Richard S. Crandall

Modeling of thin film solar cells: Uniform field approximation

A model of a p‐i‐n thin‐film solar cell is presented that can be easily used to analyze solar cell properties. The continuity equations are solved using the regional approximation, producing elementary solutions that give insight into the physics of the transport in the cell. The steady‐state solutions are compared with measurements on typical hydrogenated amorphous silicon, a‐Si:H, solar cells. The ac solutions are used to explain a new source of photocapacitance due to mobile carriers.

Stability of n‐i‐p amorphous silicon solar cells

Unencapsulated, amorphous silicon indium tin oxide/n‐i‐p/stainless‐steel solar cells were tested for stability. All cells have excellent shelf life. Changes occur during exposure to light, but can be controlled by the deposition conditions of the amorphous silicon. The changes are due to trapping and recombination of optically generated carriers in the i layer, and are reversibly annealed out above 175 °C. Preliminary life tests on two relatively stable cells showed a small initial drop to 5%, followed by a weak logarithmic decay that predicts only ∼20% further decrease in efficiency after 20 years in sunlight. Work is continuing on improving the efficiency and stability of these cells.

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.

Use of carbon dioxide in energy storage

We have investigated an energy‐storage cycle in which CO2 is electrochemically reduced to formic acid, HCOOH. The product can be used in several ways. By means of a catalyst, it can be converted to hydrgen for use as a fuel or raw material. We have obtained data on the efficiency of the process and analyzed the energetics.

Gravitational Compression of Crystallized Suspensions of Polystyrene Spheres

In a crystallized suspension of polystyrene spheres, the earths gravitational field, acting on a vertical column of material several centimeters high, produces an elastic deformation that can be readily observed through its effect on the crystal lattice constant. This effect has been used to determine that Youngs modulus for the crystalline material ranges from 1 to 3 dynes per square centimeter, depending on the concentration of spheres.

Theory and measurement of the change in chemical potential of hydrogen in amorphous HxWO3 as a function of the stoichiometric parameter x

Abstract Measurements of the variation of the chemical potential μH of hydrogen in amorphous films of HxWO3 were made at room temperature over the composition range from x=0.002 to x=0.5. The μH values were obtained from e.m.f. measurements in an electrochemical cell using H2SO4 as the electrolyte. The chemical potential is given by μ H =A+0.53x+2( RT F ) ln [ x (1−x) ] where A is an undetermined constant and μH is in eV. The term linear in x is due to interactions in the HxWO3, while the final term is the entropy contribution.

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.

Transport in hydrogenated amorphous silicon p‐i‐n solar cells

Measurements of the voltage dependence of the photocurrent in hydrogenated amorphous silicon p‐i‐n solar cells are presented along with a simple physical model of the transport. For the conditions of weakly absorbed light, the photocurrent‐voltage curve can be completely specified by the light intensity and electron and hole drift lengths. Furthermore, the carrier with the longer drift length determines the solar cell current‐voltage curve.