Cu(In,Ga)Se2 solar cells with double layered buffers grown by chemical bath deposition

In based mixture In_x(OH,S)_y buffer layers deposited by chemical bath deposition technique are a viable alternative to the traditional cadmium sulfide buffer layer in thin film solar cells. We report on the results of manipulating the absorber/buffer interface between the chalcopyrite Cu(In,Ga)Se₂ (CIGS) absorber and CdS or ZnS buffer by addition of a thin In based mixture layer. It is shown that the presence of thin In_x(OH,S)_y at the CIGS absorber/CdS or ZnS buffer interfaces greatly improve the solar cell performances. The performances of CIGS cells using dual buffer layers composed of In_x(OH,S)_y/CdS or In_x(OH,S)_y/ZnS increased by 22.4% and 51.6%, as compared to the single and standard CdS or ZnS buffered cells, respectively.     

Structural and chemical analyses of sputtered InxSy buffer layers in Cu(In,Ga)Se2 thin-film solar cells

Sputtered In_xS_y layers deposited on borosilicate glass and Si at substrate temperatures ranging from about 60 °C to 340 °C were analyzed by means of X-ray diffraction, energy-dispersive X-ray spectrometry, and optical transmission and reflection measurements. With increasing substrate temperature, the In_xS_y layers exhibit increasing sulfur concentration and also increasing absorption-edge energies. In_xS_y layers on Cu(In,Ga)Se₂(CIGS)/Mo/glass stacks were additionally studied by scanning and transmission electron microscopy. With increasing substrate temperature, Cu, Ga, and In interdiffusion between CIGS and In_xS_y becomes more enhanced. At 340 °C, CuIn₅S₈ forms instead of In_xS_y. The CuIn₅S₈ formation at elevated temperatures may be the reason for the very low efficiency of solar cells with indium sulfide buffers deposited at temperatures above about 250 °C by various techniques.     

Performance of CuIn1 − xGaxSe2/(PVD)In2S3 solar cells versus gallium content

The present work studies the influence of the Ga content (x = Ga / (Ga + In)) in the absorber on the solar cell performance for devices using (PVD)In₂S₃-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In₂S₃-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E_gCIGSe < 1.1 eV. However, the parameters of the cells buffered with (PVD)In₂S₃ are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In₂S₃ interface.     

Ultrasonically sprayed indium sulfide buffer layers for Cu(In,Ga)(S,Se)2 thin-film solar cells

Indium sulfide layers were grown by an ultrasonic spray pyrolysis method for application in Cu(In,Ga)(S,Se)₂ solar cells. X-ray diffraction measurements of layers on soda lime glass showed polycrystalline In₂S₃ with preferential orientation along the [103] direction and X-ray photoelectron spectroscopy revealed presence or absence of oxygen and chlorine impurities depending on the composition of the spray solution. For more quantitative chemical composition measurements In₂S₃ layers were sprayed on silicon substrates and analyzed with Rutherford backscattering spectrometry. The structural and chemical information on the In₂S₃ layer sprayed with different sulfur concentrations in the chemical precursor solution are correlated to the photovoltaic performance of solar cells. Best cell efficiency of 12.4% was achieved with an ultrasonically sprayed In₂S₃ buffer layer on a Cu(In,Ga)(S,Se)₂ absorber.     

Characterization of the chemical bath deposited In(OH)xSy films: Effect of the growth conditions

The In(OH)_xS_y thin films were deposited by chemical bath deposition (CBD) using three different deposition procedures: ‘hot’: starting the deposition at 70 °C, ‘cold’: starting the deposition at room temperature and pre-treatment with In³⁺ ions prior the ‘hot’ deposition. The analysis of the deposited In(OH)_xS_y layers on glass revealed that modifications in the chemical bath deposition procedure provoked significant changes in the nucleation process, the growth rate, the layer elemental composition and the layer morphology. With an additional In³⁺ pre-treatment or starting from a cold solution, the formation of a dense bottom layer has been observed, resulting in In(OH)_xS_y films with more compact structure with refractive index values of 2.6. The comparison of the measured In/S ratio with a thicker layer suggests, that the In(OH)_xS_y deposition starts with an OH-rich layer. Assuming the indirect allowed band gap transition type, an E_g of 2.2 eV was found independent of the procedure type or deposition time.     

