Earth-abundant amorphous oxide semiconductors like Zn-Sn-O and Cu-Sn-O feature electrical conductivities that can be tuned over several orders of magnitude by adjusting the metal composition and oxygen content. This study investigates the element-specific local structure and amorphous nature of Zn-Sn-O and Cu-Sn-O thin films with varying Zn/Sn and Cu/Sn ratios, grown at different oxygen partial pressures by room temperature pulsed laser deposition. X-ray absorption spectroscopy was performed at the Zn, Cu and Sn K-edges, providing local coordination and structural parameters for each metal species by in-depth analysis of near edge and extended fine structures. Except for very high copper contents, all thin films are fully amorphous with no noticeable order beyond the first coordination shell. Zinc atoms keep a tetrahedral configuration with an average bond length slightly larger than that of crystalline ZnO. Tin atoms prefer an octahedral coordination similar to crystalline SnO 2 but with reduced average coordination number and slightly smaller average bond length. Copper atoms feature a CuO-like coordination with reduced average coordination number and bond length for high copper contents, while further reduction of these structural parameters for low copper contents indicates a broad distribution of local environments. The variability of the Zn-O, Sn-O and Cu-O local building blocks is thus determined by the coordination flexibility of the respective metal, being small for zinc with only one stable oxide but large for tin and copper with two stable oxides at ambient conditions. The dominant effect of the metal composition on the electrical properties of the material is given by the changing fraction of Zn-O, Cu-O and Sn-O building blocks. Undercoordinated tin atoms provide negative charge carriers, leading to increasing n-type conductivity with increasing tin content. In contrast, CuO-like environments promote p-type conductivity for Cu-Sn-O thin films with high enough copper contents.

Here, we explore component-dependent local lattice distortions in polycrystalline, equiatomic, face-centered cubic CrMnFeCoNi high-entropy alloys and their modifications induced by dilute interstitial carbon. Multi-edge extended X-ray absorption fine structure spectroscopy combined with reverse Monte Carlo analysis reveals that the Cr component experiences the most substantial local distortions, independent of the temperature of prolonged annealing treatments (993 K or 1373 K) and the nominal carbon content (0 to 0.8 at.%). The static disorder around Cr atoms was found to increase markedly and monotonically upon carbon alloying, whereas Mn, Fe, Co, and Ni demonstrate weaker and non-monotonic tendencies. The carbon-induced lattice distortions extend over several coordination shells, indicating the pronounced effect of the carbon presence on the local environment around Cr absorbers. First-principles density functional theory and finite-temperature molecular dynamics simulations confirm the greater impact of carbon on the local lattice distortions around Cr than around the other 3d constituent elements, based on the previous finding that carbon preferentially occupies Cr-rich interstitial sites. These results provide decisive hints towards the atomistic origin of the non-monotonic diffusion behavior previously reported for carbon-doped CrMnFeCoNi alloys, and are noticeable for understanding the carbon-induced phase transitions in compositionally complex systems.

Thin film solar cell technologies are mainly based on polycrystalline absorber layers, which is also the case for kesterite-based photovoltaic devices. An alternative technology, which is promising and low cost, is based on kesterite-type Cu2ZnSn(S,Se)4 (CZTSSe) monograins as absorbers, which are fixed in a polymer matrix to form a flexible solar cell. The large band tailing observed in Cu-based kesterite-type semiconductors is believed to cause voltage losses, limiting the efficiency of kesterite-based devices. Cu/Zn disorder, which is always present in these compounds, is discussed in literature as a possible reason for band tailing. The experimental determination and quantification of Cu/Zn disorder is possible by in-depth analysis of neutron diffraction data. This work reveals that the purity of copper used in the synthesis of CZTSSe monograins has an influence on the degree of Cu/Zn disorder in the semiconductor and thus on optical and PV parameters. Comparing CZTSSe monograins, less Cu/Zn disorder was observed for the monograins synthesized using copper with higher purity; the respective monograin-based solar cell shows a higher power conversion efficiency. On the other hand, the band gap energy as well as the photoluminescence maximum (PLmax) of both monograins are the same. Applying a low-temperature annealing procedure allowed us to increase the quality of monograins synthesized using 5N copper, very close to the one grown using 6N copper. The PLmax slightly shifts into higher energy, which is most likely an indication of the decreased Cu/Zn disorder, either moving the defect states toward the valence band or that it reduces the formation of the tail states near the conduction band minimum.

