Copper Indium Gallium Diselenide Solar Cells Research Articles

Simulation of new thin film Zn(O,S)/CIGS solar cell with bandgap grading

Copper-Indium-Gallium-diSelenide (CIGS) thin film solar cell is a promising candidate for energy harvesting because of its high absorption coefficient and low cost compared to silicon-based solar cells. Absorber layer bandgap grading is a suitable method to improve CIGS thin film solar cell performance. Bandgap grading leads to a decrease in the recombination rate at the rear surface, which increases the open circuit voltage. Furthermore, bandgap grading improves the short circuit current due to the enhancement of collection probability. This paper introduces various routes for improving the performance of thin film CIGS solar cells by using bandgap grading. As a first step, both the bandgap energy and the thickness of the CIGS absorber layer of a uniform bandgap profile are optimized to get the best performance. Simulation is performed using SCAPS software and optimization results show that CIGS absorber layer with a bandgap of 1.2 eV and a thickness of 0.7 μm achieves a 22.48% efficiency. Then, bandgap grading with a parabolic distribution of various profiles is investigated and compared. It is found that with a parabolic double bandgap grading profile, which is a combination of front and back grading, an efficiency of up to 24.16% is achieved. This improvement is obtained using a gallium composition ratio of 0.1 for the minimal bandgap at 0.1 μm and 0.13 μm from the back contact and front contact, respectively. This result represents a 7.47% improvement compared to the baseline structure of a CIGS solar cell.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Effects of Layer Thickness on the Residual Stresses of CIGS Solar Cells with Polyimide Substrate

In this paper, we investigate the effect of layer thickness on the residual stresses of copper indium gallium diselenide (CIGS) solar cells with polyimide substrate caused by CIGS layer deposition at 400?C and then cooling down to room temperature using the Finite Element Method (FEM). Moreover, we also examined the effect of layer thickness on residual stress of CIGS solar cells after cooling down to room temperature from the hotspot temperatures of 200?C, 300?C, and 400?C. Our simulated CIGS is composed of five layers: ZnO, CdS, CIGS, Mo, and PI substrate. We were able to quantify the effect of each layer’s thickness and hotspot temperature on the average stresses of each layer for the CIGS solar cells. We found that the PI substrate layer has the most significant effect on the residual stress of CIGS solar cells. Our simulation results reveal that the stress type (tensile vs. compressive) and the magnitude of stress of the CIGS layer (main absorber layer) can be controlled by changing the thickness of the PI substrate while applying a heat to CIGS solar cells. Quantitative analysis of relationship between layer thickness and thermo-mechanical stress of thin film solar cells can help solar cell manufacturers design more robust and reliable solar cells. For example, fabricating PI layer thickness less than 17 μm can improve the performance of CIGS solar cells by nullifying the compressive residual stress in the CIGS absorber layer.

Effect of absorber layer bandgap of CIGS-based solar cell with (CdS/ZnS) buffer layer

This article uses computational models to evaluate the potential of copper-indium-gallium-diselenide (CIGS) thin film solar cells. The use of cadmium sulphide (CdS) renders the solar cell environmentally hazardous. A zinc sulphide (ZnS) that is non-toxic and has a large bandgap is studied as a potential replacement for cadmium sulphide in CIGS-based solar cells. The present research focuses on the impact of the CIGS-based solar cell bandgap absorber layer by increasing the absorber layer thickness (0.1-2 μm) using the solar cell simulator simulation tool SCAPS. The basic simulation produces 18.2 % efficiency with a CdS buffer layer, which is 9.95% better than the previously published work. The Simulated efficiency is 22.16% for the CIGS solar cell using ZnS. The simulation of solar cell characteristics of how the thickness of the absorber layer, the gallium grading (efficiency ranges up to 22.25 %) is demonstrated, showing the effect of buffer layer (ZnS) on the current of short-circuit density (JSC), open-circuit voltage (Voc), fill factor (FF), and efficiency (η) of the solar cell.