Cu2ZnSnS4 solar cell prepared entirely by non-vacuum processes

Cu₂ZnSnS₄ (CZTS) solar cell with superstrate structure of fluorine-doped tin oxide glass/TiO₂/In₂S₃/CZTS/Carbon was prepared entirely by non-vacuum processes. The compact TiO₂ window and In₂S₃ buffer layers, CZTS absorber layer and Carbon electrode layer were prepared by spray pyrolysis method, ball milling and screen printing combination processes and screen printing process, respectively. The short-circuit current density, open-circuit voltage, fill factor and conversion efficiency of the best fabricated solar cell are 8.76 mA/cm², 250 mV, 0.27 and 0.6%, respectively. The fabrication process for the CZTS solar cell did not employ any vacuum conditions or high-toxic materials (such as CdS, H₂Se, H₂S or Se).     

The reaction mechanism of the spray Ion Layer Gas Reaction process to deposit In2S3 thin films

The spray Ion Layer Gas Reaction (ILGAR) is a well-established, patented and commercial process used primarily to deposit In₂S₃ as buffer layers in thin film solar cells. In this paper we investigate the growth mechanism of the spray In₂S₃ ILGAR process by characterising the intermediate growth stages of films, following the growth mechanism with a quartz crystal microbalance and tracking the gaseous side-and-intermediate products during film growth, using a mass spectrometer. A basic growth mechanism model is then proposed based on an aerosol assisted chemical vapour deposition of an In(O_x,Cl_y,(OH)_z) film, as the first stage process, followed by the conversion of the intermediate film using H₂S gas to In₂S₃.     

Flexible Cu(In,Ga)Se2 on Al foils and the effects of Al during chemical bath deposition

Cu(In,Ga)Se₂ (CIGS) solar cells on aluminum foils offer the advantage to be flexible, lightweight and, because of the low cost substrate, can be used for several applications, especially in buildings, where aluminum is already commonly used. There are reports of a-Si solar cells on Al foil, but to our knowledge development of CIGS solar cells on Al foils has not been reported. We have developed CIGS solar cells on coated Al-foil samples. When using Al as substrate, CIGS layers of suitable structural and opto-electronic properties should be grown at low (< 450 °C) deposition temperatures, because of the difference in the thermo-physical properties of layers and substrates. We have grown CIGS layers by evaporation of elemental Cu, In, Ga, and Se at different substrate temperatures and investigated the properties of these CIGS layers by different methods (SEM, SIMS, and EDX). The photovoltaic properties of small area solar cells were characterized with I-V and quantum efficiency measurements. An efficiency of 6.6% has been achieved. We have also observed that some Al from the foil dissolves during chemical bath deposition (CBD) of CdS. The presence of Al in the bath seems, in some cases, to be beneficial for the electrical properties of the CIGS solar cells. Thinner and more homogenous CdS layers are obtained. Elastic Recoil Detection Analysis (ERDA) and SIMS measurements have shown incorporation of Al in the CdS.     

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.     

Optimization of AlxOy/Pt/AlxOy multilayer spectrally selective coatings for solar-thermal applications

Spectrally selective Al_xO_y/Pt/Al_xO_y multilayer absorber coatings were deposited onto corning 1737 glass, Si (111) and copper substrates using electron beam (e-beam) vacuum evaporator at room temperature. The employment of ellipsometric measurements and optical simulation was proposed as an effective method to optimize and deposit multilayer solar absorber coatings. The optical constants (n and k) measured using spectroscopic ellipsometry, showed that both Al_xO_y layers, which used in the coatings, were dielectric in nature and the Pt layer was semi-transparent. The optimized multilayer coatings exhibited high solar absorptance α ∼ 0.94 ± 0.01 and low thermal emittance ? ∼ 0.06 ± 0.01 at 82 °C. The Rutherford backscattering spectroscopy (RBS) data of Al_xO_y/Pt/Al_xO_y multilayer absorber indicated the Al_xO_y layers present in the coating were nearly stoichiometry. The scanning electron microscope analysis (SEM) result indicated that the average diameter and inter-particles distance of Pt grains were statistically about 146 ± 0.17 nm and 6-10 ± 0.2 nm respectively.     