The x-ray excited optical luminescence (XEOL) for defect and near band edge (NBE) transitions combined with simultaneous x-ray absorption measurements are experimentally and theoretically studied on single crystalline ZnO and GaN across the Zn and Ga K edges, respectively, in a wide range of sample thicknesses. Increasing the sample thickness leads to the appearance of an inverted line shape and negative edge jump for the XEOL defect response, whereas the line shape of the XEOL NBE edge remains positive. A one-dimensional transport model is developed, which includes experimental geometry, the creation of x-ray generated excitations, diffusion and recombination of the carriers, and reabsorption of x-ray fluorescence and XEOL photons. The model calculations reproduce the experimentally observed changes of the edge shape in the XEOL spectra caused by variation of the sample thickness and reveal surface recombination and optical absorption as the main factors determining the XEOL edge shape for a given sample thickness.

Stable Sb exhibits a rhombohedral structure, often referred to as distorted primitive cubic, with each Sb atom having three short and three longer first neighbor bonds. However, this crystal structure can also be interpreted as being layered, putting emphasis on only three short first neighbor bonds. Therefore, temperature-dependent extended X-ray absorption fine structure (EXAFS) spectroscopy is carried out at the Sb K-edge in order to obtain more detailed information on local structural and vibrational properties. Evaluation of the temperature-dependent bond lengths provides the temperature-dependent Peierls distortion while the temperature dependence of the variance of the interatomic distance distribution yields the EXAFS force constants. Ab initio density functional theory (DFT) calculations are used for determining projected force constants. Both EXAFS and DFT force constants are compared to those of other materials with different bonding characteristics, including two-center covalently bonded semiconductors, multicenter bonded IV-VI and V2VI3 compounds, and metallic Cu. Clearly, Sb exhibits characteristics of both localized covalent bonding and delocalized multicenter bonding. This suggests a continuous transition between these two bonding scenarios and adds to the understanding of bonding in elemental Sb in particular and in IV-VI and V2VI3 materials in general.

The chalcopyrite alloy (Ag,Cu)(In,Ga)Se-2 is a highly efficient thin film solar cell absorber, reaching record efficiencies above 23%. Recently, a peculiar behavior in the bond length dependence of (Ag,Cu)GaSe2 was experimentally proven. The common cation bond length, namely Ga-Se, decreases with increasing Ag/(Ag + Cu) ratio even though the crystal lattice expands. This is opposite to the behavior observed for Cu(In,Ga)Se-2, where all bond lengths increase with increasing lattice size. To better understand this peculiar bond length behavior, element-specific bond lengths of (Ag,Cu)InSe2 and Ag(In,Ga)Se-2 alloys are determined using extended x-ray absorption fine structure spectroscopy. They show that the peculiar bond length dependence occurs only for (Ag,Cu) alloys, independent of the species of common cation (In or Ga). The bond lengths are used to determine the anion displacements and to estimate their contribution to the bandgap bowing. Again, both behaviors differ significantly depending on the type of alloyed cation. A valence force field approach, relaxing bond lengths and bond angles, is used to describe the structural distortion energy for a comprehensive set of I-III-VI2 and II-IV-V-2 chalcopyrites. The model reveals bond angle distortions as main driving factor for the tetragonal distortion and reproduces the literature values with less than 10% deviation. In contrast, the peculiar bond length dependence is not reproduced, demonstrating that it originates from electronic effects beyond the scope of this structural model. Thus, a fundamental understanding of bond length behavior and tetragonal distortion is achieved for chalcopyrite materials, benefiting their technological applications such as high efficiency thin film photovoltaics.

Contrary to other semiconductor alloys, incorporation of Ag into CuGaSe2 increases the bandgap, even though the lattice expands, and the Ga–Se bond length is theoretically predicted to decrease rather than increase. Herein, we experimentally confirm this peculiar bond length dependence of (Ag,Cu)GaSe2 using x-ray absorption spectroscopy. We further model the different anion displacements and estimate that their combined contribution to the bandgap bowing is close to zero. These findings differ from those for Cu(In,Ga)Se2 and demonstrate the diversity of chalcopyrite alloys and their properties.

