HETEROJUNCTION TECHNOLOGY

HETEROJUNCTION
TECHNOLOGY
THE SOLAR CELL
OF THE FUTURE
G. Roters1, J. Krause1, S. Leu2, A. Richter2, B. Strahm3
Roth & Rau AG, An der Baumschule 6-8, 09337 Hohenstein-Ernstthal, Germany
Meyer Burger Technology Ltd, Schorenstrasse 39, CH-3645 Gwatt (Thun), Switzerland
3
Roth & Rau Research AG, Rouges-Terres 61, CH-2068 Hauterive, Switzerland
1
HETEROJUNCTION TECHNOLOGY
2
ABSTRACT
Wafer-based silicon photovoltaic (PV) production has only changed slightly in the last forty years. The
standard concept comprises p-type silicon wafers, fired contacts and encapsulation. Cost reduction
is necessary if PV is to survive without feed-in tariffs and be competitive with grid electricity costs.
Therefore levelised cost of electricity (LCOE) is one of the primary metrics for the cost of electricity
produced by both utility scale and distributed power systems. The fastest path to lower LCOE is to
introduce high efficiency solar cell concepts like the heterojunction technology (HJT). Photovoltaic
systems using heterojunction technology (HJT) modules outperform any other PV systems and this
paper will explain why.
Introduction
Meyer Burger develops solar technology – from wafers to complete PV systems – with the aim of promoting the widespread use
of photovoltaics and making solar power a first-choice source of
renewable energy. To this end, the company focuses strongly on
developing solar cell technologies which allows highest efficiencies
and highest energy output at lowest production costs.
Heterojunction cell technology combines the advantages of mono
crystalline silicon (c-Si) solar cells with the good absorption and
the superior passivation characteristics of amorphous silicon (a-Si)
known from a-Si thin film technology using readily available materials. The HJT design is not new. Sanyo (now Panasonic) first
pushed this technology into mass production achieving around
20% cell efficiency.
Recently Panasonic demonstrated efficiencies
of 24.7% on laboratory scale cells [1]. With the
discontinuation of the basic technology patent,
heterojunction technology was opened to the
public in 2010. Meyer Burger is now offering this
appealing technology as a high performance key
technology in the photovoltaic value chain.
The simple structure of a HJT cell is shown in
Figure 1. The thin intrinsic a-Si:H layers deposited
between c-Si wafer and doped layers are key to
achieving maximum performance from the cell
structure. They result in reduced interface state
density, decreased surface recombination losses
and lower the emitter saturation currents.
ITO
Texture
Standard process
Sel. Emitter process
MWT process
MB-PERC
HJT process
Texture
Texture
Laser Drilling
Texture
Texture
Doping / Diffusion
Doping / Diffusion
Texture
Doping / Diffusion
a-Si Front / Rear Side
Edge Isolation
Additional
4 Process Steps
Doping / Diffusion
Edge Isolation
TCO /
Metal Rear Contact
PSG Etch
Edge Isolation
PSG Etch
PSG Etch
Print Front Side
AR Coating
PSG Etch
AR Coating
SiNx Capping- & AlOx
passivation-layer
Curing
Print Rear Side
AR Coating
Print Front Grid
and p-Contact
AR Coating
Test & Sort
Print Front Side
Print Front Side
Firing
Laser contact
opening
Firing
Print Rear Side
Edge Isolation (Front)
Print Front- & Rearside
Test & Sort
Firing
Edge Isolation (Rear)
Firing
Test & Sort
Test & Sort
Test & Sort
CZ: 19 – 20%
MC: 18 – 19%
CZ: 20 – 21%
MC: 18 – 19%
CZ: 19%
MC: 16.8 – 17.5%
(p) a-Si:H
(i) a-Si:H
n-type c-Si
ITO
Ag (PVD)
(i) a-Si:H
TCO /
Metal Rear Contact
Print Front Side
Curing
(n) a-Si:H
Test & Sort
Figure 1:
Schematic drawing of the HJT cell and the process steps required
2
CZ n-type:
21 ~ 25%
Figure 2:
Process sequence of differing PV technologies
Thin wafers
The relatively straightforward HJT production process takes place
at low temperatures and requires fewer production steps compared
to other high efficiency designs as shown in Figure 2, which is economically attractive as it results in significant energy cost savings.
