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Cu(In,Ga)Se2 (CIGSe) solar cells with absorber thicknesses of

Photoelectrochemical (PEC) water splitting was performed using co-evaporated Cu(In,Ga)Se 2 (CIGS, p-type) films as the photocathode. Pt was electrodeposited on CIGS and CIGS/CdS films. The effect of the electrodeposition time was investigated to determine the optimal deposition conditions. The CIGS film was covered with a 60-nm-thick CdS layer (n-type) using a chemical-bath deposition technique, which created a p-n junction. The effect of the Pt electroplating time was again investigated for the CIGS/CdS films; thus, the effect of CdS addition could be quantitatively investigated. The effect of the pH of 0.5 M Na 2 SO 4 electrolyte was also investigated. The optimal water-splitting performance occurred at −24.16 mA/cm 2 at −0.7 V vs. Ag/AgCl with a Pt electrodeposition time of 20 min and pH 9. The CIGS/CdS films were characterized by X-ray diffraction, scanning electron microscopy, and focused-ion beam transmission electron microscopy.

A solar cell based on Cu(In 1−x ,Ga x )Se 2 (CIGS) with bandgap (E g ) of 1.5 eV is superior because of the low coefficient of power loss and resistive losses. CIGS thin-films with the E g = 1.5 eV and Cu/(Ga + In) (CGI) ratios of 0.92, 0.84, and 0.78 were deposited in this study using single-stage process. Photovoltaic (PV) parameters of the solar cell ameliorated on raising CGI values from 0.78 to 0.84, but abruptly deteriorated on further increasing the CGI value to 0.92. The PV properties of CIGS solar cell with CGI = 0.92 were poor due to the high defect density and low shunt resistance. The optimal CGI range for making efficient wide gap CIGS solar cells through the single-stage process was found to be from ∼0.80 to 0.84. The best CIGS solar cell with CGI value of 0.84 exhibited the conversion efficiency of 11.00%.

The temperature variation of solar cells due to white bias light irradiation during the spectral responsivity measurement and its effect on the spectral responsivity measurement result were investigated for various types of solar cells, such as crystalline silicon (c-Si), Cu(In,Ga)Se 2 (CIGS), and dye-sensitized solar cells (DSSCs). For the investigation, a spectral responsivity measurement system, which can employ the well-known sample temperature control methods (such as the “temperature controlled sample stage” method and the “forced air cooling” method) has been used. Hence, the availability of these sample temperature control methods has also been tested. Through the investigation, it was found that the actual temperature of the solar cells located under the AM1.5G-approximated white bias light can be increased significantly during the spectral responsivity measurement, depending on the sample temperature control methods applied. In addition, it was also found that the increase of sample temperature can lead to a significant error in the measured spectral responsivity, depending on the types of solar cells being measured. In addition, a simple analytic model based on the classical heat transfer theory was developed to understand the temperature variation of the solar cells under the spectral responsivity measurement environment.

Different from conventional photovoltaics, building-integrated photovoltaics needs not only high performance but also a high degree of transparency. Nevertheless, the Cu(In,Ga)Se2 (CIGS) solar cell has advantages in terms of the highest conversion efficiency and stability among all thin-film-based solar cells. The semitransparent (ST) CIGS solar cell using an ultrathin CIGS absorber on a transparent conducting oxide (TCO) experiences loss in fill factor and open circuit voltage due to the poor grain morphology and CIGS/TCO interface. Thus, these issues must be addressed to realize ST CIGS solar cells. Wide-bandgap (1.5 eV) submicron CIGS-based solar cells were prepared in this study unlike conventional CIGS with a bandgap of 1.2 eV, in order to enhance see-throughness. But such cells demonstrated low conversion efficiency due to the poor grain morphology and absence of back grading. Therefore, for the sake of improving grain morphology and to create back grading, a sulfurized-AgGa (AGS) layer was deposited between CIGS and the indium tin oxide (ITO) back contact. Ag from the AGS layer diffused throughout the absorber and thus ameliorated the grain morphology. However, Ga and S in the AGS layer remained confined near the back contact, therefore resulting in the creation of back grading. Consequently, a solar cell based on 230 nm thick CIGS modified with a 45 nm thick AGS layer exhibited efficiency of 5.94% with averaged visible transmittance over 25%. This is the highest reported efficiency for a ST CIGS solar cell with over 20% visible transparency. The CIGS solar cell based on this novel approach can be a competent candidate for building-integrated semitransparent photovoltaics applications.

