Phase identification and XPS studies of Cu(In,Ga)Se2 thin films

In this paper we present results concerning the effect of preparation conditions on the surface chemistry and crystalline phase of Cu(In,Ga)Se₂ (CIGS) thin films grown by a chemical reaction of the precursor species in two and three stage processes. The CIGS samples were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis. It was found that the bulk of the samples grown in a three stage process contains mainly the CIGS phase; however, secondary phases like In₂Se₃, Cu₂Se and In₂O were additionally identified at the surface of CIGS samples grown in two stages.     

Behavior of Cu(In,Ga)Se2 solar cells under light/damp heat over time

We performed accelerated tests on various sealed CIGS solar cells in a complex light/damp heat environment, and investigated the behavior of P_max and other properties over time. The half-life time of P_max in well-sealed cells was found to be 40–80 times greater than that in non-sealed cells, and the degradation of P_max was dependent on changes in V_oc and FF. The cause of these changes was examined based on the behavior of photo IV characteristics under conditions of feeble light, dark IV, and CV.     

High-efficiency cadmium-free Cu(In,Ga)Se2 thin-filmsolar cells with chemically deposited ZnS buffer layers

Cadmium-free Cu(In,Ga)Se₂ (CICS) thin-film solar cells with a MgF₂/ZnO:Al/CBD-ZnS/CIGS/Mo/SLG structure have been fabricated using chemical bath deposition (CBD)-ZnS buffer layers and high-quality CICS absorber layers grown using molecular beam epitaxy (MBE) system. The use of CBD-ZnS, which is a wider band gap material than CBD-CdS, improved the quantum efficiency of fabricated cells at short wavelengths, leading to an increase in the short-circuit current. The best cell at present yielded an active area efficiency of 16.9% which is the highest value reported previously for Cd-free CIGS thin-film solar cells. The as-fabricated solar cells exhibited a reversible light-soaking effect under AM 1.5, 100 mW/cm² illumination. This paper also presents a discussion of the issues relating to the use of the CBD-ZnS buffer material for improving device performance     

Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance

Thin‐film solar cells with Cu(In,Ga)Se₂ (CIGS) absorber layers ranging from 1.8 to 0.15 μm in thickness were fabricated by co‐evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross‐sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8–1 μm they showed a maintained high performance (η ∼ 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short‐circuit current, although the loss was not only due to a reduced absorbance. The open‐circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga+In) graded CIGS, which also resulted in an increased open‐circuit voltage of 20–30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 μm CIGS thickness, 15.0% at 1.0 μm and 12.1% at 0.6 μm (total area without anti‐reflective coating) were achieved. Copyright © 2002 John Wiley & Sons, Ltd.     

Structural Characteristic of CdS Thin films and Their Influence on Cu(in,Ga)Se2(CIGS) Thin Film Solar Cell

Deposition and structural characteristics of cadmium sulfide (CdS) thin films by chemical bath deposition (CBD) technique from a bath containing thiourea,cadmium acetate,ammonium acetate and ammonia in an aqueous solution are reported.Researches are made on the influence of the fundamental parameters including pH,temperature,and concentrations of the solution involved in the chemical bath deposition of CdS and titration or dumping of the thiourea solution on the structure characteristic of CdS thin films.The pH of the solution plays a vital role on the characteristic of the CdS thin films.The XRD patterns show that the change in the pH of the solution results in the change in crystal phase from predominant hexagonal phase to predominant cubic phase.The CdS thin films with the two different crystal phases have different influences on CIGS thin film solar cells.The crystal mismatch and the interface state density of the cCdS(cubic phase CdS) and CIGS are about 1.419% and 8.507×1e12cm－２ respectively,and those of the h-CdS(hexagonal phase CdS) and CIGS are about 32.297% and 2.792×1e12cm－２ respectively.It is necessary for high efficiency CIGS thin film solar cells to deposit the cubic phase CdS thin films.     

