Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2014.
Supporting Information
for Small, DOI: 10.1002/smll.201402781
Flexible a-Si:H Solar Cells with Spontaneously Formed
Parabolic Nanostructures on a Hexagonal-Pyramid Reflector
Wan Jae Dong, Chul Jong Yoo, Hyoung Won Cho, Kyoung-Bo
Kim, Moojin Kim, and Jong-Lam Lee*
Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013.
Supporting Information
Flexible a-Si:H solar cells with spontaneously formed parabolic nanostructures on
hexagonal-pyramid reflector
Wan Jae Dong, Chul Jong Yoo, Hyung Won Cho, Kyung-Bo Kim, Moojin Kim and Jong-Lam
Lee*
Fabrication of thin film silicon solar cells
Imprinting
+ UV Curing
ITO
De-molding
+ Curing
Back Reflector
Deposition
Incident light
ITO / Ag
ITO
Deposition
a-Si:H
Deposition
Figure S1. Schematics of device fabrication: nano-imprinting the hexagonal-pyramid
structure, followed by Ag and ITO back reflector deposition, and a-Si:H absorber layer and
ITO electrode deposition.
For the practical application of hexagonal pyramid nanostructures, the nano-imprint
lithography is demonstrated with GaN master mold. The hexagonal wurtzite structure of GaN
produces hexagonal pyramid structure when etched by KOH solution. The pyramid arrays of
GaN are utilized as master mold. Figure S1 shows the illustrations of device fabrication
process. The hexagonal pyramid back reflectors are fabricated by nano-imprint lithography
using hybrid polymer, followed by deposition of Ag and ITO as back reflector. The a-Si:H
absorber is prepared by plasma enhanced chemical vapour deposition (PECVD), providing
isotropic deposition and then parabolic nanostructures are spontaneously formed onto back
reflectors. The device fabrication is completed by deposition of transparent ITO electrode.
1
Surface morphology of parabilic nanostructures
0
(a)
(b)
20
nm
700
nm
1μm
1μm
0
Figure S2. Atomic force microscopy (AFM) images of a-Si:H solar cells fabricated on (a) flat
substrate and (b) hexagonal-pyramid nanostructures.
Table S1. Measured roughness of top surface of a-Si:H solar cells.
Roughness
Flat
Nano-patterned
Rms (nm)
2.41
84.8
Ra (nm)
1.83
67.1
Rmax (nm)
46.74
592.9
Figure S2 shows the AFM images of a-Si:H solar cells deposited on different sizes of pyramid
substrate and the roughness is summurized in Table S1. The flat device shows extremely
smooth surface having RMS roughness and average roughness (Ra) of 2.41 nm and 1.83 nm,
respectively. However, the Ra gradually increases to 67.1 nm when devices are fabricated on
hexgonal pyramid.
2
(a)
Sheet Resistance (Ohm/sq)
Mechanical flexibility of ormoclear/Ag/ITO reflector
1200
R = 0.36 cm
900
R = 0.43 cm
PES / ITO (170 nm)
PES / Ormoclear (4 um)
/ Ag (120 nm) / ITO (60 nm)
600
R = 0.47 cm
300
0
R = 0.60 cm
0.30
0.45
0.60
0.75
0.90
(b)
Sheet Resistance (Ohm/sq)
Bending Radius (cm)
1500
PET / ITO
1200
40 um
900
PES / ITO (170 nm)
600
300
PES / Ormoclear (4 um)
/ Ag (120 nm) / ITO (60 nm)
0
0
100
200
300
400
500
Bending cycles
Figure S3. (a) Sheet resistance of PES / ITO (170 nm) and PES / Ormoclear (4 μm) / Ag (120
nm) / ITO (60 nm) for various bending radius. (b) Change in sheet resistance by bending
cycles at radius of 0.41 cm.
For the application of both hybrid polymer and back reflector on the flexible solar cells,
the durability under mechanical bending is necessary. The post-bending changes in the sheet
resistance of ITO (170 nm) layer and Ag(120 nm)/ITO (60 nm) reflector on PES film is
measured as function of the bending radius (Figure S3 (a)). The sheet resistance of the ITO
layer increased greatly to about 850 Ω/sq after bending at the radius of 0.37 cm since the
compressive stress produces crack in ITO layer. On the other hand, the ITO-coated Ag
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reflector on PES/hybrid polymer (ormoclear) substrate shows any degradation of sheet
resistance at extreme bending. In Figure S3 (b), the fatigue strength is testified where the
bending radius was fixed at 0.41 cm. The sheet resistance of ITO thin film linearly increases
to ~500 Ω/sq (170 nm-thick ITO) after 200 cycles of bending due to crack propagation (inset
figure). Despite the brittleness of ITO, the 60 nm-thick ITO on Ag shows high mechanical
flexibility, without change in sheet resistance after repeated bending.
4
Optics simulation of nano-patterned a-Si:H solar cells
Absorption (a.u.)
(a)
1.5
1.2
0.9
0.6
0.3
0.0
400
Flat
Pyramid (reflector)
Parabolic (surface)
Both Patterned
500
600
700
800
Wavelength (nm)
Enhancement ratio
(b)
3.0
2.5
2.0
Pyramid (reflector)
Parabolic (surface)
Both patterned
1.5
1.0
0.5
400
500
600
700
800
Wavelength (nm)
Figure S4. (a) Simulated absorption spectra and (b) enhancement ratio of solar cells having
flat substrate, hexagonal-pyramid reflector, parabolic nanostructure and both patterned solar
cells.
The calculated absorption spectra and enhancement ratio of solar cells having flat, pyramid
reflector, parabolic surface and both patterned (pyramid reflector and parabolic surface) are
shown in Figure S4. It is notable that at long wavelength region (> 600 nm), the solar cell
with parabolic structure absorbs more photons than pyramid reflector which absorption is
attributed to light scattering by parabolic nanostructures on top surface.
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