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.     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

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%.     

Nanostructure Cu2ZnSnS4 thin film prepared by sol–gel for optoelectronic applications

Thin film of Cu₂ZnSnS₄ (CZTS) has been successfully deposited by sol–gel technique on n-type silicon and glass substrates to fabricate a heterojunction photodiode. The structural properties of the film were investigated by atomic force microscopy. The AFM image of the Cu₂ZnSnS₄ film reveals that the film is a nanostructure material formed from nanoparticles with the particle size of 50–90 nm. The optical band gap, E_g of the Cu₂ZnSnS₄ film was found to be 1.48 eV and the obtained optical band gap suggests that CZTS is very suitable for photovoltaic and optoelectronic applications. The current–voltage characteristics of the Al/n-Si/Cu₂ZnSnS₄/Al diode exhibit a good rectification behavior with ideality factor of 2.84 and barrier height of 0.738 eV. The interface states of the diode were analyzed by series resistance and conductance-voltage methods. The presence of interface states in series resistance–voltage plots was confirmed by the illumination. The interface state density D_it for the diode was found to be 3.63 × 10¹² eV⁻¹ cm⁻². The obtained results indicate that the Al/n-Si/Cu₂ZnSnS₄/Al diode is a photosensor based on controlling of interface states by illumination.     

CVD of CuGaSe2 for thin film solar cells with various transport agents

Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe₂ thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe₂ source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu₂Se and Ga₂Se₃ – in combination with I₂ and HCl as the respective transport agents yielded single-phase CuGaSe₂ thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe₂/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to V_oc=853 mV.     

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%.     

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.     

Absorption coefficient of bulk and thin film Cu2O

The optical absorption coefficient of thin film and bulk Cu₂O at room temperature is obtained from an accurate analysis of their transmittance and reflectance spectra. These absorption spectra are modeled, together with the low temperature data reported in the literature, using an analytical expression to assess and quantify the role of the different absorption mechanisms. The results suggest that direct forbidden transitions and indirect transitions play an almost equally relevant role. A table of the optical constants of Cu₂O single crystal is given for reference.     

Investigation of n-type Cu2O layers prepared by a low cost chemical method for use in photo-voltaic thin film solar cells

A low cost and simple chemical method of boiling copper plates in CuSO₄ solution is used to prepare Cu₂O layers. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES) and optical absorption have been used to characterise these layers. It has been found that the layers consist of Cu₂O phase with a thickness of about 1.4 μm for 60 minutes boiling in CuSO₄ solution. The largest grain sizes are in the order of 1 μm and the layers contain cubic Cu₂O phases. The layers are n-type in electrical conduction and the optical band gap observed is 2.2 eV.     

Depth profile analysis of CuInSe2 (CIS) thin films grown by the electrodeposition technique

In this study, we report the results obtained from the auger electron spectroscopy (AES) depth profiling of CIS thin films grown by the electrodeposition technique. This result enables one to do a comparison between the bulk and superficial elemental compositions. The AES result is also compared with that obtained by the inductively coupled plasma (ICP) spectroscopy. These results support our proposition that the electrodeposited CIS film has a Cu-rich bulk region and an In rich surface, which leads to the formation of an n-layer (CuIn₂Se_3.5) on the top of the p-type CIS (CuInSe₂) phase     

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.     

Multi-electron storage of photoenergy using Cu2O–TiO2 thin film photocatalyst

UV–vis irradiation of thin films of TiO₂ (ITO/TiO₂) and Cu₂O/TiO₂ (ITO/Cu₂O/TiO₂) coated on conducting glasses generate H₂ from H₂O and once the illumination is ceased, the H₂ production was still noticeable under dark for ITO/Cu₂O/TiO₂ at a lesser production rate for up to 2 h. No such dark reactions were observed for ITO/TiO₂ or TiO₂-coated copper metal foil (Cu/TiO₂). It was noticed that the irradiation of ITO/Cu₂O/TiO₂ leads to formation of trapped electrons and this stored energy leads to generate H₂ from H₂O in the dark.     

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.     

Temperature dependence of the optical transitions in electrodeposited Cu2O thin films

Cu₂O films on flexible copper and molybdenum (Mo) substrates were prepared by electrodeposition form an alkaline bath. The as-deposited films were p-type and the XRD analysis revealed that the film contains only the Cu₂O phase. The thickness of the films was calculated from the interference fringes in the reflection spectra. The Au/Cu₂O Schottky diodes were prepared by sputtering a 15 nm thick layer of very pure gold onto the Cu₂O films on Mo substrate. The probable optical transitions near the band edge were calculated from the spectral response of the device. The band gap calculated at various temperatures show a linear dependence on temperature and the absolute zero value of the band gap is deduced as 2.206 eV. The 2.493 eV direct transition observed in the room temperature shows a temperature dependence. Evidence of phonon assisted indirect transitions were observed at various temperature regions.     

Real-time investigations on the formation of CuInSe2 thin film solar cell absorbers from electrodeposited precursors

In this article, we present results of a detailed real-time X-ray diffraction (XRD) study on the formation of CuInSe₂ from electroplated precursors. The solid-state reactions observed during the selenisation of three different types of precursors are presented. The first type of precursors (I) consists of the nanocrystalline phases Cu_2−xSe and InSe at room temperature, which react to CuInSe₂ starting at 470 K. The second type of precursor (II) shows an inhibited CuInSe₂ formation out of the initial phases Cu_2−xSe and γ-In₂Se₃ starting at 400 K. The third precursor type (III) shows completely different selenisation behaviour. Starting from the intermetallic compound Cu₁₁In₉ and amorphous selenium, the formation of the binary selenides In₄Se₃ and CuSe is observed after the melting point of selenium at 494 K. After selenium transfer reactions, the compound semiconductor CuInSe₂ is formed out of Cu_2−xSe and InSe. This type (III) reaction path is well known for the selenisation of SEL precursors (stacked elemental layers of sputtered copper and indium and thermally evaporated selenium).     

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.     

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.     

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.     

Electrochemical deposition of Zn3P2 thin film semiconductors on tin oxide substrates

Zn₃P₂ semiconductor thin films were prepared by electrodeposition technique form aqueous solutions. The deposition mechanism was investigated by cyclic voltammetry technique. Crystal structure, morphology and composition of as deposited and annealed Zn₃P₂ thin films grown on SnO₂/glass substrates were determined by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. X-ray diffraction data indicated the formation of Zn₃P₂ as the predominant phase for both as-deposited and annealed films. The compositions of the deposited films were controlled by the bath temperature, deposition potential and Zn/P ratio in the solution.The dark current–voltage measurements of SnO₂/Zn₃P₂/C devices indicated a rectifying behavior and a reverse saturation current density of 1.7×10⁻⁷ A/cm², which is in good accordance with that obtained from films prepared using vacuum technique. Also, the capacitance–voltage measurements showed that the number of interface states and the built in potential are in the order of 5×10⁻⁹ cm⁻³ and 0.85 V, respectively. These preliminary results for Zn₃P₂ thin films reveal that, this semiconductor material can be used for solar cell applications.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.