The External Light Trapping for Perovskite Solar Cells Using
Nanoimprinted Polymer Metamaterial Patterns
Albert Lin 1, Sze Ming Fu 1, Bo-Ruei Chen 1 , Sheng-Lun Yan 1, Yan Kai Zhong 1, Ming-Hsuan Kao 2,
Chang-Hong Shen 3, Jia-Min Shieh 3, Tseung Yuen Tseng 1
1
Department of Electronic Engineering, National Chiao-Tung Univeristy, Hsinchu, Taiwan 30010
2
Department of Photonics, National Chiao-Tung Univeristy, Hsinchu, Taiwan 30010
3
National Nano Device Laboratories (NDL), Hsinchu, Taiwan 30010
Abstract — In this work, we develop the process flow for
nano-imprinted metamaterial patterns for perovskite solar cell
light trapping. Nanoimprint can be scaled for large-area
photovoltaics at low cost, compared to conventional lithography
techniques. While conventional grating is normally underneath
the solar cell active layers, the degraded electrical characteristic
can be a problem. On the other hand, the external light
trapping separates the electrical and optical designs and,
therefore, it is more promising for complex photonic
management without sacrificing the diode characteristics. We
demonstrate the concept by perovskite solar cells with
metamaterial patterns. The efficiency increases from 8.41% to
8.55% using chiral metamaterial patterns. Imprinting a deeper
metamaterial pattern is suggested to enhance the far field effect
for further efficiency improvement.
II. EXPERIMENTAL PROCEDURES
Ag
Spiro-OMeTAD
Perovskite
TiO2 porous layer
TiO2 compact layer
Ag
FTO
Glass
I. INTRODUCTION
The light trapping and photon management have been
under investigation for many years [1-4]. Conventional
practice is patterning the solar cell substrates, and the film is
deposited on the corrugated substrates. The common issue is
that the deposition of semiconductor films on the grating
substrates can lead to significant degradation of the electrical
characteristic of solar cells, including VOC and FF reduction
[5] . To overcome this problem, external light trapping [6-8]
has been proposed recently to facilitate the complex and even
high-aspect-ratio photonic design for advanced light
management in solar cells.
In addition to external light trapping, metamaterial has
been an important candidate for solar cell light trapping. The
photonic density of states (PDOS) can be increased
dramatically, to boost the solar cell absorption. Recently, the
novel photonic concepts, such as chiral and non-reciprocal
designs, have significant uses for strong photon confinement
over a broad bandwidth. In this work, we use nanoimprinted
polymer grating to investigate the effect of external light
trapping in perovskite solar cells. The ultimate goal is to
develop efficient, low-cost, large-area scalable, external light
trap schemes for thin-film perovskite PV.
Glass
Polymer (Mirco Resisit TM)
Fig. 1. The device structure for porous TiO2 based perovskite solar
cells.
Firstly, 90μl compact TiO2 solution (7.5 volume ratio
Titanium isopropoxide (TTIP) mixed in ethanol) is spincoated on ozone cleaned FTO glass substrate under 2000rpm
and 60 seconds. The film is then baked at 180°C for 5
minutes. After the substrate was cooled down, the 100μl
porous TiO2 solution is spin-coated at 2000 rpm and 10
second on the compact TiO2 layer and baked at 180°C for 5
minutes. Thirdly, the sample is sintered at 550°C for 30 min.
Afterward, the sample is transferred to a vacuum chamber.
For the active layer, an 80μl perovskite solution (1.25M
CH3NH3I+1.25M PbI2 mixed in DMSO/GBL (volume
ratio=1:1)) is spin-coated on the porous TiO2 layer at
1000rpm for 10 seconds and 5000rpm for 20 seconds. After
that, the sample is baked at 100°C for 5 minutes. The 45μl
spiro-OMeTAD solution (80mg/ml mixed in chlorobenzene)
is spin-coated on perovskite layer under 2000 rpm and 30
seconds. Finally, 100 nm silver electrode is deposited on the
perovskite cell by the thermal evaporator. Fig. 1 illustrates the
device structure of perovskite solar cells.
