Ordered Vacancy Compound Research Articles

Nanoscale insight into the p‐n junction of alkali‐incorporated Cu(In,Ga)Se2 solar cells

AbstractThe effects of alkali diffusion and post‐deposition treatment in three‐stage processed Cu(In,Ga)Se2 solar cells are examined by using atom probe tomography and electrical property measurements. Cells, for which the substrate was treated at 650°C to induce alkali diffusion from the substrate prior to absorber deposition, exhibited high open‐circuit voltage (758 mV) and efficiency (18.2%) and also exhibited a 50 to 100‐nm‐thick ordered vacancy compound layer at the metallurgical junction. Surprisingly, these high‐temperature samples exhibited higher concentrations of K at the junction (1.8 at.%) than post‐deposition treatment samples (0.4 at.%). A model that uses Ga/(Ga + In) and Cu/(Ga + In) profiles to predict bandgaps (±17.9 meV) of 22 Cu(In,Ga)Se2 solar cells reported in literature was discussed and ultimately used to predict band properties at the nanoscale by using atom probe tomography data. The high‐temperature samples exhibited a greater drop in the valence band maximum (200 meV) due to a lower Cu/(Ga + In) ratio than the post‐deposition treatment samples. There was an anticorrelation of K concentrations and Cu/(Ga + In) ratios for all samples, regardless of processing conditions. Changes in elemental profiles at the active junctions correlate well with the electrical behaviour of these devices. Copyright © 2017 John Wiley & Sons, Ltd.

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%.

Copper content dependence of electrical properties and Raman spectra of Se-deficient Cu(In,Ga)Se2 thin films for solar cells

This paper reports the Raman spectra and electrical analysis of Se-deficient Cu(In,Ga)Se2 (CIGS) films with various Cu contents. These films were deposited by low-cost two-step process consisting of sputtering of metallic precursor and subsequent selenization. Raman spectra exhibit the formation of chalcopyrite phase while signs of secondary phases like CuxSe and ordered vacancy compound are also observed. Electrical measurements show the change of main carrier source as well as the transformation of conduction type with varying Cu content. Comprehensive analysis of Raman spectra and electrical measurements qualitatively explains the relationship between electrical properties and Cu contents. Based on this relationship, the optimal range of Cu/(Ga + In) for Se-deficient CIGS solar cells is identified as 0.875–0.925, which is smaller than that for Se-sufficient CIGS solar cells due to the presence of Se vacancies. The efficiency data of CIGS solar cells fabricated with same absorber deposition process are in good agreement with this optimal range. The results of this paper could be used to estimate device performance at early process stages.

Do defects get ordered in Cu2ZnSnS4?

Formation of ordered defect compounds and anomalous grain boundary physics are unique to Cu chalcogenides CuInX2 (S/Se) and its alloys. X-ray photoelectron spectroscopy (XPS) studies were carried on Cu2-xZn1.3SnS4 (x=0.0, 0.3, 0.5 and 0.7) to determine the position of valence band edge and explore the formation of ordered vacancy compounds along with absorption studies. Conductive atomic force microscopy (C-AFM) studies were carried out Cu2ZnSnS4 (CZTS) film deposited on Si and sodalime glass substrates to understand grain boundary physics.

Optical methodology for process monitoring of chalcopyrite photovoltaic technologies: Application to low cost Cu(In,Ga)(S,Se)2 electrodeposition based processes

