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The growth of Cu2ZnSnS4 (CZTS) polycrystals from solid state reaction over a range of compositions, including the regions which produce the highest efficiency photovoltaic devices, is reported. X-ray measurements confirm the growth of crystalline CZTS. Temperature and intensity dependent photoluminescence (PL) measurements show an increase in the energy of the main CZTS luminescence peak with both increasing laser power and increasing temperature. Analysis of the PL peak positions and intensity behavior demonstrates that the results are consistent with the model of fluctuating potentials. This confirms that the polycrystals are heavily doped with the presence of a large concentration of intrinsic defects. The behavior of the main luminescence feature is shown to be qualitatively similar over a broad range of compositions although the nature and amount of secondary phases vary significantly. The implications for thin-film photovoltaic devices are discussed. [ABSTRACT FROM AUTHOR]

Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl2, which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 1014 cm−3) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell production. [ABSTRACT FROM AUTHOR]

Defects in Au-catalysed CdTe nanowires vapour-liquid-solid-grown on polycrystalline underlayers have been critically evaluated. Their low-temperature photoluminescence spectra were dominated by excitonic emission with rarely observed above-gap emission also being recorded. While acceptor bound exciton lines due to monovalent metallic impurities (Ag, Cu or Na) were seen, only deeper, donor-acceptor-pair emission could be attributed to the Au contamination that is expected from the catalyst. Annealing under nitrogen acted to enhance the single crystal-like PL emission, whilst oxidizing and reducing anneals of the type that is used in solar cell device processing caused it to degrade. The incidence of stacking faults, polytypes and twins was related only to the growth axes of the wires (111 (50%), (112) 30% and (110) 20%), and was not influenced by annealing. The potential electrical activity of the point and extended defects, and the suitability of these nanowire materials (including processing steps) for solar cell applications, is discussed. Overall they have a quality that is superior to that of thin polycrystalline films, although questions remain about recombination due to Au. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the microstructural and optical properties of thin films of CuCl grown by vacuum evaporation on amorphous silica glass, polycrystalline indium tin oxide (ITO) and Silicon (100) substrates. Room temperature X-ray diffraction measurements reveal that CuCl grows preferentially with a (111) orientation irrespective of the substrate. Atomic force microscopy measurements showed similar surface topologies and roughness for CuCl/Si, CuCl/ITO and CuCl/Glass structures. Photoluminescence measurements at 20 K revealed the bound bi-exciton N1, impurity bound exciton I1 and the free exciton Z3 occurring at 3.152 ± 0.002 eV, 3.18 eV, and 3.204 eV, respectively, for all three structures. In addition to this, temperature dependent impedance measurements using irreversible electrodes (Au) showed that the deposited CuCl is a mixed ionic and electronic semiconductor with a predominantly extrinsic cationic conduction mechanism at temperatures above ∼280 K and with electronic conduction dominant at temperatures below ∼265 K. [ABSTRACT FROM AUTHOR]

A rapid screening methodology for the development of transparent conducting oxides is presented. The methodology, based on a combination of spectrophotometry, ellipsometry and 4-point probe measurements, was used to map out the opto-electronic properties over a co-sputtered ZnO : Al2O3 film deposited from separate ceramic targets of ZnO and Al2O3. Clear distributions for the carrier density, ne, and mobility, μe, are determined as a function of wt%. Al2O3 content within the film. A minimum resistivity value of 7.6 × 10−4 Ω cm was achieved for a composition of 1.5 wt%. Al2O3. [ABSTRACT FROM AUTHOR]

Vacancy defects have been investigated in a series of CdTe thin films grown by close-space sublimation. Variable-energy positron annihilation lifetime spectroscopy measurements, performed with a high-intensity positron beam, were used to profile polycrystalline films with varying grain size. These were obtained by changing the nitrogen pressure used during deposition. Two vacancy defects were detected, with positron lifetimes of 321(7) ps and 450(30) ps, respectively. Density functional theory calculations support the assignment of the first to the Cd vacancy and provide evidence that the second is the divacancy defect. [ABSTRACT FROM AUTHOR]