Cu(In,Ga)Se2 Thin Film Solar Cells Research Articles

Water-based chemical synthesis of low cost CIGS thin films

Currently, the conversion efficiency of Cu(In,Ga)Se2 (CIGS) based solar cells is exceeding 20%. However, conventional vacuum based fabrication of thin film is expensive and not easy for mass production. In order to produce CIGS thin film solar cells in a cost effective way, we developed solution based synthetic method, which employs water as a solvent. In this work, we have successfully synthesised uniform copper indium gallium selenites as the amorphous precursor with nearly theoretical value of its chemical composition. The optimum reaction condition for the as prepared precursor was a temperature of 160°C and 25 min reaction time in a microwave. The precursor particles were dispersed, wet ground and coated on to the molybdenum substrate. Pure crystalline CIGS absorber layer of ∼0·5 μm was successfully grown by simultaneous annealing and selenisation of the precursor film. Sodium and antimony doping have been used to increase the grain size of the film significantly.

Residual stress in CIGS thin film solar cells on polyimide: simulation and experiments

Sputtering technique is used to prepare Cu(In,Ga)Se2 (CIGS) thin film solar cells on a UPILEX-S 50S polyimide substrate in order to investigate the residual stress in the Mo back contact and CIGS absorber layer after selenized annealing with various the thickness ratios of the Mo contact, RMo. A comparison between the results of numerical simulation and those obtained from X-ray diffractometry, indicate the existence of compressive residual stress in both the Mo contact and the CIGS absorber layer. The residual stress in the Mo contact was inversely proportional to the residual stress in the CIGS absorber layer. Residual stress decreased with an increase in the thickness ratio of the Mo contact. The empirical formulae for the residual stresses as a function of RMo in the Mo and CIGS films were deduced, from the results of this study.

Preparation and characterization of Cu(In,Ga)Se2 thin film solar cell by printing technique with powders of quaternary alloy and NaF

This study prepared Cu(In,Ga)Se2 (CIGS) thin films on bi-layer Mo coated soda-lime glass, by printing with mixed powders of CIGS quaternary alloy (average partial size of 680 nm) and NaF. A single-stage annealing process was performed to form a CIGS chalcopyrite phase. Experimental results show stoichiometric ratios of Cu/(In+Ga) = 0.94 and Ga/(In+Ga) = 0.26 in the CIGS film. The resistivity of the sample was 12.69 Ω cm, with a carrier concentration of 9.34 × 1016 cm−3, and mobility of 5.27 cm2 V−1 s−1. The CIGS film exhibited p-type conductivity. The incorporation of 1 wt% NaF in the CIGS powder produced a chalcopyrite structure with the best crystalinity. Raman analysis identified a number of phases, including CuInSe2 and CuIn3Se5. The CIGS solar cells with AZO/i-ZnO/CdS/CIGS/Mo/soda-lime glass structure were fabricated. The CIGS thin film solar cells conversion efficiency of 1.23 % on 1 × 1.5 cm2.

Large Scale Single-Crystal Cu(In,Ga)Se2 Nanotip Arrays for High Efficiency Solar Cell

Cu(In,Ga)Se2 (CIGS) is the best material choice for solar cell owing to the highest light absorption and tunable bandgap while the further improvement of efficiency has to rely on nanostructures, which has attracted much attention in recent years , . In this paper, we demonstrated direct formation of large area Cu(In,Ga)Se2 nanotip arrays (CIGS NTRs) by using one step Ar+ milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. Formation criteria of these CIGS NTRs have been discussed in terms of surface curvature, multiple components, and crystal quality, resulting in a highly anisotropic milling effect. The CIGS NTRs have very low reflectance < 0.1 % at incident wavelengths between 300 nm to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be ~390 mV and ~22.56 mA/cm2, yielding the filling factor and the efficiency of 59 % and 5.2 %, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2 %, the nanostructured CIGS NTRs can have efficiency enhancement of ~160 % due to the higher light absorption ability because of the nanostructure.

Quantitative analysis of Cu(In,Ga)Se2 thin films by secondary ion mass spectrometry using a total number counting method

The relative atomic fraction of Cu(In,Ga)Se2 (CIGS) films is one of the most important measurements for the fabrication of CIGS thin film solar cells. However, the quantitative analysis of multi-element alloy films is difficult by surface analysis methods due to the severe matrix effect. In this study, the quantitative analysis of CIGS films was investigated by secondary ion mass spectrometry (SIMS). The atomic fractions of Cu, In, Ga and Se in the CIGS films were measured by alloy reference relative sensitivity factors derived from the certified atomic fractions of a reference CIGS film. The total ion intensities of the constituent elements were obtained by a total number counting method. The atomic fractions measured by SIMS were linearly proportional to those certified by inductively coupled plasma mass spectrometry using an isotope dilution method. The uncertainties were determined from the standard uncertainties in the measurements and those of a CIGS thin film certified reference material.

