GB843686A - Radiant energy converter - Google Patents

Abstract

843,686. Photo-electric devices. TEXAS INSTRUMENTS Inc. Sept. 19, 1956 [Oct. 31, 1955], No. 28699/56. Drawings to Specification. Class 37. A solar battery comprises a plurality of layers of semi-conductor materials with different energy gaps between their conductive and valency bands, each of which contains a PN junction. Preferably the layers are superposed with the P, and N parts of adjacent layers electrically connected either by direct contact or by wires. The uppermost layer upon which the radiation is directed is of material with the highest energy gap and the following layers are of materials with progressively lower energy gaps. It is desirable if the layers are to be connected in series that the number of photons absorbed in each layer is the same. Since when a photon is absorbed any energy it possesses in excess of the energy gap of the absorbing material is dissipated as heat the efficiency of conversion of radiation to electrical energy for the device as a whole is improved by using a large number of layers with closely spaced energy gap. The operation is as follows: photons with energy greater than the energy gap of the first layer are absorbed therein to give electron whole pairs which separate across the PN junction to give an output current. The first layer being transparent to less energetic photons these pass to the second layer where those with energies between the energy gaps of the first and second layers are absorbed, and so on. The depth of the PN junctions which are formed by diffusion of acceptor impurity into formerly N type layers,. is chosen so that each junction is within the minority carrier diffusion length of the point at which maximum absorption in the layer occurs to prevent loss of minority carriers by recombination. The use of Group IV elements, such as Ge and Si, of compounds of Groups III and V elements such as GaAs and InSb, of compounds of Group II and VI elements such as ZnS and CdSe, and of mixed crystals of such intermetallic compounds with intermediate values of energy gap is suggested. One example uses layers of CdSe (energy gap 1À74eV), Si, ((1À08eV) and GaSb (0À67eV) connected in series and another layers of AlSb (1À88eV) InP (1À25eV) and Ge (0 À82eV) also in series. The conversion efficiencies are 17À4 and 22 % respectively against 11% for a single layer Si PN junction device. One of the layers may be electrically independent of the others which are then series or parallel connected.