Molecular beam epitaxy growth of germanium junctions for multi-junction solar cell applications

作者：Masuda T; Faucher J; Lee M L^*

来源：Journal of Physics D: Applied Physics , 2016, 49(46): 465105.

DOI：10.1088/0022-3727/49/46/465105

• multi-junction solar cells
• Ge solar cells
• lattice-matched
• molecular beam epitaxy

We report on the molecular beam epitaxy (MBE) growth and device characteristics of Ge solar cells. Integrating a Ge bottom cell beneath a lattice-matched triple junction stack grown by MBE could enable ultra-high efficiencies without metamorphic growth or wafer bonding. However, a diffused junction cannot be readily formed in Ge by MBE due to the low sticking coefficient of group-V molecules on Ge surfaces. We therefore realized Ge junctions by growth of homo-epitaxial n-Ge on p-Ge wafers within a standard III-V MBE system. We then fabricated Ge solar cells, finding growth temperature and post-growth annealing to be key factors for achieving high efficiency. Open-circuit voltage and fill factor values of similar to 0.175 V and similar to 0.59 without a window layer were obtained, both of which are comparable to diffused Ge junctions formed by metal-organic vapor phase epitaxy. We also demonstrate growth of high-quality, single-domain GaAs on the Ge junction, as needed for subsequent growth of III-V subcells, and that the surface passivation afforded by the GaAs layer slightly improves the Ge cell performance.

• 出版日期2016-11-23