[show abstract][hide abstract] ABSTRACT: Recent developments in the technology of high vacuum evaporated CdTe solar cells are reviewed. High-efficiency solar cells of efficiencies up to 12.5% have been developed on soda-lime glass substrates with a low-temperature (<450 °C) process. This simple process is suitable for in-line production of large-area solar modules on glass as well as on flexible polymer films with a roll-to-roll deposition process. Flexible and lightweight CdTe solar cells with a record efficiency of 11.4% have been developed in a superstrate configuration, and 3.5% efficiency mini-modules have been realised in a preliminary development. Deposition of high-temperature stable ITO front contact layer on polyimide is important for high-efficiency cells, as the layer should withstand processing steps maintaining its high electrical conductivity and optical transparency. Another development is an application of a transparent conducting oxide (TCO) ITO as a back electrical contact on CdTe leading to first bifacial CdTe solar cells, which can be illuminated from either or both sides. Accelerated long-term stability tests show that light soaking improves the efficiency of CdTe solar cells with ITO back contacts and performance does not degrade.Stability of CdTe solar cells has been measured after irradiation with high-energy protons and electrons of different fluences. These solar cells exhibit superior radiation tolerance compared to conventional Si and GaAs solar cells for space applications. Because of extreme stability, and high specific power (kW/kg) of flexible solar cells, CdTe has a promising potential for space applications.
[show abstract][hide abstract] ABSTRACT: Development of flexible and lightweight solar cells is interesting for terrestrial and space applications that require a very high specific power (kW/kg) and flexibility for curved shaping or rolling. Flexible CdTe/CdS solar cells of 11% efficiency in superstrate and 7.3% efficiency in substrate configurations have been developed with a “lift-off” approach. However, roll-to-roll manufacturing is desired in future.Therefore, flexible superstrate solar cells were directly grown on commercially available ∼10μm thin polyimide (Upilex™) foils. A process for the deposition of ITO (front contact) has been developed to have a stable front contact on the Upilex™ foil. Post-deposition annealing treatments of the ITO/polyimide stacks bring a significant stability to the front contact, having almost the same sheet resistance at the beginning and at the end of the cell fabrication process. Solar cells with AM1.5 efficiency of 11.4% on Upilex™ foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented.
Solar Energy Materials and Solar Cells - SOLAR ENERG MATER SOLAR CELLS. 01/2006; 90(18):3407-3415.
[show abstract][hide abstract] ABSTRACT: CdTe layers have been grown on CdS layers to produce thin-film photovoltaic devices. Because of the large lattice mismatch of roughly 10%, CdTe and CdS can only be joined at the expense of a high density of misfit dislocations. Additionally, after deposition the CdTe layer contains submicrometer sized,  oriented, columnar grains with a high density of stacking faults and microtwins resulting in a poor electrical performance of the p-n junction. The performance of these cells can be improved by depositing a CdCl2 layer on the CdTe absorber layer and subsequent annealing of the stack in air. This treatment induces interdiffusion of S and Te across the interface, which results in a better lattice match. During this anneal, CdTe is subject to grain growth, recovery and recrystallization. In samples annealed for different durations after different amounts of CdCl2 were applied, grain growth is completed during the first minutes of annealing. Subsequent diffusion of Cl is detected along the CdTe grain boundaries. The presence of Cl enhances the recrystallization of the CdTe layer, starting from the CdTe surface, while recovery of the CdTe layer, mostly by the reduction of microtwins, takes place at the interface. The simultaneous occurrence of recrystallization and recovery leads to a preferred alignment of grain boundaries in CdTe parallel to the interface. Electron beam induced current measurements show the detrimental effect of these grain boundaries on the charge carrier collection efficiency of the cell. Based on these results, a modified growth procedure is proposed.
[show abstract][hide abstract] ABSTRACT: The stability of CdTe/CdS solar cells depends on spatial changes of defects and impurities throughout the cell. Degradation effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases. However, cells with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing instability. Cell degradation due to generation of defects from external influences like particle irradiation, e.g. in space, is another reason for instability. The development of stable back contacts as well as sources of instability in the cell performance are discussed with a special focus on the CdS layer and CdTe/CdS interface, which are very sensitive to the accumulation of impurities and defects. A non-destructive method to assess the stability issue is described. An analysis of performance stability with respect to defect generation caused by high-energy protons and electrons is presented. Additionally, effects of meta-stability and the capability to recover from degradation by defect relaxation are shown.
[show abstract][hide abstract] ABSTRACT: The performance stability of CdTe/CdS solar cells is strongly determined by diffusion of impurities from the back contact into the absorber layer and hetero-junction. Impurity migration changes the effective carrier concentration and barriers in the device by compensation of donors or acceptors and by creation of defect centres. The CdS window layer is particularly affected by this phenomenon, since the impurities tend to accumulate there. This can be characterised by measuring the voltage dependent quantum efficiency (AQE) in the blue wavelength region, while the back contact can be analysed by the AQE in the IR. CdTe/CdS cells with different back contact materials have been stressed in different conditions and ambiences. When thermally stressed in presence of oxygen, enhanced AQEs were observed for cells containing Cu, while cells containing Sb showed negligible changes, in the UV range as well as in the IR range. In comparison, vacuum stressed Cu containing cells showed lower AQEs, but still higher than non-stressed cells. Results of the stressing tests for different materials and in different conditions have been analysed and interpreted using the recently developed model of a modulated barrier in the CdS bulk.
