Thickness-Dependent Physical Properties of Coevaporated Cu₄SnS₄ Films

臧 竞.

Chen Q., Dou X., Ni Y., Cheng S., Zhuang S.

► Photovoltaic properties of narrow-bandgap Cu 2 SnS 3 (CTS) are studied for the first time. ► CTS can be a promising candidate as bottom cell absorber layer for low-cost thin film tandem solar cell application. ► A p–n junction interface modification method that can enhance the performance of the studied CTS solar cell is demonstrated. Photovoltaic properties of narrow-bandgap Cu 2 SnS 3 (CTS) are studied for the first time by employing a superstrate solar cell structure of fluorine-doped tin oxide (FTO) glass/TiO 2 /In 2 S 3 /CTS/Mo. The structural, optical, and electronic characteristics of the CTS make it great potential as bottom cell absorber material for low-cost thin film tandem solar cell application. Furthermore, by inserting a thin low temperature deposited In 2 S 3 layer between the In 2 S 3 buffer layer and the CTS absorber layer, an enhancement in the performance of the solar cell can be achieved, leading to about 75% improvement ( η = 1.92%) over the unmodified device ( η = 1.10%).

kumar N., Parihar U., Kumar R., J. Patel K., J. Panchal C., Padha N.

Avellaneda D., Nair M.T., Nair P.K.

Thin films of copper sulfide (CuS, 200 nm thick) were deposited over thin films of tin sulfide (SnS, 180 nm thick) by sequential chemical deposition. The layers were heated in nitrogen atmosphere at 350 and 400°C. The grazing incidence X-ray diffraction analysis of these layers established the formation of thin films of ternary composition, Cu 2 SnS 3 and Cu 4 SnS 4 . Optical bandgaps of the films are direct, 0.95 eV for Cu 2 SnS 3 and 1.2 eV for Cu 4 SnS 4 , and the electronic transitions are of the forbidden type in both cases. The films are p-type, with electrical conductivities of 0.5-10 Ω ―1 cm ―1 and hole concentrations of 10 17 ―10 18 cm ―3 . Based on the optical absorption coefficients, the light generated current density (J L ) as a solar cell absorber was evaluated for these materials for air mass 1.5 (1000 W/m 2 ) global solar radiation. For a film thickness of 0.5 μm. Cu 2 SnS 3 and Cu 4 SnS 4 could offer J L of 34 and 27 mA/cm 2 , respectively. Corresponding optical conversion efficiencies of solar energy into electron-hole pairs are 32 and 24%. The built-in potential for CdS/Cu 2 SnS 3 and CdS/Cu 4 SnS 4 junctions would be above 0.9 V and above 1.1 V when ZnO replaces CdS as the window layer.

Lai F., Li M., Wang H., Hu H., Wang X., Hou J.G., Song Y., Jiang Y.

Niobium oxide (Nb 2 O 5 ) films with thicknesses ranging from 200 to 1600 nm were deposited on fused silica at room temperature by low frequency reactive magnetron sputtering system. In order to study the optical losses resulting from the microstructures, the films with 500 nm thickness were annealed at temperatures between 600 and 1100 °C, and films with thicknesses from 200 to 1600 nm were annealed at 800 °C. Scanning electron microscopy and atomic force microscopy images show that the root mean square of surface roughness, the grain size, voids, microcracks, and grain boundaries increase with increasing both the annealing temperature and the thickness. Correspondingly, the optical transmittance and reflectance decrease, and the optical loss increases. The mechanisms of the optical losses are discussed. The results suggest that defects in the volume and the surface roughness should be the major source for the optical losses of the annealed films by causing pronounced scattering. For samples with a determined thickness, there is a critical annealing temperature, above which the surface scattering contributes to the major optical losses. In the experimental scope, for the films annealed at temperatures below 900 °C, the major optical losses resulted from volume scattering. However, surface roughness was the major source for the optical losses when the 500-nm films were annealed at temperatures above 900 °C.