Ag (screen print)
a-Si Front / Rear Side
CZ: 20 – 21%
MC: 19 – 20%
Significant
potential for
cost savings!
An important technological advantage of HJT cells is the excellent
surface passivation of a-Si:H which results in high open-circuit voltages and high cell efficiencies. The superior temperature coefficient
of TC = -0.20%/K ensures higher energy yield during module operating conditions. As previously mentioned, low temperature processing (<250°C) saves energy during manufacturing, prevents bulk
degradation and enables the use of thin wafers. Integrated development throughout the PV value chain (diamond wire wafering, heterojunction cell technology and SmartWire Connection Technology
(SWCT) guarantees the maximum performance of HJT modules.
Meyer Burger technology for high performance solutions ensures
the perfect integration and harmonisation of all processes with the
goal of lowest LCOE.
Today, wafers with a thickness of ~180 µm are in
standard use in cell manufacturing. A wafer thickness even below 100 µm is enough for absorbing light in silicon solar cells. Using thinner wafers
leads to significantly lower material costs because
a higher number of wafers can be cut from one
brick. With decreasing wafer thickness, the influence of the bulk material quality also decreases
resulting in less tight requirements for e.g. minority charge carrier lifetimes. Diamond wire sawing
is the best approach for thin wafers. The use of
diamond wire leads to fewer microcracks and the
depth of microcracks is shallower. With decreasing wafer thickness and the corresponding higher
surface to bulk ratio the surface recombination
loss becomes dominant compared to recombination loss in the bulk. Consequently superior
surface passivation techniques are mandatory.
3
HELiAPECVD: a-Si:H coating for front and rear side
Figure 3: Impact of wafer thickness on efficiency of
heterojunction cells including the potential with improved light management
25.0
For heterojunction cell concepts, intrinsic a-Si:H layers are deposited on the front and rear sides of the wafer respectively, providing
excellent surface passivation.
24.0
cell efficiency [%]
In general there is an optimum wafer thickness for
every cell technology. However as shown in Figure 3,
the final cell efficiency for heterojunction cells is
independent of the wafer thickness at least in the
range of wafer thicknesses achievable with sawing
processes.
HELiAPVD: TCO/ Metal coating for rear side contact
23.0
Doped a-Si:H layers are deposited on the intrinsic a-Si:H layers
in order to create an emitter on the front side of the cell (boron
22.0
doped p a-Si:H) and a back surface field (BSF)
on the rear side (phosphorous doped n a-Si:H).
The emitter enables the separation of the charge
carriers while the BSF drives the minority charge
carriers away from the rear side to reduce rear
side recombination losses.
21.0
20.0
80
100
120
140
160
180
200
220
240
Fz1: 12378 µs
Fz2: 12841 µs
ST27-2205: 4472 µs
ST27-2209: 5639 µs
ST27-2222: 6769 µs
ST27-2318: 4837 µs
ST27-2324: 4752 µs
ST27-2325: 4365 µs
260
mean cell thickness [µm]
10-2
Process flow / Texturing
The saw damage removal can be optimised with the
texturing process. A final short dip in hydrofluoric
acid terminates the silicon surface until final passivation in the subsequent PECVD process. The surface
after this wet chemical treatment is crucial for the
quality of c-Si / a-Si:H interfaces and therefore for
the surface passivation.
PECVD:
a-Si:H coating for front and rear side
In order to minimise energy loss within the solar cell,
the surface must be highly passivated. Low temperature passivation with a-Si:H deposited in a temperature range between 150-250°C results in outstanding
surface recombination velocities. This a-Si:H is able
to passivate all levels of n-type and p-type silicon.