Non-destructive characterization of both single layers and completed devices are important issues for the development of efficient and low cost Cu(In,Ga)(S,Se)2 (CIGS) modules at high yields. This implies for the need of methodologies suitable for the assessment of optical, electrical, and physico-chemical parameters that are relevant for the final device efficiency and that can be used for quality control and process monitoring at different process steps. In these applications, detection of in-homogeneities in the different layers from large area modules is especially relevant, being the presence of these inhomogeneities responsible for the existing gap between the efficiencies achieved in these technologies at cell and module levels. In this context, this work reviews the different optical methodologies that have been developed in the framework of the SCALENANO European project for the advanced assessment of the different layers in high efficiency electrodeposited – based CIGS devices. This has includes different strategies as those based on Raman scattering, Photoluminescence/Electroluminescence (PL/EL) based techniques and new photoelectrochemical based tools and firstly Raman spectroscopy is very sensitive to both composition and crystal quality parameters that are determining for device efficiency. Use of resonant Raman excitation strategies allows achieving a high sensitivity of the Raman spectra to the analysed features in the different regions of the device. This involves selection of the suitable excitation wavelength (in the broad spectral region from UV to IR) for the resonant Raman excitation of the required layer in the device. The strong increase in the intensity of the Raman peaks related to the use of resonant excitation conditions allows also decreasing the measuring time to times compatible with the implementation of these techniques at online process monitoring level. Analysed parameters include the electrical conductivity of the Al-doped ZnO window layer, the thickness of the CdS buffer layer and the chemical composition (S/(S+Se) relative content) and presence of relevant secondary phases as Cu-poor ordered vacancy compounds in the surface region of the absorbers. In addition PL/EL imaging are powerful techniques that provide direct access to the optoelectronic properties of the materials and devices. Whereas EL is performed using complete devices by injecting current in analogy to the operation of a light emitting diode, PL allows the characterization of bare absorber materials without the need for any functional or contacting layers. Moreover, semiconductor photo-electrochemistry (PEC) is a versatile technique that enables many opto-electronic properties of semiconductors to be determined. Essentially, a semiconductor on a conducting substrate placed in a solution containing redox species forms a Schottky barrier junction. The formation of such a diode enables basic semiconductor properties to be measured such as doping type, doping density, band gap and the flat band position versus the vacuum energy scale. In all these cases, quality control indicators suitable for the advanced assessment of these processes have been identified and validated for the electrodeposition-based processes developed at Nexcis Company.

Ordered Vacancy Compound Formation by Controlling Element Redistribution in Molecular-Level Precursor Solution Processed CuInSe2 Thin Films

Ordered Vacancy Compound Formation by Controlling Element Redistribution in Molecular-Level Precursor Solution Processed CuInSe₂ Thin Films

Physical properties of Cu(In,Ga)Se2 film on flexible stainless steel substrate for solar cell application: A multi-layer precursor method

The process named “multi-layer precursor method”, consisting of Ga–Se/In–Se/Cu–Se stacked precursors, is proposed to prepare Cu(In,Ga)Se2 (CIGS) films on flexible stainless steel (SUS) substrates. In this work, precursor deposition temperature (TPRE), varying from about 250 to 430°C, is first scrutinized. From Raman scattering measurement, it is suggested that the increase in the TPRE enhances Cu2−xSe secondary phase during film deposition, thus enlarging CIGS grain size, while decreases ordered vacancy compound (OVC) near surface of the finished film, caused by desorption of In2Se during the precursor deposition. Suitable TPRE is in a range of approximately 370–400°C, resulting in long carrier lifetime. Next, according to the examinations of material compositions of CIGS absorbers, the optimized [Cu]/([In]+[Ga]) contributes to the absorbers with large grain size due to the help of Cu2−xSe phase during film deposition and OVC near their surfaces for pn homo-junction. The appropriate [Ga]/([In]+[Ga]) leads to optimized band-gap energy with high CIGS quality implied by long carrier lifetime. Ultimately, the effective [Cu]/([In]+[Ga]) and [Ga]/([In]+[Ga]) ratios for the flexible CIGS solar cell with high conversion efficiency under the so-called “multi-layer precursor method” are in ranges of about 0.74–0.84 and 0.28–0.35, respectively.

3-D point defect density distributions in thin film Cu(In,Ga)Se2 measured by atom probe tomography

This paper will discuss the findings of an ordered vacancy compound (OVC) phase that exists deep into the bulk of a high-efficiency Cu(In,Ga)Se2 (CIGS) absorber that is shown to be a result of many ordered defect pairs of 2VCu + (In,Ga)Cu as determined by atom probe tomography (APT). To date, literature has shown that absorbers grown with the three-step process exhibit the OVC Cu(In,Ga)3Se5 (135 phase) only within the first few nanometers from the CdS/CIGSe interface and at grain boundaries. In this contribution, we have found a small volume (100 nm × 100 nm × 300 nm) of an OVC phase to exist about 400 nm into the absorber. We show through concentration and density profiles that the concentration change from the stoichiometric Cu(In,Ga)Se2 to the OVC is indeed a result of many ordered defect pairs. We use this volume to perform point defect density distributions to give unique insight to the band structure at the nanoscale.

