as Investigated on
Siddhartha Garud
Laser Processes for
Polycrystalline Silicon
Silicon Solar Cells
on Glass
Laser Processes for Silicon Solar
Cells
as Investigated on Polycrystalline Silicon on Glass
vorgelegt von
M. Sc.
Siddhartha Garud
von der Fakultät IV - Elektrotechnik und Informatik
der Technischen Universität Berlin
zur Erlangung des akademischen Grades
Doktor der Ingenieurwissenschaften
-Dr.-Ing.-
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. Steve Albrecht
Gutachter: Prof. Dr. Bernd Rech
Gutachter: Prof. Dr. Marko Topič
Gutachter: Prof. Dr. Klaus Lips
Tag der wissenschaftlichen Aussprache: 16. Juni 2021
Berlin 2021
Abstract
Solar cells made from thin layers of polycrystalline silicon (
14 −15 µm
) are the core technology
for the work in this thesis. They are made by the deposition of amorphous/nanocrystalline
silicon on glass up to a desired thickness (
5−40 µm
), and crystallization with a line-shaped laser
source (referred to as Liquid-phase-crystallized silicon or LPC-Si). Such a bottom-up approach
does not incur the same losses as the current, industrial top-down approaches of slicing high
quality silicon into thin wafers from ingots at a material loss of approximately
40 %
. This thesis
aims to address the following three research questions regarding the power conversion efficiency
of such devices:
1.
Can charge collection be improved under the electron contact region while simultaneously
achieving a low resistance contact?
2.
Can the voltage output of such a solar cell be further improved by better passivation of
the contacting side?
3.
Can such a solar cell be monolithically integrated with a higher bandgap top cell for a
higher over-all power conversion efficiency?
Lasers as highly focused sources of energy were found to be key tools to address these
research questions. The solar cell architecture used consists of heterojunction interfaces based
on amorphous silicon and interdigitated contacts of both polarities on a single side (HJ-IBC).
Chapter 4
addresses the first research question with the implementation of a-Si:H(i) at the
electron contact and a laser firing technique to lower contact resistance in localized spots while
preserving a-Si:H(i) passivation in unfired regions. After the laser firing, open-circuit voltage
(V
OC
) was retained, while up to
14 %
absolute increase in fill-factor (FF) was obtained with
0.2−0.9 mA/cm2
loss in short-circuit current density (J
SC
). An approach was thus established
for a controllable trade-off between JSC and FF.
Chapter 5
addresses the second research question by improving the heterojunction contact
interfaces. Experimentation at the electron contact was enabled by the developments of chapter 4.
J
SC
was observed to be up to
33.1 mA/cm2
, surpassing all previously reported values for this
technology. V
OC
of up to
658 mV
also exceeded every previous value published at a low bulk
doping concentration (
1×1016/cm3
). Laser firing developed in chapter 4 reduced J
SC
by
0.6 mA/cm2
on average but improved FF by
22.5 %
absolute on average, without any significant
effect on V
OC
. Collectively, these efforts helped achieve a new in-house record efficiency for
LPC-Si of
15.1 %
and show potential to reach
16 %
efficiency in the near future with optimization
of series resistance. Suns-V
OC
measurement of the best cell showed a ’pseudo-efficiency’ of
16.8 %, i.e. efficiency if series resistance can be reduced to effectively zero.
In
Chapter 6
, high resolution, light beam induced current measurements (LBIC) were
used to analyse the approaches developed in chapters 4 and 5. Charge collection was observed to
have increased from
0.13 mA/cm2
to
1.2 mA/cm2
under the electron contact which is a ninefold
increase. Using
520 nm
,
642 nm
,
932 nm
and
1067 nm
wavelengths of incident light, external
quantum efficiency was mapped in defect regions and laser fired spots. Reduction of charge
collection in the laser-fired spots was limited to diameters of
20 −50 µm
, depending on whether
optical losses or electrical recombination dominated. Effective minority carrier diffusion length
under the majority carrier contacts was obtained from LBIC measurements across contact
fingers. It was observed to have improved from
20.5µm
without a-Si:H(i) to
30.5−44.4µm
in
the cell with the highest JSC and up to 89.0µmin the best case.