Doped a-Si:H is used to form both the emitter and
the back surface field (BSF). Additionally, doped a4
Measured lifetime [s]
16
14
12
SRV Seff [cm/s]
An optimal texturing and cleaning process forms the
basis for a successful production process for highly
efficient HJT cells. Meyer Burger process knowhow sets the foundation for optimum passivation.
The initial steps in the HJT process sequence are
wet chemistry processes for saw damage removal
(SDR), texturing, cleaning and hydrogen termination.
The subsurface damages and microcracks have to
be almost completely removed to achieve high efficiency HJT cells. Measuring the surface recombination velocity (SRV) indicates the necessary saw damage removal independent of the quality of the bulk.
Figure 4 shows the impact of saw damage removal
on the surface recombination velocity.
10
8
6
4
2
0
4
6
8
10
12
14
SDR [µm/side]
Figure 4: Impact of saw damage removal on SRV
Si:H contributes to passivation due to its field effect properties.
Roth & Rau, a member of the Meyer Burger Group, has developed the modular high throughput HELiAPECVD system specifically for wafer based silicon HJT cell concepts [2]. The heart of
the HELiAPECVD system is the patented S-Cube™, a sophisticated parallel plate plasma reactor with a box-in-box arrangement
providing ultra-pure and uniform amorphous silicon layers. A
13.56 MHz RF source is used to minimise plasma damage
during layer deposition. Thus the required quality of the amorphous silicon layers in terms of minority charge carrier lifetime
and band gap are ensured. The key parameter to determine
the quality of the passivation with the a-Si:H layer is the minority charge carrier lifetime. The HELiAPECVD system has achieved
values of more than 10 ms effective lifetime on FZ-wafers and
more than 4 ms on CZ production wafers as shown in Figure
5. These sets of data both for the polished FZ-wafers and for
the textured CZ-wafers demonstrate obviously the passivation
capability of the HELiAPECVD system.
10-3
1014
1015
Minority Carrier Density [cm-3]
1016
1017
Figure 5:
Sinton lifetime data of an i-passivated polished FZ and CZ wafers (Numbers for internal reference of experiment)
Stripe
4
Stripe
3
PM
n
PM
i
HTO
Stripe
2
Stripe
1
PM
p
PM
i
WF
In the single stripe:
1. Automatic carrier movement
2. Process sequence automatic
HTI
Automatic wafer transfer
Automatic wafer transfer
Automatic wafer transfer
between Stripes
Figure 6: Wafer flow within the HELiAPECVD system
5
Lift
WL
Double LM
BMi
TCO
BMo
TM
BMi
Metal
BMo
UM
WU
Lift
D - Unload
C - Process
Figure 7: HELiAPVD system configured for HJT cell processing
B - Process
To achieve a high quality surface passivation of a HJT cell, it is essential to avoid any cross doping. The ­HELiAPECVD system therefore
performs these process steps in dedicated process strips to use
an optimised process setting for each layer and prevent from any
surface contamination.” Figure 6 shows a schematic of a HELiAPECVD
system including the wafer flow.
The HELiAPECVD system in Figure 6 is designed for a throughput up
to 2,400 wafers per hour. As the a-Si:H layers are in the range of
only a few nanometers, the deposition process is very short and
the gas consumption is low.
PVD: TCO / Metal coating for rear side contact
In the modular HELiAPVD system, a sputter process is used in one
of the chambers to apply a transparent conductive oxide (TCO)
layer to the front and rear side of the wafer [2]. In addition to collecting the photo-generated current and forming an ohmic contact
on the cell, the TCO layer on the front side acts as an anti-reflection
layer. Especially for the front side of the HJT cell, the optical and
electronic properties of a-Si:H and TCO layers need to be adjusted
with respect to each other. Indium tin oxide (ITO) is a favorable TCO
material for HJT cells because it is very transparent and conductive
while providing a good electrical contact with the
doped a-Si:H layers. In a second chamber of the
same HELiAPVD system, a metal layer such as silver, or a stack of different metals such as a combination of silver and nickel vanadium is applied
on the TCO layer as rear side metallisation. The
composition of the metal stack is dependent on
the downstream module process. The rear side
metallisation provides an excellent electrical contact to the TCO layer as well as a good internal
reflectance of long wavelength light. This is accomplished without having to break the vacuum
or to turn the wafer between these coating processes. Figure 7 shows a HELiAPVD system configured for HJT cell processing. This configuration
enables to achieve the edge isolation simultaneously avoiding extra laser or chemical steps.
