Ordered Vacancy Compound Research Articles

False metals, real insulators, and degenerate gapped metals

This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation (“Mott insulators”) whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As the science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an “internal band gap” between the principal band edges. The significance of this electronic configuration is that it might be unstable toward the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator. The analogous process also exists for degenerate but gapped metals with the Fermi level inside the valence band, where the energy gain is defined by transfer of electrons from the donor level to the unoccupied part of the valence band. We focus here on the fact that numerous electronic structure methodologies have overlooked some physical factors that could stabilize the insulating alternative, predicting instead false metals that do not really exist (note that this is in general not a physical phase transition, but a correction of a previous error in theory that led to a false prediction of a metal). Such errors include: (i) ignoring spin symmetry breaking, such as disallowing magnetic spin ordering in CuBi2O4 or disallowing the formation of polymorphous spin networks in paramagnetic LaTiO3 and YTiO3; (ii) ignoring structural symmetry breaking, e.g., not enabling energy-lowering bond disproportionation (Li-doped TiO2, SrBiO3, and rare-earth nickelates), or not exploring pseudo-Jahn–Teller-like distortions in LaMnO3, or disallowing spontaneous formation of ordered vacancy compounds in Ba4As3 and Ag3Al22O34; and (iii) ignoring spin–orbit coupling forcing false metallic states in CaIrO3 and Sr2IrO4. The distinction between false metals vs real insulators is important because (a) predicting theoretically that a given compound is metal even though it is found to be an insulator often creates the temptation to invoke high order novel physical effects (such as correlation in d-electron Mott insulators) to explain what was in effect caused by a more mundane artifact in a lower-level mean-field band theory, (b) recent prediction of exotic physical effects such as topological semimetals were unfortunately based on the above compounds that were misconstrued by theory to be metal, but are now recognized to be stable insulators not hosting exotic effects, and (c) practical technological applications based on stable degenerate but gapped metals such as transparent conductors or electrides for catalysis must rely on the systematically correct and reliable theoretical classification of metals vs insulators.

On the Paramount Role of Absorber Stoichiometry in (Ag,Cu)(In,Ga)Se2 Wide‐Gap Solar Cells

This contribution evaluates the effect of absorber off‐stoichiometry in wide‐gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells. It is found that ACIGS films show an increased tendency to form ordered vacancy compounds (OVCs) with increasing Ga and Ag contents. Very little tolerance to off‐stoichiometry is detected for absorber compositions giving the desired properties of 1) an optimum bandgap (EG) for a top cell in tandem devices (EG = 1.6–1.7 eV) and at the same time 2) a favorable band alignment with a CdS buffer layer. Herein, massive formation of either In‐ or Ga‐enriched OVC patches is found for group I‐poor ACIGS. As a consequence, carrier transport and charge collection are significantly impeded in corresponding solar cells. The transport barrier appears to be increasing with storage time, questioning the long‐term stability of wide‐gap ACIGS solar cells. Furthermore, the efficiency of samples with very high Ga and Ag contents depends on the voltage sweep direction. It is proposed that the hysteresis behavior is caused by a redistribution of mobile Na ions in the 1:1:2 absorber lattice upon voltage bias. Finally, a broader perspective on OVC formation in the ACIGS system is provided and fundamental limitations for wide‐gap ACIGS solar cells are discussed.

Assessing the roles of Cu- and Ag-deficient layers in chalcopyrite-based solar cells through first principles calculations

Chalcopyrites are a demonstrated material platform for realizing efficient thin-film photovoltaics, with the most well known Cu(In,Ga)Se2 (CIGS)-based solar cells exceeding 23%. Several factors, including flexibility in tuning the absorber bandgap, enhanced surface treatments, and the electrically benign nature of common defects are responsible for the existing high performance and future promise in chalcopyrite-based photovoltaic devices. The introduction of Cu-poor phases (also known as ordered-vacancy compounds or OVCs) between the absorber and buffer layers in CIGS solar cells is known to enhance device performance; however, the overall properties and role of OVCs remain poorly understood. Using first principles calculations based on the density functional theory with screened hybrid functionals, we explore the electronic structure and stability of OVCs and their band offsets with defect-free chalcopyrite layers in Cu- and Ag-based compounds (ABX2 where A=Cu, Ag; B=In, Ga, Al; and X=S, Se). Using AB3X5 and AB5X8 stoichiometries as model OVC systems, we report on the variation of the bandgap with the A/B ratio and discuss the trends in other Cu- and Ag-based chalcopyrites beyond CuInSe2. We find that the valence and conduction bands are lower in energy in OVCs with respect to the parent ABX2 chalcopyrite owing to a reduced p–d interaction between X and A atoms. We additionally perform device-level simulations to assess the implications of the results, finding that the valence band offsets of OVCs are favorable, while the conduction band offsets of chalcopyrites beyond CuInSe2-based absorbers may be detrimental in conventional solar cell device designs.