Microstructure characterization of the soda-lime-glass/copper–indium–gallium–selenium interface in Cu-poor Cu(In,Ga)Se2 thin films

Abstract The microstructure characteristics of the soda-lime-glass/Cu(In,Ga)Se 2 (SLG/CIGS) interface in Cu-poor CIGS films are investigated by transmission electron microscopy and selected area electronic diffraction (SAED). The SAED patterns show very sharp and strong spots, indicating the main structure of CIGS chalcopyrite. Small dispersed crystals with size distribution from 2 to 5 nm seem to be embedded in amorphous matrix, and additional spots indicate the presence of an ordered vacancy compound (OVC). This observation is consistent with the Raman results, and the OVC phase with the nanoclusters exists in the CIGS matrix, instead of layer structure. Lattice distortion results in local changes in contrast. Some pseudo-disordered structure is observed, however, the structure is actually the chalcopyrite CIGS structure. 180° rotation twins are also observed at the SLG/CIGS interface. Lattice distortion is widely observed at the interface of the Cu-poor CIGS films, and the extra spots could be caused by different lattice orientations.

Fabrication and characterization of photovoltaic cell with novel configuration ITO/n-CuIn3Se5/p-CIS/In

A novel configuration ITO/n-OVC CuIn3Se5/p-CIS/In solar cell has been fabricated by multisource vacuum co-evaporation technique on soda lime glass substrates. The pn junction is formed with ordered vacancy compound as the n counter part for the p type CuInSe2. The structural, compositional, hall coefficient, optical and electrical properties of the p and n layers have been studied respectively by X-ray diffraction, Energy Dispersive Analysis of X rays, optical absorbance and conductivity measurements. Current density-Voltage measurements enabled the determination of efficiency of the device.

Transparent Back Contacts for Superstrate <sc>(Ag,Cu)(In,Ga)Se</sc><inline-formula><tex-math>$_{\bf 2}$ </tex-math></inline-formula> Thin Film Solar Cells

Molybdenum oxide (MoO3) and tungsten oxide (WO3) are considered as transparent back contacts for (Ag,Cu)(In,Ga)Se-2 thin film solar cells. MoO3 and WO3 films were deposited by reactive RF sputtering at room temperature in an Ar/O-2 ambient on (Ag,Cu)(In,Ga) Se-2 absorber layers with various Ga/(Ga + In) and Ag/(Ag + Cu) ratios. Determination of the valence band offsets by XPS showed that Ag-alloying of absorber layer changes the energy band alignment at the absorber-back contact interface with MoO3 and WO3 contacts. This produces a primary contact with lower valence band offset compared with Cu(In,Ga) Se-2 counterparts. The effect is less significant in films with Ga > 0.5 and Ag > 0.5 (corresponding to E-g > 1.4 eV) probably due to the different nature of ordered vacancy compounds forming near the surface phases.

Structural and Vibrational Study of Pseudocubic CdIn2Se4 under Compression

We report a comprehensive experimental and theoretical study of the structural and vibrational properties of α-CdIn2Se4 under compression. Angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy evidence that this ordered-vacancy compound with pseudocubic structure undergoes a phase transition (7 GPa) toward a disordered rocksalt structure as observed in many other ordered-vacancy compounds. The equation of state and the pressure dependence of the Raman-active modes of this semiconductor have been determined and compared both to ab initio total energy and lattice dynamics calculations and to related compounds. Interestingly, on decreasing pressure, at ∼2 GPa, CdIn2Se4 transforms into a spinel structure which, according to calculations, is energetically competitive with the initial pseudocubic phase. The phase behavior of this compound under compression and the structural and compressibility trends in AB2Se4 selenides are discussed.

A novel non-vacuum process for the preparation of CuIn(Se,S)2 thin-film solar cells from air-stable, eco-friendly, metal salts based solution ink

