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).