Budhika G. Mendis

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 ...

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.

Use of the Nye tensor in analyzing HREM images of bcc screw dislocations

The Nye tensor characterizes the strength of infinitesimal dislocations at each point in a continuously dislocated crystal, and provides a measure of the Burgers vector and the extent of dislocation dissociation. The present work employs this description to analyze ½ screw dislocations in bcc Mo obtained by Finnis–Sinclair, Bond Order Potential and first-principles simulations in an attempt to detect misfit in the core region. The spatial distribution and strength of the fractional dislocations are calculated and compared between the different simulated core structures. The Nye tensor technique is also applied to HREM images of end-on screw dislocations in Mo to detect the edge fractional dislocations predicted by simulation. The advantage of the Nye tensor for this purpose arises from its insensitivity to the Eshelby twist due to surface relaxation in a thin TEM foil (this is not the case for the more conventional methods of depicting misfit, such as direct and differential displacement maps). Although the Eshelby twist was removed from the experimental HREM images, some residual, mottled contrast structure was observed in the Nye tensor plots. This contrast was found to be due to the experimental noise, which masks the true structure of the dislocation core and precludes experimental characterization of screw dislocations in Mo by HREM.

Giant dielectric permittivity of detonation-produced nanodiamond is caused by water

We show that small (≤4%) amounts of water which detonation-produced nano-diamond powder always adsorbs spontaneously from air can increase its dielectric permittivity (e) at low frequencies from single digits to over 1019, by far the highest value observed for any system including ferroelectrics. Conversely, traces of DND drastically affect the physical properties of water, increasing its e from ∼80 to over 106 and altering sound velocity. The effect is due to proton-releasing functional groups on the diamond surface interacting with the adsorbed water monolayer, hence it does not occur in hydrogen-free DND. The observed giant dielectric permittivity makes DND a prospective material for high-performance capacitors for use in microelectronics, and for the development of large-scale capacitance-based energy-storage devices urgently demanded in the quest for green energy technology. The results are also relevant for biomedical applications of DND and for understanding the enigmatic surface conductivity of diamond and electrical spectroscopy of porous rocks, which is important in geology.

Simple and scalable route for the ‘bottom-up’ synthesis of few-layer graphene platelets and thin films

Graphene has generated much interest owing to its exceptional electronic properties and high mechanical strength. This has enabled new types of electronic devices and composite materials to be envisaged. The main problem is the availability of the material and the difficulties associated with its synthesis. Here we have used a simple, convenient and scalable chemical vapour deposition method involving sodium ethoxide in ethanol to produce few-layer graphene sheets or platelets. The process has the advantage that it can be used to grow graphene films on non-metal containing substrates such as silicon wafer and quartz glass and that all non-carbon by-products are soluble in water.

Effects of thickness on the cation segregation in epitaxial (001) and (110) La2/3Ca1/3MnO3 thin films

Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 (LCMO) films of various thicknesses grown on SrTiO3 (001) and (110) substrates. For relatively thick films (≥20 nm), epitaxial tensile strain in (001) films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed (110) films are chemically homogeneous. In contrast, much thinner (001) and (110) LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth.

A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics

A cathodoluminescence-based, contactless method for extracting the bulk minority carrier diffusion length and reduced recombination velocity of an individual grain boundary is applied to vapor grown CdTe epitaxial films. The measured diffusion length was within the range of 0.4–0.6 μm and the grain boundary recombination velocity varied from 500 to 750 cm/s. The technique can be used to investigate the effect of grain boundaries on photovoltaic performance.

Core-shell ITO/ZnO/CdS/CdTe nanowire solar cells

Radial p-n junction nanowire (NW) solar cells with high densities of CdTe NWs coated with indium tin oxide (ITO)/ZnO/CdS triple shells were grown with excellent heterointerfaces. The optical reflectance of the devices was lower than for equivalent planar films by a factor of 100. The best efficiency for the NW solar cells was η = 2.49%, with current transport being dominated by recombination, and the conversion efficiencies being limited by a back contact barrier (ϕB = 0.52 eV) and low shunt resistances (RSH < 500 Ω·cm2).

Microstructure and point defects in CdTe nanowires for photovoltaic applications.

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.

Top-down fabrication of single crystal silicon nanowire using optical lithography

A method for fabricating single crystal silicon nanowires is presented using top-down optical lithography and anisotropic etching. Wire diameters as small as 10 nm are demonstrated using silicon on insulator substrates. Structural characterization confirms that wires are straight, have a triangular cross section and are without breakages over lengths of tens of microns. Electrical characterization indicates bulk like mobility values, not strongly influenced by surface scattering or quantum confinement. Processing is compatible with conventional silicon technology having much larger critical dimensions. Integrating such nanowires with a mature CMOS technology offers an inexpensive route to their exploitation as sensors.