Development of stacked multiple bandgap solar cells

Stacked multiple bandgap solar cells utilize separate junctions or solar cells or selected properties which are combined in series both optically and electrically to obtain higher conversion efficiencies than can be achieved by any of the cells individually. Basic requirements for the successful fabrication of high efficiency tandem structures will be defined, and the apparent practical limitations on the formation of such multiple bandgap solar cells by presently available techniques will be identified. Experimental tandem or "cascaded" structures have been made using CVD, LPE and MBE techniques in various-materials combinations. Two cell tandem structures using individual cells of GaAlAs and GaAs, and individual cells of GaAs and Ge have demonstrated the greatest promise to date. A large area two cell stack involving GaAlAs/GaAs cells connected by a conducting tunnel junction and formed by MOCVD has exhibited a Vocin excess of 2.1 volts. GaAs/Ge monolithic tandem structures have been prepared by MBE and demonstrate current collection in both junctions. Experimental results in both of these systems will be described.