A photonic-plasmonic structure for enhancing light absorption in thin film solar cells
Joydeep Bhattacharya, Nayan Chakravarty, Sambit Pattnaik, W. Dennis Slafer, Rana Biswas, and Vikram L.
Dalal
Citation: Appl. Phys. Lett. 99, 131114 (2011); doi: 10.1063/1.3641469
View online: https://doi.org/10.1063/1.3641469
View Table of Contents: http://aip.scitation.org/toc/apl/99/13
Published by the American Institute of Physics
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APPLIED PHYSICS LETTERS 99, 131114 (2011)
A photonic-plasmonic structure for enhancing light absorption in thin film
solar cells
Joydeep Bhattacharya,1 Nayan Chakravarty,1 Sambit Pattnaik,1 W. Dennis Slafer,2
Rana Biswas,1,3,a) and Vikram L. Dalal1,a)
1
Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State
University, Ames, Iowa 50011, USA
2
Lightwave Power, Cambridge, Massachusetts 02138, USA
3
Department of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA
(Received 11 May 2011; accepted 21 August 2011; published online 30 September 2011)
We describe a photonic-plasmonic nanostructure, for significantly enhancing the absorption of
long-wavelength photons in thin-film silicon solar cells, with the promise of exceeding the classical
4n2 limit for enhancement. We compare identical solar cells deposited on the photonic-plasmonic
structure, randomly textured back reflectors and silver-coated flat reflectors. The state-of-the-art
back reflectors, using annealed Ag or etched ZnO, had high diffuse and total reflectance. For
nano-crystalline Si absorbers with comparable thickness, the highest absorption and photo-current
of 21.5 mA/cm2 was obtained for photonic-plasmonic back-reflectors. The periodic photonic
plasmonic structures scatter and reradiate light more effectively than a randomly roughened
C 2011 American Institute of Physics. [doi:10.1063/1.3641469]
surface. V
Long-wavelength red and near-infrared (IR) photons are
poorly absorbed in thin film silicon solar cells, due to their
long absorption lengths.1,2 Light trapping is necessary to harvest photons with k > 600 nm. A randomly textured back
reflector is commonly used that reflects light randomly
thereby increasing the photon path length in the absorber
layer.3–14 Recently, much work has been done on periodically textured back reflectors (BRs) that enhance longwavelength photon absorption through diffraction and light
concentration.15–29 Here we systematically compare randomly textured BRs with a periodic, designed, photonic plasmonic reflector, and show that the photonic plasmonic
structure is more effective in enhancing light absorption in
thin film Si solar cells.
We choose nano-crystalline silicon (nc-Si) as the
absorber material, since micro-morph tandem solar cells are
a widely accepted high efficiency thin-film cell architecture,1,2,6,9,14,29 with a top amorphous hydrogenated silicon
(a-Si:H), and a bottom nc-Si cell. The a-Si:H cell collects
short-wavelength photons, whereas the nc-Si cell collects red
and near-IR photons up to the band edge (1100 nm). It is
necessary to achieve light trapping in the bottom nc-Si cell
rather than in a-Si:H.
We identify photon management strategies that maximize the photo-current by systematically comparing nc-Si
solar cells grown on randomly roughened and periodically
textured BRs. For meaningful comparisons, the nc-Si
absorber layer thicknesses were kept the same and the device
architecture was identical in the solar cells.
Randomly roughened BRs, with features much less than
the wavelength, randomly scatter incoming light. Yablonovitch30 demonstrated that Lambertian scattering increases
the path length by 4n2, for loss-less conditions, where n(k) is
the refractive index. This enhancement can approach 50 in
silicon.30 There are significant losses, and experimental
enhancements are suggested31 to be considerably less than
4n2. Alternatively, periodic BRs have a pitch considerably
smaller than the wavelength in the absorber k/n(k). Incoming
light is diffracted by the periodic texture, with closely spaced
diffraction resonances, where absorption is maximized.15,20,27,28 Also plasmonic concentration of light near
the periodic BR increases the enhancement.20,25 We show
that this combination of photonic and plasmonic effects
works the best at enhancing light absorption in thin film
cells. The concept is widely applicable to any material system, particularly to organic cells.