In the tandem Cu(In 1− x Ga x )Se 2 (CIGS) thin film solar cell fabrication, Indium tin oxide thin film (ITO) is a promising material as back contacts of the top cell. ITO thin films were deposited by radio frequency (rf) magnetron sputtering in pure argon atmosphere at a working pressure of 9 × 10 −4 Torr with substrate temperature ( T sub ) of 300 °C. The sheet resistance of as-deposited 200 nm-thick ITO thin films was about 1.3 × 10 −4 Ω cm. The ITO thin films were subsequently annealed by rapid thermal annealing (RTA) at a temperature range between 400 and 550 °C for application of the high-temperature CuGaSe 2 (CGS) deposition process. After the annealing processes of ITO thin films, we have examined the optical and electrical properties as transparent conducting oxide (TCO) back contacts. CGS thin films were prepared for the top cell in a tandem solar cell structure with wide bandgap of above 1.6 eV as high open circuit voltage photovoltaic devices. The optical and electrical properties of CGS thin film solar cells with ITO back contacts were investigated as compared with that of metallic Mo back contacts. Also, the CGS thin film solar cells were fabricated using ITO and MO back contacts with a conversion efficiency of 5 and 8.2%, respectively.

An investigation into the effects of Se flux on absorber thin film growth at each step of a 3-stage co-evaporation process was conducted to further optimize the performance of CIGS solar cells. In ‘step I’ forming an In-Ga-Se precursor thin film during the 3-stage process, Se flux affected the preferred orientation of the CIGS crystal structure, but not the film morphology. In ‘step II’, no correlation was found between Se flux and the crystal structure, although excessively high Se flux employed throughout the 3-stage process degraded the solar cell performance. A CIGS thin film, with a (220/204) crystal orientation, minor physical surface defects and ∼20 nm thick MoSe2 at CIGS/Mo interface, was obtained by fine control of Se flux conditions (high Se flux at ‘step I’ and low Se flux at ‘step II’) at optimum substrate temperatures. The solar cell fabricated using the aforementioned CIGS thin film showed the highest conversion efficiency of 20.02 %.

High-speed spraying quickly deposits dry, solid particles at atmospheric pressure, without the use of binders, across large coating areas. We experimentally deposited Al2O3 and copper–indium–gallium–selenium (CIGS) nanoparticles on Al2O3 and molybdenum substrates and numerically replicated the results to elucidate the details of the deposition mechanisms. Thin films formed from layers of sprayed-particle impacts. Both single- and multiple-particle impacts are simulated and increases in pressure, temperature and von Mises stress are reported. Both experimentally and numerically, micron-sized particles are pulverized into flattened layers of nano-sized particle fragments. Characterizing the impact physics (particle collapse speed, energy exchange, and substrate damage) helps identify the optimum operating envelope for particle speeds less than 200 m/s that maximizes thin-film growth rates and minimizes substrate damage.

Carbon-free CuInSe 2 (CIS) thin film with a dense microstructure has been prepared using a novel non-vacuum based fabrication route. Cu x S y and In 2 Se 3 binary nanoparticles, approximately 10 nm in size, were synthesized by a low temperature colloidal process. The precursor film was deposited using the coating ink formulated with the binary nanoparticles and pyridine, and then annealed in the rapid thermal annealing (RTA) chamber at 540 °C for 15 min under selenium (Se) atmosphere. Scanning electron micrographs, X-ray diffraction patterns and Raman spectra showed a phase pure carbon-free and dense CIS thin film was prepared in this method. A solar cell device fabricated using this CIS thin film showed the following photovoltaic characteristics: V OC = 350 mV, J SC = 24.72 mA cm −2 , FF = 38.73% and η = 3.36% under standard AM 1.5 condition.