Rotating cylindrical sputter targets for TCO and
metals on magnetrons enable a high target utilisation of over 85% to be achieved, ensuring
a cost effective coating process. The HELiAPVD
and HELiAPECVD systems are perfectly matched in
terms of capacity and layer properties.
Print front side
Meyer Burger offers the busbarless SmartWire Connection
Technology (SWCT) to minimise cell-to-module (CTM) loss and
to optimise the module efficiency. Only the finger grid pattern
on to the front surfaces of HJT cells will be formed by using a
conventional screen printer and an epoxy-based low temperature paste. The busbarless design facilitates a fine-lined screen
printed grid pattern. By replacing the busbars with a lined grid
pattern with thinner and smaller fingers, silver consumption is
significantly reduced. It also increases the cell conversion efficiency by decreasing shadow loss on the cells. Figure 8 shows
the performance of ­HJT SWCT modules. The module power is
increased while silver consumption is reduced.
With the application of SWCT, the requirements for finger conductivity become less stringent compared to the requirements for conventional busbar designs. Finger resistance of up to 100 Ω/cm can
6
102.0
101.5
100.5
100.0
99.5
99.0
98.5
98.0
Screen A
Finger width: 90-100 um
110 mg of Ag paste
Screen B
Finger width: 50-60 um
40 mg of Ag paste
Figure 8: Power gain combined with reduced
silver consumption of Heterojunction SmartWire
Connection Technology modules
be combined with SWCT without causing significant power losses. The shorter the distance between two finger contacts, the lower the impact
of power losses in fingers.
CALiPSO: flexible heat treatment furnace
Curing
After printing the front side, contact firing is not required with this
HJT cell concept. The curing of printed HJT cells is a simple thermal process at temperatures <250°C in order to outgas the solvents of the low temperature paste. The temperature profile affects
the conductivity of the screen printed lines and of the TCO lay-
ers on the cells as well as the solderability of the
cells. The Roth & Rau curing system is tailored to
the HJT process in terms of process and productivity requirements [2].
Testing
HJT cells are high capacitance cells which require a measurement
time of 400-600 ms. This is significantly longer compared to standard low capacitance cells. Pasan SA [3], a member of the Meyer
Burger Group, in cooperation with the Institute of Micro Technology
(IMT) at the University of Neuchâtel in Switzerland [4], has developed a new I-V curve cell tester series known as SpotLIGHT, which
is available in two formats: SpotLIGHT 1 sec and SpotLIGHT HighCap.
With its high quality A+A+A+ 5 ms length pulsed Xe light source,
SpotLIGHT 1 sec is dedicated to the high-speed measurements that
are required for in-line applications, such as end-of-line quality control in solar cell production lines or beginning-of-line quality control
in solar module production lines.
101.0
Power [%rel]
Conventional crystalline silicon solar cell technologies use front
side collector lines (“fingers”) and busbars. Reducing the consumption of the silver paste used in the fingers and busbars is
key to reducing costs.
A - Load
SpotLIGHT HighCap is dedicated to testing solar cells with high capacitances such as heterojunction cells. The SpotLIGHT HighCap
combines light-emitting diodes (LEDs) to increase the pulse length
from 5ms to 600ms (Figure 9). This hybrid light source measurement process had been validated by Fraunhofer ISE. The result is a
system which provides dependable solar cell measurements while
maintaining the total COO of the system at the same level achieved
by the SpotLIGHT 1 sec. This is a unique characteristic among longpulse solar simulators.