Preferred orientation of Cu(In,Ga)Se2 thin film deposited on stainless steel substrate

The influences of process parameters and Fe diffusing into Cu(In,Ga)Se₂ (CIGS) films on the orientation of CIGS absorbers grown on the stainless steel (SS) foils are investigated. The structural properties, morphology, and elemental profiles are characterized using X‐ray diffraction, scanning electron microscopy, and second ion mass spectroscopy, respectively. The orientation of CIGS thin films on the SS substrates strongly depends on the texture of the (In,Ga)₂Se₃ precursor, determined by the substrate temperature at the first stage (Ts1) and the flux ratio of Se to (In + Ga). Among these factors, Ts1 is the prerequisite to achieve [300]‐oriented IGS layer, which will yield [200]‐oriented CIGS thin film in the later process. The results indicate that through the comparison of CIGS thin films on the Mo/SS substrates and on the Mo/ZnO/SS substrates and combined with simply calculation, Fe diffusing into the CIGS layer will hinder the growth of the CIGS grains along [112] orientation. The grazing‐incidence X‐ray diffraction results suggest that the surface of the [220]‐textured CIGS thin film on the SS substrate still has [220] predominance, whereas the surface texture of the [220]‐texture CIGS thin film on the Mo/soda‐lime glass substrate became [112] predominant, which is due to the different compensation ability between Fe and Na elements. Finally, the relations between the device parameters and the degrees of the preferred orientation of CIGS absorbers are investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of high growth rates on evaporated Cu(In,Ga)Se2 layers and solar cells

Thin film solar cells based on polycrystalline Cu(In,Ga)Se₂ were prepared by elemental co‐evaporation using modified three‐stage processes on soda lime glass substrates at a low substrate temperature of 450°C intended for application on polyimide foils. The growth rates in the different stages of the growth process were varied, and it was observed that the final composition profile and structural quality of the film are mainly determined by the growth rate in the third stage. Application of high growth rates in the second stage was found to have no significant impact on layer morphology and gallium grading profile, which was confirmed by scanning electron microscopy, secondary ion mass spectroscopy, and x‐ray diffraction measurements. On the other hand, scanning electron microscopy cross sections revealed that high growth rates in the third stage lead to a fine‐grained structure toward the surface as well as smaller grains toward the back contact. Secondary ion mass spectroscopy and x‐ray diffraction measurements of such layers revealed a pronounced gallium grading profile, while Raman spectroscopy showed strong occurrence of group III‐rich phases in the near‐surface region. The final device performance was found to deteriorate by about 10% relative to the baseline process efficiency when growth rates of up to 500 nm min⁻¹ were applied in the second stage or 600 nm min⁻¹ in the third stage. Copyright © 2011 John Wiley & Sons, Ltd.     

Flexible Cu(In,Ga)Se2 solar cells with reduced absorber thickness

Reduction of the absorber thickness combined with deposition on a flexible substrate is a technically viable strategy to allow lower cost manufacturing of Cu(In,Ga)Se₂ solar modules. Flexible plastic substrates, however, require a low‐temperature deposition process and appropriate control of the band gap grading for achieving high efficiencies. In this work, we developed solar cells on polyimide films using evaporated Cu(In,Ga)Se₂ absorbers with thickness of 0.8–1.3 µm. The double Ga‐grading profile across the absorber thickness was modified by varying the maximum Cu excess at the end of the second stage or by adapting the In and Ga evaporation flux profiles during the growth process. By minimizing the Cu excess during the intermediate stage of the growth process, no loss in open circuit voltage and fill factor is observed compared with a device having a thicker absorber. Efficiency of 16.3% was achieved for cells with an absorber thickness of 1.25 µm. Insufficient absorption of photons in the long wavelength region is mainly responsible for current loss. By changing the In and Ga evaporation profiles, the shape and position of the Ga notch were effectively modified, but it did not lead to a higher device performance. Modifications of the Ga compositional profile could not help to significantly reduce absorption losses or increase charge carrier collection in absorbers with thickness below 1 µm. Changes of open circuit voltage and fill factor are mostly related to differences in the net acceptor density or the reverse saturation current rather than changes of the double Ga grading. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of TiSi2 formation temperature on film thermal stability

This work is addressed to investigate thermal stability of a thin TiSi₂ film, that is its ability to resist degradation due to heat treatments at high temperatures. The study was carried out as a function of the formation RT treatment (675–750°C) at the end of a common process flow. Sheet resistance measurements were employed in order to evaluate this degradation. Electrical measures were performed on large and narrow poly-Si lines, on Van Der Pauw structures and on doped mono-Si substrates. An increase in sheet resistance value of an order of magnitude for silicide formed at temperatures below 700°C with respect to the one formed at temperatures above 700°C was found, particularly on poly-Si lines. The effect is detectable independently of the structure: it was observed also on 0.75-μm wide poly-Si lines, increasing when line width decreases. Different morphological analyses were carried out for investigating the influence of the formation temperature. We explain the increase of the final sheet resistance decreasing the formation temperature as a lower thermal stability of the TiSi₂ film, leading to a thermal grooving of the silicide grains.     

