Optical properties of Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4

The optical properties of CuInSe₂, CuGaSe₂, Cu₂ZnSnS₄, and Cu₂ZnSnSe₄ are investigated using three different first-principles methods, namely the generalized gradient approximation by Perdew, Burke, and Ernzerhof (PBE), the hybrid Hartree-Fock-like functional by Heyd, Scuseria, and Ernzerhof (HSE), and a Green's function approach (GW). The density-of-states, the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω), and the optical absorption coefficient α(ω) are determined, providing fundamental understanding of these materials. We find that even though the PBE method generates fairly accurate effective crystal potentials, the HSE and GW methods improve considerably the band-gap energies E_g and also the localization of the semicore states, thereby describing the optical properties much better. Furthermore, we also present optimized convergence parameters for the self-consistent HSE calculation in order to reduce the computational time of this orbital-dependent method.     

Band gap optimization in Cu(In1 − xGax)(Se1 − ySy)2 by controlled Ga and S incorporation during reaction of Cu-(In,Ga) intermetallics in H2Se and H2S

A large number of companies around the world are developing a variety of manufacturing approaches aimed at low-cost, high throughput, large area CIS-based photovoltaic modules that maintain laboratory-scale cell efficiencies. The most critical technological issue, which directly impacts on the cost-of-ownership of large-scale production, is the specific technology employed for the deposition of the chalcopyrite absorber film. In standard reactive annealing processes, the complex reaction kinetics during the chalcogenization of the precursor film, results in phase segregated multinary alloys. This in turn results in compositionally graded absorber films, which could adversely affect the performance of devices, if grading is not carefully controlled through proper process control. Against this background, a clear understanding of the reaction paths for the formation of the chalcopyrite multinary alloys is essential. In this paper, the details of a fast solid-state reaction process producing single-phase homogeneous Cu(In_1 − xGa_x)(Se_1 − yS_y)₂ alloys, are discussed. The most significant material properties of the resulting single-phase chalcopyrite alloys, as well as the corresponding device characteristics, are also reviewed. This technology has been successfully demonstrated in a pilot facility at the University of Johannesburg and is currently been applied on a commercial level by Johanna Solar Technology GmbH.     

Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance

The present contribution deals with the influence of the copper concentration in Cu(In,Ga)Se₂ (CIGSe) on the solar cells based on CIGSe/(PVD)In₂S₃ and CIGSe/(CBD)CdS. We find that, depending on the buffer layer, the optimum open circuit voltage (Voc) is not reached for the same copper concentration. The values of Voc for the CIGSe/(CBD)CdS solar cells are higher when the copper content is very close to stoichiometry (25%), whereas, the Voc values for CIGSe/(PVD)In₂S₃ solar cells attain their maximum for lower copper contents. On the other hand, contrary to the case of the (CBD)CdS buffer, the Jsc is strongly hindered for the (PVD)In₂S₃ buffered cells when the copper content is lowered. The study has been made for different absorber gallium contents and the evolution is coherent with the presence of a cliff at the CIGSe/(PVD)In₂S₃ interface.     

Synthesis of Cu(In,Ga)Se2 absorber using one-step electrodeposition of Cu-In-Ga precursor

One-step Cu-In-Ga electrodeposition on Mo substrate is carried out by potentiostatic method in acidic aqueous media. The applied potential, the pH and the nature of the electrolyte are determined to obtain adequate precursor composition. The electrodeposit is found highly dendritic, due to Cu diffusion-controlled deposition. Selenization at temperatures ranging from 450 to 600 °C leads to Cu(In,Ga)Se₂ (CIGS) absorber. The influence of selenization temperature and duration on Ga distribution as well as on CIGS crystallinity is discussed. Although the precursor is dendritic, relatively compact absorbers can be obtained. The best solar cell, achieved on 0.1 cm², shows 9.3% efficiency (V_oc 456 mV; j_sc 33 mA cm⁻²; FF 62%).     

Characterisation of Cu(In,Ga)Se2-based thin film solar cells on polyimide

Thin films of Cu(In,Ga)Se₂ (CIGS) were deposited at temperatures below 450 °C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.     