A compact and efficient angle-resolved X-ray fluorescence spectrometer for elemental depth profiling

An angle resolved X-ray fluorescence spectrometer based on the concept of scanning-free shallow detection with energy-dispersive area detectors is presented. The instrument is characterized with respect to energy resolution, linearity, angular discrimination and repeatability and the necessary data evaluation strategies are presented in detail. As demonstration of its capabilities and showcase for potential applications, two different copper indium gallium (di)selenide (CIGS) absorber layers, typically applied in thin film solar cells, are analyzed. In combination with quantitative (but integral) results for layer composition and layer thickness from a commercial XRF spectrometer, depth profiles of the Ga concentration of both ≈ 2 μm thin samples are obtained. The results of the novel spectrometer compare well with quantitative depth-profiles obtained from glow-discharge optical emission spectrometry. The combination of simplicity, stability and efficiency of the spectrometer concept makes it a potential candidate for industrial applications.Description of compact angle-resolved X-ray fluorescence spectrometer. Exploiting scanning-free grazing emission X-ray fluorescence approach. Energy dispersive charge-coupled device with single-photon detection. Enabling non-destructive, quantitative elemental depth profiling. Depth profiling of nano-structured stratified specimen, here CIGS solar cells.

Optimized CIGS based solar cell towardsan efficient solar cell: impact of layers thickness and doping

This paper presents a numerical simulation study of copper-indium-gallium-diselenide (CIGS) thin film solar cells. An electron back reflector layer (EBR) is added to the conventional CIGS structure to minimize the recombination of the carriers at the back contact, and then absorber thickness can be further decreased. The impacts of thickness and carrier concentration variations of the CIGS cell structure are investigated to optimize the performance of the solar cell using the simulator SCAPS. The proposed CIGS solar cell achieved a significant improvement in the conversion efficiency of 27.652%, which is more than 43% higher than those previously reported in the literature for the CIGS solar cells. The proposed work does not only greatly improve the efficiency but also the overall thickness of the CIGS layers are dramatically decreased to 505 nm. That is already 3 to 4 times less than the average thickness reported for the conventional CIGS solar cells.

Novel cost-effective approach to produce nano-sized contact openings in an aluminum oxide passivation layer up to 30 nm thick for CIGS solar cells

This work presents a novel method of local contact openings formation in an aluminum oxide (Al2O3) rear surface passivation layer by the selenization of the lithium fluoride (LiF) salt on top of the Al2O3 for ultra-thin copper indium gallium (di)selenide (CIGS) solar cells (SCs). This study introduces the potentially cost-effective, fast, industrially viable, and environmentally friendly way to create the nano-sized contact openings with the homogeneous distribution in the thick, i.e. up to 30 nm, Al2O3 passivation layer. The passivation layer is deposited by atomic layer deposition, while the LiF layer is spin-coated. Selenization is done in the H2Se atmosphere and the optimal process parameters are deduced to obtain nano-sized and uniformly allocated openings as confirmed by scanning electron microscopy images. The contact openings were produced in the different thicknesses of the alumina layer from 6 nm to 30 nm. Furthermore, the Al2O3 rear surface passivation layer with the contact openings was implemented into ultra-thin CIGS SC design, and one trial set was produced. We demonstrated that the created openings facilitate the effective current collection through the dielectric Al2O3 layer up to 30 nm thick. However, the upper limit of Al2O3 thickness in which the contact openings can be created by the described method is not established yet. The produced passivated CIGS SCs show increased external quantum efficiency response due to the optical enhancement of the passivated cells. However, the production of SCs on the Al2O3 passivation layer with the openings created by selenization of LiF is not optimized yet.

Defect states and grading induced bandgap variability analysis of CIGS solar cells through device simulations

Copper-indium-gallium-diselenide (CIGS) is one of the promising absorbing material in thin film solar cells. It is a direct band gap material with the band gap (Eg) ranging from 0.98 to 1.68 eV. The band gap of this material is dependent on the composition of the material. The band gap enhancement leads to significant reduction in the extent of exciton recombination but on the other hand it also reduces the absorption in the absorber layer. We simulated and analyzed the effect of band gap variations in CIGS layer, induced by grading of doping concentration profile and defect states over the performance of solar cell. The performance analysis has been done using SCAPS-1D (Solar Cell Capacitance Simulator) simulator. It is intended to develop specially to simulate the AC and DC characteristics of hetero-junction thin film solar cells, especially CIGS and CdTe solar cells. By the introduction of grading in the CIGS material, an enhancement of 2.48% has been observed in the overall efficiency. This enhancement can be attributed to the reduction in recombination and increase in the exciton generation.

An optimized efficient double junction CGS / CIGS solar cell with improved performance

A new solar cell structure was presented in this paper, which consists of two p-n junctions of copper indium gallium diselenide (CIGS) and copper gallium diselenide (CGS) and double anti-reflector layers of cadmium sulfide and zinc telluride. The structure of the solar cell was designed and optimized. The current matching was achieved with tuning the thickness of the anti-reflector layers. The optimization procedure was performed by adjusting the CIGS bandgap, doping concentration, and thickness of semiconductor layers. The appropriate characteristics of open-circuit voltage equal to 2.03 V and short-circuit current density (Jsc) equal to 25.568 mAcm2 were obtained with optimum efficiency of 45.23 percent.

