Fast Cu(In,Ga)Se2 precursor growth: Impact on solar cell

In order to reduce the co-evaporation time of Cu(In,Ga)Se₂ (CIGSe) thin film absorber, a sequential approach has been investigated. CIGSe layers have been grown using the three-step based CUPRO (Cu-Poor/Rich/Off) process at substrate temperature of 600 and 500 °C. The first step consists in the growth of Cu-poor ([Cu]/[In + Ga] = 0.9) precursor layers. This paper aims at investigating the impact of this layer deposition duration on the CIGSe and respective solar cell properties. It is observed that for the two substrate temperatures investigated, the morphological and structural properties of the CIGSe layers do not change with increasing precursor deposition speed, even when it is increased by ten. Furthermore, the respective device performance also appears not affected by this reduction of the precursor growth time; all cells demonstrate 15% efficiency. From this work, the duration of our standard deposition process could be decreased from 23 to 14 min without performance loss independently of the substrate temperature.     

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.     

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.     

Influence of Cu off-stoichiometry on wide band gap CIGSe solar cells

The electric properties of solar cells based on co-evaporated Cu(In,Ga)Se₂ (CIGSe) thin film show a good tolerance regarding the absorber Cu content (y = [Cu]/([In] + [Ga])) for standard Ga concentration, i.e. x = [Ga] / ([In] + [Ga]) ~ 0.3. In the present contribution, we show that this tolerance is lost when the gallium content is increased. Wide bandgap CIGSe samples (x ~ 0.55) with a variation in y from 0.97 to 0.84 have been grown. The efficiency of the cells decreases from 12.6% to 6.5% for y = 0.97 and 0.84 respectively. For the lowest y, the efficiency is harmed because of a low short-circuit current density (J_sc), an increased voltage dependency in the current collection, which affects the fill factor (FF), and a decrease of the open-circuit voltage (V_oc). For y = 0.97 and 0.84 respectively, the decrease of the activation energy (E_a) from 1.36 to 1.24 eV indicates a shift of the area of the dominant recombination from the space charge region towards the interface. There seems to be evidence that reducing the Cu-content in the CIGSe thin film will cause a decrease in the width of the space charge region. Solar cells based on Cu-rich CIGSe (1.03 < y < 1.09) have also been fabricated and characterized. A strong deterioration of their electrical properties is observed despite the KCN etch of the segregated Cu_2 − xSe binary phases at the surface, suggesting the presence of residual Cu_2 − xSe precipitates within the layer.     

Characterization of the CdS / Cu(In,Ga)Se2 interface by electron beam induced currents

The method of electron beam induced currents in junction configuration (JEBIC) has been employed to investigate carrier collection in Cu(In,Ga)Se₂ solar cells. A detailed analysis of JEBIC line-scans reveals unexpected carrier collection properties, which cannot be explained with common models. We ascribe this anomalous behavior to an electrostatic barrier effect at the Cu(In,Ga)Se₂ / CdS interface. We suggest the existence of a thin defect-layer on the surface of the Cu(In,Ga)Se₂ with high acceptor concentration and valence band edge that is energetically lower than that of the bulk. Using this model, we achieve a good agreement between experimental and simulated JEBIC line-scans. The influence of the barrier effect is considerably reduced by a metastable change of the interface properties induced by intensive electron irradiation of the interface. This effect is explained by a metastable decrease of the negative charge density in the defect-layer.     

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.     

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.     

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 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.     

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.     

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.     

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.     

Raman scattering analysis of Cu-poor Cu(In,Ga)Se2 cells fabricated on polyimide substrates: Effect of Na content on microstructure and phase structure

This work reports the Raman scattering surface and in-depth resolved analysis of Cu-poor Cu(In,Ga)Se₂ (CIGS) grown on polyimide substrates. In order to study the effect of Na on the formation and microstructure of the CIGS and the corresponding Cu-poor ordered vacancy compound (OVC) phases, a NaF precursor layer with different thicknesses was deposited on the Mo-coated substrates before growing of the samples. The Raman spectroscopy data are correlated with the analysis of the samples by Auger electron spectroscopy and scanning electron microscopy. These data corroborate the significant role of Na on the inhibition of Ga-In interdiffusion and on the formation of the MoSe₂ interfacial phase at the back region of the layers. Presence of Na also leads to an enhancement in the formation of the chalcopyrite CIGS phase and a decrease in the occurrence of the dominant OVC phase at the surface region. This study confirms the strong dependence of the microstructure and phase distribution in CIGS absorber layers on the Na available during their growth.     

CIGS thin films, solar cells, and submodules fabricated using a rf-plasma cracked Se-radical beam source

Coevaporated Cu(In,Ga)Se₂ (CIGS) film growth using a rf-plasma cracked Se-radical beam (R-Se) source leads to a significant reduction in the amount of raw Se source material wasted during growth and exhibits unique film properties such as highly dense, smooth surfaces and large grain size. R-Se grown CIGS solar cells also show concomitant unique properties different from conventional evaporative Se (E-Se) source grown CIGS cells. In the present work, the impact of modified surfaces, interfaces, and bulk crystal properties of R-Se grown CIGS films on the solar cell performance was studied. When a R-Se source was used, Na diffusion into CIGS layers was enhanced while a remarkable diffusion of elemental Ga and Se into Mo back contact layers was observed. Improvements in the bulk crystal quality as manifested by large grain size and increased Na concentration with the use of a R-Se source is expected to be effective to improve photovoltaic performance. Using a R-Se source for the growth of CIGS absorber layers at a relatively low growth temperature, we have successfully demonstrated a monolithically integrated submodule efficiency of 15.0% (17 cells, aperture area of 76.5 cm²) on 0.25-mm thick soda-lime glass substrates.     

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.     

Near-interface doping by ion implantation in Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂ absorber layers were implanted with 20 keV Cd ions in order to investigate the influence of changes in the near-interface doping profile. Modifications in this region are shown by AMPS-1D simulations to have substantial impact on solar cell properties. Ion implantation and subsequent thermal annealing steps were monitored by SIMS measurements to control the thermal diffusion of the dopant. Solar cells both with and without CdS buffer layer were made from the implanted absorbers and characterized by j-V and EQE measurements. These experimental results in conjunction with simulations of the quantum efficiency show that a well-defined type-inversion of the implanted layer can be achieved by low-energy ion implantation.     

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.     

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.