16MBE of III–V Semiconductors for Solar Cells

Takeyoshi Sugaya

National Institute of Advanced Industrial Science and Technology, Ibaraki 305‐8568, Japan

16.1 Introduction

Studies to achieve ultra‐high efficiencies (i.e. over 50%) for multi‐junction solar cells have looked to use subcells with different materials and bandgaps connected in series through tunnel junctions. Lattice‐matched monolithic InGaP/GaAs/Ge triple‐junction solar cells have been used for commercial space and terrestrial concentrator applications. A metamorphic growth technique on lattice‐mismatched materials has also been used to develop high‐efficiency triple‐ and quadruple‐junction solar cells; for example, InGaP/GaAs/InGaAs triple‐junction solar cells grown on GaAs substrates have been found to have efficiencies of 37.7% and 44.4% under 1 sun air mass (AM) 1.5G and concentrator conditions, respectively [1,2].

However, the monolithic epitaxial growth technique limits the choice of material and cell combinations, because lattice‐mismatched epitaxial growth is very difficult. An alternative approach for fabricating multi‐junction solar cells involves a semiconductor bonding technique that can be applied to two different material systems [3,4]. Quadruple‐ and quintuple‐junction solar cells can be obtained by using a technique for directly bonding GaAs‐based top and InP‐based bottom cells; the highest reported efficiencies of these cells were 38.8% and 46.0% under 1 sun AM 1.5G [5] and concentrator [6] conditions, ...

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