Brian W. Faughnan

Model for the bleaching of WO3 electrochromic films by an electric field

Measurements have been made of the current flow in amorphous WO3 films containing electrons and mobile cations. In a configuration in which electrons are extracted at one contact and cations at the other, the current decays as t−3/4 over many decades of time. By using space‐charge current flow ideas, we develop a theory that gives the correct time dependence and magnitude of the current for this double‐extraction phenomenon.

Optical properties of mixed‐oxide WO3/MoO3 electrochromic films

The electrochromic optical absorption of mixed‐oxide WO3/MoO3 amorphous films occurs at higher energy than either pure oxide alone. The systematics of the energy shifts as a function of MoO3 concentration and coloration density is determined. The data is explained by the intervalency charge‐transfer model if we assume that electrons trapped at Mo6+ ions lie 0.73 eV deeper than electrons on W6+ ions. Measurements of electron diffusion in mixed oxides support this hypothesis. The maximum absorption peak of mixed oxides is 2.15 eV compared with 1.4 eV for WO3. This is close to the peak in eye sensitivity, thereby leading to improved electrochromic display devices.

Dynamics of coloration of amorphous electrochromic films of WO3 at low voltages

Measurements of the time dependence of the coloration (formation of HxWO3) of amorphous films of WO3 are described and analyzed in terms of a barrier for current flow at the WO3‐electrolyte interface. A novel feature is that as coloration proceeds, the chemical potential of hydrogen in the HxWO3 increases, opposing further coloration.

Measurement of the diffusion coefficient of electrons in WO3 films

Measurements of the electron diffusion coefficient in amorphous films of tungsten oxide give a value of D=0.0025±0.0006 cm2 sec−1. D decreases by less than a factor of 2 between +40 and −20 °C.

Determination of carrier collection length and prediction of fill factor in amorphous silicon solar cells

Using a carrier collection length lc, defined for penetrating light, a convenient technique for measuring lc, in pin cells is demonstrated. We show experimentally that lc, predicts the fill factor measured with white light under AM1 conditions for a wide variety of pin cells. A simple model is presented which fits the experimental results. The implications of these results on the carrier collection loss for strongly absorbed light are discussed.

Carrier lifetime model for the optical degradation of amorphous silicon solar cells

The light‐induced performance degradation of amorphous silicon solar cells is described well by a model in which the carrier lifetimes are determined by the dangling bond density. Degradation will be slower in solar cells operating at lower excess carrier concentrations. This is documented with a comparison of degradation data for cells at open circuit versus load, and for single versus cascade cells. At sufficiently long times, the efficiency will decrease at approximately the same rate for all cases, with an offset in time between the individual cases which can be calculated.

Subthreshold model of a polycrystalline silicon thin‐film field‐effect transistor

A model is presented for the subthreshold current versus gate voltage of a polycrystalline silicon thin‐film field‐effect transistor. It utilizes the experimentally observed exponential density of states of polycrystalline silicon grain boundaries and is based on an earlier model of M. Shur and M. Hack [J. Appl. Phys. 55, 3831 (1984)] which they applied to hydrogenated amorphous silicon. Experimental subthreshold curves are presented along with the corresponding curves predicted by the model. In addition current activation data are shown to fit the model. The primary fitting parameter is the density of states at the valence band.

Relationship between collection length and diffusion length in amorphous silicon

We demonstrate a relationship between the diffusion length lD, as measured by the surface photovoltage (SPV) method and the collection length lco, measured on the same thin films of a‐Si:H. lco is the appropriate quantity to describe current collection in p‐i‐n cells of a‐Si:H, which is normally electric‐field dominated. We have shown previously that lco can be used to predict the fill factor of p‐i‐n cells. Therefore, this letter justifies the use of lD measurements by SPV for optimizing the quality of i layers for p‐i‐n cells. An expression for ambipolar diffusion is presented and the experimental results are used to place limits on the relative magnitudes of electron and hole mobilities and lifetimes.

Radiation-Induced Changes in Silicon Photovoltaic Cells

The effect of both electron and proton irradiation of silicon photovoltaic cells is given in terms of the loss of photovoltaic response and the decrease in the lifetime. Analysis of the spectral response shows that a simple carrier diffusion model provides an adequate description of the behavior of the shallow-diffused junctions that were investigated, and yields values for the minority carrier diffusion length before and after irradiation. Most of the photovoltaic response is shown to occur in the base region of the cells, rather than in the surface layer, and virtually all of the loss of response is caused by defects introduced in the base. The reciprocal of the lifetime is linear with the cumulative irradiation flux, and is consistent with the loss of photovoltaic response. There are significant differences between p on n and n on p cells under electron bombardment; the former damaging roughly 100 times as rapidly as the latter. Under proton bombardment the difference is roughly a factor of three. A co...

Optical and EPR studies of photochromic SrTiO 3 doped with Fe:Mo and Ni:Mo

Large reversible photochromic changes were observed in SrTiO 3 double doped with Mo and either Fe or Ni. The photochromic absorption is induced by irradiation in the vicinity of 4000 A and decays thermally in minutes or can be bleached to its original state by long wavelength radiation. A model is presented in which a charge is transferred between the Fe and Mo ions via the conduction or valence bands. Optical absorption, heat treatment, and EPR data are presented, which support the model.