H. Paul Maruska

Photoelectrolysis of Water in Sunlight with Sensitized Semiconductor Electrodes

In all previous studies of the photoelectrolysis of water, very little attention has been paid to the carrier generation and transport properties of the optically active semiconductor electrode. To gather such information the present work theoretically analyzes the spectral response of TiO/sub 2/ photoelectrolysis cells. Comparison with experimental results allows us to determine the diffusion length of minority carriers in TiO/sub 2/ for the first time. It is the hole transport that governs the spectral response curve, not the electron transport. The quantum efficiency of carrier generation in TiO/sub 2/ electrodes in the photolysis mode can be increased to 80% by doping the crystals with Al. The sunlight conversion efficiency has been raised to 1.3% from 0.4% reported earlier by others. The spectral response of the device has been extended into the visible portion of the spectrum through sensitization of the TiO/sub 2/ with Cr dopant impurities, allowing hydrogen generation with visible light. The photoelectrolytic processes associated with the impurity doped crystals are discussed. 36 references.

Photocatalytic decomposition of water at semiconductor electrodes

Although semiconductor electrodes have been studied for a long time, the problem of photocorrosion made them of limited use in practical devices. The discovery that TiO2 is stable in an illuminated electrochemical cell brought a great new interest to this topic. The photoelectrolysis of water to provide the useful fuel hydrogen was shown to be possible using a TiO2 anode. However, the efficiency of hydrogen production in sunlight has been low because TiO2 is a poor absorber of visible light. A number of techniques for improving the response are now being tried. A number of new semiconductors with smaller band gaps, such as Fe2O3, WO3 and p-GaP are being studied. Dye sensitization has only received limited attention, but decomposition problems have again occurred. Formation of heterostructures between stable wide band gap materials and corrosion-prone small band gap semiconductors needs further work, as does the area of increasing the visible light response through impurity doping.

Visible electroluminescence from porous silicon np heterojunction diodes

We report the preparation of silicon‐based visible light‐emitting diodes, configured as heterojunctions between porous silicon (formed by electrochemical etching of p‐type silicon wafers), and n‐type indium tin oxide (ITO). The transparent ITO film allows light emission through the top surface of the device, under a forward electrical bias of several volts across the junction. Photogenerated currents are observed under reverse biases. A tentative model for this electroluminescence is presented, based on injection of minority carriers through a narrow interphase region into the porous silicon structure, where radiative recombination occurs.

Transition-metal dopants for extending the response of titanate photoelectrolysis anodes

Abstract Photoelectrolysis of water with doped TiO2 and SrTiO3 electrodes is reported. Dopants used are transition metals V, Cr, Mn, Fe, Co, Ni, as well as Al. The response to visible light is greatest at Cr, then decreases across this period of elements, being absent in Co, Ni and Al. Hydrogen and oxygen generation are observed with Cr-doped TiO2 under visible light excitation. Ultraviolet light response decreases with the dopants in reverse order, being highest for Al and lowest for V. An explanation is given in terms of the minority carrier diffusion length,1which is calculated for samples with each of the dopants.

Current injection mechanism for porous‐silicon transparent surface light‐emitting diodes

We present a model for the injection of minority carriers into porous silicon films which results in visible dc electroluminescence. A thin interfacial dielectric region is postulated between the surface of the porous silicon layer and a transparent conductive oxide on the surface, which allows alignment of states between the two corresponding conduction bands of these materials under bias, and hence, overlap of electron wave functions and the passage of a tunneling current. Interface state densities are calculated and a parasitic nonradiative shunt current through such states is discussed.

Hall mobility of polycrystalline silicon

Hall mobility of polycrystalline silicon was measured in the dark and under illuminated conditions. Grain boundary potential barriers present in the dark can be eliminated with light. When the barriers are removed, the mobility between 200 and 400 K is found to vary as T−2, which is the dependence observed in single crystals for the same order of magnitude of doping. The free‐carrier concentration of 5×1015 cm−3 was not affected by illumination, and the room temperature mobility in 1‐mm grain size material after barrier elimination with light was 900 cm2/V sec. A phenomological theory of Hall mobility in polycrystallllne silicon which explains these observations is presented.

The role of polar species in the aggregation of asphaltenes

Abstract Many properties of heavy oils are Influenced by the presence of asphaltenes. According to solubility studies, asphaltenes are the most polar fraction of heavy oils, but the magnitude of the dlpole moment is not known. We quantified this parameter by applying dielectric spectroscopy to several heavy oils with different asphaitene concentrations, and the response of the permanent dlpoles was measured as a function of concentration and temperature. The asphaltenes have dielectric constants ranging from 5 to 7. Calculations Indicate more than one dlpole per asphaitene molecule. The diameter of the dlpole center was assessed to be 3 to 6 A. The probe was extended to study the Involvement of dlpoles In asphaitene aggregation. As the concentration of asphaltenes exceeds 10%, the dielectric constant exhibits substantial negative deviation from linearity, signifying the onset of Intermolecular Interactions (the pairing of dipoles in head-to-ta1l configurations to form clusters). Raising the temperature In...

A study of oxide-based heterostructure photoelectrodes

Abstract A review of the literature on solar-driven water photoelectrolysis cells employing semiconductor electrodes revealed that all devices tested suffered from three major problem areas: corrosion, poor sunlight absorption and external bias requirement. Only certain oxides are stable, but they are not good absorbers of sunlight and require biasing arrangements. The correlation between the external bias requirement and electron affinity of oxides was identified. A series of composite electrodes comprised of a stable wide band gap oxide (TiO2, SrTiO3, Al2O3) and a corrosion prone sunlight absorber (CdS, CdSe, ZnTe, Si) were fabricated and tested. Performance of heterostructure electrodes was shown to be limited either by pin hole problems or by potential barriers between the valence bands. These potential barriers can be minimized by using insulating oxide samples, with high work functions, and the photovoltaic properties of insulating SrTiO3 are presented.

Optoelectronic applications of porous polycrystalline silicon

We report visible light emission from porous structures formed in bulk and thin‐film polycrystalline silicon materials by anodic etching in an HF:ethanol solution. Our results indicate photoluminescence (PL) peaks at wavelengths between 650 and 655 nm and with intensities comparable to those typically obtained from porous samples of single‐crystal silicon. The analyses of the surface morphology of porous polycrystalline silicon (PPSI) layers suggest that the etch rate could be preferentially greater at the grain boundaries. We have illuminated PPSI films formed on quartz substrates from both the front and rear of the samples and have measured PL emission from the same corresponding sides. Luminescent polycrystalline silicon films offer the possibility of integrating a novel Si‐based flat‐panel display along with the recently developed thin‐film transistor (TFT) driver circuitry on a glass substrate. In addition, nanostructures originating from polycrystalline silicon substrates may enable low‐cost fabrica...

Properties and photovoltaic applications of microcrystalline silicon films prepared by RF reactive sputtering

Undoped, phosphorus‐, and boron‐doped microcrystalline silicon films were prepared by rf reactive sputtering, and their properties were investigated through structural, optical, and transport measurements. The merits of microcrystalline films for the p and n contacts in photovoltaic devices were demonstrated through the fabrication of single and tandem p‐i‐n solar‐cell structures with best efficiencies between 5 and 6%.