CIGS Cells Research Articles

High photo-conversion efficiency in double-graded Cu(In,Ga)(S,Se)2thin film solar cells with two-step sulfurization post-treatment

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells and to improve the open circuit voltage (VOC) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double-graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two-step sulfurization process provides an increase of VOC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm2 monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm2), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two-step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two-step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two-step-sulfurization appeared to have less of Cu2-xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Improving efficiency (optimization) of CIGS thin film solar cell using absorbent layer profile change

CIGS thin film solar cell of solid solution combination of Copper-indium-selenide (CIS) and copper -Gallium – selenide CGS is an I-III-VI2 semiconductor which is one of the options to replace silicon solar cell due to high stability and direct band gap, it also has higher yields and lower price compared to other thin film solar cells. In this article, we will initially evaluate the effect of three major factors of thickness, impurity density and band gap of absorber layer which is the most important layer in solar cells on the output parameters including efficiency using AMPS (Analysis of Microelectronic and Photonic Structures) software. Then, for the first time we will provide two important strategies to improve the efficiency of CIGC thin film solar cell. Layers of this type of solar cell are: 1-TCO made of ZnO (zinc oxide) 2- CdS layer gate (cadmium sulphate) with impurities of n 3-CIGS absorber layer with the impurity of p, 4- 5 glass substrate molybdenum layer made of soda. Reduced thickness of the absorbent layer makes the back connection to be closer to the discharge area. J sc ,V oc , FF and Quantum efficiency (QE) decrease due to absorption of electrons of electrons to the surface of back connection and their participation in recomposition. Efficiency increases from 20.3399% to 21.3721% by increasing impurity density of absorbent layer and efficiency increases to 28.9266% and the quantum efficiency (QE) doubles by increasing the band gap of absorber layer which is a result of increasing the amount of gallium in CIGS cell. Then, we will achieve the efficiency of 34.1512% which has improved by 5.2246% compared to previous state by providing a way to reduce recombination at the intersection of absorbent / final connection by adding an extra layer made of absorbent layer with more impurities. In the final step, we improved the efficiency by 3.8581% (38.0093%) for the first time by increasing the band gap of CIGS layer. The solar cell used in this cellular thesis has characteristics such as J sc = 36.7059&, V oc = 0.6721%, FF = 82.4422% and η = 20.3393%. Keywords: CIGS thin film solar cell, absorbent layer, improving efficiency, quantum efficiency QE, and recombination

Efficiency enhancement of ultra-thin Cu(In,Ga)Se2 solar cells: optimizing the absorber bandgap profile by numerical device simulations

Thin-film chalcopyrite Cu(In1−x,Gax)Se (CIGS) solar cells have recently achieved an efficiency of ∼22% at the lab scale, making the technology more promising for commercial applications than most other thin-film solar cells. Using numerical device simulations, this study provides approaches to enhance the efficiency of ultra-thin CIGS solar cells. Effects of various Ga grading profiles in the CIGS absorber and of surface bandgap modifications are simulated. Our simulation results reveal that, in ultra-thin CIGS solar cells, back grading is an effective and practical approach to increase the cell efficiency, while front grading is unfeasible due to unacceptable current and fill factor losses. The quality of the back surface is of particular importance in moderately graded cells, while interface and bulk defect properties dominate in extremely graded cells. By introducing an ordered vacancy compound (OVC) layer with a downward-shifted valence band at the CIGS surface, the interface recombination losses can be significantly suppressed due to the reduced hole concentration. The thickness of the OVC layer and the valence band offset (VBO) between the OVC and CIGS materials are critical parameters for the cell efficiency. The simulations reveal that an optimized CIGS cell with a 300 nm thick CIGS absorber, a back-graded absorber profile and a 70 nm thick OVC layer at the CIGS surface can reach a 1-Sun efficiency of over 12%.

