CIGS Surface Research Articles

Expanding the Notch Region by Adjusting the Copper Growth Profile for High-Efficiency Flexible Cu(In,Ga)Se2 Solar Cells.

Flexible CIGS solar cells, with their adjustable band gap for future flexible tandem solar cells and flexibility for roll-to-roll manufacturing, have the potential to be used in a wide range of applications. However, flexible CIGS solar cells are always manufactured at relatively low temperatures, where Cu diffusion has a substantial impact on the CIGS surface state and defect formation. To address these issues, we designed a new CIGS growth profile in this work by carefully examining the effects of different locations of excess Cu in the third stage of the CIGS deposition profile. The results showed that adding more Cu to the middle part of the third stage can enhance the crystal quality, expand the GGI grading notch region, move the GGI minimum to the CdS side, cause a JSC rise, achieve proper band alignment at the interface, and then enhance the FF. By taking advantage of these advantages, the photoelectric conversion efficiency (PCE) is significantly raised. An impressive improvement of 32.6% over the initial efficiency of 13.8% has been attained, yielding a high efficiency of 18.3% and providing a strong basis for the development of flexible CIGS solar cells.

A Novel Strategy for the Application of an Oxide Layer to the Front Interface of Cu(In,Ga)Se2 Thin Film Solar Cells: Al2O3/HfO2 Multi-Stack Design With Contact Openings

Interface recombination is one of the factors limiting the performance of Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS). Especially in the absence of band grading at the front and rear surface, interface passivation approaches become important to improve device performance. The integration of an oxide layer as passivation layer at the front surface of the CIGS requires meticulous considerations in order not to impact the further steps of the solar cell production. In this article, a novel approach is reported to try to tackle the problem of interface recombination at the front surface of CIGS without affecting further solar cell production steps. In this approach, an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> multi-stack layer with contact openings is applied. NaCl template patterning with preliminarily selected parameters was used to create a homogeneous pattern of contact opening on the CIGS surface and allow the current flow in the device. After the removal of the NaCl islands, the holes in the multi-stack (openings) were visualized by scanning electron microscopy. In addition, energy-dispersive X-ray spectroscopy (EDS) was performed before and after chemical bath deposition of the buffer layer. The EDS result confirmed that the undesired etching of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer during buffer layer deposition was prevented by using a thin HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer. Solar cells were produced by using preliminarily selected parameters for the multi-stack design. As a result, without having a significant negative impact on the solar cell parameters, a device design was achieved which is almost comparable with the reference device. In addition, options for future improvement and development are discussed.

Thin oxide buffer layers for avoiding leaks in CIGS solar cells; a theoretical analysis

The purpose of this research is the performance improvement of the CIGS/CdS/ZnO solar cells, while the CdS buffer layer is too thin. Enhancement of photocurrent by decreasing the thickness of CdS buffer layer is expected from a reduced parasitic absorption at short wavelengths. However, the formation of pinholes due to the too-thin CdS buffer layer and a non-uniform coverage of the CIGS surface degrades the solar cell performance by reducing fill factor (FF) and open-circuit voltage (Voc). This degradation is because the direct contact of ZnO and CIGS could exist in pinholes, and the formation of the cliff-like band alignment at CIGS/ZnO interface increases the recombination rate. In this work, to eliminate this destructive effect in the CIGS solar cells with a thin CdS layer, ZnO window layer has been replaced by a suitable thin oxide layer as an intermediate buffer layer. By the proposed oxides, the band alignment between the CIGS layer and the adjacent layer is optimized, while pinholes occurred. It is found that the proposed buffer-less CIGS solar cell leading to the efficiency improvement from 17.6% to 18.8%.

Numerical analysis of CdS-CIGS interface configuration on the performances of Cu(In,Ga)Se2 solar cells

One dimension solar Cell Simulation package (SCAPS) is used to analyze the impact of the CdS-CIGS interface configuration on the performances of CIGS solar cells. We simulated the current-voltage characteristic of two models of the cell: one with a donor type defect (OVC model) and the other with acceptor type defect (P+ model) at the CdS-CIGS interface. The advantages and disadvantages of these CIGS surface configuration on the electrical parameters were discussed according to their thicknesses, defect density and carrier lifetime. The simulation results show that the model with the P+ layer has poor performance when its thickness and defect density increase, due to a huge distortion on the J-V characteristic. On the other hand, the OVC layer plays a fundamental role in the performance of CIGS solar cells. Better performances are obtained with the OVC model when the density of donor defect is in the range 1013 - 1015 cm−3, the charge carriers lifetime in the range 0.02 - 1 ns, and the thickness of the OVC layer in the range 200 - 400 nm.

