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CZTSSe absorbers are synthesized by a one-step annealing process from Cu, SnS and ZnS stacks prepared by sputtering deposition. CZTSSe thin films are fabricated by a selenization process using an optimized precursor stack structure. The selenization temperature has a strong influence on the phase and reactions of the elements in the films. CZTSSe formation is initially triggered by the interaction of Cu with Sn(S, Se) to form CuSn(S, Se) compounds, which then react with ZnS. CZTSSe films obtained at 590 °C exhibit a kesterite structure. Sulfo-selenization is performed using an optimized selenization process. Based on the mass ratio of the chalcogen sources, selenium disulfide and selenium, the effect of the S/(S + Se) compositional ratio on the structural and morphological properties of the as-grown films and the optoelectronic parameters of solar cells fabricated using these absorber films are studied. Using this single-step sulfo-selenization method, CZTSSe devices with conversion efficiencies up to 9.47% are obtained.

The structural and electro-optical influence of marcasite on the properties of solution-processed iron pyrite thin films was investigated. Marcasite has a strong tendency to form simultaneously with pyrite upon sulfurization of amorphous iron-oxide precursor films, leading to a mixed-phase structure in which pyrite grains are surrounded by nanocrystalline marcasite boundaries. The optical analysis in combination with spectroscopic ellipsometry revealed that marcasite should have a bandgap of approximately 0.85–0.88 eV with a higher absorption coefficient than pyrite, differing strongly from the prior belief that marcasite has a bandgap of less than 0.4 eV. In addition, the pyrite/marcasite film has been found to have a larger diffusion coefficient for photogenerated minority carriers than the phase-pure pyrite film from electrochemical impedance analyses, resulting in a higher photocurrent density, as determined through photoelectrochemical measurement. The facile transport of a minority carrier along the marcasite boundaries is the putative origin of the observed improvement in the photoactivity of the pyrite/marcasite mixture films.

Sb2Se3 is a promising candidate for thin-film photovoltaics, with a suitable band gap, benign grain boundaries, Earth-abundant and nontoxic constituents, and excellent stability. However, the low doping density (1013 cm–3) of Sb2Se3 absorber and back contact barrier limit its efficiency. Here we introduced a PbS colloidal quantum dot (CQD) film as the hole-transporting layer (HTL) to construct a n-i-p configured device and overcame these problems. Through simulation-guided optimization, we have significantly improved the efficiency of a Sb2Se3 thin-film solar cell to a new certified record of 6.5%. The PbS CQD HTL not only minimized carrier recombination loss at the back contact and boosted carrier collection efficiency but also contributed photocurrent by its own near-infrared absorption. Furthermore, these n-i-p devices also demonstrated improved device uniformity, achieving 6.39% in a 1.02 cm2 device.

To commercialize Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells, it is necessary to improve their efficiency and to develop the technological ability to produce large-area modules. Defect formation due to the secondary phase is considered to be one of the main reasons for decreased CZTSSe thin-film solar-cell efficiency. This study explores the potential capabilities of large-area thin-film solar cells by controlling the defect formation using various CZTSSe precursor designs, and by improving the characteristic uniformity within the thin-film solar cells. Alloying the precursor as a stack of discrete layers can result in lateral segregation of elements into stable-phase islands, yielding a non-uniform composition on small length scales. It is found that the application of an indiscrete layer by minimizing the precursor-layer thickness allows avoiding Zn rich inhomogeneities in the absorber that would favor formation of detrimental ZnS-ZnSe secondary phases and deep defects. Among the various precursor layers designed by considering the reaction mechanism under annealing, a sample with 15 precursor layers is found to exhibit a shallow electron-acceptor energy level, high photovoltaic conversion efficiency, and uniform characteristics over the corresponding thin-film solar cell. Based on such improvements in both the efficiency and characteristic distribution, it is expected that the commercialization of CZTSSe thin-film solar cells can be advanced.