Influence of Ni and Cr impurities on the electronic properties of Cu(In,Ga)Se2 thin film solar cells

Deposition of Cu(In,Ga)Se₂ (CIGS) thin film solar cells on metallic substrate is an attractive approach for development of low cost solar modules. However, in such devices, special care has to be taken to avoid diffusion of impurities, such as Fe, Ni, and Cr, from the substrate into the active layers. In this work, the influence of Ni and Cr impurities on the electronic properties of CIGS thin film solar cells is investigated in detail. Impurities were introduced into the CIGS layer by diffusion during the CIGS deposition process from a Ni or Cr precursor layer below the Mo electrical back contact. A high temperature and a low temperature CIGS deposition process were applied in order to correlate the changes in the photovoltaic parameters with the amount of impurities diffused into the absorber layer. Solar cells with Ni and Cr impurities show a reduction in the device performance, whereas the effect was most pronounced in Ni containing devices. The presence of deep defect levels in the absorber layer was identified with admittance spectroscopy and can be related to Ni and Cr impurities, which diffused into the CIGS layer according to secondary ion mass spectroscopy depth profiles and inductively coupled plasma mass spectrometry. Copyright © 2014 John Wiley & Sons, Ltd.     

铜含量变化对Cu(In,Ga)Se2薄膜微结构的影响

报道了不同的铜含量(Cu/(Ga+In)=0.748~0.982)对Cu(In,Ga)Se2 (CIGS)薄膜微结构的影响.文章中的CIGS薄膜采用磁控溅射金属预置层后硒化的方法制备, 其X射线衍射谱(XRD)中一系列黄铜矿结构CIGS(CH-CIGS)相的衍射峰确认了CH-CIGS相的存在.对CIGS薄膜拉曼光谱的分析表明, 随着铜含量的上升, CIGS薄膜经历了CH-CIGS和有序缺陷化合物(OVC)混合相、CH-CIGS单相、CH-CIGS和CuxSe混合相三种状态.进一步的分析显示, CIGS薄膜拉曼峰的半高宽随铜含量变化, 并在Cu/(Ga+In)=0.9附近时达到最小值, 这说明此时CIGS薄膜具有更好的结晶度和更少的无序性.此外还得到了CIGS薄膜拉曼峰半高宽与铜含量的经验关系公式.这些研究表明拉曼光谱能比XRD更加灵敏地探测CIGS薄膜的微结构, 可望作为一种无损和快速测量方法, 用于对CIGS薄膜晶相和铜含量的初步估计.     

Modification of deposition process for Cu(In, Ga)Se2 thin film solar cells on polyimide substrate at low temperature

We fabricate polycrystalline Cu(In, Ga)Se₂ (CIGS) film solar cells on polyimide (PI) substrate at temperature of 450 °C with single-stage process, and obtain a poor crystallization of CIGS films with several secondary phases in it. For improving it further, the two-stage process is adopted instead of the single-stage one. An extra Cu-rich CIGS layer with the thickness from 100 nm to 200 nm is grown on the substrate, and then another Cu-poor CIGS film with thickness of 1.5–2.0 μm is deposited on it. With the modification of the evaporation process, the grain size of absorber layer is increased, and the additional secondary phases almost disappear. Accordingly, the overall device performance is improved, and the conversion efficiency is enhanced by about 20%.     

Study on ALD In2S3/Cu(In,Ga)Se2 interface formation

The formation of the interface between In₂S₃ grown by atomic layer deposition (ALD) and co‐evaporated Cu(In,Ga)Se₂ (CIGS) has been studied by X‐ray and UV photoelectron spectroscopy. The valence band offset at 160°C ALD substrate temperature was determined as −1·2±0·2 eV for CIGS deposited on soda‐lime glass substrates and −1·4±0·2 eV when a Na barrier substrate was used. Wavelength dependent complex refractive index of In₂S₃ grown directly on glass was determined from inversion of reflectance and transmittance spectra. From these data, an indirect optical bandgap of 2·08±0·05 eV was deduced, independent of film thickness, of substrate temperature and of Na content. CIGS solar cells with ALD In₂S₃ buffer layers were fabricated. Highest device efficiency of 12·1% was obtained at a substrate temperature of 120°C. Using the bandgap obtained for In₂S₃ on glass and a 1·15±0·05 eV bandgap determined for the bulk of the CIGS absorber, the conduction band offset at the buffer interface was estimated as −0·25±0·2 eV (−0·45±0·2 eV) for Na‐containing (Na‐free) CIGS. Copyright © 2005 John Wiley & Sons, Ltd.     

