Low-cost, deterministic quasi-periodic photonic
structures for light trapping in thin film silicon solar cells
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Citation
Xing Sheng et al. “Low-cost, Deterministic Quasi-periodic
Photonic Structures for light trapping in thin film silicon solar
cells.” Photovoltaic Specialists Conference (PVSC), 2009 34th
IEEE. 2009. 002395-002398. © 2010 IEEE.
As Published
http://dx.doi.org/10.1109/PVSC.2009.5411306
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Institute of Electrical and Electronics Engineers
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Thu May 26 00:18:47 EDT 2016
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http://hdl.handle.net/1721.1/61422
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Detailed Terms
LOW-COST, DETERMINISTIC QUASI-PERIODIC PHOTONIC STRUCTURES FOR LIGHT
TRAPPING IN THIN FILM SILICON SOLAR CELLS
1
2
2
2
1, 2
Xing Sheng , Jifeng Liu , Jurgen Michel , Anuradha M. Agarwal , and Lionel C. Kimerling
1
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139
2
Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA, 02139
ABSTRACT
Light trapping has been an important issue for thin film
silicon solar cells because of the low absorption coefficient
in the near infrared range. In this paper, we present a
photonic structure which combines anodic aluminum oxide
(AAO) and a distributed Bragg reflector (DBR) in the
backside of thin film silicon. Simulation results show that
this quasi-periodic AAO structure has a stronger light
trapping effect than perfectly periodic diffractive gratings.
As a proof of concept, we incorporated the backside
structure into thick silicon PV cells. By comparing the
measured photoconductance of silicon with different
backside structures, the enhancement of light absorption
near the band edge of silicon is demonstrated for our
proposed light trapping structure.
INTRODUCTION
Currently, most commercial solar cells are based on
silicon wafers. Cost reduction of silicon wafer based solar
cells is challenging because it is dominated by the starting
material. Thin film silicon solar cells based on inexpensive
substrates are designed to reduce the silicon consumption
by 100 fold so that the material cost becomes negligible.
However, as films become thinner, the absorption of
photons with longer wavelengths is reduced. This problem
is severe especially for silicon because of its indirect
bandgap, thus the power conversion efficiency is
decreased as well. To overcome this limit, several light
trapping schemes were proposed to increase the optical
absorption in thin film silicon. For example, Al or Ag is
used for the backside contact as well as a reflective mirror
[1]. Another example is to utilize a textured transparent
conductive oxide prepared by sputtering and a subsequent
etching step to generate a rough surface that scatters light
into the Si plane [2]. Recently, we designed a textured
photonic crystal (TPC) back reflector, which combines a
one-dimensional reflection grating and a distributed Bragg
reflector (DBR) [3]. The DBR is a multilayer stack of
alternating SiO2 and Si, which forms a one-dimensional
photonic crystal with nearly 100% reflectivity in the nearinfrared range. Meanwhile, the grating can diffract the
incident light into oblique angles, thus total internal
reflection occurs at the front surface of silicon if the
diffractive angle is larger than the critical angle between
silicon and air interface. By integrating the TPC structure
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into 5µm thick monocrystalline silicon solar cells, a
significant enhancement of the external quantum efficiency
(EQE) was observed in a wavelength range from 600 nm
to 1000 nm, leading to a 19 % increase in the cell
efficiency [4]. However, this method has its own limitation.
Since photolithography and other cleanroom facilities are
required to fabricate the subwavelength gratings, the TPC
structure cannot be scaled to large area applications.
To reduce the fabrication cost while maintaining the
same light trapping performance, in this paper we propose
a low cost and easily-controllable Deterministic Quasiperiodic Photonic Structure (DQPS) for light trapping in
thin film silicon solar cells. In the DQPS, the perfect
periodic grating is replaced by a quasi-periodic array,
which is fabricated by self-assembled anodic aluminum
oxide (AAO) and subsequent DBR deposition. Using
simulations, we investigate and compare the light trapping
effects of the DQPS and previous TPC work. Finally, we
present the experimental confirmation of the absorption
enhancement by photoconductance measurements based
on thick silicon wafers.
DESIGN AND SIMULATION
Since only a single-step anodization is used to obtain
the AAO structure instead of a two-step anodization
process [5], a relatively disordered pore distribution is
generated. To compare the light trapping effect of our
quasi-periodic structure with the perfectly periodic gratings,
the absorption spectra of thin film silicon solar cells based
on different back structures are simulated through FiniteDifference-Time-Domain (FDTD) calculations.
Both the TPC and DQPS structures used in FDTD
simulations are illustrated in Fig. 1. Perfectly periodic
grating structure (TPC) can be fabricated by interference
lithography, while single-step aluminum anodization forms
a quasi-periodic distribution with disorder to a certain
extent. Silicon is filled into the pores by PECVD to
increase index contrast. The front silicon layer is 2 µm
thick. The DBR layer in the backside is assumed to have
100% reflectivity in the red and near infrared range.
002395
J sc = eη c ∫ s (λ ) ⋅ a (λ )dλ
Fig. 1. Grating configurations used in FDTD simulation.
