Preparation of thin films by sulfurizing sol–gel deposited precursors

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing precursors deposited by the sol–gel method. Copper (II) acetate monohydrate, zinc (II) acetate dihydrate and tin (II) chloride dihydrate were used as the starting materials of the sol–gel method, and 2-methoxyethanol and monoethanolamine were used as the solvent and the stabilizer, respectively. The solution was spin coated on soda lime glass substrates and dried at . The coated glasses were sulfurized by annealing at in a hydrogen sulfide-containing atmosphere. The annealed thin films showed X-ray diffraction peaks attributed to the single phase CZTS. The chemical composition of the films was almost stoichiometric and the band gap energy was at room temperature.     

Optimized fabrication of sputter deposited Cu2ZnSnS4 (CZTS) thin films

An effective synthesis strategy is employed to fabricate Cu₂ZnSnS₄ (CZTS) thin films using radio frequency (rf) magnetron sputtering technique on soda lime glass substrates. The as-grown films are annealed at different temperatures ranging between 350 and 550 °C, and their chemical compositions and structural properties are investigated. The as-grown films have poor Cu/(Zn + Sn) ratios ranging between 0.39 and 0.44 and S/(Cu + Zn + Sn) ratios ranging between 0.97 and 1.21. The Cu/(Zn + Sn) ratio is improved by growing a thin additional Cu-capping layer on the as-grown film followed by annealing. An improved Cu/(Zn + Sn) ratio of ∼0.71 is obtained and the S/(Cu + Zn + Sn) ratio is slightly reduced to ∼0.85. The formation of a kesterite type CZTS is confirmed using X-ray diffraction and Raman spectroscopy measurements. Absorption measurements and band-gap energy determination of the CZTS films are carried out in order to confirm applicability to solar cells.     

A review on growth optimization of spray pyrolyzed Cu2ZnSnS4 chalcogenide absorber thin film

The progress of solar cell technology in the development of clean and economic quaternary compound copper zinc tin sulfide (CZTS)‐based absorber thin films using the spray pyrolysis technique are presented in this review. CZTS (Cu₂ZnSnS₄) is the only potential competitor for the existing solar thin film absorbing materials owing to its environment‐friendly Earth abundant constituents. Even though different nonvacuum thin film technologies have been developed for the large area fabrication of this nontoxic absorber material, spray pyrolysis technique offers more versatility in changing the process parameters which has a direct impact on the cell efficiency. It can be used for depositing a wide variety of materials even with complex composition with good crystallinity, and the method has the advantage of being flexible and straightforward to design and can be quickly adopted for extensive area deposition. A survey on the effects of experimental conditions as well as the nature of precursors on the structural, morphological, electrical, and optical properties on the spray pyrolyzed CZTS thin films is discussed in detail. This analysis certainly could provide a potential to obtain new insights in the fabrication of high‐efficiency CZTS‐based solar cells and to launch it into the commercial market to satisfy the ever‐growing future energy demand.     

电沉积法制备铜锌锡硫薄膜概述

Cu_2ZnSnS_4(CZTS)为锡黄锡矿结构的四元化合物,其禁带宽度为1.45 e V,与半导体太阳能电池所要求的最佳禁带宽度(1.5 eV)十分接近;该材料与目前在薄膜太阳能电池领域表现出色的黄铜矿结构的CIGS(铜铟镓硒)材料具有相似的晶体结构,且CZTS有着很好的光电性能,组成元素在地球上含量丰富,安全无毒和环境友好,因而成为太阳能电池吸收层的最佳候选材料之一。介绍了Cu_2ZnSnS_4(CZTS)薄膜材料的结构特性和光学特性,总结了电化学沉积方法制备CZTS的研究现状。最后对CZTS目前存在的挑战和今后的研究重点进行总结并展望了将来可能的突破方向。     

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.     

