Publication Date

April 2019

Advisor(s)

Meng-ju Sher

Major

Physics

Language

English (United States)

Abstract

Silicon is a commercially predominant solar cell material used in the study of hyperdoping, in which an element is added to broaden the range of light captured by the cell. The electrons in the hyperdoped silicon are excited by sending ultra-fast pulsed light through the sample and their conductivity is measured by a function of time. That conductivity decay rate is a measure of how fast the electrons are relaxing i.e. carrier lifetime. However, particular samples produce a bi-exponential decay curve with two different lifetimes as fast and slow decay. We hypothesized that the fast and slow decay stem from an uneven distribution of dopant concentration across the depth of the cell that is created by the manufacturing process of hyperdoped silicon. The silicon crystalline structure is melted to add in the dopant and then a super-cooling process re-solidifies the material. The speed of cooling affects the amount of dopant accepted into the silicon structure, subsequently causing a pile-up of dopant near the surface of the cell. We show with experimental that the fast decay correlates to a higher concentration profile by exciting at different depths where the concentration of gold dopant varies. The correlation is further proved with a numerical simulation of the fast decay based on diffusion of carriers and a depth-dependent recombination rate. Through experiment and numerical modeling, we hope to better our ability to increase cell efficiency through different doping techniques.

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