Cascade solar cells
A monolithic cascade cell for converting incident radiation,particularly solar radiation, into electrical energy at a highefficiency with at least three layers of semi-conductive GroupIII-IV material. The top layer is doped into p and n regions with ahomojunction therebetween and has a bandgap such that photons abovea predetermined energy interact with the semi-conductive materialto produce a potential across the homojunction and current flow andphotons below that energy pass through the first layer to a secondsimilar layer having a lower bandgap so that some of the radiationpassing through the first layer produces a potential across ahomojunction in the second layer to improve the overall energyconversion efficiency of the cell. The first and second layers areseparated by at least a third layer of a Group III-V material,similarly doped into p and n regions with a tunnel junction formedtherebetween to provide a low voltage electrical connection betweenthe first and second layers. In one embodiment, the first andsecond layers are connected in series to be additive so thatcurrent flow takes place between contacts associated with the firstand second layers. In another embodiment, the first and secondlayers are connected in opposition with a third terminal connectedto the third layer which, in this case, need not be a tunneljunction. Alternatively, the two layers may be light producingdevices such as light emitting diodes (LEDs), emitting light atdifferent wavelengths, or photodiodes, detecting light of differentwavelengths, or one layer can be a light producing device such asan LED while the other layer is a light receiving device such as aphotodiode, again operating at different wavelengths.
Lamorte, M. (1979). IPC No. U.S.Cascade solar cells (Patent No. 4179702.)