MSE Colloquium: Ryan France, Dislocations in Ordered III-Vs for Ultra-High Efficiency Solar Cells

Abstract

Multijunction III-V solar cells hold the promise of ultra-high power conversion efficiencies, but often require combining epitaxial materials with different lattice constants – a materials science challenge! One technique to overcome this challenge uses compositionally graded buffers, which slowly grade the lattice constant of the epitaxial material. Dislocations are intentionally introduced in a controlled fashion in order to relieve the strain in the buffer and allow thick, strain-free growth of multijunction subcells. For efficient photoconversion, the majority of the dislocations must be confined to the buffer, and not penetrate the active region of the solar cell. However, this requires understanding the driving forces for dislocation formation and glide in epitaxial III-V materials, including microstructural influences such as phase separation and atomic ordering.

We have recently studied the influence of atomic ordering on dislocation formation and glide in III-V materials. Glide through ordered material creates a jog in the order pattern, or an anti-phase boundary in the ordering. Order-APBs have previously been studied in metals, where the ordered structure is stable. In III-V materials, the ordering is metastable and is a result of surface strain during epitaxy. In this case, the disruption of the order pattern lowers bulk energy, and so dislocation glide through ordered planes is enhanced. This effect has consequences for both strained and strain-relaxed III-V devices, which will be discussed. Using this knowledge, we have improved graded buffers to allow multijunction solar cells with over 45% efficiency.

Bio

Ryan is a scientist at the National Renewable Energy Laboratory, where he has been working on III-V materials and multijunction solar cells for 7 years. His research at NREL has involved the investigation of new III-V materials for photoconversion, including GaAsBi and GaInNAs grown by MBE. His current focus is on metamorphic buffers and lattice-mismatched solar cells, where he has studied the role of atomic ordering on dislocations in III-V materials. He has integrated high quality lattice-mismatched solar cells into 3- and 4-junction metamorphic devices, which have led to ultra-high efficiency multijunction solar cells. He earned his BS from Washington University in St. Louis in 2001, after which he researched single- and multi-crystalline Si solar cells at the University of New South Wales in Sydney. He then received a MS from Boston University in 2006, where he researched III-nitride LEDs.