Modeling Thermo-Mechanical Stress of Flexible CIGS Solar Cells

Copper indium gallium diselenide (CIGS) thin-film solar cells are fabricated through several deposition and annealing processes at high temperatures, which can generate significant thermal residual stress to solar cells. Moreover, since CIGS solar cells with flexible substrates are rollable and bendable, they are susceptible to mechanical stresses during these processes. In addition, partial shading (hotspot) can exert high heat on the CIGS solar cell. In this paper, we investigate the thermo-mechanical stress of each active layer of CIGS solar cells due to annealing, external bending, and hotspot using finite element method (FEM). We found that average stress of each active layer decreases and maximum stress in the cell increases when interface crack is introduced between cadmium sulfide (CdS)/CIGS. Our overarching goal is to quantify the relationship between the fabrication/operating process and the reliability of CIGS solar cells (energy release rate and internal stresses), which could improve the reliability of flexible solar cells. It is also found that lowering the annealing temperature can reduce the stresses in the cells and lowering CIGS thickness can reduce the delamination probability of CdS/CIGS interface. Finally, we investigate the effect of the crack length of the CdS/CIGS interface on the electrical performance of CIGS solar cells through FEM simulations. We found that as the crack size between CdS and CIGS layers increases, short-circuit current density decreases, while open-circuit voltage remains almost constant.

Optical study of back-contacted CIGS solar cells.

A novel back-contacted solar cell based on a submicron copper indium gallium (di)selenide (CIGS) absorber is proposed and optically investigated. First, charge carrier collection feasibility is studied by band diagram analysis. Then, two back-contacted configurations are suggested and optimized for maximum current production. The results are compared with a reference front/back-contacted CIGS solar cell with a 750-nm-thick absorber. Current density production of 38.84 mA/cm2 is predicted according to our simulations for a realistic front-side texturing. This shows more than 38% improvement in optical performance compared to the reference cell and only 7.7% deviation from the theoretical Green absorption benchmark.

Comprehensive characterization of CIGS absorber layers grown by one-step sputtering process

We have demonstrated that the use of a one-step sputtering process allowed for the fabrication of copper indium gallium diselenide (CIGS) thin films by RF magnetron sputtering without an additional selenization process. The CIGS thin films deposited at different substrate temperatures were synthesized on soda-lime glass (SLG) substrates using a single quaternary CIGS target. The film composition ratios of ([Cu]/[In]+[Ga]), ([Ga]/[In]+[Ga]), and ([Se]/[Cu]+[In]+[Ga]) were almost consistent with those of the sputtering target. X-ray diffraction (XRD) and Raman results showed that the crystallinity of the CIGS thin films was gradually improved as substrate temperatures increased. Transmission electron microscopy (TEM) showed that the films grown at 600 °C have a columnar structure with the grain size of ~100 nm. In addition, for the CIGS films grown at 600 °C, TEM-EDX analysis revealed that the synchronized fluctuation of the Cu and Se signals was observed in the direction of the film depth, while the In and Ga signals were constant. As a result, the CIGS solar cell made using the film showed a degraded cell efficiency of 2.5%, which might be have been caused by not only Cu-rich and Se-poor compositions but the locally unstable composition in the CIGS films fabricated by one-step sputtering.

A study to improve light confinement and rear-surface passivation in a thin-Cu(In, Ga)Se2 solar cell

Reducing the absorber layer thickness below 1 μm for a regular copper indium gallium di-selenide (CIGS) solar cell lowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incomplete light absorption and high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rear surface modifications on a 430 nm thick CIGS absorber layer to reduce both these affects: an aluminium oxide passivation layer to reduce the back-surface recombination and point contact openings using nano-particles for electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.

New insights on the chemistry of plasma-enhanced atomic layer deposition of indium oxysulfide thin films and their use as buffer layers in Cu(In,Ga)Se2 thin film solar cell

A comparative chemical analysis of InxSy and In2(S,O)3 thin films grown by atomic layer deposition (ALD) and plasma-enhanced ALD, respectively, was performed to understand the challenges and issues related to the assistance of plasma, especially for the implementation of these films as ultrathin (<50 nm) interfacial buffer layers in copper indium gallium diselenide (CIGS) solar cells. The films were synthesized using indium acetylacetonate [In(acac)3], hydrogen sulfide, and an Ar/O2 plasma as indium, sulfur, and oxygen precursors. Film growth mechanisms and chemistries were studied using gas phase measurements by quadrupole mass spectrometry and x-ray photoelectron spectroscopy for surface and in-depth characterizations. Distinctive signatures of thermal and plasma processes on the overall compositions of the films were evidenced, which were further discussed and explained. Added to this, the impact of the plasma on the underlying substrate, using silicon as a reference, was further investigated to identify its modification. This extensive study has led to a readjustment of the deposition conditions of In2(O,S)3 thin films and allowed promising implementation as buffer layers in CIGS solar cells.