In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu2ZnSn(S,Se)4 (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161 eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029 eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (Voc) of 0.505 V, short-circuit current density (Jsc) of 31.61 mA/cm2, fill factor (FF) of 64.6%, and Voc deficit of 525 mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, Voc of 0.424 V, Jsc of 32.83 mA cm−2, FF of 51.9%, and Voc deficit of 576 mV, the Voc and Voc deficit of the CZTSSe cell improved considerably. The high Voc, low Voc deficit, and less loss of Jsc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film.

The defects states and carrier density of CZTSe absorber layers are two of the crucial factors that decide the photovoltaic performance of CZTSe thin film solar cells. Fine tailoring the defects and carrier density is a key to push the power conversion efficiency of CZTSe solar cells to a more competitive level. In this work, the phase properties, defect states, and carrier density of CZTSe thin film are well controlled by fine tuning the ratio of Zn/Sn in the range from 0.75 to 1.27. Capacity-Voltage measurements and Admittance Spectroscopy are used to characterize the carrier density, depletion region width, and defect states of the CZTSe solar cells. The results indicate that the defects states and carrier density of CZTSe layer are very sensitive to the ratio of Zn/Sn. Combining experimental results and numerical simulation, the statistic regularities of the photovoltaic parameters of the CZTSe solar cells with different ratios of Zn/Sn is well explained. The increase of VOC of CZTSe solar cells with the ratio of Zn/Sn is related to both the increased carrier density and the decreased deep level defects states. The decline of JSC of the Zn-rich solar cells is caused by both the shrunken depletion region width and a large barrier caused by ZnSe phase. This barrier is the cause for a low fill factor in the Zn-rich solar cells. Overall, the CZTSe solar cells with a stoichiometric ratio of Zn/Sn=1.02 have favorable defects property and carrier density, thus resulting in the highest photovoltaic efficiency of 10.21%.

In this study, we investigate methods of improving the efficiency of Cu 2 ZnSn(S,Se) 4 (CZTS)-based solar cells by comparing Cu(In,Ga)Se 2 (CIGSe)- and CZTS-based absorber layers. In particular, the CZTS-based absorber exhibits lower current characteristics than the CIGSe absorber layer in terms of the band gap alignment and electron-hole recombination at the CdS-absorber interface. Moreover, we demonstrate that defects are one of the causes of the voltage loss. In order to improve the efficiency of CZTS-based solar cells, it is important to control the band gap alignment at the CdS-absorber layer interface and to suppress the formation of secondary phases inside the absorber.

Raman spectra of few-layer MoSe2 were measured with 8 excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned. The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of intra-layer A1g, E1g, and A2u modes due to inter-layer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of inter-layer shear and breathing modes and Davydov splitting of intra-layer modes to a linear chain model, we extract the strength of the inter-layer interaction. We find that the second-nearest-neighbor inter-layer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations., 45 pages, 19 figures, ACS Nano published (2016)

To observe the superconducting current and structural properties of high critical temperature ( T c ) superconductors (HTS), we suggest the following imaging methods: Room temperature imaging (RTI) through thermal heating, low-temperature bolometric microscopy (LTBM) and Raman scattering imaging. RTI and LTBM images visualize thermal-electric voltages as different thermal gradients at room temperature (RT) and superconducting current dissipation at near- T c , respectively. Using RTI, we can obtain structural information about the surface uniformity and positions of impurities. LTBM images show the flux flow in two dimensions as a function of the local critical currents. Raman imaging is transformed from Raman survey spectra in particular areas, and the Raman vibration modes can be combined. Raman imaging can quantify the vibration modes of the areas. Therefore, we demonstrate the spatial transport properties of superconducting materials by combining the results. In addition, this enables visualization of the effect of current flow on the distribution of impurities in a uniform superconducting crystalline material. These imaging methods facilitate direct examination of the local properties of superconducting materials and wires.

