Bernhard Dimmler

II–VI compound thin films for windows in heterojunction solar cells

Highly n-type conducting wide bandgap II–VI compound thin films are mostly used as the collector/window layer in heterojunction solar cells. The preparation and properties of CdS, (Zn, Cd)S, and ZnO thin films are reported. CdS and (Zn, Cd)S films are prepared by vacuum evaporation of the binary compounds. Intrinsic doping due to S vacancies is achieved by using high source temperatures. The change of donor density with the evaporation temperature is evaluated. Extrinsic doping is provided by coevaporation of gallium or indium. By appropriately adjusting the deposition conditions and Ga doping level, highly conductive (ZnxCd1−x) S films for x < 0.5 are achieved. ZnO films are prepared by reactive evaporation and sputtering of Zn in O2 atm osphere and by sputtering from a ZnO target. Conductivities above 400 Ω-1 cm-1 are obtained without extrinsic doping. The performance of various heterojunctions with the most promising I–III–VI2 absorbers is discussed. The theoretically achievable open circuit voltage of CuGaSe2 based heterojunctions with respect to the band discontinuities is compared with experimental results.

Chemical bath deposition of CdS buffer layer : prospects of increasing materials yield and reducing waste

Abstract The CdS buffer layer for CIGS-based solar cells is grown in an aqueous solution containing a cadmium salt, ammonia, and thiourea. Bottlenecks of this technique called chemical bath deposition (CBD) are the low material yield and the production of toxic CdS-containing waste. To improve yield and reduce waste, the CdS precipitate was separated from the waste after deposition by ultra-filtration, and the permeate, which contains ammonia and thiourea, was used for the next CBD process after addition of cadmium salt. The use of permeate leads to a decrease of the CdS growth rate but has no significant influence on the CdS film composition and on the Cu(In,Ga)Se 2 /CdS/ZnO device performance. The prominent formation of guanidine and urea was identified and quantified by chemical analysis of the permeate. A decrease of the deposition rate is observed as a function of the number of runs, which is related to the enrichment of the permeate with reaction products and to hydroxide ion consumption. The growth rate can be maintained by adjusting the concentrations after each CBD run.

Large-area CIGS modules: processes and properties

Abstract For the photovoltaic (PV) power market, modules with an area in the square meter range are necessary. ZSW and Wurth Solar developed for the first time processes for a Cu(In,Ga)Se2 (CIGS) pilot line for modules of an area up to 1.2×0.6 m2, which is a common size for PV generators. The pilot line has been running since the first half of 2001 and CIGS modules for different applications are realised. This paper describes the baseline processes and discusses the loss mechanisms for modules and cells. The production statistics for the 0.7 m2 modules are given with top aperture efficiencies of over 12% and average values approximately 10%. Failures due to inhomogeneous film properties are discussed with respect to upscaling from cells to modules. New processes, like thin foil substrates, atomic layer chemical vapour deposition of In2S3 instead of CBD CdS, or modifications of the absorber layer are tested in the ZSW line. Data from outdoor measurements demonstrate that CIGS modules also perform well under real conditions.

Monolithic hybrid tandem solar cells comprising copper indium gallium diselenide and organic subcells

Combining wide and narrow band gap absorbers in tandem solar cells is a promising approach to improve the energy conversion of sun light. In this work, we present hybrid tandem devices comprising monolithically connected copper indium gallium diselenide (CIGS) bottom cells and polymer top cells. The thin polymer:fullerene bulk heterojunction absorber layers were transferred onto the rough CIGS surface by a soft-contact lamination technique. Sputtered or solution-deposited top cathodes complete the tandem devices with enhanced open circuit voltages.

Photoelectrochemical studies of copper gallium diselenide films

Thin films of copper gallium diselenide of various composition were prepared by vacuum deposition. Spectral responses at several applied potentials were obtained in a photoelectrochemical cell with an acidic electrolyte (1 M HC1) and in a 0.05 M V2+/V3+ solution. They allowed us to test the intrinsic quality of the layers to establish photovoltaic applications. Effective diffusion lengths and acceptor concentrations are determined. It is shown that the samples may be classified into four groups, depending on their photoelectrochemical properties. An attempt is made to correlate behaviour to the chemical composition of the films, by defining convenient deviations from stoichiometry and from molecularity. The results suggests that in the monophasic Cu2SeCuGaSe2 range, the deviation from stoichiometry is due to an excess of copper and to an excess of selenium, or to a defect of gallium.

