Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

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%.     

The influence of sodium on the properties of CuInS2 thin films and solar cells

The controlled incorporation of sodium into the absorber layer of CuInS₂ solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors. We also investigated the characteristics of the sodium-incorporated CuInS₂ films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS₂ was found to form mainly on the surface of each of the CuInS₂ films.     

Photoelectrochemical behavior of In2S3 formed on sintered In2O3 pellets

Microcrystals of In₂S₃ were formed on sintered In₂O₃ pellets by sulfurizing in H₂S atmosphere. The flat band potential of compound In₂S₃|In₂O₃ electrodes was evaluated as −1.0 V vs Ag|AgCl in 1 M KOH, 1 M Na₂S, 10⁻² M S. Significantly enhanced photocurrent was observed on compound In₂S₃|In₂O₃ electrodes with a lower degree of sulfurization to that of compound In₂S₃|In₂O₃ electrodes with higher degree of sulfurization. Photocurrent generation of compound In₂S₃|In₂O₃ electrodes was explained from the viewpoint of semiconductor sensitization.     

Electrodeposited CuInS2-based thin-film solar cells

CuInS₂ thin films were prepared by sulfurization of electrodeposited Cu–In precursors. Morphological improvement enabled us to fabricate the solar cells using electrodeposited Cu–In precursors. The photovoltaic property of a conversion efficiency of 1.3% was obtained.     

Sulfurization of Cu–In electrodeposited precursors for CuInS2-based solar cells

Cu–In electrodeposited layers were annealed using rapid thermal processing (RTP) in a reactive atmosphere containing sulfur vapors. The CuInS₂ formation mechanism during sulfurization of electrodeposited precursors proceeds mainly through direct sulfurization of the metallic Cu–In alloy, forming spinel CuIn₅S₈ and chalcopyrite CuInS₂ ternary phases. During the heating step, the Cu–In metallic alloy gets richer in copper as the temperature increases and transforms from CuIn₂ to Cu₁₁In₉, then Cu₁₆In₉ and finally to Cu₇In₃. The use of rapidly cooled samples stopped after different durations of the process along with ex-situ XRD analysis enabled us to differentiate the Cu₁₆In₉ and Cu₇In₃ phases. Finally, the efficiency of the solar cells made with the two-step electrodeposition and RTP low-cost process reaches 11% (active area 0.421 cm²), which is close to the results obtained for cells made with PVD precursors.     

Effect of Ga incorporation in sequentially prepared CuInS2 thin film absorbers

Thin film CuInS₂:Ga solar cell absorber films were prepared by sequential evaporation of Cu–In–Ga precursors and sulfurization in sulfur vapor. The depth distribution of Ga was found to be highly inhomogeneous caused by CuGaS₂ phase segregation at the back contact. Depending on overall Ga content and sulfurization temperature a quaternary CuGa_xIn_1−xS₂ compound formed exhibiting a shift in absorber lattice constant and band gap. Micro Raman measurements showed that crystal quality was also affected by Ga. Open-circuit voltages well above 800 mV were achieved while sustaining high fill factors of 71%.     

Structural and optical properties of sulfur-annealed CuInS2 thin films

CuInS₂ thin films were deposited by single source vacuum thermal evaporation method on substrates submitted to longitudinal thermal gradient. Some of these films were annealed in sulfur atmosphere and converted into CuInS₂ homogenous layers. Both of the as-deposited and sulfurated films were characterized by X-ray diffraction, optical transmission and reflection measurements. The optical band gap of films after sulfurization was 1.50 eV which is near the optimum value for photovoltaic energy conversion.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

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.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

Double layer CuInS2 absorber using spray pyrolysis: A better candidate for CuInS2/In2S3 thin film solar cells

In the fabrication of CuInS₂/In₂S₃ solar cell using chemical spray pyrolysis (CSP) deposition technique, one of the major problems is the diffusion of Cu towards the In₂S₃ layer affecting stability and repeatability of the CuInS₂/In₂S₃ cells. In order to ensure a Cu-free In₂S₃ layer, a ‘double layer structure’ of CuInS₂ film, having a Cu-rich first layer and In-rich second layer was deposited using manual CSP technique. In this paper, we present the difference in material properties of single and double layered CuInS₂ films and the results of characterisation of the junctions prepared using such films with β-In₂S₃ films. Better crystallinity as well as larger and densely packed grains were observed for the CuInS₂ films having ‘double layer structure’. Such samples also possessed two band gaps, which was not due to the presence of different phases, but due to the Cu-rich and Cu-poor layers. In addition, their low resistivity makes the double layered CuInS₂ film more beneficial for photovoltaic applications.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

