The influence of hydrogen in the incorporation of Zn during the growth of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a p-type semiconductor, candidate to replace Cu(In,Ga)Se₂ as absorber layer in thin film solar cells. The best solar cells based on CZTS present efficiencies up to 6.8%. These results were improved when metallic Zn was replaced by ZnS, which may imply a different chemical path for the formation of CZTS. In this study it is compared with the diffusion of Zn on Cu₂SnS₃ by introducing metallic Zn or ZnS. For this CZTS films were grown by sulphurization of Cu₂SnS₃, some with a Zn layer and others with a ZnS layer. The influence of H₂ during the annealing process is also studied and for this some sulphurizations were done in the presence of a partial atmosphere of H₂.The SEM micrographs of the samples show a columnar growth structure of the films with different degrees of compactness. The compactness is improved in the samples where a ZnS layer was present in the precursor and the sulphurization was done in the presence of H₂. EDS chemical profiling revealed regular zinc distribution for the samples with metallic Zn whilst the ones with ZnS exhibited a Zn-rich surface. X-ray diffraction (XRD) indicated the presence of CZTS and Cu_2−xS phases in all samples. These results were confirmed by Raman scattering.It was concluded that the sulphurization of Cu₂SnS₃ films with the use of ZnS layers under H₂ atmosphere produces better quality CZTS thin films, since it promotes Zn diffusion and avoids Zn losses by evaporation.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

Comparative study of Cu2ZnSnS4 film growth

We fabricated Cu₂ZnSnS₄ (CZTS) thin films using two different methods, spray pyrolysis and sulfurization of Cu-Zn-Sn metallic films. Spray pyrolysis was carried out under air ambient with modified ultrasonic spray system. Sulfurizations of metallic Cu-Zn-Sn films were done for stacked metallic films, Cu/Sn/Zn/glass, Cu/Sn/Cu/Zn/glass and Sn/Cu/Zn/glass, which were prepared by sputtering method in high vacuum chamber. The sprayed films were not observed to be grown well with good crystallinity, compared with CZTS films made by sulfurization of stacked metallic films. However, it was found that application of additional sulfurization to sprayed CZTS films induced great improvement of crystallinity to the level of the sulfurized metallic films. This implicates that spray pyrolysis with additional sulfurization is a good method for fabrication of CZTS films, especially as a low-cost fabrication technique. All CZTS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy measurements.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Preparation of CuGaS2 thin films by two-stage MOCVD method

Copper gallium disulfide (CuGaS₂; CGS) films were deposited on glass or ITO glass by two-stage metal-organic chemical vapor deposition (MOCVD) method, using Cu- and Ga/S-containing precursors without toxic H₂S gas. First, pure Cu thin films were prepared on glass substrates by using a single-source precursor, bis(ethylbutyrylacetato)copper(II) or bis(ethylisobutyrylacetato)copper(II). Second, the resulting Cu films were processed using tris(N,N-ethylbutyldithiocarbamato)gallium(III) at 410-470 °C to produce CuGaS₂ films. The optical band gap of the CGS film grown at 440 °C was about 2.53 eV. In addition, it was found that the elemental ratio of Cu and Ga elements of the CGS films can be elaborately adjusted by controlling deposition conditions on demand.     

Synthesis and characterization of co-electroplated Cu2ZnSnS4 thin films as potential photovoltaic material

Cu₂ZnSnS₄ thin films have been successfully prepared by a novel synthesis process that involves a single step deposition of Cu₂ZnSnS₄ followed by a post-annealing treatment at 550 °C for 60 min in the atmosphere of N₂+H₂S (5%). The microstructure, morphology, composition and optical property of the film have been investigated in detail. It is found that the Na₂S₂O₃5H₂O concentration in the solution has a significant effect on the Cu₂ZnSnS₄ thin films. X-ray diffraction data indicates that the annealed Cu₂ZnSnS₄ thin films have a kesterite structure with preferred orientation along the (1 1 2) plane. Uniform and compact topographies are observed in some annealed films. From the energy dispersive X-ray spectroscopy analysis, it can be seen that Cu-poor and Zn-rich Cu₂ZnSnS₄ thin films have been obtained. The direct band gap energy of the film is about 1.5 eV.     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

Effect of sodium diffusion on the structural and electrical properties of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a kesterite semiconductor consisting of abundantly available elements. It has a band gap of 1.5 eV and a large absorption coefficient. Hence, thin films made of this material can be used as absorber layers of a solar cell. CZTS films were deposited on soda lime and Na free borosilicate glass substrates through Ultrasonic Spray Pyrolysis. The diffusion of sodium from soda lime glass was found to have a profound effect on characteristics like grain size, crystal texture and conductivity of CZTS thin films. Copper ion concentration also varied during the deposition and it was observed that the carrier concentration was enhanced when there was a deficiency of copper in the films. The effect of sodium diffusion and copper deficiency in enhancing the structural and electrical properties of CZTS films are presented in this paper.     