Chapter 7
details a novel technique developed in this thesis to spatially map the impact of
the developments of chapters 4 and 5 with micrometer resolution using LBIC. The implemented
circuit model is simple and can be adapted to other solar cell architectures and materials. Each
pixel is treated as an independent one-diode model, surrounded by other one-diode models
representing the remaining (dark) cell. With this circuit model, full cell maps of the following
parameters were obtained: dark saturation current density, ideality factor, effective series
resistance, shunt resistance, V
OC
,J
SC
, maximum power points and efficiency. Recombination
losses due to grain boundaries, shunts and other defects were quantified. It was also concluded
that the laser firing described in chapter 4 does not lower the over-all V
OC
of a cell because it
is primarily limited by grain boundaries present in the cell.
Chapter 8
addresses the third and final research question using high bandgap (
>1.6 eV
)
perovskite top cells. A potential efficiency of
24 %
was estimated for a 4-terminal configuration
with a suitable optical filter and the state-of-the-art cells of chapter 5. A 2-terminal configuration
was created by successfully inserting a refractory metal (molybdenum) between glass and the
SiO
x
/SiN
x
/SiO
x
N
y
passivation layers before laser-crystallization of silicon. A Mo-Si back contact
was created by laser firing the metal through the transparent glass in localized spots while
preserving surface passivation in other areas. A 2-terminal tandem solar cell was demonstrated
with a V
OC
of
1.6 V
and FF of
80 %
but an efficiency of
8.4 %
due to limited reflection from
molybdenum.
Chapter 9
summarizes all identified limitations of the presented solar cell architectures
and future steps to resolve them.
iv
Zusammenfassung
Solarzellen aus dünnen Schichten polykristallinen Siliziums (
14 −15 µm
) bilden die zentrale
Technologie dieser Doktorarbeit. Hergestellt werden sie durch die Abscheidung von amorphen
oder nanokristallinen Siliziumschichten auf Glassubstraten bis zur gewünschten Dicke (
5−40 µm
)
und der darauf folgenden Kristallisation mittels einer linienförmigen Laserquelle. Da hierbei
die Siliziumschicht geschmolzen wird, nennt man diese Technologie flüssigphasenkristallisiertes
Silizium oder LPC-Silizium. Ein solcher Bottom-Up-Ansatz verursacht nicht die gleichen
Verluste wie die derzeit verwendeten industriellen Top-Down-Ansätze, bei denen mit einem
Materialverlust von ungefähr
40 %
hochwertiges Silizium aus Blöcken in dünne Wafer geschnitten
wird. Diese Doktorarbeit zielt darauf ab, die folgenden drei Forschungsfragen bezüglich der
Effizienz solcher Solarzellen bei der Energieumwandlung zu beantworten:
1.
Kann die Sammlung der Ladungsträger im Elektronenkontaktbereich verbessert werden,
während gleichzeitig ein geringer Kontaktwiderstand erreicht wird?
2.
Kann die Ausgangsspannung einer solchen Solarzelle durch eine bessere Passivierung der
Kontaktseite weiter optimiert werden?
3.
Kann eine solche Solarzelle zusammen mit einer oberen Zelle mit größerer Bandlücke mono-
lithisch in eine Tandemsolarzelle integriert werden, um einen höheren Gesamtwirkungsgrad
zu erzielen?
Als zentrale Instrumente zur Beantwortung dieser Forschungsfragen erwiesen sich Laser,
also stark gebündelte Energiequellen. Die verwendete Solarzellenarchitektur besteht aus
Heteroübergängen auf der Basis von amorphem Silizium und interdigitierten Kontakten beider
Polaritäten auf einer Seite (HJ-IBC).
Kapitel 4
befasst sich mit der ersten Forschungsfrage, und zwar durch die Implementierung
von a-Si:H(i) am Elektronenkontakt und durch eine Laserfeuerungstechnik zur Verringerung
des Kontaktwiderstands an lokal begrenzten Stellen unter Beibehaltung der Passivierung von
a-Si:H(i) in nicht befeuerten Regionen. Nach der Laserfeuerung wurde die Leerlaufspannung
(V
OC
) erhalten, während ein absoluter Anstieg des Füllfaktors (FF) um bis zu
14 %
erzielt
wurde. Dabei betrug der Verlust der Kurzschlussstromdichte (JSC) zwischen 0,2 mA/cm2und
Loading more pages...