Study of CdS/Cu(In, Ga)Se2 heterojunction interface using admittance and impedance spectroscopy

The interface properties of an unusual CdS/Cu(In, Ga)Se₂ solar cell have been studied by admittance and impedance spectroscopy. The current transport in this device has previously appeared to be dominated by tunnelling enhanced recombination at junction interface. The existence of this unexpected route was attributed to the presence of Cu-rich and indium depleted thin layer which might possibly be formed on the absorber surface. We have performed temperature dependent admittance and impedance measurements in order to clarify this phenomenon. An acceptor level with ionization energy of about 50 meV seems to be strongly correlated to the appearance of the CuGaSe₂ layer. The impedance spectra obtained at 100 K and 300 K exhibited single semi-circles. This indicates the dominance of the heterojunction interface without the effect of any other capacitive components. The equivalent circuit model consisting of a parallel resistor and capacitor in series with a resistor has been found to give a good fit to the experimental data.     

Cu(In,Ga)Se2 solar cells on stainless-steel substrates covered with ZnO diffusion barriers

ZnO layers were deposited as diffusion barriers by DC magnetron sputtering from a pure Zn target on stainless-steel substrates. It was found that the insertion layer of ZnO between Mo film and stainless-steel substrate had no influence on the orientation and composition of CIGS films, as identified by an X-ray diffractometer and X-ray fluorescence, respectively. However, ZnO diffusion barriers had strongly reduced the diffusion of Fe from stainless-steel substrates into the CIGS films, as investigated by secondary ion mass spectrometry. With such diffusion barriers, the efficiency, open-circuit voltage, and fill factor of CIGS solar cells all increased.     

Effects of heat treatments on the properties of Cu(In,Ga)Se2 nanoparticles

Effects of heat treatment in nitrogen or Se atmosphere on the properties of Cu(In,Ga)Se₂ (CIGS) nanoparticles were investigated to extract optimum sintering conditions for fabrication of solar cell applicable CIGS absorber films. In nitrogen atmosphere, as the temperature increases from 100 to 400 °C the intensity of X-ray diffraction (XRD) peaks corresponding to the (1 1 2), (2 2 0) and (3 1 2) planes of the chalcopyrite CIGS increases, and the peak positions shift to lower angle regions without any particle growth in scanning electron microscopy (SEM) analysis, which is in consistent with the significant In and Ga loss in the EDS data. When the temperature further goes up to 500 °C, parts of CIGS are decomposed and Cu and CuSe₂ phases are observed. From these results, the heat treatment in nitrogen atmosphere is found to have no beneficial effect on the sintering of the particles and only induces loss of In and Ga. On the other hand, heat treatment in Se atmosphere at a substrate temperature of 550 °C with Se vapor evaporated at 250 and 450 °C provided much enhanced growth of the particles, specially up to 500 nm at 450 °C, and increased crystallinity without In or Ga loss, reflecting that Se supply played a critical role in the growth of the CIGS nanoparticles.     

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.     

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.     

In situ monitoring of the growth of Cu(In,Ga)Se2 thin films

Thin films of Cu(In,Ga)Se₂ were grown by co-evaporation process in a conventional vacuum coating system at the base pressure of 10⁻⁵ mbar. The controllable of the two-stage growth process was developed. During the growth process, substrate temperature, graphite heater temperature, heating output power and temperature of the CIGS surface were monitored simultaneously. In this setup, we use the change in the thermal behavior of the substrate due to the variations in emissivity of CIGS film during the transition of Cu-rich to Cu-poor in the second stage corresponding to the change of power fed into the substrate heater as the control signal. By observing the variation of control signals, the desired final composition of the film can be obtained. Using our device fabrication, Al/ZnO(Al)/CdS/CIGS/Mo/SLG solar cells with efficiency over 14% (without AR) were achieved.     

A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se)₂ solar cells result from alloying CuInSe₂ base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.     

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.     

Formation of CuInSe2 and Cu(In,Ga)Se2 films by electrodeposition and vacuum annealing treatment

Polycrystalline thin films of CuInSe₂ and Cu(In,Ga)Se₂ (CIGS) were grown on both polished Mo substrates and Mo-coated glass substrates by one-step electrodeposition. All the as-deposited films have been annealed in vacuum at 450°C for a short time to improve the crystalline properties. The films have been characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that the crystallization of the films was greatly improved after annealing. Further more, a CIGS film with 23 at% Ga was obtained.     

8 MeV electron irradiation studies on electrical characteristics of Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂ (CIGS) solar cells are gaining considerable interest due to their high optical absorption coefficient and adjustable band gap, which enables them to achieve high conversion efficiency and also present many promising applications in space power systems. In this paper we report the results of the effect of temperature and 8 MeV electron irradiation on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se₂/Mo polycrystalline thin-film solar cells under forward and reverse bias studied in the temperature range 270-315 K. The solar cells were subjected to 8 MeV electron irradiation from the Microtron accelerator and were exposed to graded doses of electrons up to 75 kGy. I-V characteristics of the cells under dark and AM 1.5 illumination condition were studied before and after the irradiation. Capacitance measurements were also carried out at various frequencies before and after irradiation. In the measured temperature range, the dark current contribution is due to the generation-recombination of the minority carriers in the depletion region. The ideality factor is found to decrease with increase in temperature. It seems that electron irradiation has not altered the dark current conduction mechanism significantly. The effect of electron irradiation on the solar cell parameters such as fill factor (FF), conversion efficiency (η), saturation current (I_o), short circuit current (I_sc), open circuit voltage (V_oc), and ideality factor (n) was studied. They were found to be stable up to 75 kGy of electron dose as only small changes were observed in the solar cell parameters.     