Effects of Mechanical Damage and Temperature on the Electrical Performance of CIGS Thin-Film Solar Cells

In this paper, we investigate the effect of mechanical damage and temperature on copper indium gallium diselenide (CIGS) thin-film solar cells through experiments and modeling. After generating mechanical damage on CIGS solar cell with 20% increments, the electrical performance was measured (current–voltage curve) while temperature was varying from 10 to 70 °C at 20° increments. Other measured values are open-circuit voltage ( V oc), fill factor (ff), maximum power ( P max). Those electrical values ( V oc, ff, P max) are found as a function of temperature and percent damage. Moreover, the parameters of the single diode solar cell model were obtained as a function of temperature and percentage damage: light generation current ( IL ), saturation current ( I0 ), shunt resistance ( R sh), and series resistance ( R s ). Our paper contributes to the deeper understanding of the concurrent effect of temperature and mechanical damage to solar cells.

Formation of Cl-Doped ZnO Thin Films by a Cathodic Electrodeposition for Use as a Window Layer in CIGS Solar Cells.

Zinc oxide films that are prepared by radio frequency (RF) sputtering are widely used as window layers in copper indium gallium diselenide (CIGS) solar cells. To reduce their production cost, the electrodeposition method for preparing Cl-doped zinc oxide (ZnO:Cl), rather than sputtering, was studied. The electrodeposition parameters of injected current density and the pH of the electrolyte solution were studied. A moderate current density was used to yield high quality zinc oxides. The pH of the electrolyte greatly affected the formation of ZnO films. The pH value of the electrolyte that ensured that zinc oxides of high quality are obtained was close to seven. Electrodeposited ZnO:Cl films had higher transmittance than ZnO:Al films in the near-infrared region and so they can be used to improve the performance of solar cells. Our experiments revealed that the CIGS solar cells with electrodeposited ZnO:Cl films as a window layer were slightly more efficient than those with sputtered ZnO:Al films.

Preparation and characterization of micron-scale molybdenum metal mesh electrodes

A promising candidate for a top cell in a tandem solar cell setup is a superstate, high bandgap, CIGS (Copper Indium Gallium Diselenide) solar cell. To realize a tandem cell, both, front and back contact of the top cell must be transparent. In this work a molybdenum metal mesh electrode with four pattern derivatives is presented, which are designed towards this implementation in a tandem cell setup. The electrode consists of two-micron wide fingers arranged in an industry typical H-grid pattern and in a hexagonal shaped pattern, with each of these patterns being comprised of a derivative with and without tapered busbars. The electrode's micro structure, crystal orientation, surface roughness, electrical properties and optical properties are investigated. The manufacturing process as well as a comparison of this works electrodes with various competing electrode types is also presented. All electrode derivatives show excellent transmittance of up to 97% within and beyond the visible spectrum while retaining acceptable sheet resistances of about 30 Ω/□.

The Effect of Interface Cracks on the Electrical Performance of Solar Cells

Among a variety of solar cell types, thin-film solar cells have been rigorously investigated as cost-effective and efficient solar cells. In many cases, flexible solar cells are also fabricated as thin films and undergo frequent stress due to the rolling and bending modes of applications. These frequent motions result in crack initiation and propagation (including delamination) in the thin-film solar cells, which cause degradation in efficiency. Reliability evaluation of solar cells is essential for developing a new type of solar cell. In this paper, we investigated the effect of layer delamination and grain boundary crack on 3D thin-film solar cells. We used finite element method simulation for modeling of both electrical performance and cracked structure of 3D solar cells. Through simulations, we quantitatively calculated the effect of delamination length on 3D copper indium gallium diselenide (CIGS) solar cell performance. Moreover, it was confirmed that the grain boundary of CIGS could improve the solar cell performance and that grain boundary cracks could decrease cell performance by altering the open circuit voltage. In this paper, the investigated material is a CIGS solar cell, but our method can be applied to general polycrystalline solar cells.

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe₂ Growth: Indium-Gallium Selenide Co-Evaporation.