The nanoimprint lithography (NIL) experiment can be
divided into different parts. Firstly, the silicon mold is
patterned by e-beam lithography. Then the patterns were
etched by TCP 9600 dry etcher, and the resist layer on mold
is removed by ozone asher and hot sulfuric acid (H2SO4:H2O2
=1:0) for 10 minutes. In addition, the releasing agent
(Trichloro (1H,1H,2H,2H-perfluorooctyl) silane) from
AHEAD Optoelectronics company is deposited on a cleaned
glass substrate in a vacuum chamber. The 0.1 c.c releasing
agent is dipped on the silicon substrate in the petri dish with
a glass cover. The petri dish is heated at 250°C for 40
minutes until the agent gas filled in the dish and deposited on
the silicon substrates. Afterward, a glass substrate is heated at
150°C for 15 minutes to prevent moisture. The MRI-7020
resist from Micro resist technology GmbH is spin-coated on
the glass substrate at 2000rpm and 30 seconds and then
heated at 100°C for 1 minute to accelerate the evaporation of
the solvent. For imprint procedures, the silicon mold is placed
on the glass substrate, and imprint by the NIL Etire6 from
Obducat AB. Fig. 2 illustrates the NIL process.
Imprint Steps
Mold Fabrication
mold
resist
Si
Coating resist
E-beam lithography
Press mold
resist
substrate
Transfer pattern
mold
resist
substrate
mold
Dry etching
Remove mold
resist
substrate
Resist removal
Fig. 2. The fabrication procedure for the mold and the imprinting steps. The resist is MRI-7000r series from MicroResistTM.
III. RESULTS
Fig. 3 shows the SEM graphs for the silicon mold and the
etched pattern. In this study, we use chiral meta-surface and
square grating arranged in the hexagonal lattice for external
light trapping. The chiral metamaterial is non-symmetric and
is beneficial for solar cell light confinement, due to the fact
that a well-designed chiral pattern can lead to strong incoupling but prevent photon escape. On the other hand, the
hexagonal lattice can lead to enhanced photonic density of
states, which in turn boosts solar cell absorption. The physical
reason is that the photonic band-structure is tailored by the
lattice arrangement in real space and, therefore, the slope in
w-k directly determines the PDOS. Using photonic
nanostructures can be effective for the purpose of solar cell
light trapping.
Fig. 4 shows the measured J-V and EQE for the case of
external light trapping using meta-material patterns. The
enhancement of the photocurrent is not very pronounced at
this initial effort. The EQE also reveals the consistent result
where the spectral response is not enhanced significantly by
the addition of external light trap. The solar cell parameters
are summarized in Table I. The physical reason that the
enhancement is not very pronounced can be the insufficient
grating depth. In this study, a 160 nm grating depth is formed
during the NIL process for the meta-material patterns. Such a
shallow grating is more suitable for near-field effect
enhancement. In order to have a more significant far-field
effect, deeper grating depth is necessary. In the next step, we
will increase the depth of the meta-material pattern using a
hard mask and dry etching.
(a)
(b)
(c)
(d)
Fig. 3. (a) The mold for chiral patterns. (b) The mold for hexagonal
patterns. (c) The imprinted chiral meta-surface on the polymer. (d)
The imprinted hexagonal meta-surface on the polymer.
TABLE I
SUMMARY OF SOLAR CELL PERFORMANCE
Open Circuit Voltage, Voc (V)
2
Short Circuit Current, Jsc (mA/cm )
Fill Factor, FF (%)
Efficiency, h (%)
Baseline
Chiral
Hexagoanl
0.88
18.75
50.81
8.41
0.88
18.26
52.96
8.55
0.88
18.20
51.94
8.30
believe the external light trapping using NIL is the most
promising for future photovoltaics, due to the fact that it
separates electrical and optical designs completely. Further
improvement can be made by increasing the imprinted depth
on the polymer grating to strengthen the far-field effect on the
spectral photon management.
chiral
hexagonal
resist
2
Current Density (mA/cm )
25
20
15
REFERENCES
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
100
chiral
hexagonal
resist
EQE (%)
80
60
40
20
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 4. The measured current-voltage characteristics (J-V) and
external quantum efficiency (EQE) for baseline and externally light
trapped perovskite solar cells.
IV. CONCLUSION
We have developed a process flow for the external light
trapping for perovskite solar cells using nanoimprinted
metamaterial pattern. The experimental procedure is
established for nanoimprinted lithography (NIL), perovskite
solar cells, metamaterial design, and measurement. The JSC
enhancement has not very pronounced at the initial trial. We
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