Date of Award

Summer 7-18-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Charles T. Rogers

Second Advisor

Charles W. Teplin

Third Advisor

Steven T. Cundiff

Abstract

Hydrogenated amorphous silicon films (a-Si:H) have a long history of application in optoelectronic devices, in part due to their low cost and compatibility with large area substrates. Recently, crystal silicon/a-Si:H heterojunction(SHJ) photovoltaic cells have demonstrated extremely high open circuit voltages (VOC = 750 mV) and photo-conversion efficiencies (24.7%). In SHJ PVs, the amorphous-crystalline silicon (a-Si:H/c-Si) interface is the critical aspect of the device to optimize for high efficiency. The understanding of defects and transport at the a-Si:H/c-Si junction has been slow to develop due to a dearth of optoelectronic measurements able to distinguish the unique interface physics from effects in the bulk a-Si:H and c-Si volumes.

Optical second harmonic generation (SHG) has been extensively used to selectively characterize surfaces and interface in a variety of materials, including semiconductors. In SHG experiments, interfaces and surfaces can be probed selectively: One focuses a pulsed laser beam (frequency ω) onto the sample and detects second harmonic light (frequency 2ω) generated at optically accessible surfaces and interfaces in the sample. SHG elucidates the important interface properties because the bulk "background" is mostly forbidden by symmetry in cubic and amorphous materials, leaving only interface contributions.

In this thesis, I have demonstrated that SHG is a sensitive tool for probing strong electric field present in the 10 nm a-Si:H layer in SHJ solar cells. To study the electric-field induced SHG (EFISH) in a-Si:H, we measure SHG from ITO/a-Si:H/ITO sandwich structures at different biases and polarization geometries. In this "simple" structure, we quantitatively separate interface SHG and EFISH. We also directly probe carrier dynamics in the depletion region of a ITO/a-Si:H junction with time-resolved optical second-harmonic generation. Through fitting of the time-resolved SHG data and current data simultaneously, we are able to show that slow carrier dynamics that are visible in the device current are actually taking place within the ITO/a-Si interfacial region. In summary, SHG is proven to be a promising diagnostic method to characterize the interface electrostatics and charge transport at the amorphous silicon interfaces.

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