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.     

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.     

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%.     

Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers

Optimization of substrate temperature of spray pyrolysed CuInS₂ absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS₂ thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS₂/In₂S₃ bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures.     

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.     

CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors

CuInS₂ thin-films were prepared by sulfurization of Cu---In---O precursors in H₂S gas. X-ray diffraction patterns showed that In₂O₃ phases did not remain in the CuInS₂ films sulfurized in a H₂S and H₂ atmosphere, whereas In₂O₃ phase remained in the films sulfurized in a H₂S and Ar atmosphere. The performance of CuInS₂ solar cells were studied as a function of the H₂ gas pressure during sulfurization. The open-circuit voltage, short-circuit current and fill factor increased with increasing the H₂ gas pressure. The conversion efficiency of the CuInS₂ solar cells is strongly affected by the reduction of the Cu---In---O precursors.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Single crystal growth and properties of γ-phase in the CuInSe2+2CdS⇔CuInS2+2CdSe reciprocal system

The intermediate solid solution, γ-phase, exists in the CuInSe₂+2CdS⇔CuInS₂+2CdSe reciprocal system. It crystallizes in the cubic structure and has a wide homogeneity range. Single crystals of the γ-phase are grown by a modified Bridgman method and their composition, crystal structure, optical and electrical properties are studied. The band gap varies from 1.43 to 1.05 eV along the ‘Cu₃Cd₂In₃S₈’-‘CuCd₂InSe₄’ compositional section. The crystals are photosensitive, mostly p-type, with hole concentrations in the 10¹⁵-10¹⁶ cm⁻³ range and mobilities up to 18 cm²/V s. The results indicate that the γ-phase can be considered as a new absorbing material for thin-film solar cells.     

Investigation of capacitance transients in CuInS2 due to ionic migration

Transport of mobile ions in n-TiO₂/n-CuInS₂/p-CuInS₂ thin-film devices is studied with the transient ion-drift (TID) method. In contrast to the normal TID method, a mobile ion profile is created in the CuInS₂ layer, which can be described by the Gouy-Chapman theory. Activation energies for diffusion of 0.5 and 1.0 eV are found. We postulate that these activation energies are related to the associated defect, ( In_Cu)^x, which introduces a deep electronic state inside the bandgap of CuInS₂. This defect can accept or release an electron and drift out of the depletion region. This will lower the concentration of recombination centers in the depletion region, which explains the self-healing property of CuInS₂.     

Photocurrent enhancement of wide bandgap Bi2O3 by Bi2S3 over layers

Sintered Bi₂O₃ pellets exhibited insulating properties at room temperature. Partial reduction of sintered Bi₂O₃ pellets increased the conductivity. Reduced Bi₂O₃ pellets exhibited n-type semiconductor properties. Microcrystals of Bi₂S₃ were formed on sintered Bi₂O₃ pellets by sulfurizing them in H₂S atmosphere. The direct band-gap and indirect band-gap of Bi₂S₃ were evaluated as 1.2 and 0.4 eV, respectively. A high incident photon to current conversion efficiency in the near IR region was observed on Bi₂S₃|Bi₂O₃ electrodes. Photocurrent generation of Bi₂S₃|Bi₂O₃ electrodes was explained from the viewpoint of semiconductor sensitization. The flat band potential of Bi₂S₃ was evaluated as −1.1 V vs. Ag|AgCl in aqueous polysulfide redox electrolyte (1 M OH⁻, 1 M S²⁻, 10⁻² M S).     

Some physical investigations on AgInS2 sprayed thin films

AgInS₂ thin films have been prepared on glass substrates by the spray pyrolysis process using an aqueous solution which contains silver acetate (AgCH₃CO₂), thiourea (SC(NH₂)₂) and indium chloride (InCl₃) as precursors. The depositions were carried out in the range of the substrate temperature from 260 to 420 °C. The value of the concentration ratio in the spray solution of indium and silver elements x=[Ag⁺]/[In³⁺] was varied from 1 to 1.5 with [In³⁺]=10⁻² M and [S²⁻]/[In³⁺] was taken constant, equal to 4. The structural study shows that AgInS₂ thin film, prepared at 420 °C using optimal concentration ratio x=1.3 crystallizes in the chalcopyrite phase with a strong (1 1 2) X-ray diffraction line. Moreover, microprobe analysis (EPMA) shows that a nearly stoichiometric composition is obtained for these experimental conditions. Indeed, the atomic percentage of elements were. 24.5, 25.0, 49.5 for Ag, In and S, respectively. On the other hand from transmission and reflectance spectra, the obtained band gap energy is 1.83 eV for such film.     

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.     

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.     

ZnO/CuInS2 core/shell heterojunction nanoarray for photoelectrochemical water splitting

ZnO/CuInS₂ core/shell nanorods array thin film was synthesized on conducting glass substrates for photoelectrochemical water splitting via a simple hydrothermal and cation exchange reaction, using ZnO nanorods array as reactive template. Uniform CuInS₂ films were obtained on the surface of ZnO nanorods, based on the ion-by-ion growth mechanism. The optical property of core/shell nanoarray was characterized, and enhanced absorption spectrum was observed. Hydrogen generation efficiency of 3.2% at 0.29 V versus saturated calomel electrode was achieved with synthesized ZnO/CuInS₂ core/shell nanoarray electrode due to the improved absorption and appropriate energy gap structure. The synthesized core/shell nanoarray has potential application in photoelectrochemical water splitting.     

