CuIn1-xGaxSe2 Research Articles

Increasing the Effective Area of Small‐Sized CIGS Modules by Printed Ag Front Contacts

Despite the high‐efficiency potential of the Cu(In1−x Ga x )Se2 (CIGS) technology, its global market share is still relatively small. One reason for that is the comparatively low active module area caused by small cell widths and therefore the resulting amount of scribing lines for monolithic interconnection. This results in significant losses from cell‐to‐module level. One approach to overcome this issue is the application of front contacts. Throughout this study, the electrical performance of small‐sized CIGS modules with cell widths of w c = 4 mm, w c = 7 mm, and w c = 17 mm with and without front side metallization is evaluated. A low‐temperature curing Ag paste is applied onto those different CIGS module designs by flatbed screen printing and parallel dispensing. The application of Ag‐electrodes obtains a significant increase in module power of ΔP mmp,rel. = +18% for 4 mm cell widths compared to grid‐free CIGS modules. Furthermore, the homogeneous electrode shape of dispensed structures enables broad cells of w c = 17 mm in small‐sized CIGS modules. As a result, the module performance increases by ΔP mmp,rel. = +25% compared to screen‐printed Ag‐electrodes, demonstrating the great potential of the dispensing approach for the CIGS technology and potentially other thin‐film technologies.

Practical Enhancements in Current Density and Power Generation of Bifacial Semitransparent Ultrathin CIGSe Solar Cells via Utilization of Wide Bandgap Zn‐Based Buffer (Adv. Sci. 13/2022)

Semitransparent Ultrathin CIGSe Solar Cells In article number 2105436, Joo Hyung Park and co-workers develop a bifacial and semitransparent ultrathin (STUT) Cu(In1-x,Gax)Se2 (CIGSe) solar cell with a wide bandgap Zn(O,S) buffer, and establish the concept and method to compare power generation ability by defining bifacial compatible efficiency (BCE) to reflect the albedo effect advantage in bifacial application, comparing with monofacial power conversion efficiency (PCE).

A novel energy-resolved radiation detector based on the optimized CIGS photoelectric absorption layer

In this paper, we develop a novel energy-resolved X-ray detector based on the CuIn1-xGaxSe2 (CIGS) photoelectric conversion layer. The effect of Ga content on the crystal phase and the dark current of the photoelectric conversion layer has been studied. Furthermore, the photoelectric conversion capability of the CIGS thin film layer is optimized by the surface passivation process including the sulfurization treatment and Al2O3 treatment, achieving a low dark current of 90 nA/cm2 and high specific detectivity of 2.3 × 1014 Jones. The optimized CIGS photoelectric conversion layer and the Gd2O2S scintillator are used in the X-ray detector. The directions of the incident X-ray propagation and the induced fluorescence signal propagation are mutually perpendicular, thus the effective thickness of the scintillator is substantially increased without sacrificing the resolution of the sensor. In addition, the X-ray response of the detector is evaluated, the pixels at different X-ray penetration depths show obvious differences in absorption, contributing its ability of energy-resolution. The sensitivity of the prepared indirect X-ray detector is ∼8 μCGyair−1cm2 at an irradiation intensity of ∼6.5 mGyair/s. This work offers a new strategy to fabricate high performance energy-resolved X-ray detectors.

Formation of Cu(In,Ga)S2 chalcopyrite thin films following a 3-stage co-evaporation process

The study aims to investigate the origin of the crystalline and compositional bi-layered structure of CuIn1-xGaxS2 (CIGS) films we previously observed for samples deposited following the 3-stage process. Therefore, the growth of films of different gallium contents (i.e. x) was interrupted at key steps of the 3-stage, namely at the end of the first stage, after half of the second stage and at the end of third stage. Compositional, morphological and structural differences at each stage were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-Ray Spectroscopy (EDX). From these investigations, we propose a model attributing composition plateaus to the formation of diverse Cu-poor crystalline phases during the second stage of the growth process.