Cu depletion on Cu(In,Ga)Se2 surfaces investigated by chemical engineering: An x-ray photoelectron spectroscopy approach

Photovoltaic cells based on CIGS [Cu(In,Ga)Se2] absorber technology are among the most efficient thin film solar cells and already an industrial reality. Room for improvement is still possible in the manufacturing process to approach the theoretical ultimate efficiency. This not only requires an optimal absorber material but also the control of the CIGS interface chemistry, especially at the front side with the buffer layer which represents one of the main challenges. In this paper, thanks to x-ray photoelectron spectroscopy (XPS) analysis, the CIGS surface chemical composition is studied after acid (HCl) and basic (KCN) samples dipping. Both are regularly employed to prepare CIGS surfaces. XPS monitoring of the surface composition evolution under air aging at an ambient atmosphere and over a period of 120 days is presented, bringing fundamental information about the surface oxidation trends. If the HCl treatment gives a remarkable deoxidation state for the CIGS surface, it also yields a slightly Se enriched surface indicating the presence of a Cu2–xSe binary side phase, which is totally removed, as expected, by the KCN process. The present comparative study based on intentional air aging of starting HCl and KCN treated surfaces sheds light on the reorganization mechanism of this I-III-VI quaternary compound toward oxidation of clean CIGS surfaces, in ambient conditions. The oxidation process occurs concomitantly with an Na migration toward the surface, with soda-lime glass at the back contact, acting as a nonlimiting supply, asking the question of a surface mechanistic correlation during the CIGS surface oxidation.

Atomic layer deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se2 solar cells with KF post-deposition treatment

We investigate the possibility to combine Zn(O,S) buffer layers grown by atomic layer deposition (ALD) with KF post-deposition treated Cu(In,Ga)Se2 (CIGS-KF) in solar cells. It is shown that the beneficial effect on open-circuit voltage from the post-deposition treatment is essentially independent of buffer layer material. However, a wet-chemical surface treatment is required prior to ALD in order to achieve competitive fill factor values. A water rinse is sufficient to create an absorber surface similar to the one formed during a conventional CdS chemical bath deposition process. However, it is observed that CIGS-KF/Zn(O,S) devices made with water-rinsed absorbers systematically result in lower fill factor values than for the corresponding CIGS-KF/CdS references. This effect can be mitigated by decreasing the H2S:H2O precursor ratio during ALD initiation, indicating that the fill factor limitation is linked to the initial Zn(O,S) growth on the modified CIGS-KF surface. The best CIGS-KF/Zn(O,S) devices were fabricated by etching away the KF-modified surface layer prior to ALD, followed by a low-temperature anneal. The thermal treatment step is needed to increase the open-circuit voltage close to the value of the CdS devices. The results presented in this contribution indicate that the main beneficial effects from KF-PDT in our devices are neither associated with the CdS CBD process nor due to the formation of a K-In-Se rich phase on the CIGS surface.

Tunable Carrier Concentration of NaF‐Treated CIGS Solar Cells Using Heat–Light Soaking and Subsequent Heating

The combined effect of the heat–light soaking (HLS) and subsequent heat‐soaking (HS) processes is investigated on NaF‐free and NaF‐treated CIGS solar cells with CdS buffer layer. The HLS treatment improves cell efficiency slightly with increased open‐circuit voltage for NaF‐treated CIGS solar cells, whereas the cell efficiency deteriorates for NaF‐free CIGS solar cells. The different behaviors between both types of solar cells may be due to a Na‐containing new layer, which is formed at the CIGS surface region by NaF‐treatment. On the other hand, the short‐circuit current density (JSC) decreases for both types of solar cells after HLS treatment, which is attributed to the extremely high carrier concentration (NCV), leading to a narrower space charge region. However, it is found that the large NCV can be tuned to an appropriate value using the subsequent HS technique. As a result, the JSC loss is successfully reduced, therefore improving cell efficiency. The details of this improved efficiency are discussed with respect to the change of NCV in CIGS solar cells by the HLS and HS processes.

