Download Semiperiodicity versus periodicity for ultra broadband optical

Semiperiodicity versus periodicity for
ultra broadband optical absorption
in thin-film solar cells
Mandana Jalali
Hamid Nadgaran
Daniel Erni
Mandana Jalali, Hamid Nadgaran, Daniel Erni, “Semiperiodicity versus periodicity for ultra broadband
optical absorption in thin-film solar cells,” J. Nanophoton. 10(3), 036018 (2016),
doi: 10.1117/1.JNP.10.036018.
Semiperiodicity versus periodicity for ultra broadband
optical absorption in thin-film solar cells
a
Mandana Jalali,a,b,c Hamid Nadgaran,c and Daniel Ernia,b,*
University of Duisburg-Essen, General and Theoretical Electrical Engineering (ATE), Faculty
of Engineering, Duisburg 47048, Germany
b
CENIDE—Center for Nanointegration Duisburg-Essen, Duisburg 47048, Germany
c
Shiraz University, Department of Physics, Faculty of Science, Eram Square, Shiraz 71454, Iran
Abstract. We propose the use of one-dimensional semiperiodic front and back gratings based on
Thue–Morse, Fibonacci, and Rudin–Shapiro (RS) binary sequences as promising photon management techniques for enhancing ultra-broadband optical absorption in thin-film solar cells.
The semiperiodicity allows an aggregate light in-coupling into the active layer within the
range of the solar spectrum that is less weak compared to an inherently broadband random grating, but has a much larger bandwidth than the strong in-coupling via a periodic grating configuration. The proper design procedure proposed here deviates from a canonical double grating
synthesis as it adheres to an ultra-broadband design where the spectrally integrated absorption in
the active material is the proper subject to optimization, leaving the grating perturbations just a
measure to perturb and mold the trapped light field in the active layer accordingly. It is shown
that by using a well-defined RS double grating in a 400-nm thick crystalline silicon solar cell, a
110.2% enhancement of the spectrally integrated optical absorption can be achieved relative to
the reference case without grating. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)
[DOI: 10.1117/1.JNP.10.036018]
Keywords: grating; optical absorption; photon management; semiperiodicity; solar cell.
Paper 16110 received Jul. 2, 2016; accepted for publication Aug. 30, 2016; published online Sep.
22, 2016.
1 Introduction
Thin-film solar cells are promising energy resources as they have lower cost, lower weight,
higher mechanical flexibility, and a reduced recombination probability compared to conventional
first generation solar cells.1–3 As the absorption probability is substantially reduced in such structures due to their thin active layer,4 a photon management technique is vital to enhance photon
path lengths and lifetimes in the active layer. Implementing a plasmonic back grating is an effective photon management technique that increases the optical interaction within the active layer
and hence the optical absorption.5,6 Plasmonic back gratings are apt to excite plasmonic modes
with strongly localized fields, which trap light inside the active layer increasing the absorption
probability as a result of the strong field enhancement.7 It has been shown that this technique can
boost optical absorption by 26%.8
One important aspect in solar cell design is its adherence to the very large bandwidth of the
solar illumination spectrum. In the context of the actual plasmonic back grating design, a spectral
range between 350 and 1100 nm of an AM1.5 illumination spectrum has to be captured while
maximizing the spectrally integrated absorption in the active layer.9 This aggregate absorption is
further used as a figure-of-merit in all subsequent solar cell designs.
As a first design strategy, one can rely, e.g., on periodic (metallic) back gratings with optimized grating parameters.8 These periodic structures enable a spectrally distinct light coupling
due to the excitation of a few well-defined modes (and resonant states) within the thin-film
structure of the solar cell. Despite the fact that these modes (and the resulting resonant states)
*Address all correspondence to: Daniel Erni, E-mail: [email protected]
1934-2608/2016/$25.00 © 2016 SPIE
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are relatively strong, the resulting enhancement of the optical absorption is narrowband conforming to the narrow spectral range of the excited mode.8,9 On the other side, random grating structures are capable of exciting a broad range of weak modes, however, as these multiple modes are
rather faint the resulting broadband coupling is barely suitable to affect the optical absorption.