Qiao Z., Latz R., Mergel D.

Polycrystalline In 2 O 3 :Sn (ITO) films with thickness between 40 nm and 1.7 μm were prepared by direct current magnetron sputtering at 300°C substrate temperature. They were characterized by X-ray diffractograms and transmittance curves. In the range of 0.4 to 1.4 μm, the films exhibit a mass density close to the crystalline value, small lattice distortion and microstrain, similar texture of crystalline orientations, high free electron density and low electrical resistivity. Films thinner than 0.4 μm do not exhibit such favorable properties. For all films, grain size and free carrier density decrease with increasing lattice distortion. We attribute the differences in the properties of the thin and thick films to a change of the film's microstructure taking place at 0.3–0.5 μm with more additional oxygen incorporated into the thinner films.

Nair M.T., Lop z-Mata C., GomezDaza O., Nair P.K.

Copper tin sulfide (Cu4SnS4) thin films have been prepared by heating a layer of CuS thin film deposited over an SnS thin film, both obtained by chemical bath deposition. Upon heating in a nitrogen atmosphere at 300–340 °C, the CuS layer converts to Cu8S5, which reacts with the underlying SnS thin film at about 400 °C to form Cu4SnS4. The optical band gap of Cu4SnS4 has been found to be approximately 1 eV, involving direct forbidden transitions. The films are photosensitive, and the electrical conductivity in the dark is about 1 Ω−1 cm−1.

Brüggemann R., Reinig P., Hölling M.

We detail the thickness-dependent increase of optical scattering in microcrystalline silicon by analysis of specular reflection and angle-resolved scattering measurements with a goniometer. The scattering properties are related to the increase in surface roughness with increasing sample thickness. Analysis of the specular reflection in the ultraviolet spectral range allows a determination of the surface roughness. Manipulation of the surface morphology by mechano-chemical polishing results in lesser optical scattering because polishing reduces the surface roughness and thus surface-related scattering.

Kim H., Horwitz J.S., Kushto G., Piqué A., Kafafi Z.H., Gilmore C.M., Chrisey D.B.

Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm. The device with the optimum thickness of ITO anode showed an external quantum efficiency of about 0.85% at 100 A/m2.

Marcano G., Bracho D.B., Rincón C., Pérez G.S., Nieves L.

The Hall effect and electrical resistivity measurements on p-type Cu2GeSe3 crystals were measured in the temperature range from 80 to 300 K. The temperature variation of the hole concentration p from about 200 to 300 K is explained as due to the thermal activation of a shallow acceptor level with an ionization energy of around 50 meV. At low temperatures the impurity band conduction dominates the electrical transport processes. From the analysis of the p vs T data, the density-of-states hole effective mass is estimated to be of the same magnitude as the free electron mass. The temperature variation of the hole mobility in the valence band is analyzed by taking into account the scattering of charge carriers by ionized impurities and acoustic phonons. In the impurity band, the mobility is explained as due to thermally activated hopping transport. The optical absorption coefficient spectrum shows the presence of three absorption narrow bands below the fundamental gap. From the analysis of their temperature dependence, these bands are attributed as due to free–to–bound transitions related to intrinsic defect acceptor states. Activation energies of these states are estimated to be around 0.12, 0.24, and 0.30 eV. Tentative assignment of the nature and origin of these defect states were also made.

Valla T., Kralj M., Siber A., Milun M., Pervan P., Johnson P.D., Woodruff D.P.

The temperature dependence of peak widths in high resolution angle-resolved photoelectron spectroscopy from quantum well states in ultra thin Ag films on V(100) has been used to determine the electron-phonon coupling constant, lambda, for films of thickness 1-8 layers. A strong oscillatory variation in coupling strength is observed as a function of film thickness, peaking at a 2 layer film for which lambda~1. A simple theory incorporating interaction of the photo-hole with the thermal vibrations of the potential step at the adlayer-vacuum interface is shown to reproduce the main features of these results.