We used six distinct BRs including 1) A planar stainless
steel (SS) substrate; 2) A SS substrate with a Cr (5 nm)/Ag
(200 nm) layer followed by 80 nm of ZnO; 3) Annealed Ag/
ZnO on SS with annealing of Cr/Ag at 400 C followed by
80 nm of ZnO; 4) Etched ZnO using 1 lm of Al doped ZnO
deposited by sputtering, and then etched with a hydrochloric
(HCl) acid (0.185%). A 5 nm Cr layer followed by a 100 nm
Ag layer were thermally evaporated on the roughened etched
ZnO. Feature sizes between 400 and 1500 nm were observed
in roughened substrates.
We utilized SS substrates with periodic nano-structures
prepared using nano-imprint lithography9 at Lightwave
a)
FIG. 1. (a) SEM image of nano-hole back reflector. (b) SEM image of
nano-pillar back reflector. The pitch is 750 nm.
Authors to whom correspondence should be addressed. Electronic
addresses: biswasr@iastate.edu and vdalal@iastate.edu.
0003-6951/2011/99(13)/131114/3/$30.00
99, 131114-1
C 2011 American Institute of Physics
V
131114-2
Bhattacharya et al.
Appl. Phys. Lett. 99, 131114 (2011)
FIG. 2. (Color online) AFM images of
photonic plasmonic BR (a), nano-hole
(b), and (c) annealed Ag BRs. SEM
images from the top of ITO showing the
conformal growth of the solar cell on the
photonic plasmonic BR (d), nano-hole
(e), and etched ZnO/Ag (g). Schematic
cross section of p-i-n solar cell.
Power (Cambridge, MA). Nano-imprinting used a reverse
master to imprint a large area pattern onto a polymer layer
on a substrate. The pitch of the periodic pattern was 750
nm, optimized with rigorous scattering matrix simulations.15,18,32 The nano-pillars were tapered (Fig. 1), a feature
found in simulations32 to enhance light absorption with a
height of 160 nm, near the optimum. The hole arrays had
depths of 200 nm as optimized previously.17 After Ag/ZnO
deposition, the deposition of n-i-p layers at temperatures
below 250 C, was performed. The cells grow conformally
(Fig. 2), with the substrate pattern observed at the top of the
cell. The Raman spectra gave a crystalline volume fraction
as described elsewhere.33 The i-layer crystallinity was kept
constant at 55% for all devices.
Before device fabrication, we characterized the randomly roughened BRs by measuring total and diffuse reflectance. We selected annealed Ag and etched ZnO reflectors as
benchmark “best” reflectors, because they had (Fig. 2) very
high diffuse reflectance, exceeding 70% over the entire spectral range (400–1100 nm), comparable to the state-of-theart.6,7,10,29
Nc-Si solar cells were deposited using plasma enhanced
chemical vapor deposition employing H-profiling. A 250 nm
a-Si:H nþ layer was grown on the substrates, followed by a
very thin intrinsic a-Si:H seed layer (Fig. 3), and a thick ncSi layer. Optical specular reflection measurements found the
i-layer absorber thickness to be 0.95 lm.
Optical and electrical characterization on nc-Si:H solar
cells were made. We measured I-V curves (Fig. 3) and the
external quantum efficiency (EQE) to obtain the wavelength
dependent photo-current between 400 and 1100 nm (Fig. 3)
for all devices to confirm the I-V curves. A standard c-Si
photo-diode was used for calibration of EQE and I-V. The
EQE has a maximum of 90% near 530 nm. We used a
1.0 V bias measurement in QE to ensure complete collection of photo-generated carriers. There was only 2%-3%
difference between EQE at 0 V and at 1 V. The discrepancy between currents observed in I-V and from QE was
within 2%-3%. The progression of the currents estimated
from EQE is described in Table I and in Fig. 3.