Antimony sulfide (Sb2S3) and antimony selenide (Sb2Se3) compounds have attracted considerable attention for applications in different optoelectronic devices due to their notable optical and electrical properties, and due to the strong anisotropy of these properties along different crystallographic directions. However, the efficient use of these promising compounds still requires significant efforts in characterization of their fundamental properties. In the present study, Raman scattering and spectroscopic ellipsometry were used to investigate the vibrational and optical properties of Sb2Se3 and Sb2S3 bulk polycrystals grown by the modified Bridgman method. The first technique proved the presence of the desired Sb2S3 and Sb2Se3 phases in the analyzed ingots and confirmed the absence of any preferential crystallographic orientation at the measured surface of the samples. Spectroscopic ellipsometry was performed using a multi-oscillator Tauc-Lorentz dispersion model, and yielded a complex dielectric function of chalcogenides over the range 1.0-4.6 eV with a three phase model (ambient, surface and bulk materials). Finally, spectral data on the refractive index, the extinction coefficient, the absorption coefficient and the reflectivity at normal incidence, R, were obtained, which serve as a reference for the optical modeling of optoelectronic devices based on polycrystalline Sb2S3 and Sb2Se3 compounds.

A newly designed setup to perform steady-state X-ray excited optical luminescence (XEOL) spectroscopy and simultaneous XEOL and X-ray absorption spectroscopy characterization at beamline P65 of PETRA III is described. The XEOL setup is equipped with a He-flow cryostat and state-of-the-art optical detection system, which covers a wide wavelength range of 300-1700 nm with a high spectral resolution of 0.4 nm. To demonstrate the setup functioning, low-temperature XEOL studies on polycrystalline CuInSe2 thin film, single-crystalline GaN thin film and single-crystalline ZnO bulk semiconductor samples are performed.

The present study addresses to the synthesis and determination of the dielectric function of Cu2Zn(GexSi1-x)Se-4 solid solutions with x = 0.4 and 0.8 over the range 1-4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E-0 transition and high energy E-1A and E-1B transitions are identified. It is found that the band gap varies nonlinearly on composition in the Cu2Zn(GexSi1-x)Se-4 alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells.

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

The effect of Cu off-stoichiometry and Zn alloying on the fundamental absorption region of Cu2CdSnS4 (CCTS) absorbers in complete solar cells has been investigated using electroreflectance (ER) spectroscopy at room temperature. It is found that ER spectra consist of contributions from two different sources, one of which corresponds to band gap transition in the absorber layer and the other to the interference effect in the window layer. ER measurements on CCTS samples reveal a near-constant band gap energy of 1.37-1.38 eV and a relatively small broadening of 60-90 meV in the probed 0.8 < Cu/(Cd + Sn) < 0.89 compositional range, in contrast to related kesterites Cu2ZnSn(S, Se)(4). The analysis of the band gap in Cu-2(Cd1-x, Zn-x)SnS4 alloys yields a quadratic dependence on Zn content with a bowing parameter of 0.4 eV. Finally, the broadening parameters of the band gap transitions as well as their compositional dependence are evaluated and discussed.

The effect of Cu off-stoichiometry and Zn alloying on the fundamental absorption region of ${\mathrm{Cu}}_{2}\mathrm{CdSn}{\mathrm{S}}_{4}$ (CCTS) absorbers in complete solar cells has been investigated using electroreflectance (ER) spectroscopy at room temperature. It is found that ER spectra consist of contributions from two different sources, one of which corresponds to band gap transition in the absorber layer and the other to the interference effect in the window layer. ER measurements on CCTS samples reveal a near-constant band gap energy of 1.37--1.38 eV and a relatively small broadening of 60--90 meV in the probed $0.8<\mathrm{Cu}/(\mathrm{Cd}+\mathrm{Sn})<0.89$ compositional range, in contrast to related kesterites ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$. The analysis of the band gap in ${\mathrm{Cu}}_{2}({\mathrm{Cd}}_{1\ensuremath{-}x},{\mathrm{Zn}}_{x})\mathrm{Sn}{\mathrm{S}}_{4}$ alloys yields a quadratic dependence on Zn content with a bowing parameter of 0.4 eV. Finally, the broadening parameters of the band gap transitions as well as their compositional dependence are evaluated and discussed.