Thin Solid Films 01/2003; 431:421. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: CdTeyCdS thin film solar cells have been grown by high-vacuum evaporation (HVE) and close-space sublimation. To understand the role of Cl on the microstructure and composition of the CdTeyCdS layers and interfaces, the cells were annealed for different durations after different amounts of CdCl had been deposited. Transmission electron microscopy shows a loss of 2 orientational relationship between CdTe and CdS after annealing under the influence of Cl. The interdiffusion of S and Te across the interface is measured quantitatively and segregation of Cl, Te and O at the CdTeyCdS interface is detected by energy-dispersive X-ray mapping in the electron microscope. The results show a strong correlation with the diffusion of Cl along the CdTe grain boundaries, which is directly proved for the first time. It is suggested that recrystallization of CdTe grains starts from the CdTe surface and proceeds towards the interface. Cells grown by HVE show all the features expected from the predominance of recrystallization. 2003 Elsevier Science B.V. All rights reserved.
Thin Solid Films 01/2003; 431(432):262-266. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: A Synchrotron Radiation Detector measures synchrotron radiation emitted by high energetic particles in the earth magnetic field. This allows to identify cosmic ray electrons and positrons with energies in the TeV region. One possibility for such a detector outside the atmosphere uses YAP crystals to measure synchrotron photons with energies in the keV range. As such a detector can not distinguish between photons and electrons, the main problems are the diffuse cosmic ray gamma background and low energetic electrons in the vicinity of the earth. While the intensity of the diffuse gamma rays is known quite well, there exists limited knowledge about keV-electrons in low earth orbits. To measure these electrons a Prototype Synchrotron Radiation Detector (PSRD) was flown with Space Shuttle mission STS-108 (Dec.2001) and preliminary analysis of the data show very favorable results.
[show abstract][hide abstract] ABSTRACT: Polycrystalline thin film solar cells of II-VI compound semiconductors are important because of their low cost, high efficiency and stable performance. Flexible and lightweight solar cells are interesting for a variety of terrestrial and space applications that require a very high specific power (ratio of output electrical power to the solar module weight). Moreover, light modules are advantageous in terms of transport and mounting. We have previously described the development of flexible CdTe/CdS solar cells on polyimide substrates with a novel method; efficiencies of 8.6 % were reported. Further improvements in the processing have now increased the solar cell efficiency to 11%. The CdTe solar cell is fabricated in a "superstrate" configuration where the light passes through the polyimide substrate. Absorption of the incident light and possible degradation of polyimide under UV and particle irradiation are the major limitations of this configuration. To overcome these a new approach is introduced here. The CdTe/CdS/TCO stacks are grown on a NaCl/glass substrate. After the solar cell has been processed, a polymer layer is spin coated on top and the flexible solar cells are detached from the glass by dissolving the NaCl buffer layer. Solar cells of 7.3% efficiency have been obtained.
[show abstract][hide abstract] ABSTRACT: The components of the PSRD, a detector to study the flux of X-rays and charged particles in the keV–MeV energy range in outer space, are described. The device is a precursor for the Synchrotron Radiation Detector, which is one of the detector components of the Alpha Magnetic Spectrometer (AMS02). The AMS02 will be operated for several years on the International Space Station.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 02/2002; 478(s 1–2):123–124. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Prototype Synchrotron Radiation Detector (PSRD) is a small-scale experiment designed to measure the rate of low-energy charged particles and photons in near the Earth's orbit. It is a precursor to the Synchrotron Radiation Detector (SRD), a proposed addition to the upgraded version of the Alpha Magnetic Spectrometer (AMS-02).The SRD will use the Earth's magnetic field to identify the charge sign of electrons and positrons with energies above by detecting the synchrotron radiation they emit in this field. The differential energy spectrum of these particles is astrophysically interesting and not well covered by the remaining components of AMS-02. Precise measurements of this spectrum offer the possibility to gain information on the acceleration mechanism and characteristics of all cosmic rays in our galactic neighbourhood. The SRD will discriminate against protons as they radiate only weakly.Both the number and energy of the synchrotron photons that the SRD needs to detect are small. The identification is complicated by the presence of a large particle and photon background. Existing measurements of these backgrounds are insufficient for the construction of the large-scale SRD, so a measurement in space was indispensable.The PSRD was designed to fly as a Space Shuttle secondary payload, within the Shuttle Small Payloads Project. The flight on board the Space Shuttle Endeavour took place from 5 to 17 December 2001.The scientific goal, hardware and the flight of the PSRD are described in this report.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 01/2002;
[show abstract][hide abstract] ABSTRACT: To make CdTe/CdS solar cells highly efficient, a Cu containing back contact (BC) is generally used. These cells degrade due to Cu diffusion to the front contact which causes shunting; this is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get a stable but still highly efficient cell, different BC materials and etching treatments were investigated. Chemical etching creates a back surface field (BSF) due to a p+-doped Te-rich CdTe surface. To overcome the naturally existing Schottky barrier between p-CdTe and any metal, a thin buffer layer was evaporated prior to the metallization. Amongst the many investigated BC materials, the most suitable are Sb or Sb2Te3 as a buffer and Mo for metallization. These cells showed high stability under accelerated tests corresponding to 70 years.
[show abstract][hide abstract] ABSTRACT: The measurement of quantum efficiency with bias voltage is a powerful tool to characterize CdTe/CdS solar cell. As the quantum efficiency changes drastically with bias it will be referred to as Apparent Quantum Efficiency AQE. The AQE gives insight to the spectral contents of the cell current and therefore resolves the spatial carrier collection in the cell at each working point. So it is possible to understand the influence of the junctions and changing resistances in the cell. The photoconductivity of CdS facilitates AQE well above unity, i.e. up to 100, at high forward bias. The spectral sensitivity of the CdS photoconductivity affects the cell current strongly. This can explain the dependence of fill factor and roll-over of the I-V characteristics on the spectral content of illumination. Further more back contact junction influence and defect related features, such as sub band gap generation, are evident in the AQE for high forward bias.