Lokhande C.D., Ubale A.U., Patil P.S.

Thin films of Bi2S3 with different thicknesses were prepared by the chemical deposition method from an aqueous acidic bath using thiosulfate as a sulfide ion source. The effect of film thickness on the optical, structural and electrical properties was studied. A shift of 0.6 eV in the optical bandgap energy, Eg, and a decrease in electrical resistivity from 2.8 × 104 to 5 × 103 Ω cm and an increase in grain size of Bi2S3 crystallites from 5.2 to 8.0 nm were observed when the thickness was varied from 52.7 to 220 nm. These changes are attributed to the quantum size effect in semiconducting films.

Orr B.G., Jaeger H.M., Goldman A.M.

Ultrathin films of Sn have been grown incrementally on substrates held at 15 K and then studied in situ at low temperatures. This process of repeatedly increasing the thickness of a film facilitates the investigation of thickness-dependent physical properties. The superconducting transition temperature has been found to oscillate with a period which was nominally about 4 \AA{}. The latter is approximately a factor of 2 larger than the prediction of simple theoretical models. Oscillations of the normal-state resistance with thickness have not been observed.

Sadullah M., Hussain S.M., Ghosh K.

α-Cr2O3 is used as a buffer layer for the growth of α-Ga2O3 on sapphire for power devices. Presently, the growth of crystalline corundum-structured metal oxides layers, except for α-Cr2O3, is performed with metal acetylacetonates. This article investigates the development of a crystalline α-Cr2O3 thin films on c-plane sapphire with chromium acetylacetonate (CrAc) as precursor over a wide temperature range varying from 400 to 550 °C. The temperature range not only ensures the compatibility of the process with α-Ga2O3 technology but also satisfies the requirement that the window is large enough to adequately optimize the quality of crystalline α-Cr2O3 thin film. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were performed to analyse the quality of the crystalline α-Cr2O3 layer. The XPS result showed that the ratio of different oxidation states of α-Cr2O3 changes with deposition temperature. An optimal deposition temperature at 500 °C with the molarity of CrAc being 0.05 M is achieved for better quality α-Cr2O3 thin film deposition. Thin film of α-Cr2O3 of thickness 530.5 nm has been deposited at optimal condition with a deposition rate of 35.37 nm/min and has a crystallite size of 31.21 nm and root mean square value of surface roughness of 0.647 nm.

Nath R.A., Raj A., Salam J.A., Anand A.M., Jayakrishnan R.

Emerging applications in internet of things are ushering research into novel device architectures for self-powered electronics. Pure and copper-doped CH3NH3PbBr3 was synthesized using emulsion–DEemulsion process and subsequently using spin coating method, thin films were grown. Structural and optical characterization of the films were carried out before and after subjecting them to vacuum annealing. The copper-doped CH3NH3PbBr3 thin films subjected to vacuum annealing at 400 K showed highest preferential orientation along the (113) growth plane. The characteristic green luminescence of the CH3NH3PbBr3 system did not undergo any significant change in its photoluminescence emission energy indicating that copper doping or vacuum annealing of the copper-doped thin films does not alter the photoluminescence pathway. The time-resolved photoluminescence measurements of the samples showed that the electron–hole pair recombination lifetime varied from 10–6 s for the pure material to 10–10 s for the copper-doped vacuum-annealed material. We have engineered and characterized self-powered photodetectors using the copper-doped methyl ammonium lead bromide perovskite material which exhibits giant photosensitivity of 444%. Vacuum-annealed CH3NH3Pb1−xCuxBr3-based photodetectors yield improved photocurrent and spectral response both in the UV and in the visible region delivering an effective increase in responsivity bandwidth relative to their pristine counterparts. The CH3NH3Pb1−xCuxBr3-based photodetectors exhibit a self-powered ON/OFF photocurrent ratio of ~ 35, a detectivity of ~ 21 × 105 Jones and responsivity of 8.244 × 10–6 A/W.

