J.C.L. Cornish

Adsorption of N2, O2, N2O and NO on Ir(111) by EELS and TPD

Adsorption of N2, O2, N2O and NO on a clean Ir(111) surface at 84 K has been studied by high resolution EELS and TPD. N2, O2 and N2O desorb reversibly with the desorption peak for N2O shifting to lower temperature with increasing exposure, being 102 K for monolayer coverage. NO, on the other hand, decomposes on the Ir(111) surface and its desorption spectrum indicates the existence of three identifiable adsorption states two of which desorb mostly as NO with some dissociation whereas the third state dissociates entirely on heating. The first two states are proposed to correspond to NO which is bridge bonded to three and two atoms, respectively, while the third state corresponds to terminally bonded molecules.

Field and laboratory studies of the stability of amorphous silicon solar cells and modules

If photovoltaic solar cells and modules are to be used as a major source of power generation it is important to have a good knowledge and understanding of their long-term performance under different climatic and operating conditions. A number of studies of the long-term performance of commercially available photovoltaic modules manufactured using different technologies have now been reported in the literature. These have shown clear differences in the seasonal and long term performance and stability of different solar cell techniques. In addition to general module engineering factors that result in a loss of performance in all modules some types of solar cells, such as those made from thin film amorphous silicon (a-Si:H), also suffer specific losses in performance due to fundamental material changes, such as photodegradation or the Staebler–Wronski effect (SWE). A field evaluation of the long term performance of state-of-the-art crystalline and amorphous silicon photovoltaic modules in Australian conditions is currently being undertaken at Murdoch University. The initial results from this monitoring program are reported. This paper also reports on laboratory and field studies being undertaken on the nature of the Staebler–Wronski effect in amorphous silicon solar cells and how the stability of these cells is affected by different operating conditions. Based on a mechanism for the SWE in a-Si:H solar cells developed as a result of our research we propose a number of possible ways to reduce the Staebler–Wronski effect in a-Si:H solar cells.

Optimal optical design of thin-film photovoltaic devices

In this paper we report the results of an effort to develop a design tool to optimise any solar cell with thin-film structure. An optical admittance analysis which takes into account the interference effects in the multilayer thin-film system is discussed. This method provides a technique for calculating the optical properties of a thin-film device with a multilayer configuration. It also allows us to explore the nature and magnitude of optical losses in p-i-n type single or multi-junction a-Si : H solar cells, and to optimise the structure of such cells to utilise enhanced interference effects to obtain the maximum possible utilisation of incident solar radiation.

Electronic surface resonances of Cu(001)

The surface barrier resonances of Cu(001) have been measured using low energy electron scattering in the energy range from 2–50 eV. Incidence angles ranged from 45° to 68° with azimuths close to the 〈01〉 and 〈11〉 directions. The data was compared with results of calculations using theoretical models of the surfaces barrier including step, non reflecting and modified image barriers (MIB). Good agreement was found for calculations with the MIB and a barrier origin of 1.3 ± 0.3 au above the surface and a barrier height of 0.8 ± 0.05 Rydberg.

Effects of annealing on infrared and thermal‐effusion spectra of sputtered a‐Si:H alloys

Infrared spectroscopy and thermal effusion have been used to study the nature of the silicon-hydrogen bond in sputtered a-Si:H alloys. The samples were prepared by reactive sputtering under different deposition conditions to produce varying hydrogen contents. The Fourier transform infrared spectra have been analyzed using the simplex algorithm to deconvolute the component peaks. This technique has been applied separately to both the stretching- and bending-mode regions of the infrared absorption spectra. Studies have been made of the effects of annealing on both the infrared and the thermal evolution spectra of hydrogen. The results indicate a redistribution and transformation of different bonding configurations due to annealing. A comparative study is presented of the thermal-effusion spectra for partial and total degassing with the infrared spectra taken before and after each phase of degassing.