Photoelectric characterization of Cu(In,Ga)S2 solar cells obtained from rapid thermal processing at different temperatures

CuInS₂-based solar cells have a strong potential of achieving high efficiencies due to their ideal band gap of 1.5 eV. A further increase in the efficiency is expected from doping the absorber film with gallium due to enlargement of the band gap (E_g) and correspondingly the open-circuit voltage (V_OC). We investigated Cu(In,Ga)S₂ solar cells obtained from stacked metal layers sputtered from In and (Cu,Ga) targets followed by rapid thermal processing (RTP) in sulfur vapor. Depending on the actual RTP temperature profile, the films might exhibit CuInS₂/CuGaS₂ (top/bottom) segregation, which is rather detrimental for a large V_OC. We found that only precursors sulfurized at sufficiently high temperatures exhibit the desired interdiffusion of the segregated CuInS₂/CuGaS₂ layers resulting in an increased V_OC. Moreover, temperature dependent current-voltage profiling (suns-V_OC-analysis) yielded strong indications for improved current collection and reduced losses for devices with proper interdiffusion of the CuInS₂/CuGaS₂ layers. A more fundamental question is related to the variation and formation of defect states in differently processed absorber films. The studied samples were thus further investigated by means of admittance spectroscopy, which allowed us to confirm the presence of three individual defect states in both absorber configurations. Two defects exhibit activation energies, which remain unchanged upon varying the RTP temperature whereas a third state exhibits significantly increased activation energy in devices showing interdiffusion of CuInS₂/CuGaS₂ layers. According to the characteristic shift of the conduction band edge upon Ga-doping we conclude that the latter defect level corresponds with the minority carriers in the p-type absorbers.     

Optical and photoelectrical properties of β-In2S3 thin films prepared by two-stage process

β-In₂S₃ films were grown on glass as well as on quartz substrates by rapid heating of metallic indium films in H₂S atmosphere. The effect of sulfurization temperature and time on the growth, structural, electrical and photoelectrical properties of β-In₂S₃ films has been investigated. Highly oriented single-phase β-In₂S₃ films were grown by the sulfurization technique. The morphology and composition of films have been characterized. The optical band gap of β-In₂S₃ is found to vary from 1.9 to 2.5 eV when the sulfurization temperature is varied from 300 to 600 °C or by increasing the sulfurization time. The electrical properties of the thin films have also been studied; they have n-type electrical conductivity. The photoelectrical properties of the β-In₂S₃ films are also found to depend on the sulfurizing temperature. A high photoresponse is obtained for films prepared at a sulfurizing temperature of 600 °C. β-In₂S₃ can be used as an alternative to toxic CdS as a window layer in photovoltaic technology.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

A novel one-step electrodeposition to prepare single-phase CuInS2 thin films for solar cells

Copper indium disulfide (CuInS₂) thin films have been successfully prepared on Ni substrates using a novel one-step potentiostatic electrodeposition combined with a potassium hydrogen phthalate (C₈H₅KO₄) complexing agent, accompanied by annealing at 350 °C. Electrodeposition in the solution of Cu and In salts and sodium thiosulfate (Na₂S₂O₃) containing an adequate concentration of C₈H₅KO₄ (e.g., [C₈H₅KO₄]=23 mM) provides thin films comprised of a CuInS₂ single phase as the bulk composition, without forming Cu_xS secondary phases. In addition to the effect on bulk-phase compositions, the adjustment of [C₈H₅KO₄] causes variation in morphology and atomic composition of the film surface. The surface states of the films change from the Cu-rich rough surface at low [C₈H₅KO₄] (15 mM) to the In-rich smooth surface at high [C₈H₅KO₄] (23 mM). The higher [C₈H₅KO₄] induces the grains constructing the film to interconnect and form a densely packed CuInS₂ film without voids and pinholes. The single-phase and void-free CuInS₂ film shows a band gap of 1.54 eV, satisfying the requirement of the absorber layers in solar cells. The electrical properties tests denote its n-type conductivity with a resistivity of 9.6×10⁻⁵ Ω cm, a carrier concentration of 2.9×10²¹ cm⁻³ and a carrier mobility of 22.2 cm² V⁻¹ s⁻¹.     

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.     

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.