CIGS thin-film solar cells on steel substrates

Steel foil is an attractive candidate for use as a flexible substrate material for Cu(In_x,Ga_1 − x)Se₂ solar cells (CIGS). It is stable at the high temperatures involved during CIGS processing and is also commercially available. Stainless chromium (Cr) steel is more expensive than Cr-free steel sheets, but the latter are not stable against corrosion. We processed CIGS solar cells on both types of substrates. The main problem arising here is the diffusion of detrimental elements from the substrate into the CIGS absorber layer. The diffusion of iron (Fe) and other substrate elements into the CIGS layer was investigated by Secondary Ion Mass Spectrometry (SIMS). The influence of the impurities on the solar cell parameters was determined by current voltage (JV) and external quantum efficiency (EQE) measurements. A direct correlation between the Fe content in the CIGS layer and the solar cell efficiency was found. The diffusion of Fe could be strongly reduced by a diffusion barrier layer. Thus we could process CIGS solar cells with a conversion efficiency of 12.8% even on Cr-free steel substrate.     

Substitution of the CdS buffer layer in CIGS thin‐film solar cells

This contribution provides an overview of current activities in the area of alternative buffer layers for Cu(In,Ga)(S,Se)₂ (CIGS) thin‐film solar cells. Good cell and module results were achieved by replacing the standard Cds buffer with Zn(O,S), In₂S₃, (Zn,Sn)O_y or (Zn,Mg)O grown by various methods like chemical bath deposition (CBD), thermal evaporation, sputtering, atomic layer deposition, and spray ion layer gas reaction. The “dry” deposition methods like sputtering and thermal evaporation could be favorable in an industrial environment on glass substrates or application in a roll‐to‐roll coater. Significant progress was made within the last two years for various Cd‐free CIGS devices. We list current records for cells with alternative buffers, e. g. Zn(O,S)‐buffered champion cells with efficiencies between 18—20 % and In₂S₃‐buffered cells with 16—17 %. Both materials have the potential to substitute CdS with efficiencies approaching the 20 % mark already surpassed by CIGS cells with CBD CdS buffers.     

Optical properties of Cd1−xZnxS thin films for CuInGaSe2 solar cell application

The photovoltaic Cd_1−xZn_xS thin films, fabricated by chemical bath deposition, were successfully used as n-type buffer layer in CuInGaSe₂ (CIGS) solar cells. Comprehensive optical properties of the Cd_1−xZn_xS thin films were measured and modeled by spectroscopic ellipsometry (SE), which is proven to be an excellent and non-destructive technique to determine optical properties of thin films. The optical band gap of Cd_1−xZn_xS thin films can be tuned from 2.43 eV to 3.25 eV by controlling the Zn content (x) and deposition conditions. The wider-band-gap Cd_1−xZn_xS film was found to be favorable to improve the quantum efficiency in the wavelength range of 450-550 nm, resulting in an increase of short-circuits current for solar cells. From the characterization of quantum efficiency (QE) and current-voltage curve (J-V) of CIGS cells, the Cd_1−xZn_xS films (x = 0.32, 0.45) were demonstrated to significantly enhance the photovoltaic performance of CIGS solar cell. The highest efficiency (10.5%) of CIGS solar cell was obtained using a dense and homogenous Cd_0.68Zn_0.32S thin film as the buffer layer.     

Analysis of electrical properties of CIGSSe and Cd-free buffer CIGSSe solar cells

We investigated the influence of different buffer layers to the electrical parameters (J_sc, V_oc, QE and efficiency η) of solar cells. The cells with an In₂S₃ and a ZnMgO buffer layer were compared with a reference cell with a CdS buffer layer. We performed temperature and light dependent current-voltage measurements, temperature dependent capacitance measurements and quantum efficiency measurements.The cells with In₂S₃ and ZnMgO buffers differ not too much in J_sc, but they do differ in V_oc and their electrical properties — fill factor FF, diode saturation current J₀ and efficiency η. They also do differ in their spectral response, both at short and long wavelengths, and in their ideality factor. This indicates a different current transport mechanism. The device simulation program SCAPS is used for further interpretation of the measurements. After exploring the parameters we found an acceptable agreement between simulated and measured J-V and QE(λ) curves. The simulated QE curves fit well over the whole spectrum, except for the CdS buffer cell, where there is an overestimation for the intermediate wavelengths. Because of this the simulated J_sc is higher than the measured one. The simulated V_oc agrees well for all cells. For the ZnMgO buffer cell it was necessary to include a buried homo-junction.     

