<title>Monolithic Multicolor Solar Conversion</title>

Abstract

*Anaheim, California 92803AbstractThe promise of multicolor (multiple bandgap) solar converters for achieving conversionefficiencies in excess of 25% in space or 30% in terrestrial applications has been widelydiscussed. lie describe current results of efforts to reach these performance goals withtwo cell monolithic stacks containing an intercell ohmic contact. Basic requirements forthe successful fabrication of high efficiency tandem structures are defined, and theapparent practical limitations on the formation of such multiple bandgap solar cells bypresently available techniques are identified. Experimental tandem or cascaded struc-tures made by MO-CVD, LPE and MBE growth techniques in various materials combinations aredescribed and current performance results are presented.IntroductionThe current strong interest in developing solar photovoltaic systems as practical sourcesof electrical power has led to a strong related interest in improving the conversion effi-ciency of photovoltaic devices. The driving force for this efficiency improvement is thepromise of significant system cost savings resulting from reduction of the active arrayarea needed to produce a given level of output power.The ways in which such cost savings can be maximized are somewhat different for spaceand terrestrial applications. For terrestrial applications, there is substantial financialleverage gained through appropriate use of sunlight concentration. In concentrator systemsrelatively expensive cell area is proportionately replaced by relatively cheap concentratorarea, thereby significantly reducing any adverse economic impact from increases in cellcosts resulting from improved efficiency. In addition, at concentration levels of 500 to1000 SUfIs because of the significantly higher current densities encountered, there is thepotential for further enhancement of conversion efficiency by a factor of 1.1