The lowest current is obtained (Table I) for cells on SS.
Ag on SS improves the current by 13%. Etched ZnO þ silver
gives a higher current. Annealed Ag or nano-holes imprinted
substrates shows a 27% improvement over flat silver, and
finally, the photonic plasmonic structure gives the best
results, an increase of 34% over flat silver.
The EQE shows that cells on the Ag coated nano-hole,
photonic plasmonic substrate, and randomly roughened substrates are considerably enhanced over the flat Ag/ZnO and
SS substrates at long wavelengths (k > 580 nm) up to the
band edge (1100 nm). The EQE oscillations for the periodic
substrates are due to the diffraction. The EQE for the photonic plasmonic BR exceeds that of the etched ZnO and
annealed Ag BRs over a wide range (k ¼ 580–800 nm).
FIG. 3. (Color online) (a) Measured
EQE (at 1.0 V) and (b) current-voltage
characteristics for nc-Si solar cells on
stainless steel (SS), flat Ag, random
textured annealed Ag/ZnO, random textured etched ZnO/Ag, periodic nanohole, and photonic plasmonic substrates.
The device deposited on the photonic
plasmonic substrate produces the highest
EQE at long wavelengths, in agreement
with Table I.
131114-3
Bhattacharya et al.
Appl. Phys. Lett. 99, 131114 (2011)
TABLE I. Photo-currents (J) obtained from the EQE under AM1.5 illumination for a sequence of nc-Si solar cells, grown under identical device architectures on the different types of periodic, random, and flat reference BRs.
The photo-current enhancement over the reference flat Ag/ZnO and bare SS
substrates is shown. The obtained J agrees well with the I-V curves of Fig. 3.
Type of back reflector
Bare stainless steel substrate
SS þ Ag
Annealed Ag/ZnO
Etched ZnO
Nano hole/Ag/ZnO
Photonic plasmonic BR/Ag/ZnO
J (at 1.0 V)
(mA/cm2)
% Enhancement over
flat Ag/over bare SS
14.2
16.1
20.4
19.9
20.4
21.5
—
—/13
27/43
18/39
27/43
34/51
Significantly the photo-current Jsc for the photonic plasmonic
substrate is the highest (21.5 mA/cm2), exceeding that of the
randomly roughened annealed Ag BR, the etched ZnO/Ag,
by comfortable margins much greater than the experimental
uncertainties.
For maximum plasmonic enhancement the back
nþ layer should be very thin so that the reradiated photons
from the plasmonic effect are not absorbed directly into it.
Our devices had relatively thick back nþ layers, 0.25 lm
to avoid shorts. Calculations show the best effects when the
nþ layer thickness is <100 nm.
We chose absorber layer thickness of 1 lm, typical for
micro-morph cells. These conclusions should be observed
for other thicknesses. Although surface plasmons can cause
losses in random BRs, a substantial fraction of light coupled
into localized plasmonic modes at the Ag/ZnO interface may
be re-radiated in optimized textured back reflectors,14 and
dynamical effects need further understanding.
In conclusion, we have fabricated a controlled series of
nc-Si solar cells on both photonic-plasmonic back reflectors
and randomly textured back reflectors. The randomly textured BRs were chosen to have very high diffuse reflectance.
The periodic BR of photonic plasmonic nano-structures outperforms the best annealed silver randomly textured back
reflector, illustrating the viability of the photonic plasmonic
structure for advanced photon harvesting. A very interesting
direction is to combine our plasmonic structure with random
texture.
We thank Max Noack, Supriyo Das, and the entire
Microelectronics Research Center team at Iowa State University. We acknowledge support from the Iowa Powerfund,
NREL, and the NSF under grants ECCS-0824091 and ECS0601377. The Ames Laboratory is operated for the Department of Energy by Iowa State University under contract No.
DE-AC0207CH11385.
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