Performance Optimization of ZnS/CIGS Solar Cell with over 25% Efficiency Enabled by Using a CuIn3Se5 OVC Layer

Chalcopyrite CIGS solar cells have been demonstrated with the highest efficiency for thin-film solar cell families. Herein, the effects of thickness and bandgap of cadmium (Cd) free ZnS buffer layer grown on CIGS absorber with an ordered vacancy compound (OVC) at buffer/absorber interface in CIGS solar cell structure have been investigated using ADEPT 2.1 simulator. The ZnS buffer possesses a wider energy bandgap than the traditional CdS buffer layer, which substantiates the higher conversion efficiency of this solar cell, allowing the photons of lower wavelength into the CIGS layer. Besides, CuIn 3 Se 5 has been used as an OVC layer that assists to improve the conversion efficiency to an optimized level of over 25% by reducing the recombination rate. Moreover, the optimized short circuit current (J sc ) and corresponding open-circuit voltage (V oc ) yield a higher fill factor (FF) of 86.22%, which, therefore, results in an optimum efficiency of the CIGS cell estimated as 25.68% under AM 1.5 irradiance.

On the structural, electronic, optical and thermoelectric properties of CdIn2Se4 ordered-vacancy compound

In this paper, we report results of the structural, electronic, optical and thermoelectric properties of CdIn2Se4 compound. These properties are investigated using first-principles calculations based on the full-potential linearized plane-wave (FP-LAPW) technique within the framework of density functional theory (DFT) and semiclassical Boltzmann transport theory. During the structural optimization, the electron exchange-correlation potentials are described through Wu-Cohen (WC) functional, while the Tran-Blaha modified Becke-Johnson exchange potential is employed when calculating the electronic and related properties to obtain more accurate band gap. Electronic band structures reveal that the CdIn2Se4 compound is a direct gap Γ−Γ semiconductor, with a band gap of 1.834 ​eV. Results of the compound optical properties suggest this material as promising absorber to work under ultraviolet radiation. Finally, the thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor and dimensionless figure of merit are calculated and analyzed in details. With a proper hole doping level, the CdIn2Se4 compound can a promising candidate for thermoelectric applications with figure of merit close to unity.

A new ordered vacancy compound; preparation and crystal structure of Ag3In5Te9

The new ordered vacancy compound Ag3In5Te9 was synthesized by the melt and annealing technique and its structure was refined from X-ray powder diffraction pattern using the Rietveld method. The title compound crystallizes in the tetragonal space group P 2c (Nº 112), with a = 6.3453(2) Å, c = 12.5754(7) Å, V = 506.32(4) Å3. The refinement of 23 instrumental and structural parameters led to Rp = 5.4%, Rwp = 5.8%, Rexp = 5.1 %, S = 1.1. This ternary compound is isostructural with Cu3In5Te9 and have a defect adamantane structure.

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

On the crystal structure of the ordered vacancy compound Cu3In5Te9

The crystal structure of the ordered vacancy compound (OVC) Cu3In5Te9 was analyzed using powder X-ray diffraction data. Several structural models were derived from the structure of the Cu-poor Cu-In-Se compound b-Cu0.39In1.2Se2 by permuting the cations in the available site positions. The refinement of the best model by the Rietveld method in the tetragonal space group P2c (Nº 112), with unit cell parameters a = 6.1852(2) Å, c = 12.3633(9) Å, V = 472.98(4) Å3, led to Rp = 7.1 %, Rwp = 8.5 %, Rexp = 6.4 %, S = 1.3 for 162 independent reflections. This model has the following Wyckoff site atomic distribution: Cu1 in 2e (0,0,0); In1 in 2f (½,½,0), In2 in 2d (0,½,¼); Cu2-In3 in 2b (½,0,¼); in 2a (0,0,¼); Te in 8n (x,y,z).

