Dieter Bonnet

CADMIUM-TELLURIDE : MATERIAL FOR THIN FILM SOLAR CELLS

Due to its basic optical, electronic, and chemical properties, CdTe can become the base material for high-efficiency, low-cost thin film solar cells using robust, high-throughput manufacturing techniques. CdTe films suited for photovoltaic energy conversion have been produced by nine different processes. Using n -type CdS as a window-partner, solar cells of up to 16% efficiency have been made in the laboratory. Presently five industrial enterprises are striving to master low cost production processes and integrated modules have been delivered in sizes up to 60 × 120 cm 2 , showing efficiencies up to 9%. Stability, health, and environmental issues will not limit the commercial potential of the final product. The technology shows high promise for achieving cost levels of

Manufacturing of CSS CdTe solar cells

0.5/ W p at 15% efficiency. In order to achieve this goal, scientists will have to develop a more detailed understanding of defect chemistry and device operation of cells, and engineers will have to develop methods for high-throughput manufacturing.

Surface analysis of CdTe thin film solar cells

Abstract Due to its basic physical and chemical properties CdTe has become a favoured base material for thin film solar cells, using robust, high-throughput manufacturing procedures. The technology shows significant potential for attaining cost levels of p . Close-spaced sublimation (CSS) is the fastest and simplest deposition process for both semiconductors used, CdTe and CdS, permitting in-line production at a high linear speed of about 1 m/min. The individual manufacturing steps for integrated modules are explained in view of their incorporation into the production line. ANTEC solar GmbH is engaged to enter the production of CdTe thin film modules on a scale of 10 MW p (100 000 m 2 ) per annum, using CSS as the deposition procedure for the semiconductor films, and high-rate in-line sputtering for transparent and opaque contacts. Standard module size will be 60×120 cm 2 . The production line is presently under construction.

Grain boundaries and impurities in CdTe/CdS solar cells

The surface properties of CdTe thin film solar cells prepared by ANTEC using the close-space sublimation — (CSS) — technique have been analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), photoelectron emission microscopy (PEEM), high-resolution scanning electron microscopy (HRSEM) and photoelectron spectroscopy (XPS) after different pretreatment conditions. Exposure of the CdTe films to air leads to surface oxidation with the formation of TeO2 and CdO. The amount of surface oxides depends on the CdCl2 activation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indicating the formation of a surface barrier. The surface oxide can be removed by mild sputtering. The results suggest that no extra surface defects are introduced by this procedure. Before sputtering, Cl is found on the surface of the activated material, although no such contamination is found in the stoichiometric bulk material using XPS. A variation in the Fermi level position is observed for the non-activated to the activated CdTe material from weakly to higher p-doped levels. This type of conversion is evidently restricted to the near surface area as further in the bulk, weakly p-doped CdTe is found again. The results indicate that, besides the surface composition, the electronic properties of the film also depend on the different pretreatment steps.

Key Aspects of CdTe/CdS Solar Cells

Near-interface grain effects and quantitative SIMS impurity profiles for close-space sublimation-deposited CdTe/CdS solar cells and test structures are described. TEM of the interfacial CdTe grains demonstrated them to be smaller than expected from extrapolation of the bulk grain size. Computer simulation showed this to be consistent with the development of grain sizes during additive growth processes, the grain size increasing approximately as the root distance from the polycrystalline hetero-interface. TEM of the near-interfacial CdTe before and after CdCl2 processing revealed grain growth from 0.1 to 0.5 μm — even though the grain size distribution in the overlying CdTe is stable to processing. Quantitative SIMS of test structures comprising co-sputtered contacts on close-space sublimation-deposited cells (Sb–Te/CdTe/CdS/TCO/glass) is reported for the structures in their fresh, light-soaked and bake-tested states. High concentrations of impurities are present in the CdTe of the fresh cells as follows: S, 7×1019; Cl, 8×1018; Cu, 7×1017; Na, 3×1017; Sb, 4×1018; and O, 7×1018 cm−3. It is considered that the unintentionally introduced impurities are unlikely to originate from the source CdTe. The effects upon the impurity distributions of light soaking and baking in air at 400°C are reported. Accumulations of impurities near to the CdS layer are discussed.

Device analysis methods for physical cell parameters of CdTe/Cds solar cells☆

Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb 2 Te 3 in contacts to CdTe. Nominally identical solar cells fabricated using 99.999% pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher V oc , FF and efficiency, and larger grain size. Quantum efficiency and C-V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb-Te contacts to CdTe reveals that annealing in air at 400 °C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation.

IIc-3 – CdTe Thin-Film PV Modules

Abstract CdTe/CdS solar cells are thin film solar cells made of several different materials. As front contact serves a TCO, the p–n junction is an intermixed CdS/CdTe heterojunction and the metallization on the CdTe layer, which is needed for the back contact, usually shows a Schottky diode behaviour. Therefore the optoelectrical properties of the cells are complex and can often not be explained straight forward like in silicon solar cells. In order to determine the physical cell parameters like the Schottky barrier height and the minority carrier diffusion length in the absorber layer, we investigated the temperature dependence of the dark I – V characteristics and the spectral response. By modelling the temperature and wavelength dependence of the cell parameters, physical quantities such as the barrier height of the Schottky contact can be determined.

A comparative EBIC study of CdTe solar cell activation using CdCl2 and Cl2

Publisher Summary This chapter focuses on CdTe thin-film PV modules. CdTe is well suited for use as active material in thin-film solar cells due to special properties. CdTe has an energy gap of 1.45 eV, and therefore is well adapted to the spectrum of solar radiation. The energy gap of CdTe is “direct,” leading to very strong light absorption. CdTe has a strong tendency to grow as an essentially highly stoichiometric, but p-type semiconductor film, and can form a p–n heterojunction with CdS. Simple deposition techniques have been developed for CdTe suited for low-cost production. Open-circuit voltages of 880 mV, leading to AM 1.5 efficiencies of 18%, can be expected for CdTe cells made under a mature technology. This chapter begins with explaining steps for making thin-film CdTe solar cells, including film deposition, CdTe, CdS, TCOFilms, substrates, improvement of critical regions of the CdTe solar cell, the back contact, and stability issues. The chapter also explains concepts related to making of integrated modules such as interconnection of cells, contacting, and lamination.

CdTe Thin Film Modules — Progress towards Manufacture

Abstract The effect of a new Cl2 gas-based activation for CdTe/CdS thin film solar cells has been examined. Devices deposited by close space sublimation were treated using this method and compared with otherwise identical control devices activated using a conventional CdCl2 treatment. Depth-dependent carrier collection functions were obtained by measuring the current induced under electron beams of varying energy; this data was deconvoluted using carrier generation functions modelled using Monte Carlo simulations. The resultant measurements show that the performance of the chlorine treated device falls within the range of values measured for CdCl2 treated cells.

Process for producing a thin-film solar module and separating means for use in this process

Publisher Summary Antec Solar GmbH has undertaken the venture to enter production of CdTe thin film solar modules at a capacity of 100 000 m2 per annum in a plant, which is to be built in Germany. This chapter describes a survey of the state of the construction of the production line at beginning of the year 2000. From its basic physio-chemical properties, CdTe is an optimum material for use in thin film solar cells. CdTe has an energy gap of 1.45 eV and thereby, is very well adapted for efficient conversion of solar light into electricity. The energy gap is direct thus, resulting in an absorption coefficient for visible light of > 105 cm-1 so that the absorber layer needs only to be a few μm thick to absorb >90% of photons at energies above 1.45 eV.