Performance enhancement of CIGS solar cell with two dimensional MoS2 hole transport layer

This paper describes a performance analysis of solar cell based on Cu (In1-xGax)Se2 (CIGS) material as an absorber layer together with two dimensional (2d)-MoS2 as hole transport layer (HTL) inserted between absorber layer and back contact using 1D-SCAPS program tool. In this work, numerical modeling tool has used to investigate the effect of thickness, doping concentration and defect variation on the performance of solar cell. Based on optimization of the device parameters, highest power conversion efficiency (PCE) of 26.81% (Voc =0.783 V, Jsc = 40.30 mA/cm2 and FF = 84.97%), has been obtained for ZnO/CdS/CIGS/MoS2 photovoltaic cell having 1.18eV energy bandgap (Eg) of CIGS layer. Performance of proposed CIGS solar cell with 2d-MoS2 HTL is better than the conventional CIGS solar cell and provide new path for recent advanced research for fabrication of solar cell on this technology. Further, solar cell's performance has been analyzed for various series-shunt resistances, work function of metal contacts and temperature for better analysis of the cell.

Practical Enhancements in Current Density and Power Generation of Bifacial Semitransparent Ultrathin CIGSe Solar Cells via Utilization of Wide Bandgap Zn-Based Buffer.

Among many building‐integrated semitransparent photovoltaics (BISTPVs), semitransparent ultrathin (STUT) Cu(Inx,Ga1‐x)Se2 (CIGSe) solar cells are distinguishable due to their potential high power conversion efficiency (PCE) among other thin‐film solar cells, versatile applicability based on thin film deposition processes, high stability consisting of all inorganic compositions, and practical expandability to bifacial applications. However, the fundamental trade‐off relationship between PCE and transparency limits the performance of BISTPV because implementing a higher semitransparency lowers the optical budget of incoming light. To expand the available optical budget and to enhance the PCE while maintaining a suitable transparency in STUT CIGSe solar cell with single‐stage coevaporated 500‐nm‐thick absorber, an atomic layer deposited wide bandgap Zn(O,S) buffer is introduced as the replacement of conventional CdS buffer, which partially limits incoming light less than 520 nm in wavelength. As a replacement result, more incoming light becomes valid for power conversion, and the short circuit current density (J sc) has increased comparatively by 17%, which has directly lead to a large increase in PCE up to 12.41%. Furthermore, Zn(O,S) buffer in the STUT CIGSe solar cell also has enhanced the bifacial compatible efficiency (BCE), which has increased to 14.44% at 1.3 sun and 19.42% at 2.0 sun.

Performance improvement of semi-transparent ultra-thin CIGSe solar cell by transferring exfoliated WTe2 multilayered-2D flakes to ITO substrate

Cu(In1-x,Gax)Se2 (CIGSe)-based solar cells have a high absorption coefficient (∼105), by which they can be used as thin-film solar cells that have the potential to be applied in various fields. To employ such a CIGSe-based solar cell as a transparent type, the conventional Mo substrate should be replaced with a transparent conductive oxide (TCO) substrate as the back electrode, and indium tin oxide (ITO) is one of the candidate TCO materials. However, various problems need to be addressed because the utilization of ITO may hinder solar cell performance, one of which is the contact occurring at the interface between CIGSe and ITO. This can be solved by adding an appropriate material at the interface while preserving the inherent purpose fulfilled by using the ITO electrode. In this study, we attempt to improve the contact properties using semi-metallic WTe2 flakes dispersed on the surface of the ITO substrate to form an electrical path between the CIGSe and the ITO substrate, which exhibit outstanding electrical conductivity among two-dimensional transition metal dichalcogenide materials. Accordingly, the effect of applying WTe2 flakes at the ITO and CIGSe interfaces on the performance of semi-transparent ultra-thin (STUT) CIGSe solar cells is discussed.

Back Shallow Ge Gradient Enhanced Carrier Separation for CZTSe Solar Cells through a Coselenization Process.

Given the prominent success of the Ga gradient in CuIn1-xGaxSe2 (CIGSe) solar cells, Ge gradient implementation is a promising way to boost Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. However, Ge-graded CZTSSe solar cells only possess a low efficiency of 9.2%, far from that of Ge-incorporated CZTSSe without a gradient (12.3%). Herein, we demonstrated a shallow Ge gradient CZTSe solar cell with an improved efficiency over 10%. The Ge gradient was achieved through a GeSe2-Se coselenization process, where GeSe2 acts as a low-temperature fluxing agent to assist crystallization and induce Ge transport toward the back interface. The relieved band tails and improved junction quality, leading to a better carrier separation, were found to take a primary responsibility for device improvement. These results highlight a remarkable breakthrough for Ge-graded CZTSe solar cells and offer a promising way to develop Ge-involved solar cells.