We found a clear correlation between the solar conversion efficiency and the distribution of the ZnS secondary phase in Cu 2 ZnSnS 4 (CZTS) solar cells. Five CZTS solar cells were prepared by the two-step process in which sputtered precursor multilayers were sulfurized at different temperatures. In higher efficiency solar cells, the ZnS secondary phase was found to accumulate near the interface between CZTS and MoS 2 formed on top of the Mo layer. It was also found that a solar cell with a higher efficiency showed weaker ZnS Raman signal in the MoS 2 layer. The highest conversion efficiency of 7.5% was obtained from a solar cell prepared at a sulfurization temperature of 570 °C, in which no ZnS was detected in the MoS 2 layer. We suggest that in order to obtain a high efficiency CZTS solar cell, it is critical to find a condition that keeps ZnS from the MoS 2 layer.

Cu 2 ZnSnSe 4 photovoltaic absorber layers were prepared by the vapor selenization of stacked Cu/Sn/CuZn/Mo/glass precursor films with different Cu/(Zn + Sn) compositional ratios (CZT = 0.8, 0.85 and 0.9). The absorber was ZnSe-free at CZT = 0.9 but the films obtained from CZT = 0.85 or 0.8 contained a ZnSe phase mainly at the bottom or top of the absorber film, respectively. Although the solar cell fabricated from the CZT = 0.9 absorber showed poor-shunt characteristics (efficiency = 1.94%), that produced from CZT = 0.85 exhibited significantly distorted current–voltage characteristics (efficiency = 4.23%). The CZT = 0.8 ratio showed the highest efficiency of 5.8% and normal pn diode behavior. This suggests that the existence or location of the ZnSe phase within the absorber is not related directly to the solar cell performance. Instead, the ZnSe phase may alter the defect distribution near the space charge region. Several aspects of the device behavior depending on the CZT ratio are discussed with respect to the relative position of the ZnSe phase.

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

In this study, CZTSSe precursors (Se/Cu/SnS/ZnS) were deposited by sputtering and evaporation and then annealed without added chalcogen substances in an argon-filled atmospheric chamber using rapid thermal processing (RTP). A low-temperature pre-annealing treatment (PAT) was applied to induce a pre-reaction between Se and Cu/SnS/ZnS. It was found that the PAT leads to the formation of CuSe and Cu2−xSe and enhances Se incorporation during RTP chalcogenization. Scanning electron microscopy (SEM), X-ray diffractometry, Raman analysis and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) were applied to investigate the differences between the absorber with the PAT and the one without the PAT. Better morphologies on the surface and in the cross section were obtained for the CZTSSe absorber annealed using the PAT compared with the absorber annealed without the PAT. Both absorbers showed nearly pure kesterite phases in their upper regions, as confirmed by Raman analysis. However, STEM-EDS maps revealed that when the absorber was prepared without the PAT, Zn- and S-rich secondary phases and voids were easily formed near the back contact. The electrical characteristics and efficiencies of the CZTSSe thin films showed drastic changes; the CZTSSe solar cell without the PAT showed no diode response, but the cell with the PAT showed an efficiency of 6.77% with an open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of 376 mV, 31.39 mA/cm2 and 57%, respectively.

ZnSe thin films were deposited on well-cleaned glass substrates by the close-spaced vacuum sublimation technique. Various structural, sub-structural and optical properties have been investigated through scanning electron microscopy (SEM), X-ray diffraction (XRD), spectral photometry, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Electro-physical studies were performed using an ITO/ZnSe/In “sandwich” structure. The correlation between the films structure and their optical and electro-physical properties was estimated. The evaluated films were fine-crystalline, with their grain size depending on the substrate temperature. The optical band gap was estimated through optical measurements and the high optical qualities of the ZnSe films were confirmed by Raman and FTIR analyses.

The improvement of the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)-based solar cells requires the formation of high-grain-sized pure CZTSSe throughout the film. We have successfully selenized precursor samples of Cu/SnS/ZnS/Mo/Soda lime glass in an almost sealed selenium furnace. Owing to the presence of confined and high-pressure Se vapor in the furnace, Se easily diffused into the precursor samples, and high-quality Se-rich CZTSSe absorbers were obtained. To understand the effect of the growth mechanism in our precursor and annealing system, this study examines the phase evolution and grain formation. Device parameters are discussed from the perspective of a material microstructure in order to improve performance. At a selenization temperature of 570 °C, a CZTSSe film showed fully developed grains with a size of around 2 μm without noticeable pore development near the Mo back contact. Solar cells with up to 8.03% efficiency were obtained with a layer thickness of about 1.2 μm. Detailed electrical analysis of ...