Avellaneda D., Paul A., Shaji S., Krishnan B.

Three phases of the ternary Cu-Sn-S system were synthesized using thin films of chemical bath deposited tin sulfide (SnS) and thermally evaporated copper. Cu layers with different thicknesses (125–175 ​nm) were deposited on SnS thin films (250 ​nm) and the SnS: Cu stacks were sulfurized at temperatures in the range of 400–600 ​°C. Three different ternaries of Cu-Sn-S were obtained. Cu 2 SnS 3 (CTS) thin films were formed for 125 ​nm copper on SnS (250 nm) , sulfurized at 500 ​°C. Tetragonal crystalline structure was determined for CTS thin films by X-ray diffraction (XRD) analysis and confirmed by two characteristic Raman peaks at 296 and 352 ​cm -1 . Optical properties showed a direct bandgap close to 1 ​eV and electrical conductivity of 1.9 (Ω ​cm) -1 . Increasing the Cu thickness to 175 ​nm and the temperature of sulfurization to 550 ​°C, the Cu 3 SnS 4 phase (orthorhombic) was formed which was confirmed by the high intense Raman peak at 316 ​cm -1 . A direct bandgap of 1.75 ​eV and electrical conductivity in the order of 10 3 (Ω ​cm) -1 was obtained for this phase. Further increase in sulfurization temperature to 600 ​°C for the stack with the same Cu thickness (175 ​nm), Cu 4 SnS 4 phase (orthorhombic) was achieved exhibiting a Raman vibrational peak at 317/322 ​cm -1 , a direct bandgap of 1 ​eV, and electrical conductivity of 10 (Ω ​cm) -1 . The morphological analysis of these three CTS phases by scanning electron microscopy presented distinct morphologies for each phase. X-ray photoelectron spectroscopy (XPS) analysis of the samples was done for their elemental composition and chemical state information. This is the first report to synthesize all the three stable phases of the Cu-Sn-S system viz. Cu 2 SnS 3 , Cu 3 SnS 4, and Cu 4 SnS 4 by selecting the sulfurization temperature and copper layer thickness in similar SnS: Cu layer thin films. • Synthesis of Cu 2 SnS 3 , Cu 3 SnS 4, and Cu 4 SnS 4 thin films by combining physical-chemical techniques. • Different phases obtained by selecting of Cu layer thickness and sulfurization temperature. • Structure, morphology, composition, and opto-electronic properties were investigated. • Cu 2 SnS 3 and Cu 4 SnS 4 thin films exhibited suitable optoelectronic properties for solar cell applications.

Paul A., Shaji S., Krishnan B., Avellaneda D.A.

Copper tin sulfide (Cu–Sn–S) thin films have gathered great attention in the research for earth-abundant, non-toxic, and low-cost thin film solar cells. In this investigation, preferentially oriented Cu 4 SnS 4 thin films have been fabricated via a combination of chemical bath deposition and thermal evaporation for the first time. Stacked precursor layers of thermally evaporated copper and chemically deposited SnS thin films were annealed in the sulfur atmosphere at a temperature of 823 K for 1 h. The copper layer thickness was varied from 150 to 200 nm. Various characterization techniques were employed to study the structure, morphology, optical and electrical properties of the Cu 4 SnS 4 films (XRD, Raman, FE-SEM, AFM, UV–Vis spectroscopy) formed at different conditions. The orthorhombic structured films formed at the best conditions showed highly oriented growth along (102) plane. An intense peak at 316 cm −1 in the Raman spectrum further confirmed the formation of the Cu 4 SnS 4 phase. The crystalline sizes and bandgap energies of deposited films varied between 33.6 and 52.7 nm and 1.0–1.6 eV, respectively. The sample with a copper precursor layer thickness of 185 nm exhibited an optimum bandgap of 1.4 eV and larger grains implies that a copper layer of 185 nm in thickness on 200 nm SnS is an optimal precursor layer thickness for achieving highly oriented Cu 4 SnS 4 thin film with excellent properties. • First attempt to fabricate Cu 4 SnS 4 thin films via a combination of physical-chemical deposition methods. • Cu 4 SnS 4 thin films with novel preferential orientation along (102) plane. • Optimization of copper precursor layer thickness resulted enhanced crystallinity. • The optimum bandgap of 1.4 eV for photovoltaic applications.