Measurement of high efficiency modules has different constraints:
illuminating a large area during several hundreds of millisecond
would not be cost effective, neither accurate. On the other hand,
the capacity of a module is much lower than that of a cell. Therefore, Pasan has developed the DragonBack® solution which is
a dynamic sweep methodology that can be used with standard
HighLIGHT module tester with a 10 ms pulse length of A+A+A+ quality. Such industry-oriented and cost-effective solution enables to
measure the power of high capacitive modules,
taking into account production tact-time, low
TCO and high measurement accuracy. This innovative solution has been successfully validated
by PI Berlin.
High quality Xenon
flash for calibration
of LEDs
LED flash
up to 600ms
Figure 9:
Measuring methodology of the new
Pasan spotLIGHT HighCap [3]
7
VOC : 44.38 V
ISC :
9.01 A
FF :76.8%
PMPP : 307.1 W
HETEROJUNCTION TECHNOLOGY
HJT – A breakthrough in levelized cost of electricity
LCOE is one of the primary metrics used to measure
the cost of electricity produced by both utility-scale and
distributed power systems. A simulation for 100 kW PV
power systems using silicon wafer and thin film PV modules was done for two different climatic conditions based
on Germany which represents a cold climate and on India
which represents a humid, subtropical climate. The calculation used PV SOL 6.0 Expert and the following parameters: Inverter Fronius Agilo 100.0-3, Modules Poly: 245
W, HJT 290 W, CdTe 90 W, CIGS 95 W and the following
financial assumptions: 80% bank loan, 20% self invest,
capital interest: 2%, loan interest 4%, 0,5% degradation/
year, 0,7% running cost/yr., base system cost 2015 with
20% margin.
Milestone: 307 watt module
At 307 watts, Meyer Burger set a new photovoltaic record
using a standard 60 cell solar module shown in Figure 10
[5]. A high level of process integration between the wafer,
cell and module technologies made this developmental
leap possible.
The Meyer Burger HJT modules deliver an increased energy yield by combining HJT cells with a conversion efficiency of 21% and a very low temperature coefficient of
just TC = -0.20%/K, together with the revolutionary SmartWire Connection Technology. Compared to standard cells
which have a value of TC = -0.43%/K, a solar module using HJT cells from Meyer Burger can achieve >10% more
energy yield on average. This results is a significant competitive advantage for cell and module manufacturers, as
well as for end customers. SGS Fresenius Institute has
already IEC certified HJT modules with SmartWire Connection Technology.
As a result of this simulation LCOE of 9.1 $cent/kWh for
Germany and 5.2 $cent/kWh for India are calculated.
These attractive numbers underline the superiority of the
Meyer Burger heterojunction technology compared to existing PV technologies currently available on the market.
Figure 10: 307 watt record HJT module from Meyer Burger
Site: India (Delhi)
0.140
0.140
0.120
0.120
0.100
0.100
LCOE [$/kWh]
LCOE [$/kWh]
Site: Germany
Record Module!
60 cells, 156 mm x 156 mm
307 watt
8
Figure 11 shows a comparison of both regions. For both
regions, the LCOE achieved using the HJT PV system
clearly outperforms the other PV systems. The HJT PV
system benefits from its excellent temperature coefficient,
higher conversion efficiency, higher energy yield and lower
balance of system costs.
0.080
0.060
0.040
0.020
0.080
0.060
0.040
0.020
0.000
0.000
Standard Poly
HJT
(Meyer Burger)
Standard Poly
CdTe
CIGS
HJT
(Meyer Burger)
CdTe
CIGS
Figure 11: Levelized cost of electricity for a 100 kW PV power system
9
ABOUT US
AUTHORS
Meyer Burger
Meyer Burger Technology Ltd. is the technology leader at every
stage of the photovoltaic (PV) value chain – from wafers to buildingintegrated PV systems. Processes and systems from the Meyer
Burger group play a vital role in increasing overall performance and
efficiency throughout the value chain.