The performance of $\mathrm{C}{\mathrm{u}}_{2}\mathrm{ZnSnS}{\mathrm{e}}_{4}$ 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 $\mathrm{C}{\mathrm{u}}_{2}\mathrm{ZnSnS}{\mathrm{e}}_{4}$. This paper clarifies the crystal structure of ${(\mathrm{A}{\mathrm{g}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x})}_{2}\mathrm{ZnSnS}{\mathrm{e}}_{4}$ 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 reflection 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 $\mathrm{C}{\mathrm{u}}_{2}\mathrm{ZnSnS}{\mathrm{e}}_{4}$.

The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.

The pseudo dielectric function of Cu2ZnSn(SxSe1-x)(4) [x = 0.35, 0.62, 0.81] bulk polycrystals is determined over the range 1.1-4.6 eV at room temperature from the analysis of spectroscopic ellipsometry data using the Adachi model. From the analysis, the lowest E-0 transition and high energy E-1A and E-1B transitions are clearly identified, and used to follow the evolution of the pseudo dielectric function as a function of the composition. It is shown that the fundamental E-0 and high energy E-1A transitions can be tuned by increasing the sulfur content over a range of 0.3 eV. These results show the potential of the kesterite compounds for the design of efficient tailored photovoltaic solar cells. (C) 2020 Elsevier B.V. All rights reserved.

The phase mixture CsPb2Br5/CsPbBr3 has raised interest as a promising material system for light emission, light detection, and even for photovoltaic devices owing to its high luminescence yield and improved device performance when compared with pure CsPbBr3 devices. Given that CsPb2Br5 and CsPbBr3 exhibit similar formation enthalpies, thermal treatment can induce phase transformations between them. The influence of that transformation on the optoelectronic properties is still unclear. In the present work, we report the effect of elevated-temperature (>580 K) postdeposition annealing on the luminescence of a mixed-phase CsPb2Br5/CsPbBr3 thin film deposited by coevaporation. Using combined photoluminescence spectroscopy, X-ray diffraction, and scanning electron microscopy techniques, we found that deep defects at the CsPb2Br5/CsPbBr3 interface contribute to quench the CsPbBr3 green luminescence and that annealing increases the defect concentration in CsPb2Br5, leading to a strong decrease of the external photoluminescence quantum yield. Our results do not confirm any passivating effect of CsPb2Br5—as-deposited or induced by annealing—as reported by other authors.

Current state-of-the-art Cu2ZnSn(S,Se)(4) kesterite solar cells are limited by low open-circuit voltages (V-OC). In order to evaluate to what extent the substitution of Sn by Ge is able to result in higher V oc values, this article focuses on Cu2ZnGeSe4 "CZGSe" devices. To reveal their full potential, different strategies are explored that, in particular, aim at the optimization of the CZGSe/buffer heterojunction. Here, employing hard X-ray photoelectron spectroscopy, it is evidenced that only a combination of different surface treatments is able to remove all detrimental secondary phases. Further improvements are achieved by establishing a solar cell heat treatment in air. A systematic study of the impact of different annealing temperatures and durations determines the best heat treatment parameters to be 60 min at 200 degrees C. Also, Zn(O,S,OH) as a more transparent alternative to the heavy-metal compound CdS buffer layer has been realized. Combining all of the strategies, solar cells with 8.5 and 7.5% total area efficiency have been prepared, which is a record for Sn-free kesterite solar cells and any kesterite solar cell with a Zn(O,S,OH) buffer, respectively. Beyond these records, this work clearly confirms the emerging trend that Ge-for-Sn substitution is a successful strategy to improve the V-OC of kesterite solar cells.

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 clarifies the crystal structure of (Ag1-xCux)(2) ZnSnSe4 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 reflection 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.

A polarization-dependent infrared reflectivity study on oriented single crystals in the range of 100–500 cm−1 was performed to investigate the optical phonon modes in the stannite Cu2CdSnS4 semiconductor. Based on the symmetry analysis and multi-oscillator model calculation, we determine the parameters of B2 and E polar modes. The values of high frequency ε∞ and static ε0 dielectric constant are 7.69 and 10.01 for Eǁc and 6.53 and 8.81 for E⊥c polarization directions.