K. Durose

Materials aspects of CdTe/CdS solar cells

The current state of CdTe/CdS solar cell development is reviewed with emphasis on the understanding of the materials behaviour. Metallurgical and optoelectronic changes effected by the CdCl2 treatment are described with reference to the CdS–CdCl2 and CdTe–CdCl2 phase diagrams. Some studies relating to the nature, distribution and influence of important optoelectronic centres are described and some new results presented. These include PL, EBIC and CL. The potential for complex interaction of processing steps in the materials system is noted and key areas for future studies are highlighted.

Photoluminescence study of polycrystalline photovoltaic CdS thin film layers grown by close-spaced sublimation and chemical bath deposition.

Photoluminescence (PL) measurements were used to study the effect of postdeposition treatments by annealing and CdCl2 activation on polycrystalline CdS layer grown by close-spaced sublimation (CSS) and chemical bath deposition (CBD). CdS films were either annealed in a temperature range of 200–600 °C or CdCl2 treated between 300–550 °C. The development of “red,” “intermediate orange,” “yellow,” and “green” luminescence bands is discussed in comparison with PL assignments found in literature. PL spectra from CdS layer grown by CSS are dominated by the yellow band with transitions at 2.08 and 1.96 eV involving (Cdi-A), (VS-A) complex states where A represents an acceptor. Green luminescence bands are observed at 2.429 and 2.393 eV at higher annealing temperature of 500–600 °C or CdCl2 treatment above 450 °C, and these peaks are associated with zero and a longitudinal optical phonon replica of “free-to-bound” transitions. As grown CBD-CdS films show a prominent red band with four main peaks located at 1.43, ...

The role of secondary phase precipitation on grain boundary electrical activity in Cu2ZnSnS4 (CZTS) photovoltaic absorber layer material

Cathodoluminescence is used to measure the recombination velocity of the heterointerfaces between Cu2ZnSnS4 (CZTS) and CuxSnySz, SnS secondary phases precipitated along the grain boundaries as well as ZnS precipitated within the CZTS grain interiors. The CZTS/CuxSnySz and CZTS/ZnS heterointerfaces had recombination velocities smaller than the bulk carrier diffusion velocity while the opposite is true for the CZTS/SnS heterointerface. Secondary phases having crystal structures compatible with CZTS (e.g., ZnS, Cu2SnS3) are likely to form heterointerfaces with small misfit strain and hence low interfacial recombination velocity. The precipitation of such secondary phases along grain boundaries in CZTS provides a novel mechanism for grain boundary passivation. However, it is not known if grain boundary passivating secondary phases would necessarily increase the overall photovoltaic device efficiency since other factors, such as the band gap of the secondary phase compared to the Shockley-Queisser ideal value ...

Characterisation of thin film CdS/CdTe solar cells using electron and optical beam induced current

Abstract Thin-film CdS/CdTe solar cells have been imaged by EBIC and OBIC in a solar irradiation (front-wall) geometry. At low levels of injected carrier density, comparable to those due to solar irradiation, the image contrast was very low, indicating that the grain boundaries in the CdTe are electrically passivated. Further OBIC and EBIC experiments were performed as a function of injection density to investigate this in detail. The onset of unusual bright grain boundary contrast at a threshold beam current, and its decline at a very high beam currents, is interpreted in terms of the onset of high injection conditions. This, together with back- and side-wall EBIC images gives an indication of the majority carrier distribution in the CdTe. We infer that there is an enhanced concentration of holes in the vicinity of grain boundaries. That this would cause majority carriers to be repelled from the grain boundaries is consistent with our direct observation of grain boundary passivation: hence, detailed quantitative EBIC studies have enabled an electrical mechanism of grain boundary passivation in CdS/CdTe solar cells to be postulated.