Electrical and optical characterisation of CuInS2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS₂ were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The temperature dependence of the hole mobility in valence band is analysed by taking into account contributions from several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuInS₂ thin films, grainboundary conduction process was suggested. In the low temperature region, we interpret the data in terms of the Mott law and the analysis is very consistent with the variable range hopping. However, thermionic emission is predominant at high temperatures. Photoluminescence measurements have been performed on CuInS₂ crystals and the analysis has revealed that the emission is mainly due to free-to-bound and donor–acceptor pair transitions. The band gap of that compound is derived from the excitonic emission line at 1.53 eV.     

Electro deposited In2S3 buffer layers for CuInS2 solar cells

We report the electro deposition of In₂S₃ buffer layers for CuInS₂ solar cells. All materials and deposition conditions were selected taking into account environmental, economic and technological aspects of a potential transfer to large volume industrial production. Different bath compositions and electro deposition parameters were studied. The obtained films exhibited complete substrate coverage, confirmed by SEM and XPS. In/S ratio close to 2/3 was obtained. XPS measurements detected the presence of indium hydroxide, transforming into oxide upon anneal at 200 °C. Maximum photoelectric conversion efficiency of 7.1% was obtained, limited mainly by a low fill factor (51%). Further process optimization is expected to lead to efficiencies comparable to CdS buffers. So far, open-circuit voltages as high as 660 mV were demonstrated.     

Determination of Cu(In1−xGax)3Se5 defect phase in MBE grown Cu(In1−xGax)Se2 thin film by Rietveld analysis

Quantitative phase analysis of Cu(In_1−xGa_x)Se₂ (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ-2θ mode. Films were found to contain both stoichiometric Cu(In_1−xGa_x)Se₂ and defect related Cu(In_1−xGa_x)₃Se₅ phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In_1−xGa_x)Se₂ and I-42m for Cu(In_1−xGa_x)₃Se₅ phase. The effects of Ga/III (=Ga/In+Ga=x) ratio and Se flux during growth over the formation of Cu(In_1−xGa_x)₃Se₅ defect phase in CIGS was studied and the correlation between quantity of Cu(In_1−xGa_x)₃Se₅ phase and solar cell performance is discussed.     

Comparison of point defects in CuInSe2 and CuGaSe2 single crystals

Hall-effect and photoluminescence measurements have been carried out on as-grown and In/Ga-annealed CuInSe₂ and CuGaSe₂ single crystals grown by chemical vapor transport. Various defect levels in these related compounds have been identified and compared. V_Cu and V_Se show similar properties and activation energies in both materials. A tremendous difference is observed in the behavior of III_Cu antisite defects. Ga_Cu levels in CuGaSe₂ are much deeper than In_Cu in CuInSe₂, and furthermore, the formation of In_Cu is much easier compared to Ga_Cu. This is related to the higher formation energy of Ga_Cu in CuGaSe₂. Due to this difference in the defect chemistry of both compounds, it has not been possible until now, to prepare n-type CuGaSe₂ crystals, whereas CuInSe₂ is easily transformed from p- to n-type by annealing in vacuum or In-atmosphere.     

Characterization of new heterosystem CuFeO2/SnO2 application to visible-light induced hydrogen evolution

Photo-assisted H₂ evolution has been realized over the new heterosystem CuFeO₂/SnO₂ without any noble metal and was studied in connection with some physical parameters. The delafossite CuFeO₂ has been prepared by thermal decomposition from various salts. The polarity of generated voltage is positive indicating that the materials exhibit p-type conductivity whereas the electroneutrality is achieved by oxygen insertion. The plot of the logarithm (conductivity) vs. T⁻¹ gives average activation energy of 0.12 eV. CuFeO₂ is a narrow band gap semiconductor with an optical gap of 1.32 eV. The oxide was characterized photoelectrochemically; its conduction band (−1.09 V_RHE) is located below that of SnO₂ (−0.86 V_RHE) at pH ∼13.5 itself more negative than the H₂O/H₂ level leading to a thermodynamically favorable H₂ evolution under visible irradiation. The sensitizer CuFeO₂, working as an electron pump, is stable towards photocorrosion by hole consumption reactions involving the reducing agents X²⁻ (=S₂O₃²⁻ and SO₃²⁻). The photoactivity was dependent on the precursor and the best performance (0.026 ml h⁻¹ mg⁻¹) was obtained in S₂O₃²⁻ (pH ∼13.5) over CuFeO₂ synthesized from nitrate with a mass ratio (CuFeO₂/SnO₂) equal to unity. A quantum yield of 0.5% was obtained under polychromatic light. H₂ liberation occurs concomitantly with the oxidation of S₂O₃²⁻ to dithionate and sulfate. The tendency towards saturation, in a closed system, is mainly ascribed to the competitive reduction of the end product S₂O₆²⁻.     

High photocatalytic hydrogen production over the band gap-tuned urchin-like Bi2S3-loaded TiO2 composites system

A new system using Bi₂S₃-loaded TiO₂ photocatalysts (Bi₂S₃/TiO₂) was developed to enhance the production of hydrogen. The Bi₂S₃ (5, 10, 15 wt%) particles in an urchin-like morphology with a length of about 2∼3 μm and a diameter of 15–20 nm, which can absorb all wavelengths in UV–visible radiation, were prepared by solvothermal method and loaded onto nano-sized TiO₂ (10∼15 nm) for photocatalysis on hydrogen production. The evolution of H₂ from methanol/water (1:1) photo splitting over the Bi₂S₃/TiO₂ composite in the liquid system was enhanced, compared with that over pure TiO₂ and Bi₂S₃. In particular, 14.2 ml of H₂ gas was produced after 12 h when 0.5 g of a 10 wt% Bi₂S₃/TiO₂ composite was used. On the basis of cyclic voltammetry (CV) results, the high photoactivity was attributed to the increase of band gap in the Bi₂S₃/TiO₂ composite, due to the decreased recombination between the excited electrons and holes.