Search

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 (

In this work we report the results of a study where solar cells were prepared with CuIn 1− x Ga x Se 2 (CIGS) as the absorber layer deposited by a two-stage hybrid sputtering/evaporation method and finished with two different transparent conducting oxides (TCO): i-ZnO/ZnO:Al or indium tin oxide (ITO). The solar cell structure is soda lime glass/Mo/CIGS/CdS/TCO. The ZnO and the ITO window layers were deposited by RF-magnetron and DC-magnetron sputtering at room temperature respectively. The deposition of these two TCOs was optimized to obtain similar parameters. In both cases the control samples showed sheet resistivities at around 50 Ω/□ and transmission coefficients around 85% in the visible spectral range for layers 300 nm thick. The I – V characteristics of finished solar cells under simulated solar spectrum illumination have been measured and the cell parameters analyzed.

A simple process for the deposition of Cu(In,Ga)Se2 (CIGS) absorber layers is described. A low-cost CIGS precursor paste deposited by simple and fast doctor blade technique is subsequently selenized under selenium vapour in a quartz tube at 10 mbar (10 min at 550 °C). The precursor paste is prepared with metal chlorides and nitrates dissolved in alcohol. The solution is then mixed with a cellulose solution to adjust the viscosity for optimal deposition. The conversion of the precursor to the CIGS phase was confirmed by X-ray diffraction (XRD). Grain size and morphology were characterised with electron microscopy. A double-layer structure formed during selenization, with a CIGS layer on top of an amorphous carbon layer. Auger electron spectroscopy (AES) shows a decreasing Ga/In ratio from the carbon–CIGS interface towards the CIGS surface. The layer structure grown on Mo-coated glass substrates (conventional dc-sputtering) was processed to solar cells by depositing a CdS buffer layer (chemical bath deposition) and ZnO/ZnO:Al front contacts (conventional rf-sputtering). A maximum efficiency of 6.7% was achieved with approximately 0.5-μm-thick absorber layers. Quantum efficiency measurements reveal photon absorption losses for the longer wavelengths, which are attributed to the thin layers.

A set of low cost techniques with realistic potential for direct manufacturing costs reduction were developed in the last five years while the industrial Cu(In,Ga)Se2 (CIGS) solar cell production is based on vacuum processes, which require high initial investment into production machines. The common properties of these low cost techniques are the use of simple and fast non-vacuum deposition methods and the prefixing of the film-composition on a molecular level in a precursor layer, which is chemically and thermally treated to form a high quality CIGS film. The paste coating approaches use premixed inks which are applied by doctor-blade coating to yield solar cell efficiencies of 13.6%, with the potential to reach 15% and more in the next years. The choice of the precursor material has to be made with respect to the used selenization conditions to avoid detrimental impurity phases. A new precursor material is discussed, which allows fast conversion in selenium atmosphere and was used to produce solar cells with 6.7% efficiency. The CIGS film thickness has to be increased for complete absorption of the incident light.

Cu(In,Ga)Se 2 (CIGS) layers have been deposited using the three-stage process on Mo coated soda-lime glass substrates with an alkali diffusion barrier. Sodium has been incorporated in such layers by NaF coevaporation, by deposition of NaF precursor layers and by diffusion of Na from a glass substrate without barrier. Scanning electron microscope pictures showed a reduction in grain size of the CIGS films when Na was available during growth. This effect depends on the Na concentration of the finished film, but not on the incorporation method. Furthermore, the [Ga]/[In] concentration ratio is decreased in Na containing CIGS layers at depths at around one-quarter of the absorber thickness. X-ray diffraction patterns indicate a preferred orientation change only in the case of NaF precursors, which is attributed to CIGS growth on a modified surface and/or to the high Na availability during the initial stages of film growth. Solar cells with an efficiency exceeding 15% have been processed from such absorber layers containing Na.

Cu(In, Ga)Se 2 absorber layers were deposited on glass coated with a ZnO/ZnO:Al double layer for the fabrication of superstrate solar cells. Their photovoltaic performance and material properties were compared with cells in the substrate configuration which were grown on Mo-coated glass. Measurements of the capacitance–voltage characteristics were used to study the carrier density of the absorber material. Compared to substrate cells, superstrate cells revealed a low acceptor concentration. Profiling with secondary mass spectroscopy revealed a low concentration of Na, which is an effective acceptor for Cu(In, Ga)Se 2 . In contrast to the Mo layer in substrate cells, the ZnO/ZnO:Al bilayer acts as barrier against the diffusion of Na from the glass substrate. Furthermore, the carrier density analysis revealed a high concentration of trap states close to the interface in the absorber layer of superstrate solar cells. Light soaking saturates such trap states and increases the net carrier density, which improves the open circuit voltage and the efficiency of the solar cells.