Influence of H2S concentration on the properties of Cu2ZnSnS4 thin films and solar cells prepared by sol-gel sulfurization

Cu₂ZnSnS₄ (CZTS) thin films prepared by a non-vacuum process based on the sulfurization of precursor coatings, consisting of a sol-gel solution of Cu, Zn, and Sn, under H₂S+N₂ atmosphere were investigated. The structure, microstructure, and electronic properties of the CZTS thin films as well as solar cell parameters were studied in dependence on the H₂S concentration. The sulfurization process was carried out at 500 °C for 1 h in an H₂S+N₂ mixed-gas atmosphere with H₂S concentrations of 3%, 5%, 10%, and 20%. As the H₂S concentration decreased from 20% to 5%, the S content of the CZTS thin films decreased. However, when the H₂S concentration was decreased below 3%, the S content of the films began to increase. A CZTS thin film prepared with an H₂S concentration of 3% had grains in the order of 1 μm in size, which were larger than those of films prepared at other H₂S concentrations. Furthermore, the most efficient solar cell, with a conversion efficiency of 2.23%, was obtained from a sample sulfurized at an H₂S concentration of 3%.     

p-Type CuSbS2 thin films by thermal diffusion of copper into Sb2S3

We report the preparation of copper antimony sulfide (CuSbS₂) thin films by heating Sb₂S₃/Cu multilayer in vacuum. Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on well cleaned glass substrates. A copper thin film was deposited on Sb₂S₃ film by thermal evaporation and Sb₂S₃/Cu layers were subjected to annealing at different conditions. Structure, morphology, optical and electrical properties of the thin films formed by varying Cu layer thickness and heating conditions were analyzed using different characterization techniques. XRD analysis showed that the thin films formed at 300 and 380 °C consist of CuSbS₂ with chalcostibite structure. These thin films showed p-type conductivity and the conductivity value increased with increase in copper content. The optical band gap of CuSbS₂ was evaluated as nearly 1.5 eV.     

Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol-gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency

The properties of Cu₂ZnSnS₄ (CZTS) thin films deposited by sol-gel sulfurization were investigated as a function of the chemical composition of the sol-gel solutions used. The chemical composition ratio Cu/(Zn+Sn) of the sol-gel solution was varied from 0.73 to 1.00, while the ratio Zn/Sn was kept constant at 1.15. CZTS films deposited using sol-gel solutions with Cu/(Zn+Sn)<0.80 exhibited large grains. In addition, the band gaps of these Cu-poor CZTS thin films were blue shifted. Solar cells with the structure Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass were fabricated under non-vacuum conditions. The solar cell with the CZTS layer deposited using the sol-gel solution with Cu/(Zn+Sn)=0.80 exhibited the highest conversion efficiency of 2.03%.     

Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄, a potential candidate for application as absorber layer in thin film solar cells, were successfully deposited on soda lime glass substrates using spray pyrolysis and the effect of variation of precursor on the structural and opto-electronic properties was investigated. We used stannous as well as stannic chloride as precursors of tin in the spray solution. All the films exhibited kesterite structure with preferential orientation along the (1 1 2) direction. But crystallinity and grain size were better for stannic chloride based films. Also they possessed a direct band gap of 1.5 eV and the absorption coefficient was >10⁴ cm⁻¹. Carrier concentration and mobility could be enhanced and the resistivity reduced by two orders by using stannic chloride in spray solution. Junction trials were performed with CZTS films prepared using stannic chloride precursor as the absorber layer and indium sulfide as the buffer layer. XPS depth profiling of the junction was done. Formation of CZTS could be confirmed and also information about the junction interface could be obtained from the XPS results. We obtained an open-circuit voltage of 380 mV and short-circuit current density of 2.4 mA/cm².     

Development of thin film solar cell based on Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (hereafter CZTS) thin films were successfully formed by vapor-phase sulfurization of precursors on a soda lime glass substrate (hereafter SLG) and a Mo-coated one (hereafter Mo-SLG). From the optical properties, we estimate the band-gap energy of this thin film as 1.45–1.6 eV which is quite close to the optimum value for a solar cell. By using this thin film as an absorber layer, we could fabricate a new type of thin film solar cell, which was composed of Al/ZnO:Al/CdS/CZTS/Mo-SLG. The best conversion efficiency achieved in our study was 2.62% and the highest open-circuit voltage was 735 mV. These device results are the best reported so far for CZTS.     

TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Nanostructured TiO₂ thin films were deposited on glass substrates by sol-gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV-vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 ? and c = 9.5172 ?, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO₂. The room temperature electrical resistivity in the dark was of the order of 4.45 × 10⁶ ohm-cm. Photocatalytic activity of the TiO₂ films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substrates

The effects of different annealing conditions such as atmospheres, temperatures and times on the structural, morphological and optical properties of ZnS thin films prepared on ITO coated glass substrates by chemical bath deposition were studied. Aqueous solutions of zinc acetate and thiourea were used as precursors along with stable complexing agents, such as Na₂EDTA and Na₃-citrate, in an alkaline medium. X-ray diffraction patterns showed that the as-deposited and as-annealed ZnS films had an amorphous structure or poor crystallinity below the optimized annealing conditions of 500 °C and 60 min with the exception of the films annealed in N₂+H₂S annealing atmosphere. The ZnS thin films annealed in N₂+H₂S atmosphere for 1 h at 500 °C showed three sharp peaks for the (1 1 1), (2 2 0) and (1 1 3) planes of polycrystalline cubic ZnS without any unwanted secondary ZnO phases. X-ray photoelectron spectroscopy revealed Zn-OH and Zn-S bonding in the as-deposited ZnS thin film. However, the ZnS thin films annealed at 500 °C showed Zn-S bonding regardless of the annealing atmosphere. The sharp absorption edge and band gap energy of the as-deposited and as-annealed ZnS thin films varied from 295 to 310 nm and 3.5 to 3.89 eV, respectively.     

Preparation of Cu(In,Ga)(S,Se)2 thin films by sequential evaporation and annealing in sulfur atmosphere

Cu(In,Ga)(S,Se)₂ thin films with high Ga/III ratio (around 0.8) were prepared by sequential evaporation from CuGaSe₂, CuInSe₂, In₂Se₃ and Ga₂Se₃ compounds and then annealing in H₂S gas atmosphere. The annealing temperature was varied from 400 to 500 °C. These samples were characterized by means of XRF, EPMA, XRD and SEM. The S/(S+Se) mole ratio in the thin films increased with increase in the annealing temperature, keeping the Cu, In and Ga contents nearly constant. The open circuit voltage increased and the short circuit current density decreased with increase in the annealing temperature. The best solar cell using Cu(In,Ga)(S,Se)₂ thin film with Ga/(In+Ga)=0.79 and S/(S+Se)=0.11 annealed at 400 °C demonstrated V_oc=535 mV, I_sc=13.3 mA/cm², FF=0.61 and efficiency=4.34% without AR-coating.     

Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process

Cu₂ZnSnS₄ thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The Cu/(Zn+Sn) ratio of the thin film sputtered at 75 W was close to the stoichiometry of Cu₂ZnSnS₄. However, the S/(Cu+Zn+Sn) ratio was less than the stoichiometry. The as-deposited films were amorphous and annealed in the atmosphere of Ar+S₂ (g). The annealed (1 1 2), (2 0 0), (2 2 0), (3 1 2) planes were conformed to all the reflection of a kesterite structure. A preferred (1 1 2) orientation was observed with the increase of the annealing temperature. The optical absorption coefficient of the thin film was about 1.0×10⁴ cm⁻¹. The optical band energy was derived to be 1.51 eV. The optical absorption coefficient of the sputtered Cu₂ZnSnS₄ thin films was less than that of CuInS₂ thin film, however, the band gap energy was more appropriate for photovoltaic materials.     

Synthesis,characterization, and photoelectrochemical study of Cd1−xZnxS solid solution thin films deposited by spray pyrolysis for water splitting

A series of Cd_1−xZn_xS thin films were deposited onto indium-doped tin oxide (ITO) coated glass substrates by ultrasonic spray pyrolysis CdCl₂, ZnCl₂, and CS(NH₂)₂ aqueous solutions. The XRD patterns revealed that these films processed a wurtzite structure and a series of solid solutions of CdS and ZnS formed. The lattice constants decreased as the x value increased. From the transmittance and reflectance, the optical band gap was estimated to be between 2.45 eV and 3.72 eV, and the band gap increased as the x value increased according to a near linear relationship with the x value. The Mott-Schottky tests revealed that the flat potential shifted negatively as the x value increased. The photo responses agreed with the optical absorption of these films quite well. The current–potential measurements under chopped Xe lamp light irradiation show that the CdS deposited at 300 °C had best photoresponse. Its photoelectrochemical efficiency was estimated to be about 0.95% under 0.73 V bias from two electrodes current–potential tests.     

Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E---B evaporated precursors

By sulfurization of E---B evaporated precursors, CZTS(Cu₂ZnSnS₄) films could be prepared successfully. This semiconductor does not consist of any rare-metal such as In. The X-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estimated the optical band gap energy as 1.45 eV. The optical absorption coefficient was in the order of 10⁴cm⁻¹. The resistivity was in the the order of 10⁴ Ω cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.