The PassivatedEmitterandRearCell (PERC) :fromconceptiontomass Production
The PassivatedEmitterandRearCell (PERC) :fromconceptiontomass Production
The PassivatedEmitterandRearCell (PERC) :fromconceptiontomass Production
The Passivated Emitter and Rear Cell (PERC): From conception to mass
production
Martin A. Green n
Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW), Sydney
2052, Australia
art ic l e i nf o
a b s t r a c t
Article history:
Received 31 March 2015
Received in revised form
28 June 2015
Accepted 29 June 2015
Improved solar cell efciency is the key to ongoing photovoltaic cost reduction, particularly as economies
of scale propel module-manufacturing costs towards largely immutable basic material costs and as
installation costs become an increasingly large contributor to total system costs. To enable manufacturers
to move past the 20% cell energy conversion efciency gure in production, high-efciency PERC (Passivated Emitter and Rear Cell) sequences are being increasingly brought online. Most new photovoltaic
manufacturing capacity added in the second half of 2014 was PERC-based, making PERC now the cell
technology with second-highest production capacity, with the latest industry roadmap anticipating PERC
will become the dominant commercial cell technology by 2020. The rst paper describing the PERC cell
appeared in 1989, although the structure was conceived several years earlier. The attractive technical
features were the reduction of rear surface recombination by a combination of dielectric surface passivation and reduced metal/semiconductor contact area while simultaneously increasing rear surface
reection by use of a dielectrically displaced rear metal reector. The key issues in the development of
this technology and its commercial implementation are described, including a review of recent adoption
rates and the way these are likely to evolve in the future.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Silicon solar cells
PERC
High efciency solar cells
1. Background technology
The rst paper describing the PERC cell appeared in 1989 [1],
although this device was rst described in 1983 in a UNSW
(University of New South Wales) nal grant report [2] and as a
deliverable in a subsequent grant proposal [3], accompanied in
both cases by the drawing shown in Fig. 1. The attractive feature
was the elegant way in which the PERC cell incorporated three
attributes into the rear contacting scheme that earlier work at
UNSW and elsewhere had shown were important to obtaining
high efciency. These were the reduction of rear surface recombination by a combination of dielectric surface passivation and
reduced metal/semiconductor contact area, with simultaneously
increased rear surface reection by use of a dielectrically displaced
rear metal reector.
Around the time the PERC cell was proposed, the highest
conrmed efciency for a Si cell was 19.1% [4], estimated as 18.4%
efcient by present standards [5]. The cell structure was a relatively simple UNSW planar PESC cell (Passivated Emitter Solar
Cell) of Fig. 2 with the main feature responsible for its high
n
http://dx.doi.org/10.1016/j.solmat.2015.06.055
0927-0248/& 2015 Elsevier B.V. All rights reserved.
qVoc
Jo = Jsc /[exp
1]
kT
(1)
where T is the absolute temperature and kT/q is the thermal voltage (25.693 mV at 298.15 K or 25 C). This cell had a creditable Jo
of 270 fA/cm2 at 25 C, almost evenly divided between contributions from the combination of top surface and contact recombination and from bulk and rear contact recombination [7].
Apparently the rst published suggestion of reduced contact
area as a way of reducing contact recombination and its contribution to Jo was made at UNSW almost a decade earlier [8]. In
the Crowell-Sze thermionic-emission/diffusion theory of metal/
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
191
Fig. 1. First diagram of PERC cell [2,3] (PERT conguration: Passivated Emitter, Rear
Totally-diffused). More information on different PERC congurations is given subsequently in Fig. 8, with redened acronyms to better reect present usage.
Fig. 2. The PESC solar cell, the most efcient Si cell at the time of PERC cell proposal
[4].
semiconductor contacts [9], currents associated with these contacts are described in terms of effective recombination velocities
for both electrons and holes [10]. It was suggested reducing contact area as in Fig. 3 could reduce such effective velocities [8],
improving the open-circuit voltage of Schottky diode solar cells.
The specic approach shown in Fig. 3 to reduce contact area was
employed in the rst experimental PESC cells, prior to delivery of
the photolithography masks used to fabricate the 19.1% cell of
Fig. 2, with 687 mV Voc conrmed for a device with this structure
by NASA-Lewis in Sept. 1983 [2], the highest ever independently
conrmed value at this stage.