Silicon Nanowires: Growth Studies Using Pulsed PECVD

Silicon nanowires with high aspect ratios have been grown at high density using a variation of Plasma Enhanced Chemical Deposition (PECVD) known as Pulsed PECVD (PPECVD). Growth rate and morphology were investigated for a range of catalysts: gold, silver, aluminum, copper, indium and tin. The thickness of the catalyst layer was 100nm. Deposition was carried out in a parallel plate PECVD chamber at substrate temperatures up to 350°C, from undiluted semiconductor grade Silane. A 1 kHz square wave was used to modulate the 13.56 MHz RF power. Samples were analyzed using either a Phillips XL20 SEM or a ZEISS 1555 VP FESEM. The average diameter for nanowires grown using a gold catalyst layer was 150nm and the average length was 4μm although some nanowires were observed with lengths up to 20μm. Back-scattered-electron images clearly show gold present at the tips of the silicon nanowires grown using gold as a catalyst, confirming their growth by the vapor liquid solid (VLS) mechanism. Sporadic growth of nanowires was detected when using copper as a catalyst. Although gold performed best as catalyst for nanowire growth it was, however, closely followed by tin. The other catalysts produced nanowires with properties between these extremes.

Improved apparatus and procedures for isopiestic studies at elevated temperatures

A significantly improved apparatus for the isopiestic measurement of thermodynamic activities at elevated temperatures is described. Details of the new design and of an analysis of the propagation of the experimental errors are presented. The apparatus was tested by measurements of calcium chloride against sodium chloride at 353.15 K. Accuracies of better than 0.1% in the osmotic coefficient are routinely achieved. Data are also given for the osmotic coefficient of aqueous sodium hydroxide solutions at 353.15 K as a function of molality (up to 13 mol/kg), measured with respect to both sodium chloride and calcium chloride solutions as reference electrolytes.

Studies of photodegradation in hydrogenated amorphous silicon

IR absorption spectroscopy was used to study light-induced structural changes in hydrogenated amorphous silicon (a-Si:H) films. Our results suggest that illumination causes migration of H atoms from the interior of the film towards the illuminated surface. As a consequence, a transformation occurs in the bulk of the material leading to the formation of dangling bonds in the i-layer which could act as traps for minority carriers in solar cells. Using these results, we have formulated a model for the photodegradation of a-Si:H alloys.

Effect of charge on bond strength in hydrogenated amorphous silicon

We have studied the effect of excess charge on the bond strength in the silanes SiH4 and Si2H6 to assess whether charge trapping in a solid‐state lattice might promote the technologically important photodegradation of amorphous silicon alloys (the Staebler‐Wronski effect). The calculations indicate that both positive and negative charges reduce the strength of SiH and SiSi bonds considerably, to the point where they may be broken easily by visible or even infrared light.

Engineering nanocrystals of silicon

In this paper we describe the production of nanocrystals of silicon embedded in an amorphous silicon matrix. These were then modified by partially removing the amorphous phase. The band gap that defines the optoelectronic properties of silicon changes with crystallite size. Silicon nanocrystals exhibit a significant blue shift in the band gap with decreasing size. The contribution of the nanocrystals to the properties of the amorphous silicon matrix depends on their size, shape, orientation, distribution and volume fraction. These are properties that we are investigating and modifying. Such engineering of the optical and electronic properties could lead to new and improved optoelectronic devices. Thin films of silicon ranging from fully amorphous to polycrystalline have been deposited from pure silane by the Hot Wire method. The effect of deposition conditions: filament temperature, substrate temperature, silane flow rate, pressure, and time have been investigated. The conditions under which isolated nanocrystals embedded in an amorphous matrix form have been mapped. Selected films have been anisotropically etched to enhance the nanocrystalline features. The films have been investigated using UV–visible spectroscopy, Raman Spectroscopy, Fourier transform infra-red spectroscopy, transmission electron microscopy and selected area electron diffraction.