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Light management through guided-
mode resonances in thin-film silicon
solar cells

Tanzina Khaleque
Robert Magnusson

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Light management through guided-mode resonances
in thin-film silicon solar cells

Tanzina Khaleque and Robert Magnusson*
University of Texas at Arlington, Department of Electrical Engineering, P.O. Box 19016,

Arlington, Texas 76019

Abstract. We theoretically explain and experimentally demonstrate light trapping in thin-film
solar cells through guided-mode resonance (GMR) effects. Resonant field enhancement and
propagation path elongation lead to enhanced solar absorption. We fabricate nanopatterned
solar cells containing embedded 300-nm period, one-dimensional gratings. The grating pattern
is fabricated on a glass substrate using laser interference lithography followed by a transparent
conducting oxide coating as a top contact. A ∼320-nm thick p-i-n hydrogenated amorphous
silicon solar cell is deposited over the patterned substrate followed by bottom contact deposition.
We measure optical and electrical properties of the resonant solar cells. Compared to a planar
reference solar cell, around 35% integrated absorption enhancement is observed over the 450 to
750-nm wavelength range. This light-management method results in enhanced short-circuit cur-
rent density of 14.8 mA∕cm2, which is a ∼40% improvement over planar solar cells. Our exper-
imental demonstration proves the potential of simple and well-designed GMR features in thin-
film solar cells. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported
License. Distribution or reproduction of this work in whole or in part requires full attribution of the origi-
nal publication, including its DOI. [DOI: 10.1117/1.JNP.8.083995]

Keywords: guided-mode resonance; interferometric lithography; light trapping; thin-film solar
cells.

Paper 13139SS received Dec. 13, 2013; revised manuscript received Jan. 24, 2014; accepted for
publication Feb. 6, 2014; published online Mar. 6, 2014.

1 Introduction

Thin-film solar cell technology offers benefits of low-cost material usage and processing relative
to currently dominant crystalline silicon solar cells. In contrast to classic thick, wafer-based sil-
icon cells, the higher absorption coefficients of materials used in thin-film photovoltaics allow
for a film thickness of hundreds of nanometers to micrometers. The quality of material can be
relatively poor since the charge carriers only travel a distance on the order of the film thickness.
However, the low-energy photons suffer from short optical paths that ultimately cause low spec-
tral uptake in the cells near the material’s bandedge.1–3 Consequently, efficient light-trapping
mechanisms are necessary to obtain comparable performance from thin-film solar cells.

By incorporating properly designed photonic nanostructures, incoming sunlight can be
trapped inside the absorbing layer while reducing loss caused by reflection and scattering.
With a highly concentrated field, a thin absorbing layer sufficiently absorbs most of the
solar spectrum. This allows further reduction of the absorbing layer thickness and ensures min-
imal usage of materials and better collection efficiency for low-quality materials. Numerous
studies have been conducted to improve light-capture and collection efficiency of thin absorbing
layers. The application of diffractive optics,4–6 random texturing,7,8 antireflective layers,9,10 plas-
monics,11–13 photonic crystals,14–16 guided-mode excitation,6,17,18 and three-dimensional struc-
tures like nanowire, nanodome, and nanocone solar cells19–21 shows distinguished
improvements in solar absorption. Though each mechanism contributes to the manipulation
of optical path lengths inside the films, the most efficient light-harvesting scheme is yet to
be convincingly identified. Using numerical simulation tools, it is possible to design

*Address all correspondence to: Robert Magnusson, E-mail: [email protected]

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9.eps


patterns into the active region as schematically shown in Fig. 8. We fabricate a nanopattern on a
glass substrate, and the pattern is gradually transferred to the silicon regionwithout interrupting the
junctions.We consider a thin indium-tin-oxide (ITO) layer in between the glass and silicon as a top
contact. To observe the GMR-induced absorbance enhancement exclusively, we conduct the opti-
cal characterization before the bottom contact is made.