Raman scattering and atomic force microscopy measurements on Cu 2 ZnSnS 4 thin films prepared by a direct solution method revealed that metal sulfates of various morphologies (dense clusters or separated particles) were partially embedded on the surface of the Cu 2 ZnSnS 4 layer. This residue was removed during the subsequent chemical bath deposition of the CdS buffer layer. However, the removal of the residue led to poor crystallinity and reduced photocurrent near the location of the residue, which suggests that controlling the formation of the sulfates during the fabrication of the absorber layer would be critical for obtaining high efficiency solar cells by the solution method.

Compositional and structural studies of Cu 2 ZnSnSe 4 (CZTSe) thin films were carried out by X-ray diffraction, energy dispersive X-ray spectroscopy (EDS), particle induced X-ray emission (PIXE), photoluminescence, and Raman spectroscopy. CZTSe thin films with different compositions were deposited on sodalime glass by co-evaporation. The composition of the films measured by two different methods, EDS and PIXE, showed significant differences. Generally, the Zn/Sn ratio measured by EDS is larger than that measured by PIXE. Both the micro-PIXE and the micro-Raman imaging results indicated the compositional and structural inhomogeneity of the sample.

Cu₂ZnSnSe₄ thin films obtained by co-evaporation of components using an electron beam evaporation system were investigated by scanning electron microscopy, X-ray analysis, PIXI and RBS methods. The analysis of the diffraction patterns showed that the films are almost single-phased and contain mainly CZTSe compound, which has a tetragonal kesterite lattice type. The samples have textural growth of [211]. The lattice parameters of the material varied in the range of a = (0.56640-0.56867) nm, c = (1.13466-1.13776) nm, c/2a = 0.9983-1.0017 which correlate well with the reference data in a stable phase CZTSe compounds. From our PIXE analyses we assessed the influence of the growth conditions on the samples chemical composition and mapped the surface distribution.

The CdTe thin and thick films were obtained by the close spaced vacuum sublimation technique on a glass substrate under the following growth conditions: the evaporator temperature was 620 °C; and the substrate temperature was varied in the range from 250 °C to 550 °C. High purity CdTe powder was used as a charge for evaporation. The Raman spectra were measured using TRIAX 320 and TRIAX 550 spectrometers at room temperature. The 488-nm line and 514.5-nm line of an Ar+ laser and a 785-nm diode laser were used as excitation sources. The signal was collected by the liquid nitrogen cooled charge-coupled-device (CCD) detector. A number of intense Raman peaks at 140, 167, 190, 271, 332 and 493 cm-1 were observed and were interpreted as TO (140 cm-1), 1LO (167 cm-1), 2LO (332 cm-1), 3LO (493 cm-1) phonon modes and plasmon-phonon mode (190 cm-1). The presence of several phonon replicas in the Raman spectra confirms high crystal quality of the samples. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This is the Cu2ZnSnS4 (CZTS) films were fabricated on glass substrates by sputtering using compound material targets. The fabrication method combines the deposition of metallic precursors and the sulfurization annealing process using S vapor. The optimal precursor sequence for CZTS growth was Cu/SnS/ZnS/Mo. To optimize the composition and the thickness of CZTS, the thickness of Cu layer and precursor was varied. The best cell had an area of 0.185 cm2, Cu/(Zn + Sn) = 0.89, Zn/Sn = 1.04, and S/(metal) = 1.13 and showed an open-circuit voltage (VOC) of 0.66 V, a short-circuit current (JSC) of 13.60 mA cm−2, a fill factor of 43.14% and a conversion efficiency of 3.84%.