CuZnInSe3‐based solar cells: Impact of copper concentration on vibrational and structural properties and device performance

AbstractCuZnInSe3 (CZISe) is an interesting alternative for the acknowledged Cu(In,Ga)Se2 absorber layer in thin film solar cells. While the partial replacement of scarce and expensive indium and gallium by zinc decreases manufacturing costs, the solid solution between CuInSe2 and ZnSe opens interesting options for band gap tuning and grading. Its potential as an absorber layer in photovoltaic devices has been demonstrated by obtaining 7.4 and 7.6 % efficiency in CZISSe‐ and CZISe‐based devices, respectively. On the other hand, the inherent complexity of the quaternary CZISe together with a lack of fundamental insights puts a limit to its current development. We present insights on the influence of the copper content ([Cu]/([Zn] + [In]) ratio) on the structural and optoelectronic properties of CZISe as well as the formation of secondary phases. By means of XRD and Raman scattering analyses, in addition to the sphalerite CZISe structure, a chalcopyrite Cu‐In‐Zn‐Se phase was found for high copper concentrations. On the contrary, for low Cu concentrations, unambiguous indications of a new ordered vacancy compound (OVC)–like phase formation both in XRD patterns and in Raman spectra were found. Conditions of pre‐resonant Raman scattering were applied to emphasize the new found phase and to estimate its concentration. Finally, the influence of each phase on the optoelectronic parameters and performance of solar cells with efficiencies of up to 7.4 % was studied.

First-Principles Simulations of Stability, Optical and Electronic Properties of Competing Phases in Chalcopyrite-Based Photoelectrodes

Photoelectrochemical (PEC) hydrogen production is one promising approach to economically generating renewable fuels, yet the widescale deployment of leading photoelectrodes (e.g. GaInP-based devices) has been hampered by issues with durability, efficiency and cost. Chalcopyrite-based tandem photoelectrodes are attractive alternatives in that they offer a great flexibility in the choice of component materials necessary that drive the reactions for different device designs, offering ways to reduce the cost and to improve the performance. The synthesis of low-band gap chalcopyrites like CuInSe2 and alloys with CuGaSe2 (CIGSe) are known to exhibit a variety of Cu-poor phases (ordered-vacancy compounds or OVCs) that can greatly influence the resulting properties of fabricated devices, while the existence and properties of these phases in wider-band gap chalcopyrite compounds and alloys remains largely unexplored. Using hybrid functional calculations, we discuss the stability, optical and electrical properties of OVCs in a number of other chalcopyrite chemistries beyond CIGSe. We discuss the influence of such phases on the resulting absorption and band offsets that would result upon their formation, which have implications in device design. We additionally discuss experimental fingerprints that could be used to identify the existence of such phases in synthesized material. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the HydroGEN Consortium within the Department of Energy Office of Energy Efficiency & Renewable Energy (EERE) and Fuel Cell Technologies Office.

Pervasive Cation Vacancies and Antisite Defects in Copper Indium Diselenide (CuInSe2) Nanocrystals

Copper indium diselenide (CuInSe2) is a prototype ternary compound and group I–III–VI semiconductor with useful optoelectronic properties. CuInSe2 nanocrystals have been of significant interest because of their size-tunable optical properties and lack of toxic heavy metals. Because of the particular vacancy and antisite substitutional point defects in CuInSe2, large stoichiometric deviations can be tolerated, sometimes leading to the so-called ordered vacancy compounds (OVCs). Here, we use Raman spectroscopy of oleylamine-capped CuInSe2 nanocrystals and ab initio lattice dynamics modeling to study the concentration and arrangements of (2vCu– + InCu2+) defect pairs in the nanocrystals. The nanocrystals have randomly distributed defect pairs that become mobile under light excitation and accumulate, as in OVCs, along the [100] direction. Because the high concentration of vacancies in CuInSe2 nanocrystals is compensated by InCu2+ antisite defects, these nanocrystals do not exhibit an optical plasmon resonance...

Evidence of a new ordered vacancy crystal structure in the compound Cu3In7Te12

The crystal structure of the ordered vacancy compound (OVC) Cu3In7Te12 is analyzed using powder X-ray diffraction data. It is found that this OVC crystallizes with a chalcopyrite-related structure, in the tetragonal space group P 4 2c (No 112), with unit cell parameters and volume a = 6.1720(2) A, c = 12.3597(8) A, and V = 470.83(4) A3. The Rietveld refinement of 28 instrumental and structural parameters led to Rp = 9.27 %, Rwp = 10.30 %, Rexp = 6.95% and S = 1.48, for 4501 step intensities and 130 independent reflections, respectively. This compound is isostructural with Cu3In7Se12, and has a defect adamantane structure. Keywords: semiconductors, ordered vacancy compounds, crystal structure, X-ray diffraction, Rietveld.