Dislocation reduction in nitride-based Schottky diodes by using multiple MgxNy/GaN nucleation layers

We present the characteristics of nitride-based Schottky diodes with a single low-temperature (LT) GaN nucleation layer and multiple Mg_xN_y/GaN nucleation layers. With multiple Mg_xN_y/GaN nucleation layers, it was found that reverse leakage current became smaller by six orders of magnitude than that with a conventional LT GaN nucleation layer. This result might be attributed to the significant reduction of threading dislocations (TDs) and TD-related surface states. From the double crystal X-ray diffraction and photoluminescence analyses, it was found that the introduction of multiple Mg_xN_y/GaN nucleation layers could be able to effectively reduce the edge-type TDs. Furthermore, it was also found that effective Schottky barrier height (Φ_B) increased from 1.07 to 1.15 eV with the insertion of the multiple Mg_xN_y/GaN nucleation layers.     

Simulation approach for studying the performances of original superstrate CIGS thin films solar cells

In this work, we report on the performances of superstrate Cu(In,Ga)Se₂ (CIGS) thin film solar cells with an alternative SLG/SnO₂:F/CIGS/In₂Se₃/Zn structure using AMPS-1D (Analysis of Microelectronic and Photonic structures) device simulator. An inverted surface layer, n-type CIGS layer, is inserted between the In₂Se₃ buffer and CIGS absorber layers and the SnO₂:F layer is just a transparent conducting oxide (TCO). The simulation has been carried out by lighting through SnO₂:F. The obtained results show that the existence of so-called ‘ordered defect compound’ (ODC) layer in such a structure is the critical factor responsible for the optimization of the performances. Photovoltaic parameters were determined using the current density-voltage (J-V) curve. An optimal absorber and ODC layer thickness has been estimated, that improve significantly the devices efficiency exceeding 15% AM1.5 G. The variation of carrier density in In₂Se₃ layer has an influence on the superstrate CIGS cells performances. Moreover, the quantum efficiency (Q.E.) characteristics display a maximum value of about 80% in the visible range.     

Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se₂ (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In₂S₃) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In₂S₃ buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In₂S₃/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.     

Sub-Gap Modulated Photo Current Spectroscopy performed on Cu(Inx,Ga1 − x)(Sey,S1 − y)2 based solar cells

Sub-Gap Modulated Photo Current Spectroscopy (SGMPCS) is an excellent tool in order to investigate the band gap defect density of the absorber layer, directly on Cu(In_x,Ga_1 − x)(Se_y,S_1 − y)₂ (CIGSS) based solar cells. This technique is essentially sensitive to defect states located in the absorber layer, which has the lowest band gap of the heterojunction solar cell. It allows the determination of the σ · N(E) product, where σ is the defect Optical Cross Section (OCS) and N(E) is its Density Of States (DOS).We have developed an analytical model, allowing to derive the above product from the imaginary part of the ac photocurrent of the solar cell, under reverse applied dc bias. We have then applied this model to study the defect density of the co-evaporated CIGS (i.e. y = 1) absorber layer of a heterojunction solar cell. Two different defect distributions have been exhibited by SGMPCS, the properties of which vary with thermal annealing.Correlation with Admittance Spectroscopy allows us to derive an estimation of the defect OCS.     

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.     

Baseline model of graded-absorber Cu(In,Ga)Se2 solar cells applied to cells with Zn1 − xMgxO buffer layers

A baseline parameter set for electrical modelling of Cu(In,Ga)Se₂ solar cells with compositionally graded absorber and CdS buffer layer is established. The cases with and without Fermi level pinning as well as with and without a surface defect layer are considered. Simulations with a defect layer are observed to give the best correspondence to measurements. Zn_1 − xMg_xO buffer layers are introduced and initial modelling of the light soaking behaviour is performed. Simulation results are compared with experimental data.