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.     

Photoconductivity of Cu(In, Ga) Se2 films

Polycrystalline CuIn_{1 − x}Ga_xSe₂ (0 ≤ x < 0.3) films (CIGS) were deposited by coevaporating the elements from appropriate sources onto glass substrates (substrate temperature 720 to 820 K). Photoconductivity of the polycrystalline CIGS films with partially depleted grains were studied in the temperature range 130–285 K at various illumination levels (0–100 mW/cm²). The data at low temperature (T < 170 K) were analyzed by the grain boundary trapping model with monovalent trapping states. The grain boundary barrier height in the dark and under illumination were obtained for different x-values of CuIn_1−xGa_xSe₂ films. Addition of Ga in the polycrystalline films resulted in a significant decrease in the barrier height. Variation of the barrier height with incident intensity indicated a complex recombination mechanism to be effective in the CIGS films.     

X-ray fluorescence investigation of the Ga distribution in Cu(In,Ga)Se2 thin films

The efficiencies of Cu(In,Ga)Se₂/CdS/ZnO solar cell devices in which the absorbers are produced by classical two-step processes are significantly lower that those in which co-evaporated absorbers are used. A significant problem related to two-step growth processes is the reported segregation of Ga towards the Mo back contact, resulting in separate CuInSe₂ and CuGaSe₂ phases. Furthermore, it is often reported that material losses (especially In and Ga) occur during high-temperature selenization of metallic precursors. In this study, X-ray fluorescence (XRF) analysis was used to study the diffusion behaviour of the chalcopyrite elements in single-stage and two-stage processed Cu(In,Ga)Se₂ thin films. This relatively simple characterization technique proved to be very reliable in determining the degree of selenium incorporation, possible material losses and the in-depth compositional uniformity of samples at different stages of processing. This information is especially important in the case of two-stage growth processes, involving high-temperature selenization steps of metallic precursors. Device quality Cu(In,Ga)Se₂ thin films were prepared by a relatively simple and reproducible two-step growth process in which all the metals were evaporated from one single crucible in a selenium-containing environment. The precursors were finally treated in an H₂Se/Ar atmosphere to produce fully reacted films. XRF measurement indicated no loss of In or Ga during this final selenization step, but a significant degree of element diffusion which depended on the reaction temperature. It was also possible to produce Cu(In,Ga)Se₂ thin films with an appreciable amount of Ga in the near-surface region without separated CuInSe₂ and CuGaSe₂ phases.     

Effects of substrate temperature on the structural and electrical properties of Cu(In,Ga)Se2 thin films

Polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films were deposited onto soda-lime glass substrates using the three-stage process at the substrate temperature (T_sub) varying from 350 to 550 °C. The effect of T_sub on the structural and electrical properties of CIGS films has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect measurement. We found that the surface roughness, constituent phases, film morphologies, resistivity (ρ) and carrier concentration (N_P) of as-grown CIGS films indicated different change trends with increase in T_sub. The higher T_sub gives smooth surface, large grain size and single-phase CIGS films. The values of N_P and ρ have two demarcated regions at T_sub of 380 and 450 °C. At lower T_sub of 380 °C, larger N_P and lower ρ were dominated by the existence of secondary-phase Cu_xSe with lower resistivity. In the case of 450 °C, the obvious changes in N_P and ρ can be attributed to the sufficient Na incorporation diffused from the glass substrate. Finally, the correlation of cell parameters with T_sub was analyzed.     

Issues on the chalcopyrite/defect-chalcopyrite junction model for high-efficiency Cu(In,Ga)Se2 solar cells

Considering the chalcopyrite/defect-chalcopyrite junction model for Cu(In_1−xGa_x)Se₂-based devices and our previously reported findings for the Cu(In_1−xGa_x)₃Se₅ defect chalcopyrites, we have postulated that uniform high-Ga-content photovoltaic structures (with x > 0.35) do not yield acceptable device performance due to the electrical and structural differences between both types of materials (chalcopyrite and defect-chalcopyrite).In this contribution, the structural properties of the surface region of Ga containing absorber materials have been studied by grazing incidence X-ray diffraction. We find that there are significant differences between surface and bulk. A structural model is proposed for the growth of the chalcopyrite/defect-chalcopyrite junction relative to its Ga content. And we demonstrate that closely lattice matched high-Ga-content structures (x > 0.35) can produce solar cells withv acceptable performances. The high-voltage and low-current electrical outputs from high Ga structures are very desirable in module fabrication because overall resistive losses can be substantially reduced.     