Improved efficiency of Cu(In,Ga)Se2 thinfilm solar cells using a buffer layer alternative to CdS

The chalcopyrite semiconductor CuInSe2 and its constitutes Ga and/or S [Cu (InGa)Se2 or Cu (InGa)(Se,S)2], commonly referred as CIGS have been leading thinfilms for incorporation in high-efficiency photovoltaics. In conventional ZnO-N/i-ZnO/CdS/CIGS solar cells, the traditional CdS buffer is nearly optimum for the commonly used 1.15 eV (CIGS) but less optimal for higher Ga. To overcome this limitation, Cd1-yZnyS is proposed as an alternative buffer layer to replace the standard CdS in CIGS thinfilm solar cells containing an ordered vacancy compound (OVC) layer. Next, the dependence of solar cells performance on the change of Ga and Zn concentrations in absorber and buffer layers, respectively, was investigated using the AMPS-1D software. The results are potential improvement in CIGS efficiency that was obtained with replacement of CdS buffer material by its alternative in one hand, another hand by formation of OVC layer. Lastly, the optimum values of Ga and Zn concentrations were found at 0.7 and 0.6, respectively, leading to a high conversion efficiency of around 23.71%.

Study of Gallium Front Grading at Low Deposition Temperature on Polyimide Substrates and Impacts on the Solar Cell Properties

Cu(In,Ga)Se2 (CIGS) solar cells have achieved the highest efficiencies among thin-film solar technologies. At low temperatures ( V oc and FF with high Ga content at the interface. We show that the insertion of a Ga-rich layer favors the formation of ordered vacancy compound phases, and the absence of Ga during the last stage of the process impacts the overall element diffusion. Optimized Ga gradient leads to 17.8% efficiency solar cells without antireflecting coating or KF postdeposition treatment.

Crystal structure of the ternary semiconductor Cu2In14/3□4/3Se8determined by X-ray powder diffraction data

The crystal structure of the partially ordered vacancy compound Cu2In14/3□4/3Se8, belonging to the system I3-III7-□2-VI12, was analyzed using X-ray powder diffraction data. Several structural models were derived from the structure of the selenium-rich phaseβ-Cu0.39In1.2Se2by permuting the cations in the available Wyckoff positions. The refinement of the best model by the Rietveld method in the tetragonal space groupP$\overline 4 $2c(No 112), with unit-cell parametersa= 5.7487(3) Å,c= 11.5106(6) Å,V= 380.40(3) Å3, led toRp= 9.0%,Rwp= 9.9%,Rexp= 7.2%,S= 1.4 for 134 independent reflections. This model has the following Wyckoff site atomic distribution: Cu in 2e(0,0,0); In in 2b(½,0,¼), 2d(0,½,¼), and 2f(½,½,0);□ in 2f(½,½,0); Se in 8n(x,y,z).

A simple non-toxic simultaneous selenization/sulfurization process for Cu(In, Ga)(S, Se)2 solar cells

This paper proposes a non-toxic simultaneous selenization/sulfurization process to produce Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells without the need for the conventional two-stage process using toxic H2Se/H2S gases. The absorber layer is applied via sputtering with Cu-In-Ga ternary targets, whereupon non-toxic selenium and sulfur vapor are introduced simultaneously to produce chalcopyrite CIGSSe film. Experiment results demonstrate that the total sulfur content of the absorber layer increases with an increase in the S/(S + Se) ratio. When the S/(S + Se) ratio ≧ 0.12, the sulfur content of the surface absorber layer is higher than inside the sample. This contributes to an increase in the surface energy gap and Voc of the solar cell. However, an excessively high S/(S + Se) ratio can undermine the crystallinity of the CIGSSe while introducing phase segregation at the surface of the absorber layer as well as ordered vacancy compounds (OVCs) and secondary phases, which can decrease cell efficiency. A suitable proportion of sulfur in the absorber layer can inhibit excessive growth of the Mo(S,Se)2 layer and thereby enhance cell efficiency. A small number of CuGaSe2 grains were observed at the bottom of the CIGSSe absorber layer in specimens with various S/(S + Se) ratios. A maximum cell efficiency of 12.8% was achieved when the S/(S + Se) ratio was 0.12. This cell efficiency is close to the reference specimen fabricated using a two-stage selenization/sulfurization process based on toxic H2Se/H2S gases.