Gallium gradients in Cu(In,Ga)Se2 thin‐film solar cells

The gallium gradient in Cu(In,Ga)Se₂ (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe₂ and CuGaSe₂ were calculated using density functional theory. The migration barrier for the In_Cu antisite in CuGaSe₂ is significantly lower compared with the Ga_Cu antisite in CuInSe₂, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.     

High efficiency low temperature grown Cu(In,Ga)Se2 thin film solar cells on flexible substrates using NaF precursor layers

We report a total‐area efficiency of 15.9% for flexible Cu(In,Ga)Se₂ thin film solar cells on polyimide foil (cell area 0.95 cm²). The absorber layer was grown by a multi‐stage deposition process at a maximum nominal process temperature of 420°C. The Na was added via evaporation of a NaF layer prior to the absorber deposition leading to an enhanced V_oc and FF. Growth conditions and device characterization are described. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantitative luminescence mapping of Cu(In,Ga)Se2 thin‐film solar cells

We investigate photoluminescence and electroluminescence (PL and EL) emission images from Cu(In,Ga)Se₂‐based solar cells by means of a Hyperspectral Imager. Using the generalized Planck's law, maps of the effective quasi‐Fermi level splitting Δμ_eff in absolute values are obtained. A good agreement is found between the spatially averaged splitting in PL and the global open‐circuit voltage. However, from a local carrier transport discussion, we conclude that the equality does not hold locally. The spatial variations are rather attributed to local depth variations of the quasi‐Fermi level splitting due to material properties spatial fluctuations. By comparing PL and EL emissions, we discuss qualitatively the local effective lifetimes and collection efficiencies. Copyright © 2014 John Wiley & Sons, Ltd.     

Polarization photosensitivity of ZnO/CdS/Cu(In,Ga)Se2 solar cells

Results of the application of polarization spectroscopy of the photosensitivity of ZnO/CdS/Cu(In,Ga)Se₂ thin-film solar cells with different thicknesses of the CdS (50 and 100 nm) and ZnO (500 and 1000 nm) layers are considered. It is established that the induced photopleochroism coefficient is lowered while the quantum efficiency of photoconversion of the solar cells is raised by increasing the thickness of the front layer. The experimental conditions and spectral dependence of the induced photopleochroism are linked with the antireflection properties of the ZnO front layers. It is concluded that photosensitivity polarization spectroscopy can be used for rapid diagnostics of finished solar cells and to optimize their fabrication technology. Fiz. Tekh. Poluprovodn. 33, 484–487 (April 1999)     

Photosensitivity of thin-film ZnO/CdS/Cu(In, Ga)Se2 solar cells

The photoelectric properties of thin-film ZnO/CdS/Cu(In,Ga)Se₂ solar cells were studied by polarization photoactive absorption spectroscopy. It was shown that the thin-film solar cells have a high efficiency relative to the intensity of unpolarized radiation in the photon energy range from 1.2 to 2.5 eV. The induced photopleochroism coefficient P _I increases with the angle of incidence of the incident radiation as P _I ∼θ ² and at 70° it reaches 17–20% with photon energy 1.3 eV. Oscillations of the photopleochroism were also observed. These results are discussed taking into account the antireflection effect. The results obtained by us make it possible to use such solar cells as wide-band photosensors for linearly polarized radiation and for monitoring the production of high-efficiency, thin-film solar cells based on ternary semiconductors. Fiz. Tekh. Poluprovodn. 31, 806–810 (July 1997)     

Influence of Mo back contact porosity on co‐evaporated Cu(In,Ga)Se2 thin film properties and related solar cell

The present study aims at investigating the influence of Ar sputtering gas pressure on the properties of molybdenum back contact (deposited on soda‐lime glass) and consequences on co‐evaporated Cu(In,Ga)Se₂ (CIGSe) absorber layer and related solar cell. Films 300 nm thick have been grown with argon pressure between 0·75 and 11·25 mTorr; these films have been characterized by several techniques showing that the increase of the sputtering pressure yields wider amorphous areas, containing oxygen and sodium, between the molybdenum grains, thus higher sheet resistance. The volume ratio of these amorphous areas is referenced to as “porosity”. The structural and morphological properties of co‐evaporated CIGSe have not been reliably observed influenced by the molybdenum porosity; the only noticeable change is the sodium content of the absorber, which increases with the porosity of the back contact. The impact of the amount of sodium on the device performance has been observed to be very important. On the one hand, as already reported, sodium is beneficial for the open‐circuit voltage. On the other hand, a too high amount of sodium is detrimental for the fill factor (hindered shunt resistance), thus the cell efficiency; this latter observation is interpreted as a change in the grain boundary electrical properties. Copyright © 2011 John Wiley & Sons, Ltd.