Electronic properties of grain boundaries in Cu(In,Ga)Se2 thin films with various Ga-contents

We present a study on the electronic properties of grain boundaries (GBs) in polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films by means of Kelvin probe force microscopy. As grown as well as KCN-treated films were investigated comparatively. No influence of the chemical treatment on the electronic properties of GBs was found. GBs generally exhibited large variations in their electronic properties. By means of a novel method of data analysis both potential barriers for holes and electrons were found at GBs, in a range from −118mV to +114mV, as well as GBs without potential barrier. No dependence of the electronic GB-properties on the Ga-content was detected. Consequently, we conclude that there is no correlation between the electronic properties of GBs and the obtained maximum efficiencies of CIGSe thin film solar cells as a function of the Ga-content.

Development of Kesterite Cu2ZnSn(S1-x,Sex)4 (CZTSS)-Based Thin Film Solar Cells with In and Ga Free Absorber Materials

Chalcogenide-based semiconductors, such as CuInSe2, CuGaSe2, Cu(In,Ga)Se2 (CIGS), and CdTe have attracted considerable interest as efficient materials in thin film solar cells (TFSCs). Currently, CIGS and CdTe TFSCs have demonstrated the highest power conversion efficiency (PCE) of over 11% in module production. However, commercialized CIGS and CdTe TFSCs have some limitations due to the scarcity of In, Ga, and Te and the environmental issues associated with Cd and Se. Recently, kesterite CZTS, which is one of the In- and Ga- free absorber materials, has been attracted considerable attention as a new candidate for use as an absorber material in thin film solar cells. The CZTS-based absorber material has outstanding characteristics such as band gap energy of 1.0 eV to 1.5 eV, high absorption coefficient on the order of 104cm-1, and high theoretical conversion efficiency of 32.2% in thin film solar cells. Despite these promising characteristics, research into CZTS-based thin film solar cells is still incomprehensive and related reports are quite few compared to those for CIGS thin film solar cells, which show high efficiency of over 20%. The recent development of kesterite-based CZTS thin film solar cells is summarized in this work. The new challenges for enhanced performance in CZTS thin films are examined and prospective issues are addressed as well.

Quantitative elemental analysis of photovoltaic Cu(In,Ga)Se2 thin films using MCs+ clusters

In the process of optimizing solar cells, a quantitative and depth‐resolved elemental analysis of photovoltaic thin films is strongly required. Regarding Cu(In,Ga)Se2 (CIGS) thin film solar cells, depth‐dependent stoichometric changes of Ga and In are of great interest because the In/Ga ratio has a large effect on solar cell efficiencies. In this paper, we investigate the elemental composition of CIGS thin film solar cells based on secondary ion intensities in time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) depth profiling, providing high sensitivities and high spatial resolution. Quantification of the data is obtained by comparison to X‐ray photoelectron spectroscopy depth profiles. The detection of MCs+ clusters is used for semiquantitative elemental analysis of CIGS thin films. Correlation plots of the intensities of GaCs+ and InCs+ indicate that there is no relevant matrix effect for In and Ga due to changes in stoichiometry in the layer. Additional high‐resolution inductively coupled plasma mass spectrometry measurements show a strong correlation between the ratio of the bulk concentrations of Ga and In and the ratio of integrated ToF‐SIMS intensities of GaCs+ and InCs+ therefore supporting the quantitative interpretation of MCs+ data. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Analysis of laser scribes at CIGS thin-film solar cells by localized electrical and optical measurements

Laser patterning of thin-film solar cells is essential to perform external serial and integrated monolithic interconnections for module application and has recently received increasing attention. Current investigations show, however, that the efficiency of thin-film Cu(In,Ga)Se2 (CIGS) modules is reduced due to laser scribing also with ultrashort laser pulses. Hence, to investigate the reasons of the laser-induced material modifications, thin-film CIGS solar cells were laser-scribed with femto- and picosecond laser pulses using different scribing procedures and laser processing parameters. Besides standard electrical current voltage (I–V) measurements, additional electrical and optical analysis were performed such as laser beam-induced current (LBIC), dark lock-in thermography (DLIT), and electroluminescence (EL) measurements to characterize and localize electrical losses due to material removal/modifications at the scribes that effecting the electrical solar cell properties. Both localized as well as distributed shunts were found at laser scribe edges whereas the laser spot intensity distribution affecting the shunt formation. Already laser irradiation below the ablation threshold of the TCO film causes material modification inside the thin film solar cell stack resulting in shunt formation as a result of materials melting near the TCO/CIGS interface that probably induces the damage of the pn-junction.

