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Epitaxial growth of beryllium telluride BeTe and Zn0.06Be0.94Te by MBE on silicon was investigated as an approach towards multijunction solar cells. The good crystal properties, optical transparency, thermal stability and high p-type doping levels are positive attributes for a buffer layer for further growth of top solar cell junctions. AlGaAs-GaAs quantum well junctions were grown with close lattice...
Concepts of bulk sensitization of inorganic semiconductors by organic dye molecules either to increase the absorptivity of indirect semiconductors as μc-Si or to expand in a two photon process the absorption spectrum of wide gap semiconductors to photons of energy below the band gap are introduced. Hybrid composites have been prepared in vacuum by combining CVD and PVD thin film growth methods. Raman...
Many commercially-produced photovoltaic (PV) modules exhibit poor external quantum efficiency (EQE) at short wavelengths (lambda). However, the addition of a luminescent down-shifting (LDS) layer can absorb these short lambda photons that would not have contributed towards electron-hole pair generation in the solar cell, and re-emit them at longer lambda where the PV device possesses a much higher...
Hybrid solar cells consist of electroactive polymers and semiconducting nanoparticles. They will become very important for various applications in the nearer future. Their big advantages are the cheap production, the low demand on material and their flexibility. In this work, the behaviour of CdSe-nanodots in combination with new polymers was investigated. For all experiments the promising bulk heterojunction...
We are presenting porous-structured ZnO films grown by two-step chemical bath deposition (CBD), which has the potential to be used as a porous electrode in dye-sensitized solar cells (DSSCs). ZnS films were deposited on corning glass substrates by a stirred aqueous solution. Subsequent oxidization of ZnS films gave rise to ZnO thin films. The ZnO films exhibited a strong (002) diffractive peak, which...
Thin-film photovoltaic (PV) modules of CdTe and Cu(ln,Ga)Se2 (CIGS) have the potential to reach cost-effective PV-generated electricity. These technologies have transitioned from the laboratory to the market place. Pilot production and first-time manufacturing are ramping up to higher capacity and enjoying a flood of venture-capital funding. CIGS solar cells and modules have achieved 19.5% and 13%...
Cd1-xMgxTe is a potential candidate for the top cell in two-terminal tandem solar cells. The close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives flexibility to prepare material with the appropriate band gap for current-matching in a tandem solar cell. We have chosen to deposit Cd 1-xMgx...
The fundamental mechanisms governing carrier transport and in CdTe solar cells are not well established. Effects of diffusion length (L), depletion width (W), primary heterojunction vs. back junction are not well correlated with the CdTe thickness (t), or back contact. Bifacial analysis provides quantitative insight into CdTe device operation by separating the effects of front and back junction. Back...
Time-resolved photoluminescence (TRPL) computer simulations demonstrate that under certain experimental conditions it is possible to assess recombination in CdTe solar cells in spite of the junction. This is supported by experimental findings that open-circuit voltage (Voc) is dependent on lifetime in a manner consistent with device theory. Measurements on inverted structures show that the CdCl 2...
The built-in voltage (Vbi) and the charged electronic state (ES) distribution in a solar cell determine its conduction and valence band profiles. Changes in the charge state of the ES give rise to J-V curve anomalies like cross-over, roll-over, and, in some cases, long J-V and capacitance transient effects. CdTe is highly compensated containing deep donor- and acceptor-like ES, with larger densities...
Real time spectroscopic ellipsometry (RTSE) has been applied to characterize process-property relations in polycrystalline CdTe films prepared to 3000 Aring by magnetron sputtering under conditions similar to those yielding 14%-efficient solar cells. In this study, the key process variable is the deposition temperature (T: 188lesTles304 degC), which was calibrated in situ from the CdTe E0 band gap...
CdS films were deposited with chemical bath deposition (CBD) onto Cu(In,Ga)Se2/Mo/soda-lime glass structures. The basic ingredients for the CBD were ammonia (NH4OH), cadmium acetate (CdAc), and thiourea CS(NH2)2. Two recipes with very low thiourea concentration were compared to the baseline recipe. From quantum efficiency measurements we observe an increased carrier collection for longer wavelengths...
We investigated the effects of the etching processes using bromine and nitric-phosphoric acid solutions, as well as of Cu, in the bulk electrical conductivity of CdTe/CdS solar cells using conductive atomic force microscopy (C-AFM). Although the etching process can create a conductive layer on the surface of the CdTe, the layer is very shallow. In contrast, the addition of a thin layer of Cu to the...
We report on a comprehensive study of the effect of impurities in thin film CdTe/CdS PV structures. Dynamic, quantitative SIMS and ICPMS spectrometry have been used to analyze starting materials, device layers and final device structures. Source materials of different purity were used: CdCl2 (5N to 95%), CdTe (7N to 5N), CdS (4N) and TCO (5N). Structures grown on glass have been compared with those...
The band alignment at the CdS/Cu(In,Ga)(S,Se)2 interface, as derived in our earlier publications, are compared for different absorber compositions. The discussed band alignments were directly determined using a combination of UV- and X-ray photoemission and inverse photoemission. While a flat conduction band alignment can be found for low-gap material, the cell structure with a high-gap absorber shows...
Wide-gap chalcopyrite CuInS2 ("CIS")-based thin film solar cells are far behind their low-gap counterparts in terms of photovoltaic performance. To date, most chalcopyrite-based devices include a CdS layer, which is deposited in a chemical bath (CBD). In order to shed light on the effects induced by the interface formation, the buried CdS/CIS interface was investigated by X-ray emission...
We present a new thin-film solar cell structure in which the traditional transparent conductive oxide electrode (ZnO) is replaced by a transparent conductive coating consisting of a network of bundled single-wall carbon nanotubes. Optical transmission properties of these coatings are presented in relation to their electrical properties (sheet resistance), along with preliminary solar cell results...
We study the performance of CdS/CdTe thin-film devices contacted with ZnTe:Cu/Ti of various thickness at a higher-than-optimum temperature of ~360degC. At this temperature, optimum device performance requires the same thickness of ZnTe:Cu as for similar contacts formed at a lower temperature of 320degC. C-V analysis indicates that a ZnTe:Cu layer thickness of <~0.5 mum does not yield the degree...
Photo-voltage decay (PVD) is a common technique used to characterize numerous semiconductor devices. However, the technique has not been widely applied to CdTe-based solar cells. We have applied PVD to CdTe solar cells made with various fabrication conditions using a red (620 nm) LED and digital oscilloscope. We find the decay to be described by the equation v(t)=V0+A1exp (-t/tau1)+A2exp (-t/tau2)+A...
Cadmium sulfide thin films have been grown using a modified chemical bath deposition method with four innovative features: i) ethylenediamine was used as the complexing agent, enabling the use of low cadmium concentrations, ii) a rectangular bath geometry with heated glass plate walls was employed, iii) a low deposition temperature (30degC) was used and iv) nitrogen gas was flowed over the substrate...
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