Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Microelectronics-Photonics

Advisor

Gregory Salamo

Committee Member

Morgan Ware

Second Committee Member

Cynthia Sides

Third Committee Member

Shui-Qing Yu

Fourth Committee Member

Rahul Kumar

Fifth Committee Member

Matthew Leftwich

Keywords

InAs GaAs, intermediate band, quantum dot, quantum well, solar cell, Submonolayer

Abstract

Intermediate band solar cell (IBSC) is a new generation solar cell that is designed to overcome the main limitation of solar cell power conversion efficiency, which is the non-absorption of low photon energy. Experimental efforts towards fabricating high efficiency and low cost IBSC have not met the predictions of theoretical analysis. Existing implementations of the IBSC design using conventional Stranski-Krastanov (S-K) quantum dots (QDs) have resulted in a degradation in the solar cell device characteristics, which has been found to be caused by recapturing or trapping of excess carriers, low intermediate band material absorption, and increased recombination in the solar cell sub-bandgaps. The goal of this research was to investigate the effect of different quantum structures on the optical and electrical properties of IBSCs. More specifically, in this work, the performance of solar cells with different quantum structures, such as, Stranski-Krastanov quantum dots, quantum well (QW), sub-monolayer (SML) QDs, and a quasi-monolayer (1 ML) InAs quantum well, was investigated. The performance of all the grown solar cell structures were compared with a single junction p-i-n solar cell reference sample. The solar cell structures were grown using molecular beam epitaxy. The total InAs content was kept the same in all SCs. The solar cell device electrodes were patterned using the standard optical photolithography technique and deposited using an electron beam evaporator. The structural, optical, and electrical properties of the grown IBSCs were studied by X-ray diffraction (XRD), photoluminescence (PL), excitation power dependent PL, external quantum efficiency (EQE), and illuminated current-voltage (I-V) measurements. The EQE results presented the absorption near-IR of all the IBSCs indicating additional photocurrent generated from the sub-bandgap transitions in all the grown IBSCs. The EQE of the 1 ML solar cell was comparable to the one for the GaAs reference sample above the GaAs band edge and shows great improvement in the photocurrent over the 2 ML S-K quantum dot solar in the GaAs and sub-bandgap regions. The power conversion efficiency and open circuit voltage of the 1 ML solar cell are significantly improved with respect to that of the 2 ML QDSC. Different coverages of SML InAs were tested for optimum performance improvement of intermediate band solar cells. Strong confinement for 0.25 ML was indicated in the PL results. The best EQE behavior was found for 0.25 ML based SC indicating higher sub-bandgap photons absorption, and subsequent efficient carrier extraction. There was also a very clear trend that with reducing the sub-monolayers InAs coverage, the efficiency, fill factor, open circuit voltage, and short circuit current of the intermediate band solar cells increased. Near 0.25 ML InAs deposition was found best for solar cell application. The 0.25 ML IBSC structure resulted in almost 23% relative efficiency improvement and similar open circuit voltage compared to a reference GaAs solar cell and showed the best EQE behavior, indicating higher absorption of sub-bandgap photons, and subsequent efficient carrier extraction. Furthermore, superiority of the 0.25 ML SML-QD IBSC over both the SK-QD and the InGaAs QW based solar cell was found.

Available for download on Saturday, February 17, 2024

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