Science and Technology of High Efficiency Solar Cells and Materials

Solar photovoltaics are expected to contribute as one of major energy sources in the future. For this end, further development of high-efficiency, low-cost and high reliable solar cells, materials, modules and systems is necessary. This paper reviews fundamentals of solar cells and materials and present recent R&D status of various solar cells and materials and their future. In order to realize high efficiency and highly reliable solar cells, understanding and controlling imperfections (defects) are very important. There are several loss mechanisms to be solved for realizing high-efficiency and highly reliable solar cells. 1) bulk recombination loss, 2) surface recombination loss, 3) interface recombination loss, 4) voltage loss, 5) fill factor loss, 6) optical loss, 7) insufficient –energy photon loss. Key technologies for reducing the above losses are high quality film growth, reduction in density of defects, optimization of carrier concentration in base and emitter layers, double-hetero (DH) junction structure, lattice-matching of active layers and substrate, surface and interface passivation, reduction in series resistance and leakage current, anti-reflection coating, photon recycling and so forth. The multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency multi-junction (MJ) solar cells, understanding and controlling imperfections (defects) are also very important. The key issues for realizing super-high-efficiency MJ solar cells are 1) sub cell material selection, 2) tunnel junction for sub cell interconnection, 3) lattice-matching, 4) carrier confinement, 5) photon confinement, 6) anti-reflection in wide wavelength region and so forth. For concentrator applications by using MJ cells, the cell contact grid structure should be designed in order to reduce the energy loss due to series resistance, and tunnel junction with high tunnel peak current density is necessary. Because cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells, effectiveness of double hetero structure tunnel diode is also presented in this paper. Present status of various solar cells efficiencies are presented: 46.0% for concentrator III-V compound 4-junction solar cell, 38.8% for 1-sun III-V compound 4-junctiion cell, 25.6% for single-crystal Si cell, 21.7% for CIGS cell, 21.5% for CdTe cell, 20.1% for Perovskite cell, 13.6% for a-Si based 3-junction cell, 11.9% for dye-sensitized cell and 11.1% for organic cell. Efficiency potential of various solar cells is also discussed.

Optimization of the CIGS solar cell by adding a heavily doped layer and an intrinsic layer at absorber layer

Solar conversion efficiency of CIGS solar cells is investigated in this work. It is found that the efficiency can be improved if heavily doped layer is added to intrinsic absorber layer in CIGS solar cells. On the other hand, the presence of an interface between the high and low doped regions behaves like a p–n junction. As a result, an electric field for the minority electrons is created that decreases surface recombination in rear surface. In order to improve the parameters of CIGS solar cell, we investigated the effect of different thicknesses and doping concentrations of i-layer and p+ layer in the absorber layer. Finally, it had been found that the ZnO/CdS/i-CIGS/p+-CIGS structure in which at the thickness of i and p+ layers with 10 nm and 2.99 µm will have an efficiency ≈3.1 % higher than that of the basic CIGS cell.

Assessing Rare Metal Availability Challenges for Solar Energy Technologies

Solar energy is commonly seen as a future energy source with significant potential. Ruthenium, gallium, indium and several other rare elements are common and vital components of many solar energy technologies, including dye-sensitized solar cells, CIGS cells and various artificial photosynthesis approaches. This study surveys solar energy technologies and their reliance on rare metals such as indium, gallium, and ruthenium. Several of these rare materials do not occur as primary ores, and are found as byproducts associated with primary base metal ores. This will have an impact on future production trends and the availability for various applications. In addition, the geological reserves of many vital metals are scarce and severely limit the potential of certain solar energy technologies. It is the conclusion of this study that certain solar energy concepts are unrealistic in terms of achieving TW scales.

Multilayer Transparent Top Electrode for Solution Processed Perovskite/Cu(In,Ga)(Se,S)2 Four Terminal Tandem Solar Cells.

Halide perovskites (PVSK) have attracted much attention in recent years due to their high potential as a next generation solar cell material. To further improve perovskites progress toward a state-of-the-art technology, it is desirable to create a tandem structure in which perovskite may be stacked with a current prevailing solar cell such as silicon (Si) or Cu(In,Ga)(Se,S)2 (CIGS). The transparent top electrode is one of the key components as well as challenges to realize such tandem structure. Herein, we develop a multilayer transparent top electrode for perovskite photovoltaic devices delivering an 11.5% efficiency in top illumination mode. The transparent electrode is based on a dielectric/metal/dielectric structure, featuring an ultrathin gold seeded silver layer. A four terminal tandem solar cell employing solution processed CIGS and perovskite cells is also demonstrated with over 15% efficiency.