Rapid microwave synthesis of Cu2ZnSnS4 nanocrystals for photovoltaic junctions

Copper zinc tin sulfide ( CZTS) nanocrystal (NC) ink was prepared using thiourea as sulphur source by microwave assisted process. Thin films fabricated by doctor blading technique were then used to analyze the structure and morphology of the CZTS NCs. Variation in the properties of the NCs with varying Zn content in the precursor solution was studied. A Zn/Sn ratio of 1.4 with a Cu/(Sn + Zn) ratio of 0.8 in the starting solution was identified as an optimum compositional ratio to get good optoelectronic properties. Synthesis of CZTS NCs was done in ethylene glycol solvent and in a solvent which is a 1:1 mixture of isopropanol and poly ethylene glycol. The films developed from the latter ink showed better morphological and optoelectronic properties. CZTS thin films show absorption coefficient of the order of 10⁴ cm^?1 and optical band gap of 1.5 eV. The electrical resistivity was found to be 2.5 × 10² Ω/cm and hole mobility 0.051 cm²/ (Vs). Glass/FTO/CdS/CZTS multilayer structures were fabricated to form P‐N junctions. A knee voltage of 0.8 V from the I‐V characteristics of the PN junction indicates that a good V_oc can be expected from a solar cell constructed with CZTS ink as absorber layer.     

Effect of oleylamine on microwave synthesized Cu2ZnSnS4 nanocrystals for photovoltaic junctions

Cu₂ZnSnS₄ (CZTS) with its high absorption coefficient, optimal band gap, and non‐toxic, earth‐abundant elemental constituents is a promising absorber material for low cost and high‐performance photovoltaics. CZTS nanocrystals were prepared by microwave‐assisted thermolysis method using ethylene glycol solvent in a rapid and energy efficient process with and without oleylamine as additional capping agent. X‐ray diffraction data indicated crystallite sizes of 5 and 12 nm respectively. Raman spectra showed the formation of pure Cu₂ZnSnS₄ phase in both cases. With pure ethylene glycol solvent, field emission scanning electron microscopy images showed particle sizes around 250 nm, ultraviolet‐visible absorption data gave optical band gap of 1.52 eV and Hall measurements yielded electrical resistivity ~70 Ωcm, carrier density ~2.7 × 10¹⁶/cm³, and hole mobility ~3.24 cm²/Vs. Adding optimum amount of oleylamine reduced the particle size to 100 nm and lower. The optical band gap reduced to 1.48 eV and electrical resistivity, carrier concentration, and Hall mobility changed to ~5.9 × 10⁴ Ωcm, 2.57 × 10¹²/cm³, and 4.1 cm²/Vs respectively. A knee voltage of ~0.8 V shown by CZTS/CdS p‐n junction indicated that a good V_oc can be expected from a solar cell constructed with the CZTS nanocrystals as absorber layer.     

Phase transformation of solution‐based p‐type Cu2ZnSnS4 thin film: applicable for solar cell

In this present work, quaternary Cu₂ZnSnS₄ thin films were deposited on commercial glass substrates at room temperature by a novel solution growth dip coating technique. The influence of annealing temperature of the films at 300 °C in a hot air furnace without the presence of any inert gas, on structural, optical, and electrical properties was investigated and discussed. The structural analyses were analyzed by X‐ray diffraction and Raman spectroscopy, whereas optical and electrical properties were analyzed by means of ultra violet infrared (UV‐ViS/IR). The results analyzed showed that there exists a phase formation from orthorhombic to kesterite crystal structure with an increase in optical bandgap and an optical conductivity, with an increase in annealing temperature. The electrical conductivity was observed of the order of 10^?6 ohm cm^?1. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhanced hydrogen generation at designed heterojunctions of Cu2ZnSnS4-rGO-MoS2 through interface engineering

Here, we are reporting interface engineering to create designed heterojunctions in Cu₂ZnSnS₄-rGO-MoS₂ (CZTS-reduced graphene oxide-MoS₂), in which abundant high density of nanoscale interfacial contacts are formed. It has been achieved via two-step optimized electrodeposition approach. Further, as-prepared materials have been characterized by microscopic, spectroscopic and electroanalytical techniques. Finally, the concept of generation of designed heterojunctions and synergetic effect were tested for electrocatalytic and photoelectrocatalytic hydrogen generation. The electron-hole pair recombination has been reduced, since the fast transport of photoexcited electrons of CZTS to MoS₂ through reduced graphene oxide interfacial contacts. Further, the synergetic effect of increased charge separation between rGO and more catalytically active sites from MoS₂ has been attributed to enhanced hydrogen generation.     