We investigated the transport and photovoltaic properties of Cu-deficient Cu(In1 − xGax)Se2 (CIGS) thin-film solar cells containing ordered vacancy compound (OVC) layers. Raman spectra clearly revealed that the CIGS thin films with lower Cu concentrations contained larger volumes of OVC layers. The temperature-dependent inverse ideality factor showed that the CIGS film containing more (less) OVC layers exhibited tunneling-mediated bulk (interface)-dominated recombination. The capacitance–voltage characteristics and admittance spectra showed that the CIGS cells containing more OVC layers had more uniform carrier concentration near the junction and less interfacial trap states compared with cells with less OVC layers. These results suggested that the Cu-deficiency and the resulting OVC layer formation reduced the interfacial defect density and suppressed the interface recombination processes of the CIGS solar cells.

The influence of the conventional depositing processes on the optical properties of Cu(In,Ga)Se 2 (CIGS) thin films in solar cell structures was investigated by measuring the photoluminescence (PL) and Raman spectra of the CIGS layer at each stage of the solar cell deposition process. The intensities of the PL and the Raman A 1 mode increase after the CdS buffer layer is deposited, suggesting that the CdS layer either improves the optical quality of the CIGS film or protects it from degradation due to environmental factors. The temperature and excitation power dependences of the PL for the bare CIGS sample are very different from those for the samples with the CdS layer, reflecting different characters of the luminescence centers near the surface of the CIGS layer. On the other hand, the lateral homogeneity, as seen in the micro-PL and micro-Raman images, does not seem to improve. After the ZnO window layer is deposited, the overall PL and Raman intensities do not change much, although the intensity distribution becomes more inhomogeneous.

Cu 2 ZnSnSe 4 (CZTSe) thin films were deposited on Mo-coated soda–lime glass substrates by single-stage co-evaporation process and the effect of substrate temperature ( T sub ) was investigated on the CZTSe thin film growth during the process. The film thickness decreased and the grain size increased with increasing T sub . The optimum substrate temperature to obtain CZTSe films was found to be 593 K. The film grown at T sub of 773 K seems to have the phase decomposition into Cu x Se and ZnSe, probably because of Sn loss during the process. The best cell fabricated with a CZTSe absorber layer grown at T sub of 593 K produced 2.88% of conversion efficiency.

Ionic liquids of 1-butyl-3-methylimidazolium ([BMIM]) cation with different anions (Cl- , Br- , I- , and BF4- ), and their aqueous mixtures were investigated by using Raman spectroscopy and dispersion-included density functional theory (DFT). The characteristic Raman bands at 600 and 624 cm-1 for two isomers of the butyl chain in the imidazolium cation showed significant changes in intensity for different anions as well as in aqueous solutions. The area ratio of these two bands followed the order I- >Br- >Cl- >BF4- (in terms of the anion X in [BMIM]X), indicating that the butyl chain of [BMIM]I tends to adopt the trans conformation. The butyl chain was found to adopt the gauche conformation upon dilution, irrespective of the anion type. The Raman bands in the butyl C-H stretch region for [BMIM]X (X=Cl- , Br- , and I- ) blueshifted significantly with the increase in the water concentration, whereas that for [BMIM]BF4 changed very little upon dilution. The blueshift in the C-H stretch region upon dilution also followed the order: [BMIM]I>[BMIM]Br>[BMIM]Cl>[BMIM]BF4 , the same order as the above trans conformation preference of the butyl chain in pure imidazolium ionic liquids, which suggested that the cation-anion interaction plays a role in determining the conformation of the chain.

a b s t r a c t Cu(In,Ga)Se2 (CIGS) thin films were formed by a direct non-vacuum coating and a subsequent selenization of low cost precursor solutions, and their compositional, structural and optical properties were charac- terized. Selenized films showed a double-layered structure with an upper layer of chalcopyrite CIGS and an amorphous bottom layer mainly composed of carbon. For the upper CIGS layer, good compositional controllability for Cu and Ga was confirmed by linear relationship between metal ratios of the precursor solution and those of the selenized films. Effects of Cu and Ga contents on structural and optical proper- ties of the films were also characterized by X-ray diffraction (XRD), Raman and photoluminescence (PL) analyses, and the results were interpreted by defect supercluster formation (VCu-InCu) in Cu-deficient films and mass and size difference between In and Ga, respectively. Further, the bottom layer was found to be mostly composed of conductive amorphous carbon, which is the main current flow path in the completed solar cells.

