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.     

Thin films of Cu(In,Ga)Se2 and ordered vacancy compound prepared by thermal crystallization and their photovoltaic applications

Thin films of Cu–In–Ga–Se alloy system with various composition were prepared by thermal crystallization from In/CuInGaSe/In precursor. Electron probe microanalysis and X-ray diffraction study revealed that these samples were assigned to chalcopyrite Cu(In,Ga)Se₂ or ordered vacancy compound Cu(In,Ga)₂Se_3.5. Solar cell with ZnO:Al/i–ZnO/CdS/Cu(In,Ga)Se₂/Mo/soda-lime glass substrate structure was fabricated by using thermal crystallization technique, and demonstrated a 9.58% efficiency without AR-coating.     

Three-stage growth of Cu–In–Se polycrystalline thin films by chemical spray pyrolysis

Structural, optical and electrical properties of polycrystalline Cu–In–Se films, such as CuInSe₂ and ordered vacancy compounds (OVC), prepared by three-stage process of sequential chemical spray pyrolysis (CSP) of In–Se (first stage), Cu–Se (second stage) and In–Se (third stage) solutions have been studied in terms of substrate temperature at the second stage (T_S2). The films grown at T_S2420 °C exhibited larger grains in comparison with the Cu–In–Se films grown by the usual CSP method. Optical gap energy was approximately 1.06 eV for 360 °CT_S2420 °C, but increased dramatically from 1.06 to 1.35 eV when the T_S2 rose from 420 to 500 °C. Conductivity type was p-type for T_S2<420 °C, but n-type for T_S2>420 °C.     

Electrodeposition of CuInxGa1−xSe2 thin films

CuIn_xGa_1−xSe₂ (CIGS) polycrystalline thin films with various Ga to In ratios were grown using a new two-step electrodeposition process. This process involves the electrodeposition of a Cu–Ga precursor film onto a molybdenum substrate, followed by the electrodeposition of a Cu–In–Se thin film. The resulting CuGa/CuInSe bilayer is then annealed at 600°C for 60 min in flowing Argon to form a CIGS thin film. The individual precursor films and subsequent CIGS films were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and Auger electron spectroscopy. The as-deposited precursor films were found to be crystalline with a crystal structure matching that of CuGa₂. The annealed bi-layers were found to have the same basic chalcopyrite structure of CuInSe₂, but with peak shifts due to the Ga incorporation. Energy dispersive spectroscopy results show that the observed shifts correlate to the composition of the films.     

High-efficiency CIGS solar cells with modified CIGS surface

CIGS films were treated in In–S aqueous solution for high-efficiency CIGS solar cells. The In–S aqueous solution contained InCl₃ and CH₃CSNH₂ (thioacetamide). The In–S treatment modified the CIGS surface favorably for high-efficiency CIGS solar cells as evidenced by the increase in V_oc, J_sc and FF. The In–S treatment formed thin CuInS₂ layer on the CIGS surface which contributes to the high efficiency and stable performance of the CIGS solar cell. The best cell showed an efficiency of 17.6% (V_oc=0.649 V, J_sc=36.1 mA/cm² and FF=75.1%) without any annealing and light soaking before I–V measurement.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

Annealing effects on Zn1−xMgxO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn_1−xMg_xO/Cu(In,Ga)Se₂ (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn_1−xMg_xO/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn_1−xMg_xO film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn_1−xMg_xO film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn_1−xMg_xO/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn_1−xMg_xO/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn_1−xMg_xO/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn_1−xMg_xO/CIGS interface states related to the damages and diffusion of Zn.     

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.     

Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Fabrication of Cu(In,Ga)Se2 thin films solar cell by selenization process with Se vapor

CIGS films were prepared on Mo-coated glass by sputtering and selenization processes. The metallic precursors were selenized under higher pressure in selenium vapor instead of H₂Se. In order to improve the performance of CIGS thin film solar cells, the morphologies of CIGS thin films were studied carefully by various temperature profiles. The relationship between temperature decrease rate and fill factor (FF) of solar cells was investigated thoroughly. On the other hand the value of open circuit voltage (V_oc) was improved by increasing the gallium content near the surface of CIGS thin film. A glass/Mo/CIGS/CdS/ZnO cell was fabricated and the conversion efficiency of 9.4% was obtained without antireflective film.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

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.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Structural and optical properties of In-rich Cu–In–Se polycrystalline thin films prepared by chemical spray pyrolysis

Structural and optical properties of In-rich Cu–In–Se polycrystalline thin films (0.54<In/(Cu+In)<0.78) prepared by chemical spray pyrolysis (CSP) on glass substrate have been systematically studied in terms of In/(Cu+In) ratio. Lattice constants a and c of the films decrease with increase of In/(Cu+In) ratio. The films exhibit a characteristic Raman peak shifting higher frequencies as the In/(Cu+In) ratio increases. Optical bandgap energy is approximately 1.22 eV for 0.54<In/(Cu+In)<0.67, but increases from 1.22 to 1.36 eV when the In/(Cu+In) ratio increases from 0.67 to 0.78. Photoacoustic measurements reveal the existence of high concentration of nonradiative centers introduced by the deviation from the stoichiometric composition.     

Investigation of pulsed non-melt laser annealing on the film properties and performance of Cu(In,Ga)Se2 solar cells

Pulsed non-melt laser annealing (NLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed 248 nm laser beam at selected laser energy density in the range 20–60 mJ/cm² and pulse number in the range 5–20 pulses. XRD peak narrowing and SEM surface feature size increase suggest near-surface structure changes. Dual-beam optical modulation (DBOM) and Hall-effect measurements indicate NLA treatment increases the effective carrier lifetime and mobility along with the sheet resistance. In addition, several annealed CdS/CIGS films processed by NLA were fabricated into solar cells and characterized by photo- and dark-J–V and quantum efficiency (QE) measurements. The results show significant improvement in the overall cell performance when compared to unannealed cells. The results suggest that an optimal NLA energy density and pulse number for a 25 ns pulse width are approximately 30 mJ/cm² and 5 pulses, respectively. The NLA results reveal that overall cell efficiency of a cell processed from an unannealed film increased from 7.69% to 13.41% and 12.22% after annealing 2 different samples at the best condition prior to device processing.     

Electrical properties of the Cu(In,Ga)Se2/ MoSe2/Mo structure

We investigated the electrical properties of the Cu(In,Ga)Se₂/MoSe₂/Mo structure. CIGS/Mo heterocontact including the MoSe₂ layer is not Schottky-type but a favorable ohmic-type contact by the evaluation of dark I–V measurement at low temperature. A characteristic peak at 870 nm is observed in differential quantum efficiency of a solar cell with a CIGS thickness of 0.5 μm. This peak is considered with relating to the absorption of the MoSe₂ layer. The band gap of MoSe₂ is calculated to be 1.41 eV from the absorption peak. The band diagram is discussed on the basis of the electrical point of view.     

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se₂ (CIGS) thin-film solar cells using evaporated In_xSe_y and ZnIn_xSe_y buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnIn_xSe_y and the CdS buffer layers.