Abdolahzadeh Ziabari A., Mohabbati Zindanlou N., Hassanzadeh J., Golshahi S., Bagheri Khatibani A.

Earth abundant and non-toxic kesterite Cu2ZnSnS4 (CZTS) thin films have been considered remarkably as the potential alternate to the conventional, expensive, and toxic absorber layers like CdTe, c-Si, CIGS etc. Formation of secondary phases along with poor electrical mobility are the main problems in fabrication of CZTS thin films by chemical methods. In the present study, single-phase high-hole-mobility CZTS thin films have been prepared by the facile low temperature sol-gel method with emphasizing on the role of monoethanolamine (MEA) and diethanolamine (DEA) as the stabilizer and the number of dip-coating. Results show the formation of high crystallinity single-phase CZTS thin films with DEA as the stabilizer. The thickness of the films prepared by DEA was higher than that of MEA. The compositional analysis of the films showed the formation of Cu-poor Zn-rich samples that are appropriate for solar cells by using of DEA. The surface analysis of the prepared thin films revealed an appropriate RMS roughness for capturing the major portion of the incident light. The evaluated bandgap of the films was in the optimum range for solar cell applications. D.C. electrical measurements showed very high-hole mobility for the samples prepared by DEA. The functionality of the prepared samples was investigated by performing a numerical simulation.

Ho S.M.

Ternary compounds such as Cu4SnS4 thin films can be deposited onto glass substrates by various deposition methods: electrodeposition, chemical bath deposition, successive ionic layer adsorption and reaction, and evaporation techniques. Cu4SnS4 films could be used in solar cell applications because of their suitable band gap and large absorption coefficient. This paper reviews previous researches on Cu4SnS4 thin films. X-ray diffraction showed that the obtained films are orthorhombic in structure and polycrystalline in nature. Cu4SnS4 films exhibited p-type electrical conductivity and indicated band gap values in the range of 0.93 to 1.84eV.

Ito K., Nagata T., Htay M.T., Momose N., Hashimoto Y.

In this decade slow but steady progress in CZTS photovoltaics can be seen as a deep research interest and an extensive development effort in the Earth-abundant and nontoxic thin-film solar cells have continued world-wide. Sulfurization of metallic precursors is one of the most promising preparation processes for this optical absorber because it is considered favorable from a productivity viewpoint. Two types of precursors were studied in this paper. One was a multiple stack of pure metal layers and the other was an alloy layer with or without an overlayer consisting one of pure metals. By investigating about how the stacking sequence of each metallic precursor layer and a heating rate for sulfurization had an influence on the formation of secondary phases on CTZS surface and also in the bulk, we found that sulfurization process of Zn-covered metallic precursors had a beneficial effect on the improvement of device performance.

Minnam Reddy V.R., Pallavolu M.R., Guddeti P.R., Gedi S., Yarragudi Bathal Reddy K.K., Pejjai B., Kim W.K., Kotte T.R., Park C.

The rapid progress on the Cu–Sn–S (Cu2SnS3, Cu3SnS4, and Cu4SnS4) solar cells has opened a new avenue to generate the electrical energy at ultra-low-cost. Therefore, the progress in the deposition of Cu2SnS3, Cu3SnS4, and Cu4SnS4 thin films by various chemical and physical methods is reviewed comprehensively. This article briefly describes (i) the phase diagrams of Cu–Sn–S, (ii) the bulk properties of Cu2SnS3, Cu3SnS4, and Cu4SnS4, (iii) the effect of deposition conditions on the phase formation, (iv) the physical properties of Cu2SnS3, Cu3SnS4, and Cu4SnS4 thin films, and (v) the photovoltaic performance of Cu2SnS3, Cu3SnS4, and Cu4SnS4 solar cells.

