Ion Implanted Solar Cells

Abstract

<p>A topic of much interest recently is energy production, and one means of production with much to recommend it is the solar cell, a device which converts sunlight directly into electrical energy. In addition to the possible use as a general energy source, solar cells have current uses, including power generation for space stations and satellites, and the powering of remote installations such as weather stations where other power sources are not feasible.</p> <p>The solar cell is, in essence, a semiconductor p-n junction with a contact grid on the surface and a low resistance contact covering the bottom. Incident light generates electron-hole pairs, some of which diffuse across the junction and generate a voltage across it, and thus a voltage between the top and bottom contacts. The most efficient solar cells currently available (10% to 20% conversion) are single crystal silicon cells, and it is with these cells that this project is concerned.</p> <p>A number of different fabrication methods are currently used in the production of single crystal silicon solar cells, including gas and spin-on diffusion techniques, MOS structures and thin film heterojunctions. The purpose of this project is to investigate the feasibility of the use of ion implantation as a fabrication method, and to study the effect of implant parameters on solar cell efficiency with a view to optimization.</p> <p>Much work has been done on the use of implantation doping in the fabrication of devices and integrated circuits. However, little has been done to investigate its potential in the fabrication of solar cells(1,2). Several advantages of the use of ion implantation are apparent. There is greater control of the dopant distribution than with conventional diffusion techniques. In particular, very shallow junctions are possible , allowing the formation of the "blue-shifted" cell(3), which shows an enhanced response to the short wavelength end of the spectrum. This should result in increased efficiency for terrestrial applications, but of more importance for space applications. Also, the geometry control available with the implantation technique facilitates the fabrication of grating cells(4), which also have an enhanced blue response and correspondingly increased terrestrial efficiency possibilities.</p> <p>In the first part of this project, p-type silicon substrates were implanted with arsenic and phosphorous at energies from 20 keV to 120 keV, at implant temperatures from 40°K to 300°K. The samples were vacuum annealed, aluminum contacts were made, and the solar cell efficiencies were measured under a solar simulator.</p> <p>For the second part of the project, the most significant implants were duplicated in samples in which the carrier concentrations and Hall mobilities at room temperature were measured using the Hall effect.</p>