Generation of electrical defects in ion beam assisted deposition of Cu(In,Ga)Se2 thin film solar cells

Thin films of Cu(In,Ga)Se₂ (CIGS) absorber layers for thin film solar cells have been manufactured on polyimide foil in a low temperature, ion beam assisted co-evaporation process.In the present work a set of CIGS thin films was produced with varying selenium ion energy. Solar cell devices have been manufactured from the films and characterized via admittance spectroscopy and capacitance-voltage profiling to determine the influence of the selenium ion energy on the electric parameters of the solar cells. It is shown that the impact of energetic selenium ions in the CIGS deposition process leads to a change in the activation energy and defect density and also in the spatial distribution of electrically active defects.For the interpretation of the results two defect models are taken into account.     

Time-resolved photoluminescence in Cu(In,Ga)Se2 thin films and solar cells

Room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on Cu(In,Ga)Se₂ (CIGS) thin films and solar cells to clarify the recombination process of the photo-generated minority carrier. Both films and solar cells exhibited PL decay curves composed of the dominant fast (0.7-2 ns) and weak slow (3-10 ns) exponential decay curves. PL lifetime of the cell is longer than that of the thin films, indicating the longer minority carrier lifetime for the hetero-structures than in thin films. The increase of PL lifetime is consistent with the enhancement of the PL intensity and the elimination of defect-related PL as a result of the solar cell formation. These results are discussed in terms of the recombination process of carriers in films and hetero-structures. The relationship between the PL lifetime of the CIGS solar cells and the cell conversion efficiency is described.     

Impact of the Se evaporation rate on the microstructure and texture of Cu(In,Ga)Se2 thin films for solar cells

Coevaporated Cu(In,Ga)Se₂ layers on Mo-coated soda-lime glass substrates were produced by a three-stage process using various Se overpressure conditions during the three stages. Cross-sections of these samples were analyzed by electron backscatter diffraction (EBSD) in a scanning electron microscope in order to reveal the microstructures in the Cu(In,Ga)Se₂ layers. In addition, the preferential orientations of these Cu(In,Ga)Se₂ layers were studied by plan-view EBSD measurements. It was found that Cu(In,Ga)Se₂ exhibits a texture in 110 orientation for Se/(Cu + In + Ga) atomic flux ratios R which are sufficiently large (≥ 4). In one Cu(In,Ga)Se₂ layer produced with approximately R = 4, a large density of (near) Σ3 (twin) boundaries were detected which are oriented preferentially perpendicular to the substrate. By comparison of the local textures of neighboring grains and the theoretically possible changes in orientation by twinning, it is possible to retrace how the twinning occurred.     

One-step synthesis of Cu(In,Ga)Se2 absorber layers by magnetron sputtering from a single quaternary target

A one-step route was developed to fabricate Cu(In,Ga)Se₂ (CIGS) absorber layers by direct magnetron sputtering from a single quaternary target with the composition of CuIn_0.75Ga_0.25Se₂. The effects of the substrate temperature, the working pressure and the sputtering power on the morphology and phase structure of the CIGS layers were studied using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The microstructure properties of the layers, including the crystallinity, grain size, compactness and the surface evenness, were found to be strongly dependent on the deposition parameters. CIGS absorbers with compact microstructure and large grains of micrometer size were obtained at 400 °C and 160 W, showing a very strong (220)/(204) orientation preference when sputtered at a higher working pressure. Raman spectra indicated no precipitation of the Cu-Se binary phases, but revealed a slight difference in the Ga/(Ga + In) ratio of different layers. The overall composition of the as-sputtered CIGS film was confirmed to be in agreement with the target composition through energy dispersive X-ray spectroscopy study. In comparison with the conventional co-evaporation or post-selenization synthesis for CIGS, the one-step sputtering route is more simplified and economical, which shows great potential to reduce the production cost of CIGS-based solar cells.     

Interfacial chemistry control in thin film solar cells based on electrodeposited CuIn(S,Se)2

In this work, we present a study on CuIn(S,Se)₂ absorbers prepared by electrodeposition followed by rapid thermal annealing promising to lower manufacturing cost. However the annealed material contains copper sulpho-selenide of Cu(S_y,Se_1 − y) type which is harmful for the electrical properties of photovoltaic devices. These phases are removed by a cyanide etching. Because of an intrinsic variability of absorber fabrication process, the presented survey is based on statistic approach. We highlighted the influence of a cyanide treatment on surface and bulk compositions. The surface composition follows a distribution according to a Cu(S,Se)-CuIn(S,Se)₂ system and the bulk composition agrees with Cu(S,Se)₂-CuIn₃(S,Se)₅ system. Moreover, surface composition can be modified by adjusting the cyanide concentrations of etching solution without any changes in the bulk one. It ensues that Cu(S,Se) is not only present on the surface but also in the bulk of samples.     