Template synthesis of porous hierarchical Cu2ZnSnS4 nanostructures for photoelectrochemical water splitting

Environmentally friendly and low-cost Cu₂ZnSnS₄ (CZTS) is a promising light absorber for photoelectrochemical (PEC) hydrogen production from water splitting due to the earth-abundant elements, high absorption coefficient, and narrow bandgap. Herein, the hierarchical CZTS film with porous nanostructures was successfully synthesized by a template method. The hierarchical CZTS film was composed of flower-like particles, which were assembled with thin CZTS nanosheets. Macropores were generated owing to the aggregation of flower-like spheres, and mesopores were formed from the stacking of CZTS nanosheets. Compared to the dense CZTS film, the porous hierarchical CZTS film showed a much higher PEC property for water splitting. The improved performance could be attributed to three merits of the porous hierarchical morphology: enhanced light absorption, improved charge separation and transfer, and enlarged electrochemically active surface area. This study provides a useful idea to design efficient semiconductor photoelectrodes for water splitting with delicately controlled morphology.     

Preparation and characterization of Cu(In,Ga)(Se,S)2 thin films from electrodeposited precursors for hydrogen production

Semiconducting Cu(In,Ga)(Se,S)₂ thin films were made from electrodeposited Cu(In,Ga)Se₂ precursors, followed by physical vapor deposition of In₂S₃, Ga, and Se. The bandgaps of these materials were found to be between 1.6 and 2.0 eV, which spans the optimal bandgap necessary for application for the top junction in photovoltaic multijunction devices and for unassisted water photolysis. These films were characterized by electron-probe microanalysis, scanning Auger spectroscopy, X-ray diffraction, and photocurrent spectroscopy.     

Preparation and properties of CuInS2 thin film prepared from electroplated precursor

Thin CuInS₂ films were prepared by sulfurization of Cu/In bi-layers. First, the precursor layer was electroplated onto the treated surface of Mo-coated glass. Observation of the cross-section prepared by focused ion beam (FIB) etching revealed that the void-free film was initially formed on the top surface of the precursor layer and continued to grow until the advancing front of the film reached the Mo layer. The nucleation of voids near the bottom of the CuInS₂ film followed. To determine whether the condition of the Cu/In alloy influences the CuInS₂ quality we investigated the Cu/In alloy state using FIB. We found that the annealed precursor of low Cu/In ratio (1.2) has several voids in the mid position in the layer compared with Cu-rich precursor (1.6). The cross-sectional view of the Cu-rich absorber layer is uniform compared with the low copper absorber layer. Thin film solar cells were fabricated using the CuInS₂ film (Cu/In ratio: 1.2) as an optical absorber layer. It was found that the optimization of a sulfurization period is important in order to improve the cell efficiency. We have not yet obtained good results with high Cu-rich absorber because of a blister problem. This blister was found before sulfurization. So, we are going to solve this blister problem before sulfurization.     

Simulation and optimization of CdS-n/Cu2ZnSnS4 structure for solar cell applications

In this work, the performance of solar cell based on CdS-n/Cu₂ZnSnS₄-p hetero-junction is numerically simulated. The aim of the study is to investigate the influence of thickness, defects density and bandgap energy of absorber layer CZTS and the thickness of the buffer layer CdS of the solar cell on electrical parameters J_sc, V_oc, FF and efficiency η of the cell. The results of our simulation allowed us to optimize the parameters above mentioned in order to get the best efficiency at the optimal band gap which corresponds to the maximum of the solar spectrum with optimal values of the electrical performances of the cell. This results lead to develop CZTS solar cells with high efficiency and low cost and give a help full indication for fabrication process.     

