Split Spectrum Tandem Cell Systems

VI MONOLITHIC AND SPLIT SPECTRUM TANDEM CELL SYSTEMS 

The super-efficient, super thin cells described in the preceding section can of course stand alone as the building blocks of solar arrays. However, they can be used as elements in tandem cell systems. There are three principle ways in which they can be assembled to form such systems. In the first method, illustrated in Fig. 2, the cells are placed one over the other but they are electrically isolated so that each bandgap cell is connected to a separate load. Optical losses at the interfaces can be reduced by using a split spectrum system like that shown schematically in Fig. 16. [23] Here mirror M reflects that portion of sunlight which can be used by cell C, the largest E cell in this three cell tandem system. The dichroic mirror M  

The authors state that if the dichroic mirror had been a perfect filter, the efficiency would have been about 31%. Separate 

The third method for making tandem cell systems is to use a monolithic structure like that shown schematically for and m-cell system in Fig. 17 [25]. In this case, the individual cells have the electronic structure n - n/p-p , which as we have already noted is compatible with fabrication of super-efficient, thin cells. The n /p regions adjacent to each other provide ohmic contact between cells in the stack and, therefore, the doping level must be close to that required for degeneracy. The thickness of a monolithic stack need not be great. For example, if the individual cells are of the type described in the preceding section their total thickness would not exceed about 5pm so that a 12 cell stack would have a total thickness of only 60pm. 

At this point, however, we turn to discussions of the current state of the art of preparation of semiconductors for the ideal cell of Fig. 6 which can be viewed as the building blocks of future tandem solar cell systems. 

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