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Probing silicon heterojunction photovoltaic interfaces

Event Details

Event Dates: 

Thursday, October 11, 2012 - 6:00am

Speaker Name(s): 

Long He

Speaker Affiliation(s): 

University of Colorado - Boulder
Seminar Type/Subject

Event Details & Abstract: 

Hydrogenated amorphous silicon films (a-Si:H) have a long history of application in opto-electronic 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 = 743 mV) and photo-conversion efficiencies (23%). In SHJ PVs, the amorphous-crystalline silicon (aSi: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 talk, I will demonstrate how SHG can be used to study the interface electrostatics and charge transportation in SHJ solar cells. In addition, I will detail the method to quantitatively separate interface SHG and electric-field induced SHG (EFISH) from a-Si:H interfaces and bulk.