CuIn1-xGaxSe2 Research Articles

Spectroscopic evidence for a surface layer in CuInSe2:Cu deficiency

The near-surface region of thin-film polycrystalline (PX) CuIn1−xGaxSe2 (CIGS) is considered important because it is the region where the electrical junction forms in a CIGS photovoltaic device. Spectroscopic ellipsometry measurements of polycrystalline CuInSe2 films reveal that there is a thin layer at the surface which has different optical and electronic properties from those of the bulk film. This surface layer of thin-film CIGS has a larger band gap and greater spin-orbit interaction energy than the bulk film. These properties indicate that the surface layer is more Cu deficient than the bulk in the nearly stoichiometric thin-film PX-CIGS used in photovoltaic devices. This work provides an insight into the importance of surface layer engineering for photovoltaic device design.

Influence of different precursor surface layers on Cu(In1-xGax)Se2 thin film solar cells

The properties of Cu(In1-xGax)Se2 (CIGS) thin films obtained by selenization of the precursors with different surface layers have been studied, and photovoltaic devices based on the absorbers were measured and analyzed. The devices constructed by the absorbers obtained by selenization of the precursors with CuGa-rich surface layers are improved, compared with those with In-rich surface layers. Through XRD, SEM, SIMS, illuminated J–V, QE and Raman spectra measurements, it was found that the increased Ga contents within the surface region of films and the graded Ga distribution can be realized in the selenized thin films fabricated by the precursors with the CuGa-rich surface layer. Consequently, the performances of the photovoltaic devices based on these thin films are further improved.

Improved Electrodeposition of CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> Films

CuIn1-xGaxSe2(CIGS) precursor films are fabricated on Mo foil by coelectrodeposition. The influence of the applied potential and the electrolyte additive in the process of electrodeposition are discussed. The precursor films are annealed in Ar for a short time to synthesize the polycrystalline thin film. The annealed layers are only phase-pure CuIn0.7Ga0.3Se2 and show a good crystallinity.

Study of Recombination in Cu(In,Ga)Se2 Solar Cells by Time-Resolved Photoluminescence

AbstractTime-resolved PL (TR-PL) measurements have been carried out on Cu(In1-x,Gax)Se2 (CIGS) thin films and solar cells to study the recombination of photo-generated minority carriers . Room temperature PL of both CIGS thin films and solar cells exhibited band-to-band transition for alloy composition x of 0-0.4, and PL decay curves have been utilized as a measure of carrier lifetime. The solar cell fabrication leads to the enhancement of band-to-band PL intensity, reduction of defect-related PL, and increase of PL decay time. It is noted that the increase in PL decay time has been observed in CIGS films after CBD-CdS deposition. Solar cells with high conversion efficiency are characterized by both high PL intensity and long PL decay time. The PL decay curve of CIGS solar cell is studied in terms of the alloy composition x. For small x, PL decay times of CIGS solar cells are almost independent of the emission photon energy. For large x, PL decay time depended on PL emission energy. he result suggests the change of the recombination mechanism in the CIGS solar cells with the change in the alloy composition x.

In-situ Characterization of As-grown Surface of CIGS Films

AbstractIn-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In1-xGax)Se2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10-8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In0.65Ga0.35)Se2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.

High efficiency thin-film CuIn1−xGaxSe2 photovoltaic cells using a Cd1−xZnxS buffer layer

The authors have fabricated 19.52% thin-film CuIn1−xGaxSe2 (CIGS)-based photovoltaic devices using single layer chemical bath deposited Cd1−xZnxS (CdZnS) buffer layer. The efficiency equals the world record for any thin-film solar cell and is achieved with reduced optical absorption in the window layer. Using current-voltage, quantum efficiency, and capacitance-voltage measurements, the CIGS/CdZnS device parameters are directly compared with those of CIGS/CdS devices fabricated with equivalent absorbers.

Thin-film ZnO/CdS/CuIn1-xGa xSe2 solar cells: anomalous physical properties of the CuIn1-xGa xSe2 absorber

Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells are manufactured with similar to 20% solar-cell conversion efficiency. The CuIn1-xGaxSe2 (CIGS) absorber exhibits rather different physical properties co ...