Cu (In x Ga1-x )Se2 Thin Films Mechanical and Structural Properties

Due to Cu (In x Ga1-x )Se2 (CIGS) thin films are used as an absorbing material in solar cells because their direct bandgap overlaps the red edge of the solar spectrum and they have a relatively high absorption coefficient (α~105 cm−1) [1]. The efficiency of CIGS solar cells reached 22.9% for laboratory scale devices and up to 15.7% for commercial modules [2], which is the highest conversion efficiency among all commercial polycrystalline solar cells [3]. Structural/nanomechanical properties of Cu(In x Ga1-x )Se2 films deposited on Si(100) by one-step radio frequency magnetron sputtering are discussed. The films were sputtered at different substrate temperatures (25-500 °C). X-Ray diffraction revealed dominant diffraction peak of (112). Surface morphology was examined by field emission scanning electron microscopy and atomic force microscopy. The film composition was examined by electron probe microanalysis. The Cu/(In+Ga) and Ga/(In+Ga) ratios were 0.9 and 0.3, respectively. Nanoindentation, in conjunction with the scanning electron microscopy, was conducted and revealed that the films suffered severe delamination type fracture due to poor adhesion to the Si substrate. The hardness of the films was not influenced by the substrate temperature increase except for the film grown at 500 °C. The hardness results were largely affected at depths larger than the film thickness due to poor adhesion of the film. At deep indents, the hardness results agreed closely with the Si substrate hardness.

Key factors governing the device performance of CIGS solar cells: Insights from machine learning

Cu(In1-x,Gax)Se2 (CIGS) solar cells are a kind of highly efficient thin film solar cells, further breakthrough in their device efficiency relies on the development of advanced methods and/or deep insights into the factors governing the device efficiency. Herein, we use the machine learning (ML) algorithms to explore the key factors governing the device performance of the CIGS solar cells and the underlying correlations. The datasets for ML are obtained from the experimental reports, which enables the results more referable for experimental optimization. Key factors governing the device performance are screened based on the correlation studies. The ML algorithms including linear regression, neural network, random forest (RF) and extreme gradient boosting are employed, among which RF performs best in predicting the efficiency of the CIGS solar cells with high accuracy (root mean square error of 0.9% and1.8%, Pearson coefficient r of 0.9 and 0.88 for validation and test sets, respectively). Furthermore, the factors and their optimal scales for high device efficiency are predicted, which provides essential guidance for experimental device optimization.

Atomic-Scale Interface Modification Improves the Performance of Cu(In1-xGax)Se2/Zn(O,S) Heterojunction Solar Cells.

Cadmium-free buffer layers deposited by a dry vacuum process are mandatory for low-cost and environmentally friendly Cu(In1-xGax)Se2 (CIGS) photovoltaic in-line production. Zn(O,S) has been identified as an alternative to the chemical bath deposited CdS buffer layer, providing comparable power conversion efficiencies. Recently, a significant efficiency enhancement has been reported for sputtered Zn(O,S) buffers after an annealing treatment of the complete solar cell stack; the enhancement was attributed to interdiffusion at the CIGS/Zn(O,S) interface, resulting in wide-gap ZnSO4 islands formation and reduced interface defects. Here, we exclude interdiffusion or island formation at the absorber/buffer interface after annealing up to 200 °C using high-resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX). Interestingly, HR-STEM imaging reveals an epitaxial relationship between a part of the Zn(O,S) buffer layer grains and the CIGS grains induced by annealing at such a low temperature. This alteration of the CIGS/buffer interface is expected to lead to a lower density of interface defects, and could explain the efficiency enhancement observed upon annealing the solar cell stack, although other causes cannot be excluded.

A Study on Formation of Aluminum-Doped Zinc Oxide Films by Pulsed-Direct Current Sputtering for CIGS Solar Cells.