Ga2Se3 treatment of Cu-rich CIGS thin films to fabricate Cu-poor CIGS thin films with large grains and U-shaped Ga distribution

Cu-rich CIGS thin films were prepared from quaternary CIGS and Cu target, and then converted into Cu-poor CIGS by depositing the Ga2Se3 layer on the Cu-rich CIGS with the subsequent annealing. In this paper, we name the process of converting Cu-rich CIGS into Cu-poor CIGS as Ga2Se3 treatment. The Ga2Se3 treatment brings out two benefits for the CIGS absorbers. It can remove the excess CuxSe phase in the Cu-rich CIGS and don't reduce the grain size of CIGS. That is to say, the Cu-poor CIGS has almost the same size of grain of the Cu-rich CIGS thin films. What's more, the Ga2Se3 treatment can also improve the content of gallium on the surface of CIGS, resulting the U-shaped gallium distribution along the depth direction. The thickness of Ga2Se3 layer has been optimized which can result in the highest conversion efficiency of 10.6% in Cu-rich CIGS based solar cells.

Passivation effect of Cl, F and H atoms on CuIn0.75Ga0.25Se2 (1 1 2) surface

Using the first-principles calculations within the density functional-theory (DFT) framework, we theoretically investigated the surface reconstruction, surface states near the Fermi level and their passivation on CuIn0.75Ga0.25Se2 (112) (CIGS) surface by chlorine, fluorine and hydrogen. Surface reconstruction appears on CIG-terminated CIGS (112) surface and it is a self-passivation. For the locations of Cl, F and H atoms adsorbing on Se-terminated CIGS (112) surface, four high symmetry adsorption sites: top sites, bridge sites, hexagonal close-packed (hcp) sites and faced centered cubic (fcc) sites were studied respectively. With the coverage of 0.5 monolayer (ML), Cl, F and H adatoms energetically occupy the top sites on the CIGS (112) surface. The corresponding adsorption energies were −2.20 eV, −3.29 eV, −2.60 eV, respectively. The bond length and electronic properties were analyzed. We found that the surface state density near the Fermi level was markedly diminished for 0.5 ML Cl, F and H adsorption on Se-terminated CIGS (112) surface at top sites. It was also found that H can more efficiently passivate the surface state density than Cl and F atoms, and the effect of adsorption of Cl atoms is better than that of F.

Slow-muon study of quaternary solar-cell materials: Single layers and p−n junctions

Thin films and $p\ensuremath{-}n$ junctions for solar cells based on the absorber materials $\mathrm{Cu}(\mathrm{In},\mathrm{G}\mathrm{a}){\mathrm{Se}}_{2}$ and ${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ were investigated as a function of depth using implanted low energy muons. The most significant result is a clear decrease of the formation probability of the ${\mathrm{Mu}}^{+}$ state at the heterojunction interface as well as at the surface of the $\mathrm{Cu}(\mathrm{In},\mathrm{G}\mathrm{a}){\mathrm{Se}}_{2}$ film. This reduction is attributed to a reduced bonding reaction of the muon in the absorber defect layer at its surface. In addition, the activation energies for the conversion from a muon in an atomiclike configuration to a anion-bound position are determined from temperature-dependence measurements. It is concluded that the muon probe provides a measurement of the effective surface defect layer width, both at the heterojunctions and at the films. The CIGS surface defect layer is crucial for solar-cell electrical performance and additional information can be used for further optimizations of the surface.

Damage and repair of organic and inorganic surfaces by Ar+ ion and gas cluster ion beam sputtering

X-ray photoelectron spectroscopy has been used to study the interactions of argon ion and argon gas cluster ion beams with the surfaces of organic and inorganic materials. These methods of sputtering surfaces are commonly used to remove surface material to construct depth profiles by surface-sensitive techniques like XPS and SIMS. Standard argon ion sputtering has been known to damage polymer surfaces. This is demonstrated here on poly(ethylene terephthalate) and polytetrafluoroethylene. Subsequently, the damaged surfaces could be returned to their pristine state by sputtering with GCIB, which does not damage polymer surfaces. While GCIB can remove surface polymer material without damaging the underlying material, it is shown here to cause damage to the chalcogenide material, copper indium gallium selenide. Sputtering with a gas cluster ion beam was shown to selectively remove selenium from the copper indium gallium selenide structure. Additionally, chemical reduction of copper, indium, and gallium was observed. Subsequent to inducing damage to the CIGS surfaces with gas cluster ion beam, the surface could be returned to its pristine state by sputtering with argon ions to remove the damaged layer. This work demonstrates the applicability of various sputtering methods for organic and inorganic materials, suggests the route of damage formation, and shows the ability to remove the surface damage by a compatible sputtering method.

