Abstract

Photovoltaics (PV) are an enabling technology in the field of aerospace, allowing satellites to operate far beyond the technological limitations of chemical batteries by providing a constant power source. However, launch costs and payload volume constraints result in a demand for the highest possible mass and volume specific power generation capability, a proxy for which is device power conversion efficiency. Enhancing the efficiency of III-V PV devices beyond the single-junction Shockley Queisser (SQ) limit has been a driving goal in PV development. Two competing loss mechanisms are thermalization, where photon energy in excess of the absorbing material’s bandgap is lost to heat, and transmission or non-absorption, where a photon has too little energy to generate an electron-hole pair in the semiconductor. A further complication regarding the longevity of PV on satellites is damage due to exposure of high energy particle radiation limiting the operational life of the satellite via gradual degradation in efficiency.

In this work, two approaches to achieving higher power conversion efficiency are explored. The first, for devices at beginning of life, is towards the development of a prototype intermediate band solar cell (IBSC) where the spectrum is split into three optical transitions via the formation of an intermediate band between the conduction and valence bands of a wide bandgap host material. Towards this goal, an InAs/AlAsSb quantum dot solar cell (QDSC) capable of enabling sequential absorption is demonstrated via a two-step photon absorption measurement and photoreflectance is used to demonstrate the presence of intraband optical transitions. The second approach, focusing on power generation at end of life, utilizes multijunction photovoltaics where successively higher bandgap materials are stacked in series to optically split the solar spectrum to reduce both thermalization and transmission loss. The addition of InAs/GaAs QDs to a GaAs subcell and InGaAs strain balanced quantum well superlattices to inverted metamorphic multijunction (IMM) devices are explored in order to improve device current retention as material is damaged due to knock-on events displacing atoms from the crystalline lattice. A third section of this work focuses on reducing costs by demonstrating a model for performance of III-V devices grown on polycrystalline virtual substrates considering two primary extended defects: the effects of crystal grain boundaries and the effects of antiphase boundaries induced by growing polar III-V materials on nonpolar Ge substrates.

Publication Date

12-19-2018

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Seth M. Hubbard

Advisor/Committee Member

Ryne P. Raffaelle

Advisor/Committee Member

Karl Hirschman

Campus

RIT – Main Campus

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