School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Large Area Ultrapassivated Silicon Solar Cells Using Heterojunction Carrier Collectors
by
Stanislau Yur’yevich Herasimenka
Thursday, November 21, 2013
10am – 12pm
GWC 487
Committee:
Dr. Christiana Honsberg (chair)
Dr. Stuart Bowden (co-chair)
Dr. Clarence Tracy (member)
Dr. Dragica Vasileska (member)
Dr. Zachary Holman (member)
Dr. Ron Sinton (member)
Abstract
Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si
heterojunction (SHJ) have a potential to overcome the limitations of the conventional
diffused junction solar cells and become the next industry standard solar cells
manufacturing technology. A brand feature of SHJ technology is ultrapassivated surfaces
with already demonstrated 750 mV open circuit voltage (VOC) and 24.7% efficiency on
large area solar cell.
Despite very good results achieved in research and development, large volume
manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main
objectives of this work were to develop a SHJ solar cell fabrication flow using industry
compatible tools and processes in a pilot production environment and use it to study the
interactions between the used fabrication steps, to identify the minimum set of
optimization parameters and characterization techniques needed to achieve 20% baseline
efficiency and to analyze power losses in SHJ cells by characterization and modeling.
This manuscript presents a detailed description of a SHJ solar cell fabrication
flow developed at ASU Solar Power Laboratory which allows large area ultrapassivated
solar cells with >750 mV VOC. Full size SHJ cells on 140 and 50 um wafers were
fabricated and fully characterized. Passivation quality of (i)a-Si:H film, bulk conductivity
of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of
all films were identified as the minimum set of optimization parameters necessary to set
up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured
wafers to minimize the concentration of surface impurities and to avoid epitaxial growth
of a-Si films was found to be a key challenge in achieving a repeatable and uniform
passivation. This worked resolved this issue by using a multi step cleaning process based
on sequential oxidation of textured wafers in nitric/acetic acids, Piranha and RCA-b
solutions. The developed process allowed state of the art surface passivation with perfect
repeatability and negligible reflectance losses.
The second part of this dissertation considers a model describing optical,
recombination, and series resistance losses in SHJ cells based on the measured material
properties and device parameters. Using the breakdown of loss mechanisms predicted by
the built model a potential roadmap to 25% efficiency silicon solar cell is presented.
One additional study considered application of the developed (i)a-Si:H film as a
chemical passivation layer in (i)a-Si:H/SiO2/SiNx stack to be used at the front surface of
interdigitated back contact solar cells. < 1 fA/cm2 J0 was demonstrated using <5 nm (i)aSi:H film to provide both perfect passivation and low parasitic absorption.