Identifying parasitic current pathways in CIGS solar cells by modelling dark J–V response

AbstractThe non‐uniform presence of shunting defects is a significant cause of poor reproducibility across large‐area solar cells, or from batch‐to‐batch for small area cells, but the most commonly used value for shunt parameterisation (the shunt resistance) fails to identify the cause for shunting. Here, the use of equivalent circuit models to describe dark current–voltage characteristics of ZnO:Al/i‐ZnO/CdS/CIGS/Mo devices in order to understand shunting behaviour is evaluated. Simple models, with a single shunt pathway, were tested but failed to fit experimental data, whereas a more sophisticated model developed here, which includes three shunting pathways, yielded excellent agreement throughout the temperature range of 183–323 K. The temperature dependence of fitting parameters is consistent with known physical models. Activation energies and contact barriers are determined from the model, and extracted diode factors are unique across the voltage range. A case study is presented whereby the model is used to diagnose poor reproducibility for CIGS devices (efficiency ~3–14% across a 100 cm2 plate). It's shown that lower efficiencies correlated with greater prevalence of Ohmic and non‐Ohmic shunt currents, which may form due to pinholes in absorber and buffer layers respectively, whereas the quality of the main junction was constant for all cells (diode factor ~1.5–2). Electron microscopy confirmed the presence of ZnO:Al/i‐ZnO/Mo and ZnO:Al/CIGS/Mo regions, supporting the multi‐shunt pathway scheme disclosed by modelling. While the model is tested with CIGS cells here, this general model is a powerful diagnostic tool for process development for any type of thin‐film device. Copyright © 2015 John Wiley & Sons, Ltd.

Graded Carrier Concentration Absorber Profile for High Efficiency CIGS Solar Cells

We demonstrate an innovative CIGS-based solar cells model with a graded doping concentration absorber profile, capable of achieving high efficiency values. In detail, we start with an in-depth discussion concerning the parametrical study of conventional CIGS solar cells structures. We have used the wxAMPS software in order to numerically simulate cell electrical behaviour. By means of simulations, we have studied the variation of relevant physical and chemical parameters—characteristic of such devices—with changing energy gap and doping density of the absorber layer. Our results show that, in uniform CIGS cell, the efficiency, the open circuit voltage, and short circuit current heavily depend on CIGS band gap. Our numerical analysis highlights that the band gap value of 1.40 eV is optimal, but both the presence of Molybdenum back contact and the high carrier recombination near the junction noticeably reduce the crucial electrical parameters. For the above-mentioned reasons, we have demonstrated that the efficiency obtained by conventional CIGS cells is lower if compared to the values reached by our proposed graded carrier concentration profile structures (up to 21%).

C112 スマートスタック多接合太陽電池の集光条件下における発電特性評価(OS-06:ふく射輸送制御(1))

This paper describes the effect of using the low-concentration to the multi-junction solar cell module fabricated by the smart stacking technology. In this module, a high-efficiency cell (e.g. III-V MJ cell) is mechanically stacked on a larger-sized low-cost cell (e.g. Si or CIGS cell); the current matching between the stacked cells is managed by the area ratio of the cells and the irradiance distribution controlled by the concentrator. Simulation and experimental result shows that the control of irradiance distribution assists the current matching of the cells and reduces the top cell size without decreasing the module efficiency, resulting in higher efficiency and lower cost.

CIGS，CZTS形成用非真空プロセス

CIGS，CZTS形成用非真空プロセス

Space Environments and Effects on CIGS Solar Cells and Modules

ABSTRACTCu (In, Ga) Se2 (CIGS) thin-film solar cells are optimal solar cells for spacecraft, since they have high efficiency, lightweight, flexible and high radiation tolerance. The CIGS solar module without a coverglass to prevent degradation in space has been demonstrated with a small satellite and its electrical performance indicates no degradation as predicted from ground tests. However, the cells need to prevent the damages from other effects in space. The paper introduces some space environment tests and how to improve performance in CIGS cells for spacecraft.

Performance Limits and Status of Single-Junction Solar Cells With Emphasis on CIGS

Limitations in performance and the status of single-junction solar cells are reviewed. Conversion efficiency in single-junction solar cells is systematically analyzed in terms of energy conversion efficiency, the Shockley-Queisser (SQ) efficiency limit, and two remaining efficiencies, i.e., optical efficiency and electrical efficiency normalized to the SQ limit of performance parameters. There is a weak dependence of the SQ efficiency limit in single-junction solar cells on the bandgap of the absorber and much stronger dependence of the electrical efficiency. In the case of Cu(In,Ga)(Se,S) 2 (CIGS) solar cells, the bandgap can be varied through In/Ga and/or Se/S content from 1.04 to 1.7 eV, which allows considerable opportunity to optimize the bandgap. Although the record CIGS cells have a bandgap below 1.2 eV, the SQ efficiency limit predicts slightly higher efficiencies for bandgaps near 1.34 eV.

CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: A comparison of efficiency, stability and device topology

Sustainable solar hydrogen production could be accomplished along several, somewhat different routes. In this paper, the focus is directed towards photoelectrochemical cells and PV-electrolyzers. In particular, the differences and similarities between these two seemingly different approaches are investigated. In a previous article, a theoretical analysis of absorption, charge carrier separation, charge carrier transfer, and catalysis demonstrated that the difference between PEC-cells and PV-electrolyzers for solar hydrogen production is so small that they can be perceived as one unifying concept, termed PDC, or photodriven catalytic hydrogen production. In this paper this perspective is explored further by experimentally constructing a series of devices that stepwise and gradually are spanning from classical PEC-cells to traditional PV-electrolyzers. In each step, the difference and similarities are measured and discussed in terms of the underlying physical processes, the efficiency, the stability, and the device geometry. All the constructed devices were based on CIGS, CuInxGa1−xSe2, which was demonstrated to be a highly interesting material for the purpose of solar hydrogen production. Devices were constructed for both the hydrogen evolution half-reaction, as well as for the full reaction. Among the most notably of these devices is a stable monolithic device based on CIGS cells interconnected in series, which reached 10% solar-to-hydrogen efficiency for unbiased water splitting. Experimentally, it is found that small changes in the device geometry can transform one device concepts into another with only minute changes in the underlying physics. The series of constructed devices thus bridge and merge the notions of photoelectrochemical cells and PV-electrolyzers. It is further demonstrated that small topological differences in the device architecture can have a profound impact on both the stability and the efficiency of the devices.

Numerical Simulation of Quantum Efficiency of Cd0.8Zn0.2S /CIGS Solar Cells

The paper presents a simulation study using the numerical simulator SCAPS-1D to model ZnO/Cd0.8Zn0.2S/CuIn(1-y)GaySe2/CuInSe2 structures. Effects of thickness of graded and ungraded CIGS absorbers and buffer layers on cell performance have been investigated with the aim to reach a higher efficiency. Quantum efficiency (QE) as function of wavelength and thickness of these layers was studied. The high efficiency of CIGS cells, in order of 22.05%, has reached with the absorbers thickness between 2μm and 3.5μm and with acceptor concentration of about 2.1016 cm3. Other hand, we investigate the effect of Cd0.8Zn0.2S ternary compound buffer on the top of the p-CIGS cell. These simulation results give some important indication to enable further development of multilayer thin-film solar cells based on CuInGaSe2 with Cd0.8Zn0.2S as buffer layer instead of CdS

Behavior of deep level defects on voltage-induced stress of Cu(In,Ga)Se 2 solar cells

Abstract The behavior of deep level defects by a voltage-induced stress for CuInGaSe 2 (CIGS) solar cells has been investigated. CIGS solar cells were used with standard structures which are Al-doped ZnO/i-ZnO/CdS/CIGSe 2 /Mo on soda lime glass, and that resulted in conversion efficiencies as high as 16%. The samples with the same structure were isothermally stressed at 100 °C under the reverse voltages. The voltage-induced stressing in CIGS samples causes a decrease in the carrier density and conversion efficiency. To investigate the behavior of deep level defects in the stressed CIGS cells, photo-induced current transient spectroscopy was utilized, and normally 3 deep level defects (including 2 hole traps and 1 electron trap) were found to be located at 0.18 eV and 0.29 eV above the valence band maximum (and 0.36 eV below the conduction band). In voltage-induced cells, especially, it was found that the decrease of the hole carrier density could be responsible for the increase of the 0.29 eV defect, which is known to be observed in less efficient CIGS solar cells. And the carrier density and the defects are reversible at least to a large extent by resting at room-temperature without the bias voltage. From optical capture kinetics in photo-induced current transient spectroscopy measurement, the types of defects could be distinguished into the isolated point defect and the extended defect. In this work, it is suggested that the increase of the 0.29 eV defect by voltage-induced stress could be due to electrical activation accompanied by a loss of positive ion species and the activated defect gives rise to reduction of the carrier density.