Grain and crystal texture properties of absorber layers in MOCVD-grown CdTe/CdS solar cells

The microstructure of 4?13 ?m thick CdTe absorber layers in CdTe/CdS/ITO/glass solar cell structures grown by metal-organic chemical vapour deposition (MOCVD) at 350 ?C has been studied. The crystalline texture, lattice parameter and grain size were measured as a function of thickness for the as-grown layers, and as a function of annealing temperature and time for annealing in both nitrogen (N2) and cadmium chloride (CdCl2) environments. The average grain sizes developed with thickness as r (?m) = 0.050x ? 0.10 (4 < x < 12 ?m), and this behaviour is contrasted with that for close-spaced sublimation material grown at 500 ?C. Annealing in both ambients promoted grain growth (with Rayleigh grain size distribution functions and Burke?Turnbull exponents being n = 7 at 440 ?C and ~4 at 400 ?C), a development of the grown-in preferred orientation from [1?1?1] to [2?1?1], and relief of the grown-in compressive stress. A growth mechanism by which development of the [2?1?1] preferred orientation may accompany grain growth is described. It is concluded that MOCVD growth at temperatures higher than 350 ?C used here will be required to produce the larger grain sizes required for photovoltaic applications.

Preparation of ZnO films by spray pyrolysis

Decomposition of zinc acetylacetonate monohydrate (Zn(acac) 2 .H 2 O) to produce undoped ZnO films by spray pyrolysis has been investigated between 100 and 400°C. Spray trials were conducted in both dry conditions and with excess water. Films with enhanced adhesion, optical transmission and conductivity were deposited above 200°C. With excess water this temperature correlated with a change in reaction mechanism from intermolecular and intramolecular at T 200°C. Water enhanced the reactant utilisation and yielded films with lower resistivity. Use of lamps to heat the spray served only to decrease surface roughness. XRD assessment of preferred orientation and grain size is also presented. Annealing in a reducing atmosphere reduced the resistivity of the films by three orders of magnitude, the lowest obtained being ∼10 -4 Ω.m.

Growth, disorder, and physical properties of ZnSnN2

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

Impedance spectroscopy of unetched CdTe/CdS solar cells—equivalent circuit analysis

A detailed study of electric and structural properties has been carried out on CdTe/CdS solar cells which deliberately were not subjected to etching by a nitric-phosphoric (NP) or bromine-methanol (Br-Me) acids, conventionally employed for the formation of Te-rich layer before back contacting. In the previous work [J. Appl. Phys. 101, 014505 (2007)] we have shown that cells that were not etched provide more extensive information on sample/material properties than the etched ones, as analyzed by admittance spectroscopy. Although seemingly being able to describe the distribution of defect energy levels, the admittance spectroscopy approach has a significant drawback because the underlying theoretical formulation does not take into account the frequency-dependent contribution from the back contact together with its influence on the trap contributions. In this work we use an alternative methodology for analysis of impedance data measured in dark conditions, which applies an equivalent circuit model to the exp...

Improved electrical mobility in highly epitaxial La:BaSnO3 films on SmScO3(110) substrates

Heteroepitaxial growth of BaSnO3 and Ba1−xLaxSnO3 (x = 7%) lanthanum doped barium stannate thin films on different perovskite single crystal (SrTiO3 (001) and SmScO3 (110)) substrates has been achieved by pulsed laser deposition under optimized deposition conditions. X-ray diffraction measurements indicate that the films on either of these substrates are relaxed due to the large mismatch and present a high degree of crystallinity with narrow rocking curves and smooth surface morphology while analytical quantification by proton induced X-ray emission confirms the stoichiometric La transfer from a polyphasic target, producing films with measured La contents above the bulk solubility limit. The films show degenerate semiconducting behavior on both substrates, with the observed room temperature resistivities, Hall mobilities, and carrier concentrations of 4.4 mΩ cm, 10.11 cm2 V−1 s−1, and 1.38 × 1020 cm−3 on SmScO3 and 7.8 mΩ cm, 5.8 cm2 V−1 s−1, and 1.36 × 1020 cm−3 on SrTiO3 ruling out any extrinsic contribution from the substrate. The superior electrical properties observed on the SmScO3 substrate are attributed to reduction in dislocation density from the lower lattice mismatch.

The absolute determination of CdTe crystal polarity

Abstract The absolute and discriminatory methods of identifying the {111} polar surfaces of CdTe are reviewed and the division of authors between the conventions of Warekois (1962) and Fewster (1983) is noted. In this work electron microdiffraction patterns were recorded which support the findings of Fewster and we note that the Te{ 1 1 1 } surface of CdTe is the best for CdTe or (Hg,Cd)Te epitaxy. Finally, reliable unambiguous polarity revealing etchants are recommended for CdTe.