Rear interface modification for efficient Cu(In,Ga)Se2 solar cells processed with metallic precursors and low-cost Se vapor

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) material have demonstrated excellent power conversion efficiency. However, developing a low-cost fabrication process with less safety risk that compatible with large-scale production still stands as a major challenge. Using cheap and low toxic element Se vapor to replace the expensive and highly toxic H2Se which is widely used in sputtering-based industrial manufacture is an attractive solution. Received considerable attention, yet the devices fabricated from metallic precursor and element Se vapor still suffer from an inferior performance. In this work, we disclosed the main culprit behind this limitation—the ordered vacancy compounds (OVC) phase at the CIGS/Mo rear interface, which is induced by the drastic out-diffusion of Cu during the selenization. The detrimental OVC phase at the rear interface acts as a blocking barrier, and also results in enhanced rear interface recombination, and weakens the band bending in the hetero-junction, as manifested by the temperature-dependent current-voltage (J-V) measurements, KPFM measurements, and SCAPS-1D simulation. We found that the out-diffusion of Cu and the associated formation of OVC phase at the rear interface can be efficiently suppressed by turning the element Cu and In in as-sputtered precursor into Cu9(In,Ga)4 and Cu9Ga4 alloys using a co-sputtered metallic precursor with a thermal alloying treatment. With the rear interface modification, we significantly improved the device performance, with pronounced increase of Voc and fill factor. Our results indicate even higher efficiency can be achieved by further improving the doping density of the rear interface and optimizing the band gap.

Wet Pretreatment-Induced Modification of Cu(In,Ga)Se2/Cd-Free ZnTiO Buffer Interface.

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se2 (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NH4OH, H2O, and/or aqueous solution of Cd2+ ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NH4OH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NH4OH treatment. However, the addition of Cd2+ ions to the NH4OH aqueous solution suppressed the etching of the OVC by NH4OH, explaining why such a negative effect of NH4OH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.

Changes in charge density vs changes in formal oxidation states: The case of Sn halide perovskites and their ordered vacancy analogues

Shifting the Fermi energy in solids by doping, defect formation or gating generally results in changes in the charge density distribution, which reflect the ability of the bonding pattern in solids to adjust to such external perturbations. In the ionic limit, such changes are described by the formal oxidation states (FOS) whereby a single atom type is presumed to absorb the full burden of the perturbation (change in charge) of the whole compound. In this picture, when the solar absorber CsSnI3 loses 50 % of its Sn atoms thereby forming the ordered vacancy compound Cs2SnI6, the Sn is said to change from Sn2+ to Sn4+. In reality, the charge redistribution is more complex. To understand the electronic properties of these two groups and to gauge the trends in s-electron charge on the nucleus, we studied the 113/216 compound pairs CsSnCl3/Cs2SnCl6; CsSnBr3/Cs2SnBr6, and CsSnI3/Cs2SnI6. We have calculated the electronic structure of these three compound pairs, complementing them by CsSnF3/Cs2SnF6 in the hypothetical cubic structure for completing the chemical trends. These materials were also synthesized by chemical routes and characterized by X-ray diffraction, Mossbauer spectroscopy and K-edge X-ray Absorption Spectroscopy. We find that in going from 113 to 216 there is a decrease in the s charge on Sn, in agreement with the FOS picture. However, at the same time, we observe an increase of the p charge via downshift of the otherwise unoccupied p level, an effect that tends to replenish much of the lost s charge. This effect is theoretically explained as a self-regulating response [H. Raebiger, S. Lany, and A. Zunger, Nature 453, 763 (2008)] whereby the system re-hybridizes to minimize the effect of the charge perturbation created by vacancy formation. Our work clarifies the relation between FOS and charge transfer that can be applied to a wide variety of materials.

Investigation on the Performance of CIGS/TiO2 Heterojunction Using SCAPS Software for Highly Efficient Solar Cells

In this study, Cu (In, Ga) Se2 (CIGS) material with the non-toxic titanium dioxide TiO2 as an n-type buffer layer and indium tin oxide as the window layer are numerically simulated using a solar cell capacitance simulation software package. This numerical analysis has been carried out with the aim of boosting the performances of CIGS/TiO2 solar cells by tuning the defect density and band gap energy of the ordered vacancy compound (OVC) layer and by using a Back-electron reflector (EBR) layer, namely Al2O3. Solar cell performance is investigated as a function of absorber thickness. It is found that there exists an optimal thickness. The effect of OVC compounds on the performance of the device structure are discussed leading to an optimal band gap energy and defect density of about 1.17 eV and 4.97 × 1013 cm−3, respectively. The matching solar cell conversion efficiency reached a maximum value of 12.38% by introducing the OVC layer. It is also shown that, in spite of a decrease in thickness, the external quantum efficiency (EQE) of ultrathin CIGS solar cells can be enhanced owing to the employment of EBR. The significant improvement of EQE, mainly in the near-infrared part of the solar spectrum, can be ascribed to the low parasitic absorption loss in the ultrathin CIGS layer (∼570 nm).