Structure and morphology of evaporated bilayer and selenized CuInSe/sub 2/ films

Polycrystalline CuInSe/sub 2/ films for solar cells were prepared by thermal evaporation of the single elements and by selenization of the metal films. The morphology and structure of the films were studied in detail by scanning electron microscopy and X-ray diffraction. It was found that the formation of the films is governed by interdiffusion of Cu-rich and In-rich phases. This was confirmed by conductivity and optical measurements. The formation of bilayer films and the selenization process exhibit similarities. Heterojunctions with evaporated and selenized absorbers and (Zn,Cd)S windows show active area efficiencies of 10.1% and 4.1%, respectively.<<ETX>>

Applications of ZnO in Cu(In,Ga)Se/sub 2/ solar cells

The influence of preparation conditions of the transparent conductive oxide (TCO) ZnO in a magnetron sputtering process on the properties of solar cells based on Cu(In,Ga)Se/sub 2/ absorbers is examined. Criteria for optimized ZnO layers for high efficiency applications are given. Preparing at substrate temperatures below 200/spl deg/C, the sputtering conditions have only a minor effect on the device properties. A novel high rate reactive DC-sputtering process has been developed leading to highly transmissive conductive ZnO layers with no additional substrate heating. TCO layers suitable for module applications have been deposited in less than 5 minutes. Solar cell device properties using ZnO prepared with this process are comparable to cells with standard RF-sputtered ZnO. This demonstrates the possibility to apply a scalable high deposition rate ZnO process to high efficient Cu(In,Ga)Se/sub 2/ solar cells. A significant cost reduction for large area applications is expected with this novel process.

Thin film solar modules based on CIS prepared by the co-evaporation method

CuInSe/sub 2/ (CIS) and related compounds have demonstrated their high potential for high efficiency thin film solar cells. A key issue for the development of modules is upscaling of the CIS absorber deposition. Different preparation methods have been applied. For transferring all process steps on large area with high throughput and process yield, the strategy is to optimize the methods available in view of highest device performance. For the deposition of the different films, standard techniques for optimized small area devices have been applied. For the upscaling of CIS deposition, a quasi in-line system for 10 cm/spl times/10 cm substrates was constructed. In a second step, a real in-line system for 30 cm/spl times/30 cm substrates with linear evaporation sources was developed. This system is already in operation and uniform Cu(In,Ga)Se/sub 2/ (CIGS) films have already been produced. Based on laboratory processes, static and dynamic, about 50 modules with an aperture area up to 90 cm/sup 2/ and 10 to 15 interconnected cells have been prepared. For the 30 cm/spl times/30 cm technique, all film depositions from the gas phase are based on in-line processes. Mo and ZnO films are sputtered from linear magnetron targets and CIS is deposited by co-evaporation onto moving substrates. All process steps including patterning, electrical contacts and encapsulation have been set up and films with sufficient quality and uniformity for 30 cm/spl times/30 cm solar cell modules have been produced.

CIGS and CdTe based thin film PV modules, an industrial r/evolution

CIGS and CdTe industrial activities started at the beginning of this century after 20 years of comparatively low R&D intensity when contrasted to other PV materials. Fundamental R&D has shown good progress in recent years, concentrated more on CIGS and less on CdTe. Champion efficiencies around and slightly beyond 20% have been shown for CIGS by several groups; thus CIGS has reached efficiency levels equal to multicrystalline Silicon. After years of stagnation, CdTe could recently also show new champion efficiencies well beyond 17%; still about 3% less than CIGS. High volume manufacturing has started within the last decade in a fast growing PV market. Nevertheless both need intensive R&D to develop further innovations and to realize successful transfer into high volume manufacturing to stay competitive. Both materials have the ability to approach production costs well below 0.4 US-

Cadmium-free copper indium gallium diselenide hybrid solar cells comprising a 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole buffer layer

/W in the long term and to become the main material in the PV market.