or electrons. If a minority carrier enters the region of an n-p junction, it is swept across by the electric field, separated from its counterpart, and is said to be collected (figure IV.B.l.a.3). The collected electrical carriers (electrons and holes) flow from the semiconductor to metal contacts on the front and back of the device and provide power to an external load. The distance a hole or electron travels before it interacts with other atoms is defined as its diffusion length. The objective is to generate electrons or holes such that they are near enough to the n-p junction to be collected before they are recombined. If this is not accomplished, the holes and electrons will be recombined, which releases heat to the semiconductor material instead of producing electrical energy and results in a net loss of efficiency. The recombination centers are impurities and crystal defects within and on the surface of the semiconductor material. (3) SOLAR CELL EFFICIENCY CONSIDERATIONS The efficiency (n) of a solar cell is the ratio of the electrical power output to the incident solar energy impinging upon its surface. There are many characteristics which contribute to solar cell inefficiencies and include absorption coefficients, specular reflections, temperature, electrode masking, ohmic resistance, and radiation effects, among others. The sun provides 1353 W/m2 of radiant energy in outer space (AMO-air mass zero), and it is distributed over the spectrum which is shown in figure IV.B.l.a.4. However, only a small portion of this energy is available for conversion to electricity by solar cells because of limitations in the spectral response of available semiconductor materials. Figure IV.B.l.a.5 shows a comparison of the response of silicon, gallium arsenide, and cadmium sulfide with the AMO solar spectrum. Note that in each case, the peaked response of the materials does not coincide with that of the sun. Thus, complete absorption of all of the available energy is not possible. In an effort to improve the efficiency of the silicon solar cell, certain modifications have been made to its physical structure. These modifications enable the cell to make greater use of the light in the short wavelength end of the solar spectrum and to increase the total light absorption at all frequencies. Figure IV.B.l.a.6 shows a response comparison of two recent solar cell developments which have a substantially higher efficiency than does a conventional cell. The violet cell obtains its name because it has a significantly improved response in the violet region. CNR (Comsat Non-Reflective) cell also has this characteristic, plus a special surface treatment which reduces reflected light to almost zero. The maximum theoretical efficiency for silicon solar cells is predicted to be approximately 22% at 298K. Special small-area experimental silicon cells have achieved 19.7% under laboratory conditions. The conventional silicon cells in production volumes have an
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