D. P. Halliday

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 ...

Electroluminescence from porous silicon using a conducting polyaniline contact

Abstract We have fabricated a p-n junction using a p-type conducting polyaniline layer deposited on an n-type porous silicon layer. This junction shows rectifying behaviour. The I – V curves can be fitted to a Schottky barrier model with a barrier height of 0.8 eV. the porous silicon shows an orange photoluminescence band due to recombination in the silicon quantum structures both before and after coating with polyaniline. The junction emits visible electroluminescence when a forward bias is applied. The emission band is very broad extending from 500 to 1 000 nm with a peak at 790 nm. Mechanisms for charge transfer between the porous silicon and polyaniline are discussed.

A study of the effects of varying cadmium chloride treatment on the luminescent properties of CdTe/CdS thin film solar cells

Photoluminescence (PL) and electroluminescence (EL) spectroscopy have been used on a series of chemically bevelled CdTe/CdS thin film solar cells to study the effects of different CdCl2 annealing times. An anneal time of 10 min was found to produce the most efficient (8.4%) cells. Evidence was found for the CdCl2 anneal promoting the formation of a CdSxTe1−x alloy near the interface. Emission at the back surface of the cells was identified as being due to the ‘Y’ luminescence band and a donor-acceptor pair (DAP) recombination involving cadmium vacancies and chlorine donors. Sulphur diffusion into the CdTe was estimated based on changes to the energy gap obtained from the PL data.

Luminescence of Cu2ZnSnS4 polycrystals described by the fluctuating potential model

The growth of Cu 2ZnSnS4 (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.

Electrical and optical properties of a polymer semiconductor interface

Abstract We have fabricated a stable electrical interface between a range of n-type, p-type and undoped GaAs epitaxial layers and a film of conductive p-type polyaniline. The IV characteristics of this device show rectifying behaviour consistent with a potential barrier formed at the interface. We have used simple thermionic emission theory for a Schottky type barrier to model the results. This has shown no significant change in the barrier height as the fermi level moves through the semiconductor bandgap by changing from n-type to p-type substrates. We have measured the temperature dependence of the barrier height. Using n-type GaAs we can observe electroluminescence centred at 680 nm with a FWHM of 160 nm at a current density of 5 mA cm −2 . Our data suggests that the interface characteristics are controlled by surface states on the GaAs layer.

The distribution of impurities in the interfaces and window layers of thin-film solar cells

We report a systematic multielement study of impurities in CdS window layers by dynamic and quantitative secondary-ion-mass spectrometry (SIMS) with high depth resolution. The study was carried out on CdTe∕CdS solar cell structures, with the glass substrate removed. The analysis proceeded from the transparent conductive oxide free surface to the CdTe absorbing layer with a view to examining the influence of the CdCl2 heat treatment on the distribution and concentration of impurities in the structures. Special attention was paid to the impurities present in the CdS window layer that may be electrically active, and therefore affect the characteristics of the CdTe∕CdS device. It was shown that Cl, Na, and Sb impurities had higher concentrations in CdS following cadmium chloride (CdCl2) heat treatment while Pb, O, Sn, and Cu conserved the same concentration. Furthermore, Zn, Si, and In showed slightly lower concentrations on CdCl2 treatment. Possible explanations of these changes are discussed and the results...

A study of the depth dependence of photoluminescence from thin film CdS/CdTe solar cells using bevel etched samples

Abstract We present a photoluminescence study of CdTe/CdS thin film photovoltaic devices. Post growth treatment is known to increase the efficiency of the devices from a few percent to over 10%. Using a bevel etch technique, we are able to study the photoluminescence emission originating in the CdTe layer as a function of depth from the CdTe/CdS interface to the surface. By studying the intensity variation of the observed luminescence emission bands, we are able to correlate the changes in the impurity distribution deduced from the photoluminescence data with the changes in the photovoltaic efficiency. The air anneal does not significantly alter the impurity distribution whereas the anneal following CdCl2 deposition produces a redistribution of impurities. We clearly observe the diffusion of sulphur from the CdS into the CdTe layer affecting the impurity distribution. There is evidence for incorporation of oxygen into the lattice as a substitutional impurity with a greater concentration at the CdTe/CdS interface rather than the surface. A change in the concentration of cadmium vacancy–chlorine defect complexes may be related to the change in the photovoltaic efficiency.

A model for some defect-related bound exciton lines in the photoluminescence spectrum of GaAs layers grown by molecular beam epitaxy

The sharp defect-related bound exciton lines originally reported by Kunzel and Ploog (1980,1981) in the low-temperature photoluminescence spectrum of the MBE-grown GaAs are interpreted in terms of excitonic recombination at a pair of shallow acceptors. It is proposed that the various lines in the spectrum correspond to different separations of the two defects in the pair with the lowest-energy line arising from nearest-neighbour pairs. A pronounced optical polarisation of the lines is also reported.