We investigated the transport and photovoltaic properties of Cu-deficient Cu(In1 − xGax)Se2 (CIGS) thin-film solar cells containing ordered vacancy compound (OVC) layers. Raman spectra clearly revealed that the CIGS thin films with lower Cu concentrations contained larger volumes of OVC layers. The temperature-dependent inverse ideality factor showed that the CIGS film containing more (less) OVC layers exhibited tunneling-mediated bulk (interface)-dominated recombination. The capacitance–voltage characteristics and admittance spectra showed that the CIGS cells containing more OVC layers had more uniform carrier concentration near the junction and less interfacial trap states compared with cells with less OVC layers. These results suggested that the Cu-deficiency and the resulting OVC layer formation reduced the interfacial defect density and suppressed the interface recombination processes of the CIGS solar cells.

The influence of the conventional depositing processes on the optical properties of Cu(In,Ga)Se 2 (CIGS) thin films in solar cell structures was investigated by measuring the photoluminescence (PL) and Raman spectra of the CIGS layer at each stage of the solar cell deposition process. The intensities of the PL and the Raman A 1 mode increase after the CdS buffer layer is deposited, suggesting that the CdS layer either improves the optical quality of the CIGS film or protects it from degradation due to environmental factors. The temperature and excitation power dependences of the PL for the bare CIGS sample are very different from those for the samples with the CdS layer, reflecting different characters of the luminescence centers near the surface of the CIGS layer. On the other hand, the lateral homogeneity, as seen in the micro-PL and micro-Raman images, does not seem to improve. After the ZnO window layer is deposited, the overall PL and Raman intensities do not change much, although the intensity distribution becomes more inhomogeneous.

DC and pulse-reverse electrodeposition mode were employed for the deposition of polycrystalline Cu(In,Ga)Se 2 thin films. In comparison with DC electrodeposition mode, films obtained by pulse-reverse electrodeposition were smoother, denser and more uniform with good adhesion. The Ga content in final composition of CIGS thin film was improved in pulse-reverse electrodeposition mode. In addition, pulse-reverse electrodeposited CIGS thin films were more crystalline with chalcopyrite structure. The compact morphology without pores in the deposit was achieved in the pulse-reverse electrodeposited CIGS thin films by varying duty cycle.

CuInSe2 (CIS) thin films were formed on a Mo-coated soda lime glass with a newly formulated hybrid ink composed of binary Cu2−xSe nanoparticles and an In precursor solution. The thin films were fabricated by the spin coating method and a subsequent selenization process, and the compositional and structural properties of the CIS thin films were characterized. Deposition of a hybrid ink provides the unique advantage of producing centers composed of pure material-based nanoparticles that can promote stress-relief and crack-deflection. Additionally, hybrid inks offer efficient binding with nanoparticles by using precursor solutions without other organic binders. Binary Cu2−xSe nanoparticles were synthesized by a low-temperature colloidal process, and an In precursor solution was prepared by using a non-toxic chelating agent for dispersing the In component stably. The band gap of the CIS thin film was measured to be 1.14 eV by the external quantum efficiency (EQE) technique, and the best conversion efficiency of the fabricated device was determined to be 4.19%.

We investigated the transport and photovoltaic properties of Cu(In 1- x Ga x )Se 2 (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the current–voltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The low-temperature characteristics of the diode ideality factor supported bulk-dominated recombination in the same cell. This suggests that shunt-current subtraction can provide the proper diode parameters of CIGS solar cells.

a b s t r a c t Cu(In,Ga)Se2 (CIGS) thin films were formed by a direct non-vacuum coating and a subsequent selenization of low cost precursor solutions, and their compositional, structural and optical properties were charac- terized. Selenized films showed a double-layered structure with an upper layer of chalcopyrite CIGS and an amorphous bottom layer mainly composed of carbon. For the upper CIGS layer, good compositional controllability for Cu and Ga was confirmed by linear relationship between metal ratios of the precursor solution and those of the selenized films. Effects of Cu and Ga contents on structural and optical proper- ties of the films were also characterized by X-ray diffraction (XRD), Raman and photoluminescence (PL) analyses, and the results were interpreted by defect supercluster formation (VCu-InCu) in Cu-deficient films and mass and size difference between In and Ga, respectively. Further, the bottom layer was found to be mostly composed of conductive amorphous carbon, which is the main current flow path in the completed solar cells.