Considering the relationship between thin film thickness of transparent conductive oxide (TCO) materials and the reversed pulse time in pulsed-direct current (DC) sputtering, aluminum-doped zinc oxide (AZO) films were deposited on glass substrates at different reversed pulse times by changing oxygen/argon (O₂/Ar) gas ratios for window layers of large area CuIn1-xGaxSe₂ (CIGS) solar cells. As a result of the reduced sputtering time, the thickness of AZO film was decreased when the reversed pulsed time was increased. The higher resistance and resistivity of the AZO film was obtained at a higher reversed pulse time. From the structural investigations of AZO such as transmittance and X-ray diffraction (XRD), it was possible to observe the relationship between the crystallinity of AZO and transmittance. Even at the short reversed pulse time of 0.5 µs, it can be concluded that the accumulated charges on the AZO target are completely cleared and the AZO layers show the highest figure of merit (FOM) with low sheet resistance and high transmittance.

Effect of cesium fluoride treatment in bifacial Cu(In1-x,Gax)Se2 solar cell

Bifacial Cu(In1-x,Gax)Se2 (CIGS) thin film solar cells were fabricated on titanium-doped indium oxide (ITiO) coated soda-lime glass substrate, and the effect of cesium fluoride (CsF) treatment on solar cell performance was investigated. CsF treatment improved the photovoltaic properties of CIGS/ITiO-based solar cells not only under front-side illumination but also under rear-side illumination. The quantum efficiency for rear-side illumination improved significantly in CsF-treated CIGS solar cells. As a result, the bifaciality improved in the CsF-treated CIGS solar cell. ITiO exhibited good contact with the CIGS thin film, regardless of Cs incorporation. In addition, the effect of heat-light soaking treatment was also investigated in CIGS/ITiO-based solar cells.

Inkjet printed CuIn(1-X)GaXSe2 thin film by controlled selenium distribution for improved power conversion efficiency in chalcopyrite solar cells

Selenium (Se) vapor pressure is a key factor during the selenization of CuIn1-XGaX (CIG) film to obtain a high-quality CuIn1-XGaXSe2 (CIGS) absorber layer. To investigate the effect of Se vapor distribution on the grain growth of inkjet printed precursor film, two geometries of graphite box (square and circular) are used. The results revealed that selenization in the round graphite box give rise to uniform surface coverage and suppressed fine-grained layer due to adequate and uniform distribution of Se vapor. In contrast, film selenized in a square graphite box exhibits high strain and low crystallinity with a thick fine-grained layer. Probable Se vapor distribution inside the graphite box based on internal geometrical constraint and its impact on crystal phase and microstructure is discussed. Finally, CIGS devices fabricated using films selenized in a round graphite box demonstrates higher power-conversion efficiency of 5.2%, owing to high light absorption and efficient carrier separation. Based on J-V and EQE results, probable losses and recombination in the devices are examined and discussed.

Van der Waals heterojunction interface passivation using ZnS nanolayer and enhanced photovoltaic behavior of semitransparent ultrathin 2D-MoS2/3D-chalcogenide solar cells

Along with highly efficient Cu(In1-x,Gax)Se2 (CIGSe)-based thin-film solar cells, versatile solar cells are urgently required to meet a variety of functional needs, such as photovoltaic (PV) power conversion, transparency, ultrathinness, and light weight. To meet these needs, a semitransparent ultrathin (STUT) CIGSe solar cell with two-dimensional (2D) transition metal dichalcogenide (TMD) as the buffer layer is proposed as a prospective next-generation solar cell structure that exhibits advantages based on both the distinguishable optical properties of 2D TMDs and the high efficiency of CIGSe-based solar cells. However, the application of 2D TMDs on top of CIGSe is challenging because the surface of 2D TMDs is nominally and supposedly flat and smooth, while that of CIGSe absorbers is typically rough, and their combination is prone to defect formation at the interface. To overcome degradation due to interfacial defects, a 2D-MoS2/ZnS/CIGSe semitransparent ultrathin solar cell was designed by inserting a ZnS nanolayer for heterojunction interface passivation, and the photovoltaic properties of the device were found to be improved. Furthermore, based on the binding energies of each layer, the energy band structures of n-MoS2/p-CIGSe with and without a ZnS passivation layer are suggested, and the related implications for device performance are discussed.