Increase in conversion efficiency of above 14% in Cu(In,Ga)3Se5 (β-CIGS) solar cells by Na2S incorporation through the surface of β –CIGS film

Little work has been reported on the performance improvement of the β-CIGS solar cell itself even though the β-CIGS phase can have an ideal band gap for high-conversion efficiency solar cells. We incorporated Na2S to β-CIGS film by supplying Na2S to three different stages: on the (In,Ga)2Se3 layer, on the α-CIGS layer, and on the β -CIGS layer in the three-stage co-evaporation process. The purpose of Na2S incorporation was to control the carrier concentration and passivate grain boundaries in β–CIGS film. With Na2S incorporation on the β–CIGS surface, both the Cu and Se concentrations at the β–CIGS surface were greatly reduced and the Na-depleted subsurface area that existed in the referenced β–CIGS film without Na2S was eliminated. The carrier concentration determined at 100kHz was lowest with Na2S incorporation on the β–CIGS surface, while that determined at 1MHz was similar with various Na2S supply stages. The open-circuit voltage and fill factor greatly increased in the β–CIGS solar cell with the Na2S incorporation. The cell conversion efficiency with Na2S incorporation on the β-CIGS layer improved from 10.3% to 14.2% without AR coating, which is a record efficiency in β-CIGS solar cells at this time.

Chemical engineering of Cu(In,Ga)Se2 surfaces: An absolute deoxidation studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy signatures

CIGS (Cu(In,Ga)Se2) absorbers are among the most efficient for photo-conversion. As part of their manufacturing cycle, absorber surface preparation is crucial for both the success of processes and the outcome of characterizations actions. This work tackles the question of an efficient deoxidation of CIGS surfaces. Chemical composition of surface is investigated thanks to XPS (X-ray Photoelectron Spectroscopy) characterizations. Present study focuses on the possible control of the CIGS absorber surface associated with simple HCl treatment and subsequent atmosphere ageing up to 20days. Using 1.0M HCl concentration solution, remarkable oxide elimination is achieved. Starting from an air aged as-grown surface (Cu and Se-poor), this treatment enables the generation of a deoxidized chalcopyrite reference sample. Furthermore, the reoxidation process of the treated surface at ambient atmosphere is studied considering the global evolution of the XPS signals as the best probe of the ageing process. The surface composition evolution represents a key parameter for the achievement of the following interfaces engineering. The main result is that the reoxidation process is not instantaneous (no significant oxide content before 2days) and involves In, Ga and Se elements while the Cu seems less affected, leading again to a Cu-poor surface different from the initial one.

Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe2Heterojunctions

The Cu migration behavior in PVD-CdS/PVD-Cu(In,Ga)Se2 (CIGS) heterojunctions is investigated by high-resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS). Incorporation of Cu into the CdS forms Cu-rich domains but has no effect on epitaxy of the CdS. Epitaxy is commonly observed in the CdS studied. Secondary ion mass spectroscopy depth profiles confirm the presence of Cu in the CdS. In some cases, Cd is completely replaced by Cu, resulting in a Cu–S binary compound epitaxially grown on the CIGS and fully coherent with the surrounding CdS. This is most likely to be cubic Cu2S, based on lattice spacing measurements from HREM images and EDS elemental quantification. In addition, we find that the buffer layer crystal structure influences the extent of Ga depletion at the CIGS surface, which is more pronounced adjacent to zinc-blende CdS than wurtzite CdS. Density functional theory calculations reveal that Cu clustering and different Ga depletion widths can be attributed to the inherent anisotropy of wurtzite CdS and differences in CIGS point-defect migration barriers. Understanding the influence of these effects on device properties is a critical step in developing more efficient CdS/CIGS-based photovoltaics.

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%.

Room-Temperature Chemical Solution Treatment for Flexible ZnS(O,OH)/Cu(In,Ga)Se2 Solar Cell: Improvements in Interface Properties and Metastability.

We demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on Cu(In,Ga)Se2 (CIGSe) surface to engineer the ZnS(O,OH)/CIGSe interface and junction quality, leading to enhanced efficiency and minimized metastability of flexible solar cells. The control device without treatment reveals a relatively low efficiency of 8.15%, which is significantly improved to 9.74% by In-S treatment, and 10.39% by S treatment. Results of X-ray photoelectron spectroscopy suggest that S is incorporated into CIGSe surface forming CIGSSe by S treatment, whereas a thin In-S layer is formed on CIGSe surface by In-S treatment with reduced amount of S diffusing into CIGSe. PL spectra and TRPL lifetime further reveal that S incorporation into CIGS surface may substitute the OSe and/or directly occupy the vacant anion site (VSe), resulting in the effective passivation of the recombination centers at CIGSe surface. Moreover, reducing the concentrations of VSe may thereby decrease the density of (VCu-VSe) acceptors, which can minimize the metastability of ZnS(O,OH)/CIGSe solar cells. With S treatment, the light soaking (LS) time of ZnS(O,OH)/CIGSe device is reduced approximately to one-half of control one. Our approach can be potentially applied for alternative Cd-free buffer layers to achieve high efficiency and low metastability.