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.     

Highly efficient Cu(In,Ga)Se2 mini-modules

In this contribution, we present results and the philosophy of our mini-module efforts. These efforts have achieved world record levels as well as a reproducible process. Various mini-module designs are tested using two different baseline Cu(In,Ga)Se₂ deposition recipes. Gridded mini-modules achieve highest efficiencies and are much less demanding on the ZnO:Al top contact than their conventional counterpart. For all of the designs tested, our experimental results are in the order of the expectations from our modeling. Gridded modules can achieve efficiency levels very close to those of the cells.     

Investigation of rapid thermal annealing on Cu(In,Ga)Se2 films and solar cells

Rapid thermal annealing (RTA), with fast ramp rate, was performed on several Cu(In,Ga)Se₂ (CIGS) films and solar cells under various peak annealing temperatures and holding times. Characterizations were made on CIGS films and cells before and after RTA treatments to study effects of RTA on the CIGS film properties and cell performance. In addition, AMPS-1D device simulation program was used to study effects of defect density on the cell performance by fitting the experimental data of RTA-treated CIGS cells. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties of CIGS films and cell performance while preserving the film composition and microstructure morphology.     

Design of grided Cu(In,Ga)Se2 thin-film PV modules

Results from modeling designs of Cu(In,Ga)Se₂ thin-film PV modules show that grided modules, at standard test conditions as well as at low-concentrated light, exhibit significantly improved performance when compared with conventional designs. It is further discussed that a grided design is advantageous from a synthesis and manufacturing point of view, since it provides higher front contact process tolerance and throughput as well as improved degrees of freedom of the module geometry.     

The effect of NaF on Cu(In, Ga)Se2 thin film solar cells

This work investigates NaF, on Mo coated sodium barrier glass, as a sodium precursor for the growth of Cu(In, Ga)Se₂ for thin film solar cells. These precursor layers are investigated by X-ray photoelectron spectroscopy (XPS) before and after annealing, and after exposure to selenium. XPS is also performed on the Cu(In, Ga)Se₂ layer, deposited with NaF. The influence of the NaF on the absorber growth is studied by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The electrical properties are investigated by cell fabrication and characterization. Cell results are comparable when NaF or out-diffusion of sodium from the soda lime glass are used.     

Preparation of Zn doped Cu(In,Ga)Se2 thin films by physical vapor deposition for solar cells

Cu(In,Ga)Se₂ (CIGS) surface was modified with Zn doping using vacuum evaporation. Substrate temperatures and exposure times during the Zn evaporation were changed to control a distribution of Zn in the CIGS films. Diffusion of Zn in the CIGS films was observed at the substrate temperature of over 200°C. The diffusion depth of Zn increases with increasing the exposure time at the substrate temperature of 300°C. Solar cells were fabricated using the Zn doped CIGS films. A distribution of the efficiencies decreases with increasing the exposure time of Zn vapor. The doping of Zn at the film surface improved reproducibility of a high fill factor and efficiency. A solar cell fabricated using the CIGS film modified with Zn doping showed an efficiency of 14.8%.     

Cu(In,Ga)Se2 solar cells with superstrate structure using lift-off process

Cu(In,Ga)Se₂ (CIGS) solar cells with a superstrate structure were fabricated using a lift-off process. To widen the variety of substrate choices for CIGS solar cells, a lift-off process was developed without an intentional sacrificial layer between the CIGS and Mo back-contact layers. The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to an alternative SLG substrate. The conversion efficiency of the CIGS solar cells with the superstrate structure was 5.1%, which was almost half that of the CIGS solar cells with a substrate structure prior to the lift-off process. The low conversion efficiency was caused by the high series resistance and low shunt resistance, which would be due to the junction resistance between the CIGS/back contact and cracks introduced during the lift-off process, respectively.     

Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%

An efficiency of over 18% have been achieved in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J₀ of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.     

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.     

Influence of the light source on the low-irradiance performance of Cu(In,Ga)Se2 solar cells

Illumination intensities in indoor environments are usually given in terms of lux, a unit of measurement which only takes into account the spectral distribution of the light source in the sensitivity range of the human eye (380–780 nm). At a given level of illuminance, however, the performance of a solar cell will strongly depend on the spectral distribution of the incoming light and on its spectral response. This work considers the spectral distribution of some typical light sources encountered in indoor environments (natural daylight/AM 1.5, fluorescent tube, halogen lamp with and without cold reflector, and the common incandescent lamp) and calculates the actual amount of light available to a generic solar cell. We then calculate the performance for different indoor illumination levels and spectra of a Cu(In,Ga)Se₂ solar cell especially optimised for low-irradiance conditions. Considering as a reference the performance of the cell under a AM 1.5 spectrum at a given level of illuminance, we can expect the performance of the cell to be reduced by a factor 3 and 2, respectively, when using a fluorescent tube and a halogen lamp with cold reflector as the light source, and to be increased by a factor 2 to 3 if the cell is operated under a halogen/incandescent lamp.     

Quality control and characterization of Cu(In,Ga)Se2-based thin-film solar cells by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/ Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect “hard failures” following CIGS deposition, and both “hard” and “soft” failures following CdS deposition and following ZnO deposition. In addition, a semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS/CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO/CIGS structures is found.     

Lift-off process reducing crack formation and its Cu(In,Ga)Se2 thin film solar cell applications

To reduce cracks caused by the lift-off process in a Cu(In,Ga)Se₂ (CIGS) layer, we focused on increasing the transferred layer thickness. We investigated the relationship between crack formation and the transferred layer thickness which is controlled by a Mo back electrode thickness. We found that the cracks were reduced by increasing the back electrode thickness. We suggest that the dominant factor of the crack reduction is attributed to the increase of the film hardness by increasing the Mo back electrode thickness. Next, we applied this crack reduction method to the solar cell fabrication. From the comparison of the 0.2-μm-thick Au single and 0.2-μm-thick Au/1.6-μm-thick Mo stacked back electrode lift-off CIGS solar cells, we investigated advantages of our crack reduction method. The crack formation was reduced only for the stacked back electrode lift-off solar cell. From the spatial distribution evaluation of an external quantum efficiency (EQE), we found that the crack reduction leads to not only the increase of an average EQE but also the decrease of EQE dispersion. In the solar cell parameters, the stacked back electrode lift-off solar cell without cracks showed the short-circuit current density and fill factor higher than those of the single back electrode lift-off solar cell with cracks. As a result, the conversion efficiency improvement as high as approximately 1% (an absolute value) was obtained. Moreover, the stacked back electrode lift-off solar cell showed the diode parameters (the diode ideality factor, the saturation current density, and series resistance) better than those of the single back electrode lift-off solar cell in the dark current density-voltage characteristics. We concluded that this high fill factor was attributed to the better diode performance. We therefore found that the stacked back electrode structure was very effective for improving the solar cell performance using the lift-off process.     

Cu(In,Ga)Se2-based thin-film photovoltaic modules optimized for long-term performance

In this contribution we give an overview of the mechanisms behind degradation of Cu(In,Ga)Se₂-based modules. Based on the results from a detailed analysis of power losses in modules, prior to and after extended damp heat exposure, we discuss to what extent modules can be designed to achieve enhanced long-term performance. For conventional modules, we show that the stability can be improved by optimizing the interconnect and the front contact. Furthermore, we argue that gridded modules are better from a long-term performance point of view. A novel interconnect structure, specifically designed for long-term durability, is briefly discussed.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).