Cu2ZnSnSe4 thin films with different substrate temperature and Cu flux were grown by thermal co-evaporation. Raman scattering, photoluminescence, and contactless electroreflectance (ER) measurements were performed. The Raman spectra of Cu2ZnSnSe4 show two main peaks at 170 and 192 cm− 1. The photoluminescence spectrum shows a peak below 1.0 eV. Franz–Keldysh oscillations (FKOs) were observed in the ER spectra. From the analysis of the FKOs, the bandgap energy of Cu2ZnSnSe4 thin films is estimated to be 1.07 eV at 90 K and 0.99 eV at room temperature. We conclude that the bandgap energy of Cu2ZnSnSe4 thin films is around 1.0 eV.

Raman investigation of MoSe2 was carried out with eight different excitation energies. Seven peaks, including E1g, A1g, E2g1, and A2u2 peaks are observed in the range of 100-400 cm-1. The phonon modes are assigned by comparing the peak positions with theoretical calculations. The intensities of the peaks are enhanced at different excitation energies through resonance with different optical transitions. The A1g mode is enhanced at 1.58 and 3.82 eV, which are near the A exciton energy and the band-to-band transition between higher energy bands, respectively. The E2g1 mode is strongly enhanced with respect to the A1g mode for the 2.71- and 2.81-eV excitations, which are close to the C exciton energy. The different enhancements of the A1g and E2g1 modes are explained in terms of the symmetries of the exciton states and the exciton-phonon coupling. Other smaller peaks including E1g and A2u2 are forbidden but appear due to the resonance effect near optical transition energies., 16 pages, 3 figures

ZnSe thin films were deposited on well-cleaned glass substrates by the close-spaced vacuum sublimation technique. Various structural, sub-structural and optical properties have been investigated through scanning electron microscopy (SEM), X-ray diffraction (XRD), spectral photometry, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Electro-physical studies were performed using an ITO/ZnSe/In “sandwich” structure.The correlation between the films structure and their optical and electro- physical properties was estimated. The evaluated films were fine-crystalline, with their grain size depending on the substrate temperature. The optical band gap was estimated through optical measurements and the high optical qualities of the ZnSe films were confirmed by Raman and FTIR analyses. This work is supported by the Ukraine State Agency for the Science, Innovation and Informatization and by the NRF grant funded by the MSIP of Korea (2011–0019204) and by the Ministry of Education and Sciences of Ukraine (Grant no.0113U000131).

We prepared CZTSe thin films by the co-evaporation method. In order to investigate the presence of various kinds of secondary phases, we also prepared reference ZnSe, SnSe, CuSnSe, and CuSe thin films in the same way. The depth dependence of the secondary phases was investigated by preparing samples that are sputtered to various depths. We used Raman spectroscopy measurement with several lasers to detect secondary phases. CZTSe films were compared with the reference films, and only ZnSe and MoSe2 phases were detected. The relative amount of ZnSe varied with the depth from the sample surface. The ZnSe Raman peak seems to originate from the top and the bottom of the sample since the signal decreased initially and then increased as deeper parts of the film were probed. Intensity of CZTSe peak did not change much until the bottom part of the sample was reached and the MoSe2 peak appeared.

Understanding of grain boundary (GB) is critical for photovoltaic applications since electron-hole recombination at GBs determines the conversion efficiency. However, our local electrical and optical analysis shows positive potential at GBs in Cu(In,Ga)Se 2 (CIGS), which suppresses the recombination at GBs. We report on a direct measurement of potential distribution and local electrical transport on the surface of photovoltaic CIGS using a nano-scale electrical characterization of Kelvin probe microscopy and conductive atomic force microscopy. This reveals that the positively charged surface potential at GB is expected to increase the minority carrier collection and the enhanced current at GB leads to large carrier mobility and electron-hole separation at the GBs. Micro-Raman scattering results helps to analyze electrical behavior from defect analysis.