Fabrication and characterisation of Cu(In,Ga)Se2 solar cells on polyimide

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se₂/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se₂ (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E_g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.     

Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S₂ have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93-0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn₅S₈ thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn₅S₈ formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm², fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S₂.     

Cu(In,Ga)3Se5薄膜的结构及其缺陷研究

讨论了Ga含量对四元有序缺陷化合物Cu(In,Ga)3Se5薄膜结构的影响.Ga含量的增加引起晶格扭曲系数η近似按抛物线形式增加,而其晶格常数a与c呈线性减小趋势,同时(112),(220)/(204)等主衍射峰的位置和强度呈现显著的改变.而样品厚度的改变会导致薄膜中存在不同的内应力,最终对薄膜的结构产生了显著影响.     

Evaluation of electron beam induced current profiles of Cu(In,Ga)Se2 solar cells with different Ga-contents

The measurement of electron beam induced current profiles in junction configuration (JEBIC) is a settled method for several semiconductor devices. We discuss the JEBIC method in the light of the special conditions present in the case of thin film Cu(In,Ga)Se₂ solar cells.Our previously published results indicate that the charge state of defects close to or at the Cu(In,Ga)Se₂/CdS interface depends on the minority carrier distribution, which changes strongly during a scan of the cross section with an electron beam. The charge distribution influences the electrostatic potential and therewith the collection of minority carriers.Here, we present an evaluation method of JEBIC profiles that accounts for this effect. Monte Carlo simulations of the carrier generation help us to consider in detail the influence of surface recombination. We determine the diffusion length, space charge width, surface- and back contact recombination velocity of Cu(In_(1-r),Ga_r)Se₂ devices with different Ga-contents r from JEBIC line scans.     

Optimization of ALD-(Zn,Mg)O buffer layers and (Zn,Mg)O/Cu(In,Ga)Se2 interfaces for thin film solar cells

(Zn,Mg)O films, fabricated by atomic layer deposition, ALD, are investigated as buffer layers in Cu(In,Ga)Se₂-based thin film solar cells. Optimization of the buffer layer is performed in terms of thickness, deposition temperature and composition. High efficiency devices are obtained for deposition at 105-135 °C, whereas losses in open circuit voltage are observed at higher deposition temperatures. The optimal compositional region for (Zn,Mg)O buffer layers in this study is for Mg/(Zn + Mg) contents of about 0.1-0.2, giving band gap values of 3.5-3.7 eV. These devices appear insensitive to thickness variations between 80 and 600 nm. Efficiencies of up to 16.2% are obtained for completely Cd- and S-free devices with (Zn,Mg)O buffer layers deposited with 1000 cycles at 120 °C and having a band gap of 3.6 eV.     

Characterization of Cu(In,Ga)Se2 photovoltaic modules

In this study copper indium gallium diselenide photovoltaic (PV) modules were subjected to a thorough indoor assessment procedure. The assessment is to be used as a baseline for future evaluation of the modules deployed outdoors as part of an ongoing evaluation of device performance and degradation. The main focus of the study is the long term monitoring of the devices to determine service lifetime. In this paper we will present initial results of the baseline evaluation, namely I-V characteristics, thorough visual inspection and an analysis of performance parameters. The results obtained revealed that the performance of one of the modules was inferior to the others evaluated. In order to further investigate this, laser beam induced current (LBIC) measurements were conducted on regions that had a non-uniform appearance as observed visually.     

Local fluctuations of absorber properties of Cu(In,Ga)Se2 by sub-micron resolved PL towards “real life” conditions

We analyze Cu(In,Ga)Se₂ absorber layers for solar cells in a confocal microscope setup by photoluminescence (PL) experiments. We present results on lateral inhomogeneities of absorbers in terms of local fluctuations of the splitting of quasi-Fermi levels (E_Fn − E_Fp), which determines the local open circuit voltage (V_oc) of the polycrystalline cell. These results can be extracted from spectrally resolved PL scans across several tens of microns. Excitation fluxes amount to 10² − 5 × 10⁴ suns equivalent at 83-300 K. We analyze the statistical distribution of the occurring fluctuations of (E_Fn − E_Fp) which we plot in histograms, seemingly showing Gaussian-like shapes. The width of these — showing substantial dependence on excitation flux and temperature — has been extrapolated towards 1 sun equivalent light fluxes. Furthermore, we use these results to correct the absolute values (E_Fn − E_Fp) which can be derived from non-laterally resolved, calibrated PL-studies at 300 K and 1 sun equivalent on comparatively large areas (1 mm²). The latter ones provide access to the spatially averaged PL-yields (∑Y_PL,xi) and their respective quasi-Fermi level splitting ((E_Fn − E_Fp)~ ln(∑Y_PL,xi)), while the average of the (E_Fn − E_Fp) from laterally resolved measurements reads (∑ln(Y_PL,xi)). We show a comparison of the two magnitudes and thus strongly appeal for sufficient high spatial resolution for a consistent quantitative interpretation of luminescence experiments.     