Almost contemporaneously with the UNSW paper [8], Lindmayer and Allison of COMSAT Laboratories, inventors of the violet
cell that led to substantial efciency improvements in the early
1970s, also suggested use of reduced contact area in a subsequently published patent application [11]. The shallower emitters
(top diffused layers) in these devices had directed attention to
emitter surface recombination, negligible in earlier generations of
cells because of their deep emitter diffusions. Independently, these
192
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
almost ideal 98%. Knowledge of these large optical benets provided additional incentive for implementing the PERC structure.
Successful implementation of the PERC cell was not immediate.
Introducing top-surface texturing of the PESC cell of Fig. 2, also
suggested in the previous grant documents [2,3], gave more rapid
progress with 20% cell efciency demonstrated in 1985 [5].
Attention was then diverted to applying these developments to
silicon concentrator cells to meet contractual requirements, with
this work resulting in the rst 20% efcient photovoltaic module
[18]. For the PERC cell, boron diffusion capability needed to be
established and perfected, with this proving more challenging
than for the phosphorus diffusions already established. Some tips
from the solar cell group at Stanford University regarding the
advantages of chlorine-based furnace processing proved most
helpful here, allowing the whole family of PERC cells (Fig. 8) to be
experimentally investigated. The rst high efciency PERC cells
were fabricated in 1988, with 21.8% efciency conrmed at Sandia
in October 1988 (20.9% by present standards).
These initial results fuelled the ongoing improvements in silicon cell efciency on p-type monocrystalline substrates to 25%
(Fig. 9). Applying the approach to multicrystalline substrates led to
the rst multicrystalline cell of efciency above 20%, by present
standards, in 1998 [5]. Initial UNSW application to n-type substrates using reversed doping polarities gave lower efciency than
on p-type substrates, with an efciency of 21.9% (22.1% by present
standards [5]) demonstrated in 1991, with the lower efciency
arising from the increased challenges involved in performing
large-area B diffusions. Much later in 2005, an inverted rear
emitter structure increased this efciency to 22.7% (22.9% by
present standards), equalling the efciency of the best ever n-type
solar cell at that time. A signicant subsequent independent
development was the recognition of the excellent surface passivation properties of Al2O3. This material had long been used as the
low-index layer in double-layer AR coatings for space cells. An
early report [19] of its excellent passivation properties for p-type
surfaces went largely unnoticed until a new efciency mark of
23.2% (23.4% by present standards) on n-type substrates [20] was
established in 2006. This dielectric has proved important for
subsequent PERC commercialisation.
Fig. 8. The Passivated Emitter and Rear Cell (PERC) family. (a) Simple PERD cell (Passivated Emitter, Rear Directly-contacted); (b) PERL cell (Passivated Emitter, Rear Locallydoped); (c) PERT cell (Passivated Emitter, Rear Totally-diffused); (d) PERF cell (Passivated Emitter, Rear Floating-junction). The PERC congurations now most widely
implemented are the PERL and PERT.
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
193
4. Commercialisation
Since the mid-1980s, most manufacturers have used a common
manufacturing process for fabricating silicon solar cells based on
the use of phosphorus diffused, boron doped silicon wafers with
screen-printed silver paste top contacts and rear contacts based on
screen-printed Al pastes [30]. The latter are alloyed to form an AlBSF under the rear contact. Multicrystalline wafers and plasma
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
100%
New Si Capacity, %
Other n- type
HJT/Rear J
80%
PERC
60%
Al-BSF
40%
Source:
Solarbuzz
20%
0%
2013 2014 2015 2016 2017 2018
100%
Total Si Capacity, %
194
80%
60%
Si-Tandem
Rear J
40%
HJT
20%
PERC
0%
Al-BSF
share of total new capacity additions in 2013 and accounted for the
majority of new additions by the second half of 2014 (Fig. 13). This
displacement of the standard Al-BSF approach is expected to
continue, with no new Al-BSF capacity expected to be added after
2017.
This view is reiterated in the independent April 2015 photovoltaic industry roadmap (ITRPV) [39]. As indicated in Fig. 14, the
rapid growth of PERC production capacity in 2014 made PERC,
after the standard Al-BSF technology, the cell technology with the
second highest established production capacity by year-end
(ahead of rear junction and HIT/HJT a-Si heterojunction silicon
cells, as well as CdTe, CIGS and a-Si thin-lm cells). Over the next
few years, the dominance of new production capacity by PERC
(Fig. 13) will steadily increase its share of total capacity with the
most recent ITRPV roadmap [39] showing PERC likely to become
the dominant cell production technology by 2020 (Fig. 14).