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.     

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.     

Temperature-dependent quantum efficiency analysis of graded-gap Cu(In,Ga)Se2 solar cells

Despite the high solar cell efficiencies achieved with Cu(In,Ga)Se₂ (CIGS) absorbers, key parameters such as the carrier diffusion length and recombination lifetime are still under investigation. Here, we extract lifetime and diffusion length from temperature-dependent internal quantum efficiency (IQET) spectra of state of the art high efficiency CIGS solar cells. Two-parameter fits to the measured IQE curves using a model for double-graded gap solar cells show very good agreement in the studied temperature range T=146–293 K, allowing the extraction of the electron recombination lifetime in the absorber and the collection probability in the front region of the cell. The obtained results agree with current literature values obtained by other characterization techniques. Furthermore, the temperature dependence of the recombination lifetime is explained by Shockley–Read–Hall recombination through a single bulk defect level with an activation energy of 200 meV.     

Investigations on electron beam evaporated Cu(In0.85Ga0.15)Se2 thin film solar cells

CIGS bulk with composition of CuIn_0.85Ga_0.15Se₂ was synthesized by direct reaction of elemental copper, indium, gallium and selenium. CIGS thin films were then deposited onto well-cleaned glass substrates using the prepared bulk alloy by electron beam deposition method. The structural properties of the deposited films were studied using X-ray diffraction technique. The as-deposited CIGS films were found to be amorphous. On annealing, the films crystallized with a tetragonal chalcopyrite structure. An intermediate Cu-rich phase precipitated at 200 °C and dissociated at higher annealing temperatures. Average grain size calculated from the XRD spectra indicated that the films had a nano-crystalline structure and was further corroborated by AFM analysis of the sample surface. The chemical constituents present in the deposited CIGS films were identified using energy dispersive X-ray analysis. CIGS based solar cells were then fabricated on molybdenum and ITO coated glass substrates and the efficiencies have been evaluated.     

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.     

High efficiency cadmium free Cu(In,Ga)Se2 thin film solar cells terminated by an electrodeposited front contact

The possibility to reach up to 14.7% efficiency with Cu(In,Ga)Se₂ (CIGS) solar cell, using a cadmium free buffer layer (indium sulphide:In₂S₃) and an electrodeposited front contact (chloride doped ZnO:ZnO:Cl) is demonstrated in this article. This is the first time that costly gas phase deposition processes for ZnO, by high vacuum sputtering, can be replaced by an efficient low cost atmospheric technology, representing an important breakthrough in further cost reduction for photovoltaic application. In addition, the compatibility with cadmium free buffer layers brings this new approach at the cutting edge of strategic evolution of the CIGS technology. In this study the influences of the In₂S₃ buffer layer thickness, the presence of an intrinsic ZnO layer and a soft annealing treatment are studied. It is shown that the growth behavior of the electrodeposited ZnO:Cl is controlled by nucleation phenomena on different surfaces, with a unique morphology on indium sulphide. Finally the best performances have been achieved with a cell annealed at 150 °C under atmospheric conditions containing a very thin In₂S₃ layer (15 nm) but without intrinsic ZnO (CIGS/In₂S₃/ZnO:Cl).     

Effect of gallium grading in Cu(In,Ga)Se2 solar-cell absorbers produced by multi-stage coevaporation

We investigate Cu(In,Ga)Se₂ thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both back-side gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced.     

Near-band-edge photoluminescnce in Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) and PL decay characteristics of the near-band-edge (NBE) PL at room temperature have been studied on the Cu(In,Ga)Se₂ (CIGS) solar cells. The carrier recombination process has been discussed with emphasis on the photovoltaic properties of the solar cell. It has been found that: (i) PL intensity of the CIGS solar cells is much stronger than that in the corresponding CIGS thin films, (ii) the PL decay time of the cell is longer than that of the CIGS film, and (iii) the PL decay time of the CIGS solar cell exhibits strong dependence on the PL excitation intensity. In the CIGS solar cell, intense PL is obtained under the open circuit condition (oc), in contrast to the very low PL yield under the short circuit (sc) condition. The PL decay time under the sc condition is much shorter than that under the oc condition. Excitation intensity dependence of PL intensity and the PL decay time have been studied, and they are discussed with relation to the photo-voltage due to the PL excitation light. PL and injection EL under the external DC bias have been studied. The mapping image of NBE-PL intensity has been compared with that of the laser beam induced current (LBIC), and the PL intensity image reflects the photovoltaic properties of the CIGS solar cells. We demonstrated that NBE-PL of the CIGS solar cell reflects the photovoltaic effect, and it can be utilized as a powerful characterization method.     

Stability of Cu(In,Ga)Se2 solar cells and evaluation by C–V characteristics

To confirm the long-term reliability of Cu(In,Ga)Se₂, (CIGS) solar cells, we investigated the I–V and C–V characteristics during tests under irradiation or dark condition. Under irradiation, the test samples showed a little increase in efficiency (η) and open-circuit voltage (V_oc) which showed their electrical durability to light irradiation. But the diode factor (n) and series resistance (R_s) showed large changes in value. Also, the built-in voltage (V_b) and density gradient (dN_A/d_x) in the CIGS layer calculated from the C–V characteristics showed distinct changes during the test. After 4 SUN irradiation, two samples in the same fabrication-lot showed new light absorption in the lower-energy range than sun the energy gap of CIGS. We explain the change of C–V characteristics for the samples under strong irradiation with a new model named “Junction retrograde” which can treat defect generation by irradiation to reduce the acceptor density in graded p-n junction. This model for C–V analysis can be used to investigate the long-term reliability of CIGS solar cells under irradiation.