A facile solution-based non-vacuum process for deposition of CuIn(Se,S)2 (CISeS) absorber layers is presented in this work, which indicates a promising way for the low-cost applications in thin-film solar cells. Firstly, low-boiling-point solvents Monobutylamine C4H11N and Carbon disulfide CS2 are selected as the complexing and thickening agents and added into the Cu/In metal salts based solution. Thus the air-stable, eco-friendly solution ink is successfully synthesized through a simple solution synthesis route. The detailed chemical reaction mechanism and the influence of the composition of precursor solution have been discussed intensively as well. After sequential spin-coating, hot-treatment and selenization process, the high-quality CISeS films are obtained and then characterized by XRD, Raman, SEM, EDS, Metallographic microscope, Hall Effect measurement and UV–vis–NIR spectroscopy, respectively. It is found that the compact CISeS films with chalcopyrite α-phase possess a double-layer structure, and also incorporate with a little ordered vacancy compounds (OVCs) and Cu2−xSe impurities. The typical near stoichiometric CISeS films without Carbon residuals have superior photoelectric properties with carrier concentration of 3.46 × 1016 N cm−3 and band gap of 1.15 eV. Finally, the original first-made PV devices provide a power conversion efficiency (PCE) of 4.25%, which can be further improved by increasing the thickness of CISeS films and/or optimizing the selenization and sulfuration technologies.

Raman scattering analysis of electrodeposited Cu(In,Ga)Se2 solar cells: Impact of ordered vacancy compounds on cell efficiency

This work reports the detailed Raman scattering analysis of Cu-poor Cu(In,Ga)Se2 (CIGS) electrodeposited solar cells using different excitation wavelengths. The systematic assessment of cells fabricated with Cu-poor absorbers that were synthesized with different Cu contents has allowed identifying the existence of a quasi-resonant excitation of a Raman peak characteristic of an Ordered Vacancy Compound (OVC) secondary phase when using a 785 nm excitation wavelength. The enhanced sensitivity of the spectra measured with these conditions to the presence of the OVC phase provides with a suitable tool for the non destructive assessment on the occurrence of this Cu-poor secondary phase in the surface region of the CIGS absorbers from measurements performed on finished cells. The correlation between the Raman scattering data and the optoelectronic parameters of the devices shows the existence of an optimum OVC content range leading to devices with highest open circuit voltage and efficiency. These data provide with a clear experimental evidence on the impact of the OVC phases on the device efficiency.

Structural and Vibrational Properties of CdAl2S4 under High Pressure: Experimental and Theoretical Approach

The behavior of defect chalcopyrite CdAl2S4 at high pressures and ambient temperature has been investigated in a joint experimental and theoretical study. High-pressure X-ray diffraction and Raman scattering measurements were complemented with theoretical ab initio calculations. The equation of state and pressure dependences of the structural parameters of CdAl2S4 were determined and compared to those of other AB2X4 ordered-vacancy compounds. The pressure dependence of the Raman-active mode frequencies is reported, as well as the theoretical phonon dispersion curves and phonon density of states at 1 atm. Our measurements suggest that defect chalcopyrite CdAl2S4 undergoes a phase transition above 15 GPa to a disordered-rocksalt structure, whose equation of state was also obtained up to 25 GPa. In a downstroke from 25 GPa to 1 atm, our measurements indicate that CdAl2S4 does not return to the defect chalcopyrite phase; it partially retains the disordered-rocksalt phase and partially transforms into the spin...

Cu(In, Ga)Se2 thin films prepared from stacked precursors by post-selenization process

The copper–indium–gallium metallic precursors were fabricated corresponding to the sequence CuGa/CuIn/CuGa/soda-lime-glass by sequential direct current magnetron sputtering. The as-sputtered precursors were comprised of Cu11In9, Cu16In9, In, Cu9Ga4, and CuGa2 phases, which may be closely correlated to the deposition sequences of multi-layered metallic precursors. Cu(In, Ga)Se2 (CIGS) absorber films were prepared from the stacked precursors by post-selenization process with solid Se powder, and the morphological, structural, and compositional properties were investigated. The as-selenized CIGS film exhibited a smooth, compact, and densely packed morphology with well-defined and faceted crystal grains. The film was Cu-deficient and had a low Ga content. X-ray diffractometer results indicated the formation of single-chalcopyrite structure CIGS absorber layers. Depth-resolved Raman patterns showed the formation of a dominant CIGS phase in the as-selenized layer, and an ordered vacancy compound phase like Cu(In, Ga)3Se5 at the surface and inner region. With increasing the sputtering time, the full width at half maximum of the chalcopyrite A1 Raman peak increased. Band broadening can be interpreted as a result of a higher density of defects within the chalcopyrite CIGS phase. The A1 peak shifts with increasing sputtering depths were not apparent, which was related to a uniform distribution of Ga in the CIGS thin film.