Rahaman S., D. V.G., Shenoy Y.M., Krishna V.N., Shekhar S., Shaik H.

Copper Tin Sulphide is a leading prospect in thin film hetero-junction solar cells owing to its germane electrical and optical properties. It is a ternary direct band gap p-type amalgam with substantial potential significance in thin film solar cells. Its components are innocuous, copious in earth crust and inexpensive. This treatise seeks to delineate the deposition of CTS thin films on soda-lime glass substrate by a jejune sol-gel spin coating technique at distinct temperatures. The upshot of annealing temperatures on the development and properties of the films are examined by exploring their optical, structural, morphological properties using apposite characterization methods. XRD analysis avers genesis of cubic structure of CTS. SEM investigation reported adequate facet for solar cells. The optical quantifications reveals that the gap between the energy bands of the films decline from 1.61eV to 1.45eV after annealing. Suchlike energy gap magnitudes are optimal for semiconducting materials as an imbiber layer of thin film solar cells.

Kaphle A., Hari P.

In this study, we report results on coating silicon solar cells with undoped and cobalt-doped zinc oxide (5%, 10%, 15%, and 20%) nanoparticles ranging in diameter from 46 nm to 87 nm, synthesized by a simple low-temperature precipitation method. The morphology and structure of nanoparticles have been characterized by transmission electron microscopy and X-ray diffraction spectroscopy. The average size of nanoparticles was found to increases with the increase in cobalt concentration. Doped and undoped zinc oxide thin films were deposited by a simple spin coating technique on silicon solar cells. Optical properties of the cobalt-doped zinc oxide layer have been investigated. We estimated the thin film stress by adding nanoparticles layers and found that the stress is lowest for the 139 nm cobalt-doped zinc oxide layer. Bandgap was found to increase with an increase in zinc oxide nanoparticle thin film thickness. Photoluminescence spectra show a blue shift in the near band edge emission peak with increase in thickness. Current-Voltage measurement confirms that there is an enhancement in power conversion efficiency as the thickness of zinc oxide nanoparticles varied from 58 nm to 139 nm. In general, the conversion efficiency shows an increasing trend up to 139 nm of thickness in silicon solar cells coated with doped and undoped zinc oxide nanoparticles. In addition, there is an enhancement of external quantum efficiency with an increase in thickness of zinc oxide layer. The optimal thickness of nanostructured zinc oxide film layer to enhance the efficiency of the silicon photovoltaic cell was experimentally determined to be 139 nm.

Ravi Dhas C., Jennifer Christy A., Venkatesh R., Panda S.K., Subramanian B., Ravichandran K., Sudhagar P., Moses Ezhil Raj A.

CuInGaS2 (CIGS) thin films were coated using nebulizer spray technique for different solvent volumes (10, 30, 50 and 70 ml) at the substrate temperature of 350 °C. The structural, optical and electrical properties were studied for the prepared CIGS thin films. CIGS thin films exhibited tetragonal structure and the maximum crystallite size was calculated for the film deposited using 50 ml solvent volume. The surface morphology of CIGS thin films was analyzed from scanning electron microscopy and atomic force microscopy studies. The electrical parameters of CIGS thin films such as resistivity, carrier concentration and mobility were examined using four probe method and Hall measurements. Electrocatalytic activities of the CIGS films towards redox couple (I−/I3−) were analyzed by cyclic voltammograms, electrochemical impedance spectroscopy, and Tafel polarization measurements. The high photocurrent efficiency was obtained for the CIGS counter electrode prepared using 50 ml solvent volume.

Maheskumar V., Gnanaprakasam P., Selvaraju T., Vidhya B.

