Date of Graduation

12-2014

Document Type

Thesis

Degree Name

Master of Science in Microelectronics-Photonics (MS)

Degree Level

Graduate

Department

Graduate School

Advisor

Gregory Salamo

Committee Member

Morgan Ware

Second Committee Member

Shui-Qing Yu

Third Committee Member

Vasyl Kunets

Fourth Committee Member

Ken Vickers

Keywords

IBSC, Intermediate Band Solar Cells, Open Circuit Voltage reservation in IBSC, QD, Quantum Dot Size, Quantum Dot Solar Cells

Abstract

The goal of this thesis is to understand possible mechanisms for the reported decrease of the open circuit voltage and solar cell efficiency in quantum dot (QD) intermediate band solar cells (IBSCs). More specifically, the effect of indium arsenide (InAs) QD height on the open circuit voltage and solar cell efficiency was studied in a systematic way. To explore this effect in QD solar cells, several solar cells (SCs) were grown with varying InAs QD heights. All experimental characteristics of the QD solar cells were compared to a reference structure without QDs. All samples were grown by Molecular Beam Epitaxy (MBE), and self-assembled InAs QDs were formed using the Stranski-Krastanov (SK) growth method. Using a QD truncation technique, the height of the QDs was accurately varied between 2 nm and 5 nm, while maintaining both lateral size and areal density of the QDs. The intermediate band (IB) of each solar cell was constructed from 10 layers of InAs QDs of the same size and density.

All samples were fabricated as solar cell devices using standard optical photolithography, for electrical characterization and solar cell efficiency studies. Optical and structural characterization was done for all samples. The following characterizations were performed: Transmission Electron Microscopy (TEM), Low Temperature Photoluminescence (PL), Power Dependent PL, External Quantum Efficiency (EQE), Temperature Dependent Solar Power Conversion Efficiency, and Current-Voltage measurements. The efficiency measurements demonstrate the critical role of QD size on the performance of QD IBSCs. The EQE measurement indicates a change in the position of the band edge, due to carrier confinement, consistent with a QD size variation as verified by TEM and PL. Measurements demonstrate that the EQE in the NIR range of the spectrum is enhanced in the QD IBSCs devices due to light absorption by the QDs. This work also demonstrates that open circuit voltage (Voc) decreases with an increase of the QD height, which leads to significant degradation of the solar cell conversion efficiency for QD sizes above 3 nm. In addition, for samples with QD heights of 4 nm and above, the EQE spectra in the GaAs region decreases, indicating a loss of photocurrent, most likely due to traps introduced by the large QDs. These experimental results suggest that the open circuit voltage in QD IBSCs degrades with the increase of QD height as a result of (i) a decrease of the effective band gap of the absorber media and (ii) enhanced Shockley-Read-Hall recombination in the presence of traps in the solar cell space charge region.

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