Grain size enlargement of Cu(InGa)Se 2 thin films by Hot Filament Melting process

Hot Filament Melting (HFM) process has been newly proposed to enlarge the grain size of Cu(In1–xGax)Se2 (CIGS) films grown by a mechanochemical process (MCP). When the substrate temperature was 500 oC and 600 oC and the current through the hot filament was 15A, we confirmed encouraging the sintering of CIGS grains. Additionally, when the substrate temperature was 700 oC, the melt of the CIGS grains at the surface was observed. The crystal structure of CIGS films after the process were analyzed by X-ray diffraction and Raman spectroscopy, and it was found that the CIGS films kept the chalcopyrite structure and that the formation of hetero-phase was not observed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Present status and future prospects of CIGSS thin film solar cells

Efficiencies of CuIn1−xGaxSe2−ySy (CIGSS) modules are comparable to those of lower end crystalline-Si modules. CIGSS layers are prepared by reactive co-evaporation, selenization/sulfurization of metallic or compound precursors, reactive co-sputtering and non-vacuum techniques. CuIn1−xGaxS2 (CIGS2) layers are prepared by sulfurization of Cu-rich metallic precursors and etching of excess Cu2−xS. Usually heterojunction partner CdS and transparent-conducting bilayer ZnO/ZnO:Al layers are deposited by chemical bath deposition (CBD) or RF magnetron sputtering. CIGSS solar cell efficiencies have been improved by optimizing Cu, Ga and S proportions and providing a minute amount of Na. This paper reviews preparation and efficiency improvement techniques for CIGSS solar cells.

Performance comparison of hybrid sputtering/evaporation CuIn1−xGaxSe2 solar cells with different transparent conducting oxide window layers

In this work we report the results of a study where solar cells were prepared with CuIn1−xGax Se2 (CIGS) as the absorber layer deposited by a two-stage hybrid sputtering/evaporation method and finished with two different transparent conducting oxides (TCO): i-ZnO/ZnO:Al or indium tin oxide (ITO). The solar cell structure is soda lime glass/Mo/CIGS/CdS/TCO. The ZnO and the ITO window layers were deposited by RF-magnetron and DC-magnetron sputtering at room temperature respectively. The deposition of these two TCOs was optimized to obtain similar parameters. In both cases the control samples showed sheet resistivities at around 50Ω/□ and transmission coefficients around 85% in the visible spectral range for layers 300nm thick. The I–V characteristics of finished solar cells under simulated solar spectrum illumination have been measured and the cell parameters analyzed.

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

AbstractPolycrystalline CuIn1−xGaxSe2 (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the VOC and the short circuit current density JSC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and VOC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.

Thin-film photovoltaics

CdTe and CuIn1−xGaxSe2−ySy (CIGSS) are ideal candidates for thin-film solar cells. Present photovoltaic (PV) conversion efficiencies of champion thin-film solar cells are: CuIn1−xGaxSe2 (CIGS) 19.5%, CdTe 16.5%, and a-Si:H 12.4%. Thin-film PV modules could spearhead production growth of photovoltaics in the United States because of their added production capacity. For this purpose, module efficiencies must be improved to the 13%–15% range. Obtaining Ohmic contacts is difficult, especially for CdTe, because of the inherently low p-type doping level. Therefore, increasing the p-type doping level is important. Growth of CIGSS film must be controlled carefully as it transitions from Cu-rich to In-rich composition. Other issues for CIGSS cells are minimizing indium consumption, and increasing process throughput of selenization∕sulfurization and transparent conducting oxide deposition. Development of all-dry processing for CdS deposition would be beneficial for both cells. This paper discusses basic devices and related issues.

Ellipsometric spectroscopy on polycrystalline CuIn1–xGaxSe2: Identification of optical transitions

AbstractBulk materials have been synthesized by the Bridgman technique using the elements Cu, Ga, In, Se. Bulk samples have been characterized by EDS (Energy Dispersive Spectrometer), hot point, X‐ray diffraction, photoluminescence and spectroscopic ellipsometry (SE). The samples used were well crystallized and lended strong support to the achievement of a good stoechiometry. Energy levels above the gap in the band scheme were determined by measuring the dielectric function at ambient temperature for energies lying between 1.5 and 5.5 eV. Many transitions were observed above the gap for different samples of CuIn1–xGaxSe2 (0 ≤ x ≤ 1) alloy. Spectroscopic ellipsometry gave evidence for the interpretation of the choice of gap values which were compatible with that obtained from solar spectrum [1]. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Cu Deficiency on CuIn1-xGaxSe2 and High-efficiency Photovoltaic Solar Cells

AbstractWe report the results of our studies on the optical and electronic structure of a wide range of polycrystalline thin-film CuIn1-xGaxSe2 (CIGS) alloys. The composition range includes CIS and nearly stoichiometric (slightly Cu-poor) (24.3±0.3 at.% Cu) CIGS with x values located around the value that has the best efficiency (x ∼ 0.28). Relative to nearly stoichiometric CIS and CIGS, we find a reduction in the absorption strength in the spectral range 1-3 eV. This reduction can be explained in terms of the predominance of Cu 3d and Se 4p states at the valence band maximum (VBM). In addition, Cu-poor CIS and CIGS materials show an increase in bandgap because the p-d repulsive interaction in Cu-poor CIGS is less than that in nearly stoichiometric CIGS. High efficiency is discussed in terms of optical properties.