The material in blue region is alumina (n=1.72), while the
material in green is silicon (n=3.5)
With the refractive indices of different materials given
in the literature [6], we calculate absorption of the silicon
layers with different backside structures as a function of
wavelength (see Fig. 2.). At wavelengths between 0.6 µm
and 1.0 µm, both TPC (blue color) and DQPS (red color)
have higher absorption compared to the reference silicon
layer without any backside structure. The light trapping
effect is more remarkable for longer wavelength where the
silicon absorption coefficient decreases dramatically. For
incident light above 0.9 µm, the absorption of 2 µm-thick
bare silicon is negligible, while both TPC and DQPS show
strong absorption peaks, and the peak intensity of DQPS
is even higher. The oscillations are due to thin film Fabry
Perrot oscillations.
(1)
where e is the electron charge, η c is carrier collection
efficiency which depends on the surface recombination
and material quality (here we simply assume it is 100% for
monocrystalline silicon). The total number of absorbed
photons can be calculated by integrating over the product
of the absorption spectrum a (λ ) and solar spectrum
s (λ ) . Calculated results are summarized in Table 1. In an
ideal situation, the DQPS provides 53% absorption
enhancement under AM1.5G illumination, which indicates
a 53% enhancement of cell efficiency, even greater than a
49% enhancement obtained from the perfectly periodic
TPC.
Table 1. Simulated short-circuit current density of solar
cells with different backside structures and enhancement
factor
Relative
J sc (mA/cm 2 )
enhancement
Bare 2 µm Si
12.3
49%
TPC
18.3
53%
DQPS
18.8
EXPERIMENTAL RESULTS AND DISCUSSION
The process steps in the fabrication of DQPS are
shown in Fig. 3. A 200nm thick aluminum film is deposited
on silicon substrates by e-beam evaporation. Then the Al
thin film is anodized in a 4 wt% phosphoric acid at a
voltage of 140 V until the aluminum film is completely
oxidized to aluminum oxide. During the anodization, the
temperature is kept at 5 ºC to prevent electrical breakdown.
Afterwards, the samples are dipped in 5 wt% phosphoric
acid for about 2 hours at room temperature to widen the
pore size and remove the barrier layer. Under this
condition, a structure with a pore diameter of about 200nm
and a pore spacing of 300nm is formed, consistent with
literature reports [5]. The DBR consisting of 5 pairs of
Si/SiO2, which can achieve 99.9% reflectivity in the near
infrared range [7], is fabricated by Plasma Enhancement
Chemical Vapor Deposition (PECVD).
Fig. 2. Simulated absorption spectra for 2 µm thick silicon
with different backside structures
To predict the solar cell performances, the short-circuit
current density can be calculated by
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002396
Fig. 3. A schematic of the fabrication process and light
trapping effect of DQPS.
In order to measure the effects of the backside
structures we use photoconductance measurements. A
lock-in amplifier is used to precisely collect the
photoconductances of difference samples under
illumination. Light of the wavelength range of interest
between 700 nm and 1200nm was generated by a white
light source in combination with a monochromator.
To demonstrate the light trapping effect of the DQPS
as a proof of concept, we implement the DQPS on the
backside of a 300 µm thick, double side polished silicon
wafer. Thick silicon wafers are chosen because of the
simplicity of process. However, similar responses will be
obtained for different backside structures at short
wavelengths because of complete absorption by the thick
active silicon layer.
The photoconductivity of semiconductors is directly
related to the excess carrier density generated by photon
absorption:
∆σ = ∆n ⋅ eµ
(2)
Therefore, light trapping effects can be demonstrated by
an enhanced photoconductive response. The measured
photoconductive spectral responses for DQPS and a
reference sample without backside structure at
wavelengths between 0.7 µm and 1.2 µm are shown in Fig.
4. Compared to the reference sample, an increase in the
photoresponse can be clearly observed above 1000 nm
for the DQPS sample, in agreement with our calculations.
This result experimentally confirms that DQSP structures
can be applied for optical path length enhancement.
Fig. 4. Photoconductive spectral responses of a 300 um
thick silicon sample with and without DQPS on the
backside of the wafers.
CONCLUSIONS
In this paper, we propose a novel quasi-periodic
photonic light trapping structure, which can be easily
fabricated through aluminum anodization and DBR
deposition. Numerical calculations indicate that the DQPS
is capable of improving the solar cell efficiency by more
than 50 percent for thin film crystalline silicon. Moreover,
photoconductance measurements based on thick silicon
wafers clearly confirm the enhancement of photon
collection near the silicon bandedge, in good agreement
with the simulation results. Considering a thin-film silicon
cell with a thickness of several microns, the efficiency
increase would be much more significant. Furthermore,
the fabrication process of this DQPS can be readily scaled
to large areas for practical applications with significant cost
reduction. These preliminary results provide a path to
achieve low cost and strong efficiency enhancement for
thin film silicon solar cells by using our novel DQPS
structure.
ACKNOWLEDGMENT
This work was supported by Robert Bosch LLC
through MIT Energy Initiative.
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[2] J. Muller et al., “TCO and light trapping in silicon thin
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978-1-4244-2950-9/09/$25.00 ©2009 IEEE
002397
[3] L. Zeng et al., “Efficiency enhancement in Si solar cells
by textured photonic crystal back reflector”, Appl. Phys.
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[4] L. Zeng et al., “Demonstration of enhanced absorption
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[7] Y. Fink et al., “A Dielectric Omnidirectional Reflector”,
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