Interface-Barrier-Induced J–V Distortion of CIGS Cells With Sputtered-Deposited Zn(S,O) Window Layers

Sputtered-deposited Zn(S,O) is an attractive alternative to CdS for the window layer of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells due to its higher band gap, which allows greater blue-photon collection. However, distortions to current-voltage ( J-V) curves are observed in some cases. A straightforward photodiode model with a secondary barrier at the Zn(S,O)/CIGS interface is employed to explain the physical mechanisms of the experimental J-V distortions. The primary contributor to the secondary barrier is the conduction-band offset (CBO), whose magnitude is determined by the oxygen fraction in Zn(S,O) and by the carrier density of Zn(S,O); the latter may be increased with indium-doping. Comparison of experimental and simulated J-V characteristics with variation in oxygen fraction, window carrier density, and temperature, allows a reasonably compelling test of the secondary-barrier model for this system.

Substitution of the CdS buffer layer in CIGS thin‐film solar cells

AbstractThis contribution provides an overview of current activities in the area of alternative buffer layers for Cu(In,Ga)(S,Se)2 (CIGS) thin‐film solar cells. Good cell and module results were achieved by replacing the standard Cds buffer with Zn(O,S), In2S3, (Zn,Sn)Oy or (Zn,Mg)O grown by various methods like chemical bath deposition (CBD), thermal evaporation, sputtering, atomic layer deposition, and spray ion layer gas reaction. The “dry” deposition methods like sputtering and thermal evaporation could be favorable in an industrial environment on glass substrates or application in a roll‐to‐roll coater. Significant progress was made within the last two years for various Cd‐free CIGS devices. We list current records for cells with alternative buffers, e. g. Zn(O,S)‐buffered champion cells with efficiencies between 18—20 % and In2S3‐buffered cells with 16—17 %. Both materials have the potential to substitute CdS with efficiencies approaching the 20 % mark already surpassed by CIGS cells with CBD CdS buffers.

Efficiency loss prevention in monolithically integrated thin film solar cells by improved front contact

AbstractFor thin film solar cells, there is a large gap between the record efficiencies and panel power output. It was found that for a “typical industrial” CIGS cell efficiency of 15.5%, the efficiency drops to 11.7% when it is operating under the circumstances of a monolithically integrated solar panel. Part of this gap is due to limited conductivity and transmittance of the front contact. By application of a metallic grid, the conductivity can be improved by over two order of magnitude at a transmittance loss of only a few percent as was shown experimentally. In addition, modeling was used to quantify the impact of such approach on the power output of monolithically integrated solar panels. This model includes optical and resistive losses, as well as related losses caused by the inhomogeneity of the operating voltage over the surface. Both power output and the different types of losses are mapped out for various cell configurations. Optimization of transparent conductive oxide resistance, cell length, finger width, and finger spacing of grids was performed and led to an efficiency improvement from 11.7% to 13.8% when the front contact is upgraded with a metallic grid consisting of 20 µm wide parallel fingers positioned perpendicular to the interconnect. Further optimization for a wide variety of cell and grid configurations show that for a technically more feasible size of 100 µm wide fingers, the calculated efficiency is still 13.5%. Finally, the power output is mapped out for a large number of configurations as to create an overview and insight in the interdependencies of cell configuration and finger dimensions. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of Zn Diffusion from (Zn,Mg)O Buffer to CIGS Film on the Performance of Cd-Free Cu(In,Ga)Se2 Solar Cells

A diffusion of Zn into the CIGS film and its effects on the device performance were investigated when the atomic layer deposited (Zn,Mg)O film was used as a buffer layer of CIGS solar cells. SIMS and ICP mass analysis showed that the Zn concentration in the CIGS film was increased with increasing ALD process time. The hole concentration in the CIGS film was decreased by Zn incorporation. It was suggested that open circuit voltage degradation in CIGS cell with Zn incorporation was mainly due to enhanced recombination by Zn defects instead of Fermi-level change. The results suggest that the ALD process should be as short as possible to avoid Zn diffusion. On the other hand, a small amount of Zn incorporation on the surface of the CIGS film by post rapid thermal annealing of a (Zn,Mg)O/CIGS cell at 200°C showed an increased cell performance. However, excess Zn incorporation on the surface of the CIGS film caused degradation of cell performance due to the generation of a new defect. The generation of a deep-level defect and the change of the minority carrier lifetime were characterized by low-temperature PL and time-resolved PL, respectively. The cell performance was correlated with these results.