This trend is being accelerated by the consolidation policies of
the Chinese government whereby manufacturers without access
to good technology are being closed down by imposing cell efciency standards [40].
1.5
1
Q3
Q4
Q2
Q4
2014 Q1
Q3
Q2
2013 Q1
Q3
Q4
Q2
0.5
2012 Q1
PERC capacity, GW
2.5
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
195
Fig. 15. Simplied process ow for Al-BSF (upper) and PERC (lower) sequences.
6. Performance potential
In mid-2015, the best performing near-commercial PERC multicrystalline cells have efciencies in the 2021% range, while the
best PERC monocrystalline cells have slightly higher values in the
2122% range. There were condent expectations for both values
soon to surpass the top of these respective ranges, but how far can
the technology go?
Several recent studies throw some light on this issue [5255].
The most recent [55] shares the author's view that, now the
industry has invested in a transition to PERC technology, this
provides a path for manufacturers for incremental improvements
to values of cell efciency to close to the 25% value demonstrated
in the laboratory. Commercial PERC cells still perform well short of
the best laboratory devices in many ways, particularly in terms of
Voc as determined by total cell Jo (Eq. (1)) and resistive losses [56].
The 25% efcient PERL devices have a total Jo of only 50 fA/cm2
while a typical 2021% efcient commercial PERC cell may have a
value over 300 fA/cm2 with over half of this coming from the
emitter region [53,55,56].
This makes attention to emitter design important for realizing
the highest efciencies in the near term. The 25% devices have a Jo
contribution from this region of only 15 fA/cm2 [56], showing
what is ultimately feasible. Values below 80 fA/cm2 are suggested
as feasible for commercial devices in the near term using a
selective emitter approach [53], as in the PERC cells of Fig. 8,
although advanced homogeneous emitters are also regarded as an
option [55]. Reducing emitter contributions to Jo will then bring
the rear contact of the PERC cell into focus as the performancelimiting feature.
Attention to the surface recombination velocity along the noncontacted regions of the rear surface [52] as well as to the formation of the doped region in the contact areas [53,55] is expected
to bring the rear surface contribution to Jo below that of the
emitter, then elevating recombination in the bulk of the device to
being the major contributor to Jo, determined by the combination
of minority carrier lifetime and doping level in these regions.
B-doped, p-type wafers presently dominate commercial production. A key advantage is the near-unity liquid to solid segregation
coefcient of B that means its concentration remains reasonably
constant along the manufactured ingots, whether mono- or multicrystalline. A disadvantage is that B forms a complex with O that is
activated under illumination, restricting the bulk minority carrier
lifetime in elded devices [5759].
For this reason, there have been suggestions that the industry
needs to move as a whole to P-doped, n-type monocrystalline
wafers, which is also a feasible substrate for high-performance
PERC cells. Since P does not form such detrimental defects with O,
this allows higher lifetimes in standard Czochralski (CZ) grown
196
M.A. Green / Solar Energy Materials & Solar Cells 143 (2015) 190197
7. Conclusion
As manufacturers move past the 20% efciency mark in production, the standard Al-BSF approach that has been the dominant
commercial approach for the last 30 years is in the process of
being surpassed by a fundamentally higher efciency approach.
Although high silicon cell efciency has been obtained in production with both rear junction and heterojunction approaches,
these require specialized high-quality, n-type monocrystalline
wafers. The PERC cell approach has the advantage of being able to
tolerate both monocrystalline and multicrystalline substrates of
either polarity, while demonstrating similar efciencies to the
above approaches on good quality substrates.
This robustness and compatibility with existing product production lines is considered likely to see the approach surpass the
standard approach by 2020 in terms of capacity share [39]. PERC
uptake is likely to lead to an era of accelerated silicon solar cell
performance increase as the full capabilities of this technology are
exploited. Just as commercial solar cell efciencies have approached the performance of the best laboratory Al-BSF solar cells, it is
anticipated that the PERC technology will also approach that of the
best laboratory cells with efciency of 25% [5] through a process of
Acknowledgement
The Australian Centre for Advanced Photovoltaics is supported
by the Australian Government through the Australian Renewable
Energy Agency (ARENA) (SRI-001). Responsibility for the views,
information or advice expressed herein is not accepted by the
Australian Government. The author thanks the many colleagues
who contributed to PERC cell development within his group,
notably Aihua Wang, Andrew Blakers, Jianhua Zhao and Stuart
Wenham.
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