Passivated Emitter Rear Locally Diffused Solar Cells

Passivated Emitter Rear Locally Diffused Solar Cells
Passivated Emitter Rear Locally Diffused
Solar Cells
李 玉 飞
ABSTRACT High efficiency passivated emitter, rear locally-diffused (PERL) solar cells and Silicon solar cells with
passivated emitter and rear contacts (PERC) are both developed by ARC (Australian Research Council) Photovoltaics
Centre of Excellence, University of New South Wales. The former technology has a significant enhancement in the
energy conversion effi ciency of mono-crystalline silicon solar cells up to 25% under the standard global solar spectrum
which is the world record in photovoltaic area. (Used to be 24.7%, however there is a 0.3% improvement due to the new
standard calculation of Air Mass 1.5) Under monochromatic light which wavelength is 1.04 , energy conversion effi ciency
is 46.3% [1] . The passivated emitter cell series (PESC, PERC and PERL), which are invented or incorporated by UNSW,
have made a key contribution to increase the energy conversion effi ciency in photovoltaic area.
1 PERL Cell Structures used in
this Research
The PERL solar cell got a conversion efficiency of 23.1%
in 1990 [2, 3] . And it had been redesigned in 1993, the
structure of cell shown in Fig. 1. [4]
Fig.1. Schematic of a PERL (or LBSF) cell on a p-type Fz
Si wafer.
The key features of PERL cells are: The top surface of the
solar cell is textured using inverted-pyramid structures
and covered by double-layer anti-reflection coating (ARC)
which brings extremely low top surface reflection. The
front metal finger grids are defined by photolithography
technology to be very thin therefore minimising metal
shading loss. Both inverted-pyramid texturing and fine
metal fingers decrease the optical losses which contribute
to higher current for the solar cell. A selective emitter
(heavily phosphorus diffused regions underneath the
metal contacts) whilst the rest of the top surface is lightly
diffused to keep excellent “blue response” (absorbing the
short-wavelength photons), it can minimise both contact
resistance and contact area recombination.
Thermal oxide passivation of the silicon/ silicon dioxide
interface which can reduce surface recombination.
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This thermally grown oxide process was taken in
trichloroethane (TCA) ambient. Reduction of the emitter
saturation current density and improvement of the cell
open-circuit voltage to above 700-mV are two major
advantages of this technology. This method was not only
used to improve the quality of this SiO 2 layer but also
used to maintain the high carrier lifetime through the cell
processing [5] . In order to further improve the quality of the
surface passivation therefore reducing the thickness of the
thermally grown oxide to below 300Å, this SiO 2 layer was
treated with an “alneal” process, “which is performed by
coating a layer of aluminum on top of SiO 2 , sintering in
forming gas at about 370℃ for 30 min, and then removing
the aluminum layer [6] ”. Last but not least, a point contacted
rear surface (local boron diffusions) with thermal oxide
passivation in the non-contacted rear surface regions which
is covered with Al, further reducing surface recombination
due to the Alneal effect and the band bending due to work
function differences between the Al and the p-Si wafer [7] .
Hence, the combination of several mechanisms gave the
new PERL cells better performance.
Table 1:The performance of PERL cells with different oxide
thicknesses. The oxide has been grown in a TCA ambient and
annealed in forming gas. [1]
Cell ID Oxide thickness (Å) J sc (mA/cm 2 )
V oc
(mV)
W4-19-2E 200 36.5 682
Z4-16-2E 600 37.5 697
W4-6-1H 1100 40.7 703
Reflective losses have been decreased by the implement of
double anti-reflection (DLAR) coating, (a ZnS and MgF 2
DLAR coating is deposited onto the surface processed
with the “alnealed” thin oxide) which gave 3% higher
current density than SiO 2 single layer anti-reflection
(SLAR) coated cells. [1]