4 Fabrication

We fabricate the 1-D nanograting patterns on a glass substrate; Fig. 9 summarizes the fabrication
steps. First, a 1 × 1 in:2 glass substrate is cleaned with acetone, isopropanol, and deionized water
and dried with blown nitrogen. Then an 80-nm bottom antireflection coating (BARC: DUV30J-
6) is spin-coated over the glass substrate at 1200 rpm and baked for 60 s on a heating plate

Fig. 8 Schematic view of a GMR solar cell without bottom contact.

Fig. 9 Schematic view of nanopatterned solar cell fabrication steps.

Khaleque and Magnusson: Light management through guided-mode resonances in thin-film silicon solar cells

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adjusted to 205°C. A 350-nm thick photoresist coating (PR: SEPR-701) is spin-coated at
1100 rpm and baked for 90 s at 110°C. A 1-D grating pattern with a 300-nm period is transferred
into the PR by UV laser interferometric lithography using a laser with λ ¼ 266 nm. The purpose
of using the BARC layer beneath the PR is to reduce the reflection from the glass substrate and
ensure uniform pattern exposure over the PR. After developing the PR, the BARC is removed
from the open area of the pattern by reactive ion etching (RIE) using oxygen plasma. The glass
substrates are etched using an argon (Ar) and trifluromethane (CHF3) gas mixture. The remain-
ing PR and BARC are removed by RIE using oxygen plasma. In a systematic experimental
process, gratings with 50-, 60-, and 70-nm depths are fabricated on glass substrates. Here,
we present the results obtained from solar cells with 60-nm grating depths since the results
are similar to those obtained with 50- and 70-nm grating depths.

The patterned glass substrates are coated with a 140-nm thick film of ITO by sputtering.
From the scanning electron microscope (SEM) images, we confirm that ITO film deposition
over the 60-nm deep grating area conforms to the grating structures and serves well as the
top contact. The ITO-coated glass substrates are annealed in a rapid thermal annealer in a vac-
uum chamber at 490°C for 15 min. The resistivity of the film is 50 Ω∕sq. The average trans-
mittance of the annealed ITO thin film is around 90% over the 450- to 750-nm wavelength range.
Figure 10 shows the measured transmittance of the annealed ITO film. Using a multichambered
plasma-enhanced chemical vapor deposition system, a complete p-i-n single-junction solar cell
with an approximate thickness of 10-nm p-type, 290-nm i-type, and 20-nm n-type a-Si:H is
deposited over the ITO pattern. Finally, consecutive sputter deposition of 130-nm thick ITO
and 300-nm thick aluminum films constitutes the bottom contact. Thin-film solar cells on planar

Fig. 10 Transmittance of an annealed 140-nm thick ITO film deposited on a glass substrate.

Fig. 11 Atomic force microscope surface images of an ITO-coated patterned glass substrate
where Λ ¼ 304 nm, dg ¼ 60 nm, and F ¼ 0.5.

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planar reference cell. GMR phenomena strongly depend on dispersion properties and thickness
of the absorbing photovoltaic materials. Enhanced optical absorbance leading to an increased
conversion efficiency holds the same potential for micro-crystalline and organic thin-film solar
cells with optimal structural and material device parameters.

Acknowledgments

The authors thank MVSystems, Inc. for a-Si:H deposition and Wenhua Wu for computational
assistance. Partial support was provided by the UT System Texas Nanoelectronics Research
Superiority Award funded by the State of Texas Emerging Technology Fund as well as by
the Texas Instruments Distinguished University Chair in Nanoelectronics Endowment.

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Tanzina Khaleque is a graduate research assistant in the Department of Electrical Engineering
at the University of Texas-Arlington. She received her BS and MS degrees in electrical and
electronic engineering from Bangladesh University of Engineering and Technology in 2006
and 2009, respectively. She is pursuing her PhD degree in electrical engineering at UT-Arlington.

Robert Magnusson is a professor of electrical engineering and the Texas Instruments
Distinguished University Chair in Nanoelectronics at the University of Texas-Arlington. He
received his PhD degree from the Georgia Institute of Technology. He is the author of ∼400
journal and conference papers. Current research interests include periodic nanostructures, nano-
lithography, nanophotonics, nanoelectronics, nanoplasmonics, and optical bio- and chemical
sensors. He is a fellow of SPIE, OSA, IEEE, and the National Academy of Inventors.

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