Leaching and re-synthesis of CIGS nanocrystallites from spent CIGS targets

The original target utilization is only about 30% in the CIGS (Cu(In, Ga)Se2) thin film solar cell sputtering process. The remaining 70% of the target must be recycled to achieve sustainable use of the rare metals, In, Ga, and Se. It is therefore very important to establish spent CIGS target recycling technology to reduce environmental damage.High CIGS recovery was obtained in this study by autoclave leaching CIGS powder in a sulfate solution using H2O2 as the strong oxidant. The CIGS leaching mechanism was studied using XRD and SEM. H2O2 was first adsorbed onto the CIGS surface. Then copper–selenium compound was formed on the CIGS surface. The copper–selenium compound decomposition is the rate-controlling step in the CIGS leaching process. Nearly complete CIGS dissolution can be obtained by autoclave leaching at 140°C for 4h using 3M H2SO4 as the leaching agent and H2O2 as the oxidant. In this study, a direct recycling process combining leaching process and re-synthesis of CIGS nanoparticles was developed to recover CIGS materials for reinsertion into the CIGS supply chain. Mono-dispersed CIGS nano-particles with the mean particle size of 9nm can be obtained using spent CIGS target as raw materials.

Surface sulfurization on MBE-grown Cu(In1−x,Gax)Se2 thin films and devices

Molecular beam epitaxy (MBE) grown Cu(In1−x,Gax)Se2 (CIGS) thin films were sulfurized at temperatures of 450–550 °C for 30 min in a 10% H2S–N2 mixture gas. The micro-roughness together with the S diffusion in the CIGS surfaces increased with increasing sulfurization temperature. Both near-band-edge PL intensity and decay time of the CIGS absorber layer enhanced after sulfurization. PL sub-peak around 80 meV below the main peak almost disappeared after sulfurization above 500 °C, which is expected due to the occupation of Se vacancies (Vse) with S. The open-circuit voltage (Voc), hence conversion efficiency, improved after sulfurization. The photovoltaic performance of the solar cells was consistent with PL intensity. Moreover, it is found for the first time from the SIMS analysis that the Cu atoms were depleted at the surface of CIGS layer after sulfurization, which could result in the improved Voc.

Photoluminescence Analysis of CdS/CIGS Interfaces in CIGS Solar Cells

In this study, the element diffusion behavior and defect states at the CdS/CIGS interface and their effect on solar cell performance were investigated after post-annealing treatment in an air environment at 150°C and 200°C, respectively. The results showed that the defect states (VSe and VCu) were passivated by the elements of O, S, and Cd forming a new compound CIGSSe, which led to an increase in the band-gap on the CIGS surface and a high carrier concentration at the p-n junction. There was an improvement in the photovoltaic performance of the solar cells from 3.71% to 7.25% through post-annealing treatment at 200°C for 30 min.

Raman scattering peak position of Cu(In,Ga)Se2 film to predict its near-surface [Ga] / ([Ga] + [In]) and open-circuit voltage

Abstract Cu(In,Ga)Se 2 (CIGS) solar cells were fabricated on both rigid soda-lime glass (SLG) and flexible stainless steel (SUS) substrates. Their absorbers, with several [Ga] / ([Ga] + [In]) profiles, were deposited using the so-called “multi-layer precursor method”, consisting of Ga–Se/In–Se/Cu–Se stacked precursors. It was revealed that the open-circuit voltage ( V OC ) of the CIGS solar cell is well correlated with the near-surface GGI, defined as the average [Ga] / ([Ga] + [In]) ratio within 200 nm of the CIGS surface. The near-surface GGI could be predicted when the Raman scattering peak position of the CIGS film was known. The estimated penetration depth of Raman scattering light into the CIGS layer was approximately 100–140 nm from its surface. Moreover, a relationship between V OC and the Raman scattering peak position was observed. Ultimately, the Raman scattering peak position, corresponding to the near-surface GGI of below 0.45, could be utilized as an indicator of V OC of CIGS solar cells on both rigid SLG and flexible SUS substrates without cell fabrication, which is measured by the rapid and non-destructive method.