Randall Ross

Band-gap profiling for improving the efficiency of amorphous silicon alloy solar cells

We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

Physical microstructure in device-quality hydrogenated amorphous silicon

Abstract The presence of columnar morphology in hydrogenated amorphous silicon in general is well known. However, the existence of such microstructures in films with superior transport properties is not well established. Scanning electron microscopic investigation of intrinsic layers producing high-efficiency solar cells and xerographic devices indicates columnar features and intercolumnar boundaries. The dependence of these features on film thickness and substrate roughness is examined and discussed.

High efficiency multi-junction solar cells using amorphous silicon and amorphous silicon-germanium alloys

Using a novel cell design the authors have achieved a 13.7% conversion efficiency with amorphous silicon and amorphous silicon-germanium alloys in a three-cell stacked-junction configuration. 13.0% conversion efficiency was achieved in the tandem configuration. The efficiency value was measured using a triple-source solar simulator adjusted for global AM1.5 test conditions. This device has a structure of stainless steel/textured silver/zinc oxide/ni/sub 1/p/ni/sub 2/p/ni/sub 3/p/ITO/grid. The authors used an amorphous silicon-germanium alloy with the new design in the i/sub 1/ layer, and amorphous silicon alloys in the i/sub 2/ and i/sub 3/ layers. The J-V characteristic shows J/sub sc/=7.66 mA/cm/sup 2/, V/sub oc/=2.55 V, and fill factor= 0.70 with an active area of 0.25 xcm/sup 2/. The quantum efficiency of this device is 60% collection at 400 nm, 93% at the peak, 55% at 800 nm, and 21% at 850 nm. The total photocurrent density obtained by integrating the quantum efficiency envelope with the global AM1.5 spectrum is 23.5 mA/cm/sup 2/.<<ETX>>

A novel design for amorphous silicon alloy solar cells

The authors have developed an amorphous silicon alloy-based solar cell with a novel structure. Computer simulation studies show that for a given short-circuit current, it is possible to obtain a higher open-circuit voltage and fill factor than in a conventional cell design. For a nominal 1.5 eV a-SiGe alloy, the fill factor under red illumination can be improved from 0.55 to 0.64 for the same short-circuit current. Experimental cell structures confirm the theoretical prediction. The novel cell design shows a considerable improvement in efficiency. Dynamic internal collection efficiency measurements show reduced recombination in these cells, which gives rise to the observed higher fill factors. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.<<ETX>>

Optical and electronic properties of plasma deposited a-Ge:H

Films of a-Ge:H deposited at substrate temperatures of 100°C<Ts<400°C from rf plasmas of pure GeH4 and H2-diluted GeH4 have been studied optically using photothermal deflection spectroscopy and infrared absorption spectroscopy, and electrically by measurements of photoconductivity, σph, and dark conductivity, σd, versus temperature. A correlation is found between the optical and electrical properties of the film and the incorporation of hydrogen as dihydride or monohydride. Differences in these trends for the a-Ge:H deposited from pure GeH4 versus H2-diluted GeH4 are explained in terms of differences in physical microstructure.

Crucial parameters and device physics of amorphous silicon alloy tandem solar cells

Abstract We have previously shown that tandem solar cells exhibit higher efficiency and better long-term stability than single junction cells. We shall discuss crucial parameters for achieving high performance solar cells and certain aspects of device physics associated with the tandem structure. We will present data on what effect changes in intrinsic layer thicknesses, p + and n + layer thicknesses, back reflector quality, and solar spectral variation have on these multijunction devices.

Comparison of fast transient response between crystal and amorphous silicon pin photodiodes

Abstract We report on a comparison between the fast transient response of crystalline and amorphous silicon pin photodiodes. We studied time of flight, forward bias transients and reverse recovery and find that the response of the crystalline and amorphous devices are qualitatively the same except that charge storage after forward bias is more pronounced in the crystal than in the amorphous material.