Differential in-depth characterization of co-evaporated Cu(In,Ga)Se2 thin films for solar cell applications

In this paper we report an alternative approach to perform in-depth characterisation of Cu(In,Ga)Se2 (CIGS) absorber layers. While usually groups stop their co-evaporation process at different points and analyse the precursor or intermediate phases, we perform in-depth analysis on the finished absorber layer as it will be used in the solar cell. A co-evaporated CIGS layer was cut to several samples, which then were chemically etched to different thicknesses. Compared to sputtering ablation techniques, this avoids the selective abrasion of atoms with different binding energies. The samples were analysed by Raman spectroscopy and X-ray diffraction. In-depth information is obtained by differentiating the signals of samples with different thicknesses after etching and a first order correction for absorption losses was executed. The Ga/(Ga+In) ratio extracted from X-ray diffraction measurements is in good agreement with the double gradient observed by glow discharge optical emission spectroscopy. A slight variation might indicate residual stress in the CIGS layer. A preferred (112) orientation across the whole film together with changing (220), (116) and (312) orientation preferences is reported and explained on the basis of the CIGS crystal structure. Raman signals attributed to ordered vacancy compounds are found throughout the whole sample thickness and not only close to the surface, as often reported in the literature.

Buried p-n junction formation in CuGaSe2 thin-film solar cells

CuGaSe2/CdS interfaces and the mechanism behind the hetero p-n junction formation were investigated using solar cell devices which demonstrated about a 10% energy conversion efficiency. It was found that the CuGaSe2/CdS interface could be described as a CuGaSe2/Cu-deficient Cu-Ga-Se layer (CDL)/CdS structure and the p-n junction was located at the CuGaSe2/CDL interface that was present 50–100 nm from the CDL/CdS interface. While the difficulty of the realization of equilibrium n-type CuGaSe2 has been generally recognized, this result suggests that CDL consisting of ordered vacancy compound phases such as CuGa3Se5 can play the role of an n-type material.

Theoretical Study on the Diffusion Mechanism of Cd in the Cu-Poor Phase of CuInSe2 Solar Cell Material

We have employed first-principles static and molecular dynamics (MD) calculations with semilocal and screened-exchange hybrid density functionals to study the diffusion of Cd in bulk CuIn5Se8, a copper-poor ordered vacancy compound of CuInSe2. The diffusion mechanism and the underlying kinetics/energetics were investigated by combining ab initio metadynamics simulations and nudged elastic band (NEB) calculations. We found that the migration of Cd occurs via a kick-out of Cu atoms, assisted by the pristine vacancies that are constitutive of this compound, and follows a double-hump energy profile. The rate-limiting step has a barrier of about 1 eV at 0 K but reduces to 0.3 eV at 850 K, pointing out non-negligible dynamical effects. Hybrid functional calculations reveal that Cd impurities are doubly positively charged (Cd2+) in p-type and intrinsic conditions. The position of the 0/2+ charge transition level explains why Cd impurities do not constitute deep traps for carriers, making them not harmful for the...

Order/disorder phenomena in Zn1−xMnxGa2Se4 ordered vacancy compounds: high temperature neutron powder diffraction experiments

We present a study of order–disorder phenomena in the series of tetrahedral ordered vacancy compounds Zn1−xMnxGa2Se4 by means of time-of-flight neutron diffraction at high temperature together with dc magnetic susceptibility, Raman spectroscopy, differential thermal analysis and optical absorption experiments. Samples of nominal composition x = 0, 0.24, 0.5, 0.77 and 1 have been studied. An order–disorder phase transition has been detected, with Tc ranging from 472 to 610 ° C, which involves a structural change from a defect chalcopyrite phase, with space group (s.g.) and three different cation sites, to a partially disordered defect stannite, in which Zn, Mn and half of the Ga ions share the 4d site in s.g. Neither the vacancies nor the Ga ions occupying site 2a are involved in the phase transition. An additional ordering process is observed on approaching the phase transition from below, which is attributed to several factors: the activation of cation diffusion at ∼300 ° C, the partially disordered cation distribution exhibited by the as-grown single crystals and the preference of Mn atoms for the 2d crystallographic site in the structure. The reversibility of the phase transition is analysed with the aid of magnetic, optical and Raman experiments.