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

(Invited) Electrodeposition for Large Scale Photovoltaic Deployment : A Prospective Trial

Photovoltaics is entering a new age. With more than 300 GW of cumulative capacity by the end of 2016, while it was almost zero ten years ago, which represents about 1.5% of the world electricity production, photovoltaics is becoming a key component in the energy transition. The situation is even more evident when considering the contribution of photovoltaics country by country, reaching about 8% in Germany, Italy, Greece. it is also emerging very rapidly in USA, China, India, Chile for instance. It is anticipated that the level of 500 GW will be reached by 2020 and the symbolic one of one TW in the next decade. The reason is that photovoltaics is becoming economically competitive with the traditional energy sector with negociated prices as low as 3 cents per kWh in some regions. However to go futher in this direction will need an disruptive evolution of all the production chain to lower the costs, increase the efficiency and increase the throughput. Up to now the photovoltaic industrial sector is mostly based on wafer based silicon technology and on vaccuum based thin film solar cells of cadmium telluride or copper indium gallium diselenide (CIGS). Electrodeposition is almost absent in this industrial lanscape, excepted for the emerging copper plating technology for the contact in silicon solar cells, and the other experiences for implementing the electrodeposition in thin film technologies failed. Our vision is that in the new context there will be a unique opportunity to demonstrate that electrodeposition can be implemented as a disruptive technology for larger scale PV depoyment. We will recall the results obtained for the electrodeposition of CIGS solar cells from the laboratory to the industrial pilot line in Nexcis ( a spin off of our laboratory). Using a two stage approach from the electrodeposition of metallic precursors followed by thermal selenization, a record cell efficiency of 17.3 % was reached, and modules with full size (120 x60 cm2) at 14% efficiency in 2015, which was equivalent to the state of the art for CIGS modules prepared by vaccuum methods. Unfortunately for economic reasons, as many CIGS companies at that time, the company was stopped in 2015. However the unique features of the CIGS electrodeposition technology for large area and high throughput and robustness was successfully demonstrated. This research line has been maintained in our laboratory and is presently reactivated taking the lessons of the Nexcis experience. We will discuss a new process for the electrodeposition of CIGS absorbers, via an intermediate step consisting in single bath electrodeposition of mixed oxide hydroxyde copper indium gallium layers followed by thermal hydrogen reduction to form the metallic layer before selenization. The deposition of transparent conductive oxide layers by photoelectrodeposition has been successfully developped too and may become a new standard to replace more costly sputtering methods [2-4. We will also present applications and devices which specifically take benefit from a unique advantage electrodeposition approach, that is localized deposition on structured substrates. Two examples will be given: one for the electrodeposition for photovoltaic semitransparent windows and the second one for the deposition of microcells and microlines for a new concept of PV modules working under concentration. In that case we will also discuss of the mechanisms of electrodeposition of multicomponent CIGS films in conditions of microelectrode array configurations, and the challenges for upscaling. We will conclude with some prospective views at longer term as for instance for using electrodeposition in the next generation of PV modules with higher efficiencies (>30%) based on tandem devives, in that case electrodeposition of thin film solar cells could be combined with classical silicon bottom cells. References F. Tsin, et al. Electrodeposition of ZnO window layer for an all-atmospheric fabrication process of chalcogenide solar cell, Scientific Reports 5(2015)8961A. Duchatelet, G. Savidand, N. Loones, E. Chassaing, D. Lincot, Mechanism of Electrochemical Deposition of Cu-In-Ga Mixed Oxide/Hydroxide Thin Films for Cu(In,Ga)Se-2 Solar Cells, J. Electrochem. Soc. 161(2014)D3120 F. Tsin, D. Hariskos, D. Lincot, J. Rousset, Photo-assisted electrodeposition of a ZnO front contact on a p/n junction, Electrochimica Acta 220(2016)176 A. Duchatelet, K. Nguyen, PP Grand, D. Lincot, M. Paire, Self-aligned growth of thin film Cu(In, Ga)Se-2 solar cells on various micropatterns, Applied Physics Letters, 109 (2016) 253901J. Rousset, F. Tsin, M. Guc, J. Vidal, A. Lebris, A. Thomere, V. Izquerdo Roca, D. Lincot, Perchlorate-Induced Doping of Electrodeposited ZnO Films for Optoelectronic Applications J. Physical Chem. C 120(2016)18953 F. Tsin, J. Rousset, A. Lebris, D. Lincot, Electrodeposited zinc grid as low-cost solar cell front contact, Progress in Photovoltaics 24(2016)1123