8.01% CuInGaSe2 solar cells fabricated by air-stable low-cost inks

CuInGaSe(2) (CIGS), a promising thin film solar cell material, has gained lots of attention in decades due to its high energy conversion efficiency and potential lower manufacture cost over conventional Si solar cells. As a cheaper processing method compared to vacuum-based techniques, solution-based deposition has been successfully applied to fabricate electronic devices, such as transistors and solar cells. In this paper, we reported CIGS thin film solar cells with an energy conversion efficiency reaching up to 8.01% using air-stable, low-cost inks. The newly developed inks consist of commercially available, low-cost compounds and solvents and can be processed using a variety of printing and coating techniques. More importantly, the inks can produce CIGS films free of copper selenides and amorphous carbon, two common by-products from solution-based CIGS processes. The mechanism for the transformation from metal salt precursor films to CIGS absorber thin films and the influence of selenium vapour pressure on absorber film quality and photovoltaic device performance were investigated and discussed. High-quality CIGS films with micrometer-sized crystals were obtained by using higher selenization partial pressure.

Pulsed Laser Deposition ZnS Buffer Layers for CIGS Solar Cells

Polycrystalline ZnS films were prepared by pulsed laser deposition (PLD) on quartz glass substrates under different growth conditions at different substrate temperatures of 20, 200, 400, and 600 C, which is a suitable alternative to chemical bath deposited (CBD) CdS as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. X-ray diffraction studies indicate the films are polycrystalline with zinc-blende structure and they exhibit preferential orientation along the cubic phase -ZnS (111) direction, which conflicts with the conclusion of wurtzite structure by Murali that the ZnS films deposited by pulse plating technique was polycrystalline with wurtzite structure. The Raman spectra of grown films show A1 mode at approximately 350 cm1, generally observed in the cubic phase -ZnS compounds. The planar and the cross-sectional morphology were observed by scanning electron microscopic. The dense, smooth, uniform grains are formed on the quartz glass substrates through PLD technique. The grain size of ZnS deposited by PLD is much smaller than that of CdS by conventional CBD method, which is analyzed as the main reason of detrimental cell performance. The composition of the ZnS films was also measured by X-ray fluorescence. The typical ZnS films obtained in this work are near stoichiometric and only a small amount of S-rich. The energy band gaps at different temperatures were obtained by absorption spectroscopy measurement, which increases from 3.2 eV to 3.7 eV with the increasing of the deposition temperature. ZnS has a wider energy band gap than CdS (2.4 eV), which can enhance the blue response of the photovoltaic cells. These results show the high-quality of these substitute buffer layer materials are prepared through an all-dry technology, which can be used in the manufacture of CIGS thin film solar cells.

Activity and current status of R&amp;D on space solar cells in Japan

Japan's Research and Development (R&D) activities on high-performance III–V compound space solar cells are presented. Studies of new CuInGaSe2 thin-film terrestrial solar cells for space applications are also discussed. Performance and radiation characteristics of a newly developed InGaP/GaAs/Ge triple-junction space solar cell, including radiation response, results of a flight demonstration test of InGaP/GaAs dual-junction solar cells and CuInGaSe2 thin-film solar cells, and radiation response of three component sub-cells are explained. This study confirms superior radiation tolerance of InGaP/GaAs dual-junction cells and CuInGaSe2 thin-film cells by space flight experiments. Copyright © 2005 John Wiley & Sons, Ltd.