This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In + Ga) ratios. Regarding the solar cell parameters, the short circuit current density (JSC) decreased linearly with Ga/(In + Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage (VOC) increase with those. However, VOC values at high Ga/(In + Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In + Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with VOC, JSC and FF of 0.625 V, 35.03 mA cm−2 and 0.71, respectively.

A non-vacuum process for Cu(In,Ga)Se 2 (CIGS) thin film solar cells from nanoparticle precursors was described in this work. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI 3 , GaI 3 and Na 2 Se) in organic solvents, by which fine CIGS nanoparticles of about 15 nm in diameter were obtained. The nanoparticle precursors were then deposited onto Mo/glass substrate by the doctor blade technique. After heat treating the CIGS/Mo/glass layers in Se gas atmosphere, a complete solar cell structure was fabricated by depositing the other layers including CdS buffer layer, ZnO window layer and Al electrodes by conventional methods. The resultant solar cell showed a conversion efficiency of 0.5%.

A non-vacuum process for fabrication of Cu(In, Ga)Se2 (CIGS) absorber layer from the corresponding nanoparticle precursors was described. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI3, GaI3 and Na2Se) in organic solvents, by which fine CIGS nanoparticles of about 20 nm in diameter with chemical composition of Cu0.9In0.64Ga0.23Se2.00 were obtained. The nanoparticle precursors were mixed with organic binder material for the rheology of the mixture to be adjusted for the doctor blade method. After depositing the mixture of CIGS with binder on Mo/glass substrate, the samples were preheated on the hot plate in air to evaporate remaining solvents and to burn the organic binder material. Subsequently, the resultant (porous) CIGS/Mo/glass sample was selenized in a two-zone Rapid Thermal Process (RTP) furnace in order to get a solar cell applicable dense CIGS absorber layer. The selenization at 550 °C for 15 min with Se gas evaporated at 400 °C resulted in the growth of CIGS particles up to a few hundred nanometers in grain size in the upper parts of the layer with remaining unreacted small particles in the lower parts.

Effects of heat treatment in nitrogen or Se atmosphere on the properties of Cu(In,Ga)Se 2 (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 CuSe 2 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.

To make a dense CIGS absorber layer, spray deposited CIGS films were annealed in the two-zone RTP furnace in Se atmosphere. More Se supply by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in the larger CIGS grains. However, a thick MoSe2 layer was formed between CIGS and Mo, as the Se supply increased, results in partial detachment of CIGS/MoSe2/Mo layers from the glass substrate. From the result, it was found that the short heat- treatment with high Se vapor pressure is better than the long heat-treatment with low Se vapor pressure. The large CIGS grains without peeling off, can be obtained from the following conditions; Se evaporation temperature of 450oC, substrate temperature of 550oC, annealing time of 5 min, and flow rate of carrier gas of 30 sccm.

The photovoltaic properties of CIGS cells on an alumina substrate were improved through the use of Na-doped Mo as the bottom layer of a Mo back contact. Na was supplied to the CIGS bulk region from an alumina/Na-doped Mo/Mo structure, similar to the Na diffusion from soda-lime glass. The diffusion of Na from the Na-doped Mo was controlled effectively compared to that from Soda-lime glass (SLG). The present results indicate that Na-doped Mo acts as a Na source material and that the Na amount can be controlled by adjustment of thickness of Na-doped Mo layer, without the use of an alkali barrier layer. The highest conversion efficiency of 13.34% ( J sc = 34.62 mA/cm 2 , V oc = 0.58 V and FF = 66%) for an active area of 0.45 cm 2 on an alumina substrate was obtained for 100 nm Na-doped Mo/1000 nm Mo.

Cu(In,Ga)Se2 (CIGS) nanoparticles as a precursor material of a thin absorber layer for the CIGS based solar cells were synthesized by the relatively simple colloidal process. CIGS nanoparticles of 15–80 nm in diameter were obtained by reacting a mixture of CuI, InI3 and GaI3 in pyridine with Na2Se in methanol at 0 °C in nitrogen atmosphere with mechanical stirring. As the reaction time increased from 0 to 1 min. the size of nanoparticle decreased, showing a minimum at 1 min. (about 15 nm in diameter with very small size distribution). Further increase in the reaction time resulted in an increase in the nanoparticle size. The change in the size of CIGS nanoparticles with the reaction time was analyzed in terms of the effects of mechanical stirring on the concentration distribution of solute, i.e., CIGS nuclei, in the colloidal solution, and hence on the nucleation and the subsequent growth processes of the CIGS nanoparticles.

