Abstract

Thin-film III-V semiconductor-based photovoltaic (PV) devices, whose light conversion efficiency is primarily limited by the minority carrier lifetimes, are commonly designed to minimize the formation of crystalline defects (threading dislocations or, in extreme cases, fractures) that can occur, in particular, due to a mismatch in lattice constants of the epitaxial substrate and of the active film. At the same time, heteroepitaxy using Si or metal foils instead of costly III-V substrates is a pathway to enabling low-cost thin-film III-V-based PV and associated devices, yet it requires to either use metamorphic buffers or lateral confinement either by substrate patterning or by growing high aspect ratio structures. Mismatched epitaxy can be used for high-efficiency durable III-V space PV systems by incorporation of properly engineered strained quantum confined structures into the solar cells that can enable bandgap engineering and enhanced radiation tolerance. One of the major topics covered in this work is optical and optoelectronic modeling and physics of the triple-junction solar cell featuring planar Si middle sub-cell and GaAs0.73P0.27 and InAs0.85P0.15 periodic nanowire (NW) top and bottom sub-cells, respectively. In particular, the dimensions of the NW arrays that would enable near-unity broad-band absorption for maximum generated current were identified. For the top cell, the planarized array dimensions corresponding to maximum generated current and current matching with the underlying Si sub-cell were found to be 350 nm for NW diameter and 450 – 500 nm for NW spacing. For the GaAs0.73P0.27, resonant coupling was the main factor driving the absorption, yet addressing the coupling of IR light in the transmission mode in the InAs0.85P0.15 nanoscale arrays was challenging and unique. Given the nature of the Si and bottom NW interface, the designs of high refractive index encapsulation materials and conformal reflectors were proposed to enable the use of thin NWs (300 – 400 nm) for sufficient IR absorption. A novel co-simulation tool combining RSoft DiffractMOD® and Sentaurus Device® was established and utilized to design the p-i-n 3D junction and thin conformal GaP passivation coating for maximum GaAs0.73P0.27 NW sub-cell efficiency (16.5%) mainly impacted by the carrier surface annihilation. Development of a highly efficient GaAs solar cell enhanced with InxGa1-xAs/GaAsyP1-y quantum wells (QWs) is also demonstrated as one of the key parts of the dissertation. The optimizations including design of GaAsP strain balancing that would support efficient thermal (here, 17 nm-thick GaAs0.90P0.10 for 9.2 nm-thick In0.10Ga0.90As QWs) and/or tunneling (4.9 nm-thick GaAs0.68P0.32) carrier escape out of the QW while maintaining a consistent morphology of the QW layers in extended QW superlattices were performed using the principles of strain energy minimization and by tuning the growth parameters. The fundamental open-circuit voltage (V¬oc) restraints in radiative and non-radiative recombination-limited regimes in the QW solar cells were studied for a variety of InxGa1-xAs compositions (x=6%, 8%, 10%, and 14%) and number of QWs using spectroscopic and dark current analysis and modeling. Additionally, the design and use of distributed Bragg reflectors for targeted up to 90% QW absorption enhancement is demonstrated resulting in an absolute QW solar cell efficiency increase by 0.4% due to nearly doubled current from the QWs and 0.1% enhancement relatively to the optically-thick baseline device with no QWs.

Publication Date

10-2021

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Seth Hubbard

Advisor/Committee Member

Parsian K. Mohseni

Advisor/Committee Member

James E. Moon

Campus

RIT – Main Campus

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