Novel In(OH)3:Zn2+ Buffer Layer for Cu(InGa)Se2 Based Solar Cells

A novel In(OH)3:Zn2+ buffer layer was proposed as an alternative for fabricating high-efficiency Cu(InGa)Se2 (CIGS) solar cells. The buffer layers were deposited by chemical bath deposition using ZnCl2, InCl3·4H2O and thiourea. The structural, electrical and optical properties of In(OH)3:Zn2+ thin films on quartz substrates were determined. Films with resistivity as high as 108 Ω·cm and a transmittance of more than 80% have been obtained by regulating the growth conditions. CIGS thin-film solar cells with a ZnO/In(OH)3:Zn2+/CIGS/Mo structure were fabricated. The CIGS absorbers used in this study were prepared using two different deposition processes: three-stage co-evaporation and selenization/sulfurization. The influence of growth conditions of the buffer layer on device performance and device metastability was investigated. The presence of Zn2+ ions in the deposition bath was found to be an important parameter for achieving both device efficiency and metastability. A cell efficiency of 14% without the light-soaking effect was achieved using the In(OH)3:Zn2+ buffer layer.

Nano-structural investigations on Cd-doping into Cu(In,Ga)Se2 thin films by chemical bath deposition process

Inter-diffusion at the Cu(In,Ga)Se2 (CIGS)/CdS interface of high-efficiency CIGS thin film solar cells has been investigated using energy dispersive X-ray spectroscopy (EDX) and field emission transmission electron microscopy (FETEM). CdS thin layers were epitaxially grown by chemical bath deposition (CBD) at a maximum bath temperature of 80°C on CIGS thin films grown with a molecular beam epitaxy (MBE) system. EDX analysis revealed that Cd was present in the thin CIGS layer approximately 100 Å from the interface boundary. In contrast to the diffusion of Cd, the Cu concentration decreased near the surface of the CIGS film, suggesting substitution of Cd for Cu atoms. These results are direct evidence of Cd-doping into CIGS thin films during the CBD process. On the other hand, no Cd was detected at the CIS single crystal/CdS interface within the detection limit of EDX. This is presumably because of a lack of Cu deficient surface layer in which Cd-diffusion easily occurs. Details of the CIGS/CdS interfacial nano-structure, which may play a role in diffusion properties are also presented.

Direct evidence of Cd diffusion into Cu(In, Ga)Se2 thin films during chemical-bath deposition process of CdS films

The diffusion behavior at the Cu(In, Ga)Se2 (CIGS)/CdS interface of high efficiency CIGS thin film solar cells has been investigated using energy dispersive x-ray spectroscopy (EDX) and transmission electron microscopy. CdS layers were deposited on CIGS thin films using the chemical bath deposition (CBD) process. EDX analysis revealed that Cd was present in the CIGS layer approximately 100 Å from the interface boundary. In contrast to the diffusion of Cd, the Cu concentration decreased near the surface of the CIGS film, suggesting substitution of Cd for Cu atoms. These results are direct evidence of Cd diffusion into CIGS thin films during the CBD process.

Characterization of ZnInxSey Thin Films as a Buffer Layer for High Efficiency Cu(InGa)Se2 Thin-Film Solar Cells

The structural, optical and electrical properties of ZnInxSey (ZIS) thin films on Cu(InGa)Se2 (CIGS) thin films and glass substrates were characterized. Polycrystalline ZIS thin films were grown by the coevaporation method using three constituent elements. We confirmed the formation of ZnIn2Se4 from the X-ray diffraction patterns of the ZIS thin films on glass substrates. From the transmittance and reflectance measurements of these films, the bandgap of ZIS is estimated at around 2.0 eV in this study. In addition, the ZIS films on glass substrates show low dark conductivity and high photosensitivity, which are suitable for the buffer layer in CIGS thin-film solar cells. We also fabricated the CIGS thin-film solar cells with a ZnO/ZIS/CIGS structure, and investigated the relationship between the cell performance and the beam intensity ratio of zinc to indium.

Polycrystalline Cu(InGa)Se2 Thin-Film Solar Cells with ZnSe Buffer Layers

A ZnSe buffer layer has been applied as an attractive alternative to a CdS buffer layer in the development of polycrystalline Cu(InGa)Se2 (CIGS) thin-film solar cells, thus eliminating entirely the use of cadmium by employing the ZnO/ZnSe/CIGS structure. Moreover, we propose the use of a new deposition method for ZnSe buffer layers, the atomic-layer deposition (ALD) method. This method is basically the same as an “atomic-layer epitaxy” method but is applied to polycrystalline materials. Currently the best efficiency of CIGS thin-film solar cells with an about 10-nm-thick ZnSe buffer layer is 11.6%. Applying irradiation with a solar simulator under one-sun (AM-1.5, 100 mW/cm2) conditions, the efficiency of these cells was improved from about 5% to over 11% due to increased open-circuit voltage and fill factor with no change in short-circuit current density even after six-hour irradiation.