Cu(In 1−x Ga x )Se (CIGS) based thin film solar cells are usually built by depositing CdS as a buffer layer and ZnO as a window layer on top of the CIGS absorber layer. In order to optimize their performances, it is essential to understand the interactions between the layers. In this study, we have investigated the interactions between the layers by examining the optical properties of the CIGS solar cell structure at each step of the fabrication process―CIGS, CIGS/CdS, and CIGS/CdS/ZnO― using photoluminescence and Raman spectroscopic imaging techniques. The images of the intensity of the Raman peak at 175 cm−1 due to the A 1 vibration mode show that the homogeneity improves after CdS deposition. Micro-PL intensity imaging also confirmed this observation. Furthermore, the PL peak intensity and the energy position vary after each layer deposition.

Raman scattering and photoluminescence on Cu2ZnSnSe4 thin films grown by thermal co‐evaporation were performed. The photoluminescence spectrum shows a peak below 1.0 eV. The Raman spectra of Cu2ZnSnSe4 show two main peaks at 170, 192–195 cm−1 and additional Raman mode at 260 cm−1, which is ascribed to the Cu2−xSe phase. The lateral distribution of the Cu2−xSe phase in Cu2ZnSnSe4 thin films is investigated by scanning micro‐Raman scattering. In Cu‐rich samples, the distribution of the Cu2−xSe phase is found to be uneven.

The influence of a prealloying process on the formation of MoSe2 and thus on the performance of Cu2ZnSnSe4 (CZTSe) solar cells is investigated using sputtering deposition and post-annealing approaches. The dense alloy layer, which is made by a low-temperature prealloying process, acts as a temporary Se diffusion barrier during a subsequent high-temperature selenization process. The formation of thick interfacial MoSe2 can be suppressed effectively by this temporary barrier, cooperating with subsequent quick formation of compact CZTSe layer. The thickness of interfacial MoSe2 layer in CZTSe solar cells can be tailored by adjusting the preannealing process during selenization. As a consequence, the series resistance of CZTSe solar cells is reduced to a low level (≈0.6 Ω cm2), and the performance of CZTSe solar cells is improved significantly. A CZTSe solar cell with efficiency of 8.7% is fabricated.

Cu 2ZnSnSe4 (CZTSe) is a p-type semiconductor which has been developed as an absorber layer of polycrystalline thin film solar cells. Generally, Cu-poor and Zn-rich compositions tend to give the highest solar conversion efficiencies. Raman spectroscopy has been used to detect secondary phases such as ZnSe and Cu 2SnSe3 in CZTSe thin films. However, the fundamental phonon modes in single-crystal CZTSe with a composition matching that of high-efficiency thin film solar cells have not yet been fully understood. We performed polarized Raman measurements on Cu-poor and Zn-rich single-crystal CZTSe and identified 12 peaks, including two low-frequency peaks. By comparing the polarization dependence of the Raman peaks with a group theoretical analysis, we concluded that the crystal structure of CZTSe single-crystal is kesterite and made appropriate peak assignments.

Microstructural alteration induces non-uniform device characteristics in polycrystalline thin films. In thin-film solar cells based on Cu(In,Ga)Se2 (CIGS), local electrical properties are investigated by Raman scattering spectroscopic imaging and scanning probe microscopic tools. Localized and uneven intensity of phonon modes, which represent different orientation and phases, elucidate the nature of non-uniformity of crystallinity, composition and defects in the films. Surface potential mapping at nanoscale is performed by Kelvin probe force microscopy, showing 40mV of band-bending at the grain boundaries. Externally biased-current mapping, which is obtained by conductive atomic force microscopy, shows preferred current path in the films.

By controlling the heating temperature of a mixture of In and Ge powders, we have obtained monoclinic In2Ge2O7 nanowires at 600–700 °C, whereas we have produced cubic In2O3 nanowires at 900 °C. The In2Ge2O7 nanowires grown at 600 °C were terminated by Au-containing nanoparticles, giving evidence that the vapour–liquid–solid model is the major growth mechanism. With the growth process at 700–900 °C being dominated by a vapour–solid process, we have discussed the temperature-induced change in growth mechanisms. Photoluminescence measurements at 10–300 K revealed a broad visible emission centred at around 2.5 eV.