Enhancement of photoconversion efficiency and light harvesting ability of TiO2 nanotube-arrays with Cu2ZnSnS4

We report an underpotential deposition (UPD) route of Cu₂ZnSnS₄/TiO₂ nanotube arrays (TiO₂-NTAs) in which the Kesterite (Cu₂ZnSnS₄) was employed as a sensitizer to enhance the photoconversion efficiency of the TiO₂-NTAs. Cu₂ZnSnS₄ was simultaneously coated on TiO₂-NTAs by depositing its constituent metals from the precursor ions via electrochemical atomic layer deposition (EC-ALD) and subsequent annealing. The detailed synthesis process, the surface morphology, crystalline structure, photoelectrochemical properties and hydrogen production rate of the as-prepared Cu₂ZnSnS₄/TiO₂-NTAs were discussed. Thickened TiO₂ nanotubes were observed, suggesting that the Cu₂ZnSnS₄ coating was about 5 ± 0.5 nm. The results showed that the light harvesting of TiO₂-NTAs has an obvious improvement after sensitizing them with Cu₂ZnSnS₄. In comparison with pure TiO₂-NTAs, a two-fold increment in photoconversion efficiency was achieved using the composite of Cu₂ZnSnS₄/TiO₂-NTAs. The novel photoanode of CZTS/TiO₂ NTAs achieved a maximum hydrogen generation rate of 49 ml h^?1 cm^?2.     

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.     

The formation of CuInSe2 thin film solar cell absorbers from electroplated precursors with varying selenium content

We present results from real-time X-ray diffraction experiments on the formation of CuInSe₂ solar cell absorbers by annealing precursors, produced by simultaneous electrodeposition of copper, indium and selenium. The investigations reveal, that a reduced amount of electrochemically deposited selenium is the decisive parameter in order to realise a chalcopyrite formation behaviour as observed for sputtered stacked elemental layer (SEL) precursors. A simultaneous electrodeposition of the elements copper, indium and selenium in the molar ratio 1:1:2 of the chalcopyrite CuInSe₂ leads to the formation of binary copper and indium selenides during the electrodeposition process. The existence of binary selenides besides the intermetallic phase Cu₁₁In₉ as initial phases leads to an unfavourable absorber morphology. This can be explained by the observed semiconductor formation mechanism. A reduction of the deposited amount of selenium favours the formation of the intermetallic compound Cu₁₁In₉ and reduces the amount of binary selenides. These precursors show a formation behaviour and resulting absorber morphology as known for sputtered SEL precursors.     

Preparation and some semiconducting properties of CuInSe2 thin films

Thin films of CuInSe₂ were prepared by d.c. sputtering and vacuum evaporation from the synthesized bulk material. It is usual for p-type films to be obtained by d.c. sputtering. Both p- and n-type vacuum-evaporated films are obtained by controlling selenium vapor pressure or substrate temperature. The vacuum-evaporated films show a fundamental absorption similar to that of the bulk crystals. The obvious shift of the fundamental absorption edge is observed for p-type evaporated films depending upon the hole concentrations. Photoconductivities are observed for the n-type evaporated films. Some samples show high photoresponse beyond the fundamental absorption edge.     

Preparation and characterization of chemically deposited CuInS2 thin films

The structural and electrical properties of thin films of CuInS₂ prepared by the chemical deposition technique are described. The composition of the polycrystalline films produced deviate from the ideal composition as shown by energy dispersive X-ray spectrometer (EDS) analysis. Transmission electron diffraction (TED) and X-ray powder analyses showed that the films were single phase with chalcopyrite structure. The films were p-type and the resistivities were in the range of 0.1 to 10 μ cm. Influence of deposition parameters on the surface morphology was observed by SEM.     

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.