Benjamin Strahm graduated in Material Sciences from Swiss Federal Institute of Technology (EPFL) in 2004 and
All core expertise under one roof
Comprehensive, integrated technology portfolio
Sylvère Leu was born and educated in Switzerland, and graduated from ETH Polytechnic with a degree in Electronic
achieved his PhD in industrial plasma physics in 2007 in the field of silicon deposition in large area reactors. In 2008,
he joined Roth & Rau Research (formerly Roth & Rau Switzerland) working in the field of plasma reactor design and
crystalline silicon surface passivation by amorphous silicon for heterojunction solar cells. Since 2012, he is heading R&D
activities in Roth & Rau Research.
Engineering and in Business administration at the University of St.Gallen (HSG). Sylvère Leu began working in Photovoltaics 25 years ago. At the end of 2005 he was charged with building up an integrated 250 MWp facility. In 2008 Sylvère
Leu joined 3S Industries AG as COO. The company merged in 2010 into Meyer Burger Technology, where he is working
as CIO/CTO of the group.
Meyer Burger covers all important technology stages in the photovoltaic value chain. The interdisciplinary research and development
work from wafers to the installed modules results in new standards
in production procedures and processes and guarantees efficiency
and performance throughout the entire system. We rapidly incorporate research results into commercial, integrated solutions.
USA
Hillsboro (OR)
Colorado Springs (CO)
Columbia (NJ)
Europe
Moscow (RU)
Dresden (DE)
Hohenstein-Ernstthal (DE)
Zülpich (DE)
Eindhoven (NL)
Reichelsheim (DE)
Umkirch (DE)
Thun (CH)
Neuchâtel (CH)
Jens Krause studied semiconductor physics & technology at the University of Technology in Chemnitz, Germany.
After spending three year at the Center for Microtechnologies of TU Chemnitz, he worked for more than ten years in the
semiconductor industry. In 2009 he joined Roth & Rau, a member of the Meyer Burger Group, working now as head of
strategic product management.
Asia
Georg Roters studied physics at the Westfälische Wilhelms – Universität Münster, Germany. He received his PhD
in physics from the Ruhr – Universität Bochum, Germany and an Executive Master from the Boston Business School,
Zürich. Georg Roters worked as Process Engineer, Project Manager and Product Marketing Manager in the semiconductor industry for 12 years. He joint Roth & Rau AG, a member of the Meyer Burger Group, in 2009 were he was
building up and heading the Product Management department until 2012. Currently he is heading the Sales department
within the Roth & Rau B.V.
Tokyo (JP)
Seoul (KR)
Zhubei City (TW)
Shanghai (CN)
Pune (IN)
Singapore (SG)
André Richter has a degree in electronic engineering (communication engineering, process measuring and control
technology and environmental measurement) and he managed his own company specialising in electronic education
systems for eleven years. Since 2001 his focus has been on photovoltaics. At Conergy AG he was involved in development and third level support of solar plants where he held the position of Technical Director in the Conergy solar plant
located in Frankfurt (Oder). He then assumed the role of CEO for Conergy Electronics GmbH. In 2008 Andre Richter
joined the Geneva based company, SES, as a consultant for the planning and construction of module lines in the United
States. Since 2010 he has held the position of Senior Technical Developer at Meyer Burger Technology Ltd where he
focusses on the establishment and realisation of strategic photovoltaic projects.
References
With around 1800 employees worldwide, Meyer
Burger maintains close proximity to its customers.
10
[1] Panasonic (http://panasonic.co.jp/corp/news)
[2] Roth & Rau AG (http://www.roth-rau.com)
[3] Pasan SA (http://www.pasan.ch)
[4] IMT, Photovoltaics and Thin Film Electronics Laboratory
(PV-Lab), EPFL, University of Neuchâtel, Switzerland
(http://pvlab.epfl.ch)
[5] Meyer Burger AG (http://www.meyerburger.com)
11
Meyer Burger Technology Ltd
Schorenstrasse 39 / 3645 Gwatt (Thun) / Switzerland
Phone +41 33 221 28 00 / Fax +41 33 221 28 08
[email protected] / www.meyerburger.com
HJT-EN-01
PASSIONATE ABOUT PV