The work described in this thesis is part of an ongoing project to improve the performance of hydrogenated silicon (a-Si:H) based triple junction solar cells. With the
supply of fossil fuels diminishing it is imperative that cost effective alternative energy
options are developed and implemented by utility companies, businesses, and the general
public. Solar-generated electricity (photovoltaics) has a promising future as a source of
energy that is both clean and inexhaustible. With an improved understanding of
component layer structure and how it relates to solar cell performance, a-Si:H based
triple-junction solar cells can be optimized for higher efficiency and, thereby become
more cost effective – making them a viable option as an alternative source of energy.
The analysis of the structures in this study was performed using ex-situ, in-situ,
and real-time spectroscopic ellipsometry. The instrument used to collect the data was a J. A. Woollam Co. model M2000-DI ellipsometer that utilizes a rotating compensator
design with a fixed polarizer and analyzer. The spectral range of this instrument is 0.75
to 6.5 eV. The light collected by the InGaAs photodiode array and Si CCD detectors is
split into 706 wavelength channels and can be collected in a time as short as 50 ms.
The thin film structure upon which the a-Si:H based solar cell is deposited is
known as a back-reflector. It is composed of two layers on a stainless steel substrate,
including silver at the bottom and zinc oxide at the top. Ex-situ measurements of the
back-reflector were performed at different angles of incidence (AOI) in order to eliminate
degenerate solutions, which allowed for accurate determination of the sample structure.
The software used to determine the thicknesses and compositions of the back-reflector
layers, as well as interface and surface roughness layers was J. A. Woollam Co.'s
WVASE32 version 3.438.
The phase composition of the undoped layer, which serves as the intrinsic or i-
type region of the n-i-p device exhibits strong correlations with the performance
parameters of the overall solar cell. RTSE data were collected in-situ during the
deposition of all three intrinsic layers, and the optical properties as well as the bulk layer
and surface roughness thicknesses were determined from these data using custom
software developed at Penn State University (Gridb) and J. A. Woollam Co.'s EASE 2.3.
The intrinsic layers were co-deposited on side-by-side substrates of the textured back
reflector and crystal silicon. The crystal silicon was used for the RTSE analysis because
of its well-known dielectric function as well its smooth, specularly-reflecting surface.
The solar cell structure was co-deposited on the crystal silicon and textured back
reflector, the later being the actual substrate for actual devices that enable performance
parameters to be measured such as fill factor (ff) and open circuit voltage (Voc). The
solar cell performance can then be correlated with the composition as indicated on the
phase diagram to determine the effect of intrinsic layer phase structure on the cell's
ability to function as a component of a triple junction solar cell.