Lifetime prediction method for moisture-induced degradation of glass-to-glass solar cell modules using spatially distributed diode model

A Glass-to-Glass (G2G) CuInxGa1-xSe2 (CIGS) solar cell module is a promising thin-film solar technology; however, it is highly sensitive to moisture-induced degradation. Many studies have been conducted on the lifetime prediction model of this technology. However, none of these studies considered the spatial effects of moisture propagation on degradation. This study introduced a novel method, the spatially distributed diode model (SDDM), and predicted the lifetime of G2G CIGS solar modules for moisture-induced degradation. We analyzed the diffusion of spatial moisture concentration on the CIGS solar modules and estimated the model parameters using degradation data. The proposed method can predict the lifetime of any size or shape of G2G CIGS module for moisture-induced degradation. Furthermore, the method can be used to envision a possible longer lifetime for the solar modules depending on the selection of physical appearance of the modules and installation location.

Effect of combined treatment of cesium fluoride as precursor and post-treatment on Cu(In,Ga)Se2 thin film solar cell

In this study, we investigate the combined effect of cesium fluoride (CsF) as a precursor and post-treatment on a Cu(Inx,Ga1-x)Se2 (CIGS) thin film solar cell fabricated onto sodalime glass substrates. The combined treatment improves the open-circuit voltage, fill factor, and cell efficiency of the solar cell compared with its single counter treatment. Secondary ion mass spectroscopy measurements show a higher concentration of Cs diffusion in the surface regions in the combined treatment, which change the electronic properties of the buffer/CIGS interface by reducing interfacial recombination. Solar cells fabricated using CsF as a precursor indicate a deterioration in device performance owing to the formation of bulk defects, as confirmed from admittance spectroscopy. This study unveils device performance improvement by combining a heavy alkali metal as a precursor and post-treatment on CIGS thin film solar cells.

Heterojunction interface passivation strategy for Cu(In1-x,Gax)Se2 solar cell with nano-level engineering of Zn-based buffer structure via atomic layer deposition method

The global needs for ecological friendliness and sustainable development in the photovoltaic (PV) market have led to an increased demand to fabricate solar cells in an environmentally sound manner using nontoxic elements. In response to these requirements, in chalcogenide solar cell technology, researchers are attempting to replace the conventional CdS buffer. A potential replacement buffer material is ZnOS due to its direct, large, and tunable bandgap. Atomic layer deposition (ALD) can be effectively used to deposit ZnOS on Cu(In1-x,Gax)Se2 (CIGSe) absorbers, as this process allows the highly controlled thickness and composition, conformal coverage, inline compatibility, and relatively easier maintenance. However, CIGSe solar cells with an ALD grown ZnOS buffer usually suffer from the deficit of open circuit voltage (Voc), which limits power conversion efficiency (PCE). To overcome the limitation, ZnOS buffer structure with ZnS nanolayer is engineered for heterojunction interface passivation (HIP) and the role of ZnS nanolayer is investigated. The insertion of the ZnS nanolayer between ZnOS and CIGSe exhibits passivation effects at the heterojunction interface, thereby to enhance the solar cell performance owing to the realization of a more refined bandgap structure. Wide bandgap ZnS passivation thus demonstrates considerable potentials for being applied in other material systems.

Solar Energy Harvesting through Photovoltaic and Photoelectrochemical Means from Appositely Prepared CuInGaSe2 Absorbers on Flexible Substrates by a Low-Cost and Industrially Benign Pulse Electrodeposition Technique

CuIn1–xGaxSe2 (CIGS) semiconductors are promising candidates for thin-film photovoltaics and photoelectrochemical applications. In this work, CIGS absorbers prepared from an industrially benign low...

The effect of substrate temperature on Cu(In,Ga)Se2 layers deposited by dual thermal evaporation

This paper focuses on the preparation of CuInxGa1-xSe2 (CIGS) by a dual thermal evaporation method only without post-salinization treatment. The effect of substrate temperature on the deposition of stoichiometric layers has been explored. Fabricated CuInxGa1-xSe2 layers were described by energy-dispersive X-ray (EDX), diffraction of X-ray, Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), Ultraviolet-Visible-Near IR Spectroscopy and the four-point probing technique. CIGS layers of chalcopyrite structures were fabricated successfully with a bandgap of 1.35 eV. The stoichiometry, the grain size and roughness of layers are affected dramatically by the substrate temperature due to re-evaporation of the selenium content during deposition at high temperature. Electrical measurements explain the relationship between electrical conductivity and the value of metal/Se.