Built-in Potential and Open Circuit Voltage of Heterojunction CuIn1-xGaxSe2 Solar Cells

AbstractThe reasons why the open circuit voltage (Voc) of high-x CuIn1-xGaxSe2 (CIGS)/ZnO solar cells remain low are discussed. Here it is shown that the Voc ceiling can be interpreted simply on the basis of a model that the valence-band energy (Ev) of CIGS is almost immovable irrespective of x. When the conduction-band energy (Ec) of ZnO is lower than that of high-x CIGS (DEc<0), the built-in potential (Vbi) of a CIGS/ZnO junction is equivalent to the flat-band potential (Vbi) that arises from the separation between the Fermi energies of the two materials. If the Ev (and therefore the Fermi energy) of p-type CIGS is constant with increasing x, the Vbi and Voc that follows the Vbi remain unchanged since the Fermi energy of ZnO is constant. This unchangeable Voc reduces the conversion efficiency of high-x CIGS cells in cooperation with reduced photocurrents due to a larger bandgap. A positive offset, ΔEc>o gives rise to a photoelectrons barrier in the conduction-band that partially cancels Voc, thus the Voc of a low-x CIGS cell is governed by the Ec of CIGS. Based upon this concept, a material selection guideline is given for the windows and transparent electrodes appropriate for high-x CIGS absorbers-based solar cells.

In Situ Sensors for CIGS Deposition and Manufacture

AbstractIn situ sensors are an important tool for process control, optimization, and documentation, both in the laboratory and industrial environments. Their further application to deposition of CuInxGa1-xSe2 (CIGS) for photovoltaics is particularly important, as record device efficiencies produced in the laboratory have yet to be replicated in manufacturing. This paper provides an overview of the current state of the art of in situ diagnostics for devices based on coevaporated CIGS.

Fabrication of wide-gap Cu(In1−xGax)Se2 thin film solar cells: a study on the correlation of cell performance with highly resistive i-ZnO layer thickness

Abstract The correlation of the cell performance of wide-gap Cu(In1−xGax)Se2 (CIGS) solar cells with the thickness of highly resistive i-ZnO layers, which are commonly introduced between the buffer layer and the transparent conductive oxide (TCO) layer in CIGS solar cell devices, was studied. It was found that cell parameters, in particular, the fill factor (F.F.) varied with the thickness of the i-ZnO layers and the variation of the F.F. was directly related to cell efficiency. A 16%-efficiency was achieved without use of an anti-reflection coating from wide-gap (Eg∼1.3 eV) CIGS solar cells by adjusting the deposition conditions of the i-ZnO layers.

Preparation and characterization of CuIn1−xGaxSe2 thin films obtained by sequential evaporations and different selenization processes

The aim of this work was to study the effect of thermal treatment (vacuum or Ar) on the main properties of CuIn1−xGaxSe2 (CIGS) thin films prepared by selenization of identical metallic alloys. All the CIGS films showed chalcopyrite structure with preferential orientation in the (112) plane. Ar thermal treatment led to better crystallinity, lower electrical conductivity and an increase in the grain size. It has been found that the Se diffusion and the distribution of the elements depend on the selenization atmosphere. After Ar selenization, an increase in band gap energy of 0.09 eV was observed, which is associated with a reduction in defects.

Electrodeposition of CuIn1−xGaxSe2 precursor films: optimization of film composition and morphology

In this paper, we report the preparation of CuIn1−xGaxSe2 (CIGS) films using electrodeposition. The influence of individual salt concentration on film composition is discussed in detail. We observed that CuCl2 and H2SeO3 in the deposition bath strongly influence the film composition compared to GaCl3 and InCl3 salts. The deposited film morphology was also strongly influenced by the film composition. We also observed microcracks in the films when the films were deposited at low concentrations of CuCl2, InCl3, and GaCl3 salts and at high concentrations of H2SeO3.

Impact of substrate roughness on CuInxGa1−xSe2 device properties

Abstract In comparison to the traditional use of glass substrates, CuInxGa1−xSe2 (CIGS) deposited onto metal substrates offers improved device cooling under concentration, economical large-scale roll-to-roll processing, and applicability towards lightweight as well as flexible products. However, unlike glass, metal foils tend to exhibit rough surfaces. This study quantifies the effect of substrate roughness on CIGS device performance. Several substrate types with differing average roughnesses were examined. The frequencies of the surface features contributing to roughness are also examined via several different analytical techniques. Devices were prepared and characterized on steel foils, Mo foil, and glass.