Cu(In 1−x Ga x )Se (CIGS) based thin film solar cells are usually built by depositing CdS as a buffer layer and ZnO as a window layer on top of the CIGS absorber layer. In order to optimize their performances, it is essential to understand the interactions between the layers. In this study, we have investigated the interactions between the layers by examining the optical properties of the CIGS solar cell structure at each step of the fabrication process―CIGS, CIGS/CdS, and CIGS/CdS/ZnO― using photoluminescence and Raman spectroscopic imaging techniques. The images of the intensity of the Raman peak at 175 cm−1 due to the A 1 vibration mode show that the homogeneity improves after CdS deposition. Micro-PL intensity imaging also confirmed this observation. Furthermore, the PL peak intensity and the energy position vary after each layer deposition.

The formation characteristics of MoSe2 by selenization of Mo and nanoparticle derived Cu(ln,Ga)Se2(CIGS)/Mo layers were investigated with a two-zone RTP (rapid thermal process) furnace to elucidate why the CIGS/Mo layer peels off the glass substrate when selenized under the high Se vapor pressure condition. It was found that the thickness of MoSe2 linearly increased with Se evaporation temperature and substrate temperature. At a substrate temperature of 550degC and Se evaporation temperature of 450degC, 1.6mum thick MoSe2 layer formed in 5 minutes with an expense of 0.4mum thick Mo. The formation of very thick MoSe2 layer revealed that c-axis of hexagonal MoSe2 is parallel to the Mo substrate. When the nanoparticle derived CIGS/Mo layers are selenized at a substrate temperature of 500degC and Se evaporation temperature of 450degC for 5 minutes, about 1.3mum thick MoSe2 formed at CIGS and Mo interface without significant growth of CIGS particles, demonstrating that reaction rate of Se with Mo is much faster than that with CIGS nanoparticles

Spray deposited nanoparticle derived Cu(In,Ga)Se2 (CIGS) layers were selenized with a two-zone RTP furnace in order to get a solar cell applicable dense CIGS absorber layer. Effects of various selenization parameters including Se evaporation temperature, flow rate of carrier gas and substrate temperature on the growth of CIGS nanoparticles were investigated. The experimental results revealed that higher amount of Se vapor generated and transported to CIGS nanoparticles, i.e., higher temperature of Se evaporation and/or higher flow rate of carrier gas, resulted in the larger CIGS grains. High substrate temperature also enhanced the growth of individual nanoparticles. However, direct contact of Se vapor with bare Mo surface through the large numbers of pores in the layer gave rise to a formation of very thick MoSe2 layer which at times caused the CIGS/MoSe 2/Mo layers to peel off the glass substrate, presumably due to the significant molar volume difference between Mo and MoSe2

We report an efficiency limiting factor of CuInGaSe2 (CIGS) thin film solar cells fabricated by rapid thermal annealing (RTA) of sputter-deposited Cu-In-Ga-Se films without an additional supply of Se vapor. The CIGS thin films show a single-phase chalcopyrite structure without Ga segregation, and there is no apparent Se deficiency in overall composition measured by routine compositional analysis. However, detailed investigation on properties of the CIGS films and devices reveals that the formation of Se vacancies on the CIGS film surface during Se-free RTA is a main limiting factor of the device efficiency.

Spray deposited porous CIGS nanoparticle-derivedthin films were selenized in a two zone rapid thermal annealing furnace and effects of various selenization parameters including Se evaporation temperature, flow rate of carrier gas, and substrate temperature on densification of the CIGS layers were investigated. It was found that higher Se supply to CIGS nanoparticles either by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in larger CIGS grains with higher degree of crystallinity, while it also induced formation of a thicker MoSe 2 layer in-between CIGS and Mo which resulted in partial detachment of CIGS / MoSe 2 / Mo layers from the glass substrate. Densification of CIGS layer by growth of nanoparticles and formation of thick MoSe 2 were explained by a liquid Se assisted reaction rather than by a vapor phase Se assisted reaction.