Preparation of wide gap Cu(In,Ga)S2 films on ZnO coated substrates

We have prepared Cu(In,Ga)S₂ films at growth temperatures from 300 °C to 580 °C with a homogeneous gallium depth distribution (estimated band gap 1.67 eV) onto soda lime glass (SLG) substrates with one of three different kinds of back contact: Mo(1000 nm), ZnO(500 nm), and Mo(30 nm)/ZnO(500 nm), respectively. We have also investigated the depth profiles of Zn and Na (diffused from SLG) in Cu(In,Ga)S₂ films by secondary ion mass spectroscopy (SIMS). The efficiency of solar cells on Mo increases with increasing growth temperature. It is higher on Mo/ZnO than on ZnO, and increases from 350 °C to 450 °C, then decreases above 450 °C. It was observed by SIMS that the amount of Zn in Cu(In,Ga)S₂ on Mo/ZnO is lower than it is on ZnO up to 450 °C, and a large amount of Zn diffuses into absorbers over 450 °C, which contributes to decreasing efficiency. The amount of Na in the back contact increases with growth temperature. The depth distribution of Na in Cu(In,Ga)S₂ films on Mo is almost constant in the order of 10¹⁷-10¹⁸ cm^− 3, on ZnO and Mo/ZnO the Na concentration increases towards the surface and is in the range of 10¹⁵-10¹⁷ cm^− 3.     

Ensemble analyses by Minkowski-operations for spatially resolved structural and optoelectronic features of Cu(In,Ga)(Se2,S2) absorbers

Polycrystalline chalcopyrite semiconductors like Cu(In,Ga)(Se₂,S₂) as physically and technologically promising thin film solar cell absorbers show a considerable degree of spatial inhomogeneity of structural, optical and optoelectronic properties above and below the length scale of grain sizes. The dilution of magnitudes such as splitting of quasi-Fermi levels with a certain distribution of lower energies, introduced e.g. by local fluctuations, introduce an additional reduction of the free energy of the photon field and drop thus the solar light conversion yield. For a comprehensive interpretation of the influence of such inhomogeneity effects on the solar cell efficiency we perform lateral scans of various magnitudes recorded with a confocal setup with high lateral resolution (≤ 1 µm), like splitting of quasi-Fermi levels, AFM-surface contours with different scan sizes and at different sample positions. We compare these sets of magnitudes by their respective Minkowski-opening-operations, extract correlation coefficients, determine average values and their respective statistical momenta (variances), and extrapolate minimum scan sizes for the collection of statistically representative data.     

Influence of sodium compounds at the Cu(In,Ga)Se2/(PVD)In2S3 interface on solar cell properties

The present contribution deals with indium sulfide buffer layers grown by thermal co-evaporation of elemental indium and sulfur. It has been found necessary to deposit these buffer layers at low substrate temperatures in order to reach V_oc values similar to those with (CBD)CdS. However, such deposition conditions lead to the formation of a highly recombinative Cu(In,Ga)Se₂/indium sulfide interface. This behaviour may be associated to the presence of sodium carbonates/oxides at the interface even though the Cu(In,Ga)Se₂ surface was cleaned in NH₃ (1 M, room temperature) prior to the indium sulfide deposition. An explanation is that, despite the chemical etch, sodium carbonates/oxides remain in the air exposed Cu(In,Ga)Se₂ grain boundaries and can migrate towards the surface when the Cu(In,Ga)Se₂ is heated under vacuum. These polluted interface areas act as recombination zones and thus inferior devices. A possibility to improve the device performance (i.e. improve the interface quality) is to sulfurize the remaining sodium carbonates/oxides. The resulting Na₂S can then leave the interface by formation of a solid solution with the indium sulfide. By adapting the buffer layer deposition process, 13.3% efficiency devices with co-evaporated indium sulfide are realized, performance which is close to that reached with (CBD)CdS.