Vol. 5 No.8/ Aug. 2011
Table 2:The measured effective minority carrier lifetimes
at different processing stages for an 1.5 cm FZ wafer with
1100 Å TCA grown oxide. “Alneal” and forming gas alneal
significantly improved the carrier lifetimes. [1]
Processing stage
After TCA oxidation
Tau
14 s
Table 4:
Cell Technology Diagram of Rear Surface Design Voc (mV) Reference
PERC
(0.2 .cm, 696 (Blakers et al. 1989)
p-type, FZ)
After sinter in forming gas
Cell ID
After “alneal”
Substrate
Resistivity
( cm)
V oc
(mV)
Jsc
(mA/cm 2 )
40 s
400 s
Table 3: The performance of 4 cm 2 PERL FZ cells tested at
Sandia National Laboratories under the standard global
AM1.5 spectrum (100mW/cm 2 ) at 25℃. [5]
FF
(%)
Effic.
(%)
Wh20-2b 1.0 706 42.2 82.8 24.7
PERC cell structures and key feature:
The passivated emitter and rear cell (PERC) structure
was developed by Blakers et al. (UNSW) in 1988 and it
has only a slightly lower efficiency potential than a more
complex structure with rear local diffusions (PERL) [7] .
Fig 2. The PERC cell structure. The surface texturing is not
shown. [8]
It has a lightly phosphorous doped emitter with heavier
diffusions under the front contacts (selective emitter),
a silicon/silicon dioxide interface passivation and antireflection
coating, and inverted pyramids on the top
surface. Rear contact is connected with the substrate by
contact holes through the rear oxide covering about 1% of
the rear surface, without the use of boron diffusion (p ++ )
by comparing with PERL solar cell model. The design is
simpler than the PERL structure, avoids boron diffusion
which keeps recombination at the rear contacts low.
Owing to minimise contact recombination, the area of the
holes is larger than the cell thickness. “The substrate of
the record efficiency cell was moderately doped (0.2–0.5
cm) in order to reduce series resistance due to the widely
spaced contacts and to allow low resistance contact to be
made with aluminum to the substrate [7] .”
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PERL
(2 .cm, 696 (Wang et al. 1990)
p-type, FZ)
=============================================================
Various low-area localised rear contacting schemes used
in the fabrication of high efficiency Si solar cell (the front
surfaces are omitted from the diagrams) [9] .
In the PERC structure, the rear comprised of SiO 2 -
passivated p-type surface with
photolithographically defined localised metal directly
contacting the substrate. From passivation point of view,
the PERL cell was almost the same with the PERC
passivation except for the reduction of dark saturation
current owing to the metal contact regions where PERL
were passivated with heavy boron diffusion, which built
high-low junction for it [9] . The passivated emitter and
rear contacts solar cell (PERC) structure produced record
efficiency silicon cells 1980s.
2 Conclusion
PERL and PERC solar cell both are first generation
silicon solar cell (using silicon as substrate), which
have dominated photovoltaic industry for decades.
Some industrial technologies are based those laboratory
techniques, such as Pluto technology of Suntech Power,
the world's largest photovoltaic (PV) module manufacturer.
They announced that their Pluto technology which is based
on the PERL technology produce PV cells with conversion
efficiencies of approximately 19% on mono-crystalline PV
cells and 17% on multi-crystalline PV cells. It means that
this technology is used in large scale production now.
References
[1] J. Zhao, A. Wang, Stuart Wenham, Martin Green, “24% Efficient
PERL silicon solar cell: recent improvements in high efficiency
silicon cell research”, Solar Energy Material and Solar Cells,
1996.
[2] A. Wang, J. Zhao and M.A. Green, Appl. Phys. Lett.57 (1990)
602.
[3] M.A. Green and K. Emery, Progr. Photovoltaics. 2 (1994) 27.
[4] Martin A. Green, “Silicon Solar Cells: Advanced Principles
& Practice”, Centre for Photovoltaic Devices and Systems,
University of New South Wales, Sydney, 1995
[5] J. Zhao, A. Wang, M.A. Green, “High-efficiency PERL and
PERT silicon solar cells on FZ and MCZ substrates”, Solar
Energy Material and Solar Cells, 2001.
[6] P. Balk, in: The Si-SiO2 System, Elsevier, Amsterdam, 1988, p.

Passivated Emitter Rear Locally Diffused Solar Cells
234.
[7] A.W. Blakers, A. Wang, A.M. Milne, J. Zhao, M.A. Green, Appl.
Phys. Lett. 55 (13) (1989) 1363.
[8] K.R. Catchpole, A.W. Blakers, “Modelling the PERC structure
for industrial quality silicon”, Solar Energy Material and Solar
Cells, 2002.
[9] Utama R. Y. Ph.D Thesis, “Inkjet Printing for Commercial High-
Efficiency Silicon Solar Cells”, University of New South Wales,
2009.
研 究 助 理 ,2010 年 毕 业 于 澳 大 利 亚 新 南 威 尔
士 大 学 光 伏 与 太 阳 能 专 业 , 获 得 硕 士 学 位 。
主 要 研 究 单 晶 硅 , 多 晶 硅 第 一 代 太 阳 能 电 池
制 造 及 封 装 工 艺 , 设 计 光 伏 并 网 发 电 和 独 立
光 伏 发 电 系 统 , 并 积 累 丰 富 经 验 , 对 第 二 代
电 池 中 的 多 晶 硅 沉 淀 在 玻 璃 上 的 PECVD 工
艺 有 清 晰 认 识 和 理 解 , 了 解 第 三 代 电 池 中 利
用 量 子 点 阵 控 制 hot carrier 和 制 造 tandem
cell 提 高 电 池 效 率 的 概 念 和 工 艺 过 程 。 参 与
“540KwP Grid connected PV system
in Mae Hong Son Thailand” 项 目 ,
“Stand-alone PV system in Australia”
项 目 。
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