiometric kesterite-type CZGSe with decreasing Cu/(Zn + Ge).

Interplay between off-stoichiometry and intrinsic point defects in compound semiconductors

The sun provides earth with a staggering amount of energy, but if solar energy is to become a practical alternative to fossil fuels, there must be an efficient way to convert photons into electricity. Photovoltaics (PV) has developed into a mature technology during recent past – thin film PV is an emerging alternative technology because of short energy payback time and minimum use of high purity materials, addressing the urgent need for cost-competitive renewable energy technologies. Compound semiconductors, like chalcopyrite type Cu(In,Ga)(Se,S)2 (CIGSe), are the most advanced and most efficient absorber materials. Such solar cells show present record lab efficiencies of >22% [1]. Since the availability of indium is an object of concern regarding the large scale production of solar cells, its replacement with Zn and Sn is beneficial in this sense. Compounds like Cu2ZnSn(S,Se)4 (CZTS, CZTSe) are an alternative. One of the reasons for the success of CIGSe based thin film solar cells is the remarkable flexibility of its chalcopyrite type crystal structure. This flexibility is a key also for the quaternary kesterite type compounds CZTS,Se because the thin film growth is in fact a non-equilibrium process. The absorber layers of high efficient solar cells exhibits an overall off-stoichiometric composition, thus the existence of intrinsic point defects is strongly correlated with the chemical potential and therefore dependent on the composition of the material. These structural defects influence the electronic properties of the final device sensitively. A high density of bulk defects and structural disorder (Cu/Zn disorder) will cause an extreme band tailing which could account for a significant part of the open circuit voltage (Voc) loss, the main limitation for the performance of CZTS,Se-based PV devices. Our research focuses on the correlation between off-stoichiometry, point defects and physical properties of kesterites. We have demonstrated, that kesterite type CZTSe can self-adapt to Cu-poor and Cu-rich compositions without any structural change except the cation distribution [2]. The ability to accept deviations from stoichiometry, which can be categorized in off-stoichiometry types (A-L), is correlated to a Cu/Zn disorder (see figure) and the formation of intrinsic point defects. Using synchrotron X-ray and neutron diffraction data in a complementary approach we have established correlations between stoichiometry deviations and occurring intrinsic point defects (type and concentration) in CZTS,Se kesterites. We were able to show for the first time quantitatively that the Cu/Zn disorder in kesterites causes shifts in the energy band gap [3] giving raise to band tailing a possible performance limiting parameter for thin film solar cell devices based on kesterite-type absorber layers. The presentation will underline the role of crystallographic materials research to provide the basis for the understanding of structure-property relations of energy materials.