Cu(In,Ga)Se2 (CIGS) absorber layers for thin-film solar cells were grown without sodium. Na was diffused into some of the absorbers after growth, which led to strongly improved device performance compared with Na-free cells. Efficiencies of 13.3% and 14.4% were achieved at substrate temperatures as low as 400 and 450 °C, respectively. With the post-deposition treatment, the effects of Na on CIGS growth are excluded, and most of the Na is expected to reside at grain boundaries. The dominating cause for Na-induced device improvements might be passivation of grain boundaries.

It is known that high-efficiency thin film solar cells based on Cu(In,Ga)Se2 (CIGS) can be obtained using CdS buffer layers grown by chemical bath deposition (CBD). The highest efficiencies achieved with CdS buffer layers produced by physical vapor deposition (PVD) are significantly lower. To find reasons for this difference, structural and chemical properties of CBD- and PVD-CdS buffer layers and their interfaces with CIGS were investigated by means of bright-field (BF-TEM), high-resolution (HR-TEM) and energy-filtered transmission electron microscopy (EF-TEM), and also by energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). PVD-CdS grains were shown to be clearly larger than the CBD-CdS grains. Also, a large defect density was detected at the PVD-CdS/CIGS interface, which is attributed to the larger lattice mismatch than at the CBD-CdS/CIGS interface. Cu diffusion from CIGS into CdS was found for the CBD- and the PVD-CdS sample. The PVD-CdS/CIGS interface turned out to be quite abrupt, whereas the CBD-CdS/CIGS interface is rather diffuse. The differences in efficiencies of solar cells with CBD- and PVD-CdS buffer layers can partly be explained by referring to the higher defect density and the probable absence of an inversion of the near-interface region from p- to n-type at the PVD-CdS/CIGS interface.

Development of Cu(In,Ga)Se/sub 2/ (called CIGS) solar cells on polymers is challenging because of the thermo-physical properties of layers and substrates. CIGS layers of suitable structural and opto-electronic properties should be grown at low temperature (< 500/spl deg/C) as polyimides tend to degrade at higher deposition temperatures. Additionally, a method for controlled incorporation of an optimum amount of Na in CIGS is needed for high-efficiency cells since polyimides do not contain Na. Solar cells were developed on commercially available Upilex foils. CIGS layers were grown by evaporation of elemental Cu, In, Ga and Se at different substrate temperatures. Na from a NaF film was incorporated into CIGS layers with a post-deposition diffusion method that is suitable for in-line production of solar cells. Independent measurements have confirmed 14.1% efficiency under simulated AM1.5 standard test conditions. This is the highest efficiency reported to date for any kind of solar cell grown on polymer films. An average reflectance loss of about 13% was measured for these cells. Application of a commonly used anti-reflection coating would enable more than 15% efficiency flexible CIGS solar cells on polyimide foils.

The formation of MoSe2 has been studied on polycrystalline Mo layers and on Mo single crystals in dependence of the Mo orientation, the Na concentration, and also as a function of the Se source and the substrate temperatures. The Mo substrates were selenized by evaporation of Se. The samples were analyzed by means of X-ray diffraction, Rutherford backscattering spectrometry, elastic recoil detection analysis, and by conventional and high-resolution transmission electron microscopy. It was found that the crystal structure and orientation of the MoSe2 layer change with increasing substrate temperature. However, the texture of MoSe2 does not depend on the orientation of the Mo substrate. It was also found that the MoSe2 growth is significantly influenced by the Na concentration at substrate temperatures of 450°C and 580°C.

To characterize the influence of sodium diffusion on the Cu(In,Ga)Se2 (CIGS) layer, samples with and without an A12O3 diffusion barrier were investigated by transmission electron microscopy. The soda lime glass acts as a sodium source for samples without diffusion barrier while a post deposition treatment with NaF adds sodium to the CIGS layer for samples with diffusion barrier. For low deposition temperatures in the three-stage growth process for CIGS slightly columnar CIGS grains near the Mo back contact are found and larger grains near the top surface. Samples subject to high-temperature post deposition treatments show slightly larger grain sizes in the CIGS layer than samples prepared at the same temperature but with sodium from the soda lime glass. The surface smoothness of the CIGS layer and therefore the continuity of the following layers is improved for samples with higher deposition temperature. Occasional defects are found on the surface of the ZnO layer which can locally alter reflectivity and absorption properties of these solar cells. Despite the high CIGS surface roughness, the CdS layer with an average thickness of only 50 nm appears continuous and even fills trenches between CIGS grains near the interface.

