Molecular Beam Epitaxy

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, ...