The development of non-precious metal based electrocatalysts is essential for Hydrogen Evolution Reaction (HER) from the splitting of water. Recently copper tin sulphide (CTS) has gained attention due to its favourable properties in photovoltaic devices and photocatalysis. In this work, CTS has been explored as an electrocatalyst in HER. Cu 2 SnS 3 (CTS (A)) and Cu 4 SnS 4 (CTS (B)) are prepared by a simple solvothermal method. From the XRD analysis, tetragonal and orthorhombic phase is confirmed for CTS (A) and CTS (B), respectively. Morphology and composition of the prepared samples has been confirmed by SEM and EDAX analysis. Optical study reveals that the band gap energy is around 1.55 and 1.20 eV for CTS (A) and CTS (B), respectively. From photoluminescence studies, CTS (B) has been observed to have a greater recombination of charge carriers than CTS (A). The prepared materials were tested and compared for its performance as electrocatalysts for HER, where the current density of CTS (A) is higher than that of CTS (B). Furthermore, the calculated Tafel slope was 98 mV/dec and 110 mV/dec for CTS (A) and CTS (B), respectively. On the other hand, CTS (A) and CTS (B) have showed good stability even after 500 cycles in acidic medium.

Robles V., Trigo J.F., Guillén C., Herrero J.

Cu x SnS y thin films with thicknesses of 1.5 μm and x atomic ratios ranging between 3 and 4 have been prepared by co-evaporation at substrate temperatures varied from 150 °C to 450 °C and at evaporated copper rates of 10⿿16 nm/min. The evolution of the structural, chemical, optical and electrical properties as a function of the deposition conditions has been analyzed for the various samples as-grown and after annealing at 500 °C in sulfur atmosphere. The layers prepared at T sus  ⿤ 350 °C have shown a mixture of hexagonal CuS and cubic Cu 2 SnS 3 phases, which reacted by the annealing to give orthorhombic Cu 3 SnS 4 (for r Cu  = 10 nm/min) or orthorhombic Cu 4 SnS 4 (for r Cu  = 16 nm/min). The increment of the substrate temperature to 450 °C allowed also the crystallization of orthorhombic Cu 3 SnS 4 , coexisting with a secondary CuS phase for r Cu  = 16 nm/min. The application of a chemical treatment in KCN solution has been effective to remove the copper sulfide excess in the samples. The orthorhombic Cu 3 SnS 4 has evidenced a gap energy of 1.4 eV and an electrical resistivity of 7.9 y 10 ⿿4  Ω cm, whereas the orthorhombic Cu 4 SnS 4 has shown a lower gap energy of 1.2 eV and a higher electrical resistivity of 8.2 y 10 ⿿2  Ω cm.

Wu S., Chang C., Chen H., Shih C., Wang Y., Li C., Chan S.

A method for fabricating high-efficiency Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is presented, and it is based on a non-explosive, low-cost, and simple solution process followed by a two-step heat treatment. 2-Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as-coated Cu2ZnSnS4 (CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as-coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void-free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine-grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain-to-grain transport of carriers and consequently the short-circuit current (Jsc) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open-circuit voltage was 450 mV, the short-circuit current was 36.5 mA/cm2, and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.

Lui T., Hung F., Lui T., Chen K.

Cu2Sn3S7(CTS) can be used as the light absorbing layer for thin-film solar cells due to its good optical properties. In this research, the powder, baking, sulfur, and sintering (PBSS) process was used instead of vacuum sputtering or electrochemical preparation to form CTS. During sintering, Cu and Sn powders mixed in stoichiometric ratio were coated to form the thin-film precursor. It was sulfurized in a sulfur atmosphere to form CTS. The CTS film metallurgy mechanism was investigated. After sintering at 500°C, the thin film formed the Cu2Sn3S7phase and no impurity phase, improving its energy band gap. The interface of CTS film is continuous and the formation of intermetallic compound layer can increase the carrier concentration and mobility. Therefore, PBSS process prepared CTS can potentially be used as a solar cell absorption layer.