Cu(In,Ga)Se2 and CdTe heterojunction solar cells grown on rigid (glass) or flexible foil substrates require p-type absorber layers of optimum optoelectronic properties and n-type wide-bandgap partner layers to form the p–n junction. Transparent conducting oxide and specific metal layers are used for front and back electrical contacts. Efficiencies of solar cells depend on various deposition methods as they control the optoelectronic properties of the layers and interfaces. Certain treatments, such as addition of Na in Cu(In,Ga)Se2 and CdCl2 treatment of CdTe have a direct influence on the electronic properties of the absorber layers and efficiency of solar cells. Processes for the development of superstrate and substrate solar cells are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.

Cu(In, Ga)Se2 (CIGS) layers were grown with different vacuum evaporation recipes and the effects of sodium on the grain growth were investigated by scanning electron microscopy. A reduced grain size mainly in the lower part of films grown with the 3-stage process was observed when Na was available during growth. The growth kinetics were found to be influenced during the second stage of the process. In contrast, no clear change in grain size could be observed for CIGS layers grown with constant evaporation rates, with a bi-layer and with an ’extreme’ bi-layer process. A mechanism for the action of Na is proposed, which is based on the assumption that Na passivates CIGS grain boundaries and that this results in a surfactant effect. The mechanism explains the different influences of Na on CIGS growth for different evaporation recipes, the occurrence of reduced grain size in the 3-stage process and the surface smoothing effect observed mainly for CIGS grown with the bi-layer process.In-diffusion of Na after CIGS growth is shown to be an alternative Na incorporation method that does not reduce the CIGS grain size. A conversion efficiency improvement from 10.4% to 14.2% was achieved due to the post-deposition treatment on CIGS absorbers grown without Na at a maximum substrate temperature of 450°C.

Low substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al2O3), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm2 area) has been achieved.

Multijunction stacked (tandem) solar cells can increase the overall photovoltaic conversion efficiency by optimal utilization of the solar spectrum in individual cells. We demonstrate that a photovoltaic tandem cell comprising a nanocrystalline dye-sensitized solar cell as a top cell for high-energy photons and a copper indium gallium selenide thin-film bottom cell for lower-energy photons produces AM 1.5 solar to electric conversion efficiencies greater than 15%.

Incorporation of a small amount of sodium into Cu(In,Ga)Se2 (CIGS) absorbers for thin-film solar cells is well known to enhance conversion efficiencies. Usually, Na is added in a way such that it is present during CIGS growth and therewith influences the growth kinetics. We have used post-deposition Na in-diffusion into as-grown, Na-free absorbers and observed typical efficiency improvements. This suggests that in general the main positive Na effect originates from changes in the electronic absorber properties rather than from modification of CIGS growth kinetics. At low substrate temperatures, Na impedes CIGS phase formation. This may explain why absorbers grown at substrate temperatures below 450°C in the presence of Na yield inferior cells compared with post-deposition-treated CIGS. We have developed post-deposition Na incorporation for the processing of flexible CIGS solar cells on polyimide substrates. A conversion efficiency of 14.1% under AM1.5 standard test conditions was independently measured. T...

Microstructural and chemical properties of the interfaces between Cu(In,Ga)Se2 (CIGS) and In2S3 layers in dependence on the In2S3 deposition temperature and Na concentration were investigated. The In2S3 layers were deposited by atomic layer deposition on CIGS layers at substrate temperatures ranging from 140°C to 240°C. Interfaces were investigated by means of scanning electron microscopy, bright-field and high-resolution transmission electron microscopy, electron diffraction, and energy-dispersive x-ray spectrometry. An orientation relationship between CIGS {112) and In2S3 {103) planes was found for the sample deposited at 210°C, whereas no orientation relationship was detected for the 240°C sample. Cu diffusion from CIGS into In2S3 was detected, as well as Cu depletion and In enrichment on the CIGS side of the interface. All three effects are enhanced with increasing deposition temperature. These results indicate the formation of a buried junction in the CIGS layer. In addition, a Na-free solar cell was...