Multijunction Solar Cells

41.7.1 Advantages of Multi-junction Solar Cells

A-Si:H solar cells can be fabricated in a stacked structure to form multi-junction solar cells. Figure 41.7(1) shows a tandem cell with two junctions (i. e. two pin photodiodes) in series. These multi-junction cells can have higher solar conversion efficiency than single-junction cells and are presently used in most commercial mod­ules. Mostly multi-junction a-Si:H based solar cells were tandem or triple junctions with an a-SiGe low band-gap absorber. Since the early 70’s, tandem “micromorph” cells and cells with a-Si/nc Si multiple band-gap tandem modules are widely used Multi-junction solar cells are spectrum splitting. The thickness of the top pin junction is adjusted so that it filters out about half of the photons that would other­wise have been absorbed in the bottom pin junction. The top material has a larger band-gap to absorb photons at larger energies, and produces a larger open-circuit voltage than the bottom part of the cell. In this two-terminal multi-junction cell the same electrical current flows through the series-connected cells. Aside from the

spectrum-splitting effect, the upper cell is thinner than in single-junction cells, caus­ing an improved fill factor, and since a multi-junction cell delivers its power at a higher voltage and lower current, the resistive losses are reduced. Most desirable, is a higher stabilized output for multiple-junction cells (Minemoto et al. 2007).

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