In this paper, we propose a solution that lies somewhere between full periodicity and randomness, which we are going to refer as semiperiodicity. Semiperiodic grating structures can theoretically excite several modes and mode groups10,11 that are not as weak and broadband as in random
structures although the resulting light coupling and optical absorption enhancement might not be as
spectrally distinct as in periodic gratings, yet define a valuable compromise regarding the efficient
capture of the solar illumination spectrum. The definition and proper design of such semiperiodic
structures is not an easy task. Some researchers proposed designs for semiperiodic structures
capable of better photon control. The proposed structures are either two-dimensional (2-D)
quasi-random patterns just on top of the active layer,10 or one-dimensional (1-D) quasi-periodic
plasmonic front gratings,11 which we believe are not optimal choices for the intended solar cell
design. A straightforward idea to implement semiperiodicity is to add defects into a fully periodic
structure in order to introduce a corresponding degree of randomness. This simple approach is far
less controllable than assumed because it may introduce very distinct defect resonances with localized defect fields. In this paper, we use semiperiodic gratings based on deterministic mathematical
sequences,12 such as Thue–Morse (TM),13 Fibonacci (FB),14 and Rudin–Shapiro (RS)15,16 binary
sequences that are easily parameterized to the grating scenario for further numerical evaluation
with a standard computational electromagnetic solver. In the framework of this numerical analysis,
the material stacks of the solar cell including the corresponding semiperiodic sequences for the
back (and front) grating are introduced into a supercell, where the latter is assumed to be large
enough to reproduce the spectral characteristics of the semiperiodic grating structure. The simulation then analyzes the optical absorption in the active layer yielding its spectrally integrated
version as the most relevant figure-of-merit. Besides the metallic back grating, the study also
includes scenarios with extra semiperiodic front gratings (made of the same material as the active
layer) that are added on top of the material stack.17,18 Hence, the type of this semiperiodic front
grating and its phase relative to the back grating have a significant effect on the overall performance
of the solar cell and are, therefore, subject to optimization.
The remainder of the paper is organized as follows. In the second section, the mathematical
representations of different semiperiodic sequences are introduced and the corresponding realizations as binary gratings are presented. The numerical modeling of the bare thin-film solar cell
is described in the third section, while the fourth section is devoted to the case where a metallic
semiperiodic back grating is added to the layer structure as a first measure to optimize the overall
broadband absorption. The inclusion of a front grating is analyzed in the fifth section and conclusions are drawn in the sixth section.
It is worth emphasizing that the grating design presented here significantly differs from the
canonical (multiple) grating synthesis with commensurable perturbations, where grating orders
and mode coupling are vital aspects of a straightforward narrow-band design.19 Here, we adhere
to a strict ultra-broadband design, where the spectrally integrated absorption is the sole figure-ofmerit to be (numerically) optimized, leaving the underlying grating structures just as perturbations (without meaningful scale) that are apt to generate localized fields and hot-spots in the
active layer while progressing along the spectral range and cashing up for the desired thinfilm absorption. This particular design methodology is further elucidated in conjunction
with Fig. 7.
2 Semiperiodic Sequences
One of the approaches to construct a semiperiodic grating out of a corresponding semiperiodic
binary sequence is to map this sequence in parallel onto a physical periodic grating of commensurable length, where the zeroes of the semiperiodic mathematical binary sequence are labeling
the locations of the missing teeth in the physical periodic grating, giving rise to the intended
semiperiodic structure. Prior to that, the parameters of the underlying periodic grating regarding
the spectrally integrated absorption have been optimized8 using a breeder genetic algorithm. In
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the framework of this setup process, three different mathematical binary sequences are applied,
namely the TM,13 the FB,14 and the RS15,16 sequences, yielding three different semiperiodic
gratings, which are validated in the following for their applicability in thin-film solar cells.
2.1 Thue–Morse
TM is a semiperiodic binary sequence, which starts from zero and the next term is created
through bitwise negation of the pervious term and adding it to the pervious term itself. By bitwise
negation, we mean that 0 → 1 and 1 → 0. Therefore, when the first term is 0, the second term
will be 01 and the third will be 0110, and so on. As it can be easily concluded, each term has
twice the length of the previous term.
2.2 Fibonacci
FB sequence is another semiperiodic binary sequence. The zeroth term is 0 and the first term is 1.
From this initial condition, the later terms evolve iteratively according to the recurrence relation
Fn ¼ Fn–1 Fn–2 , in which Fn is the n’th term and it is defined by putting the two previous terms
together. The length of each term is the sum of the lengths of the two previous terms leading to a
quick nonlinear evolution of the length of the overall sequence.
2.3 Rudin–Shapiro
RS is the last sequence considered here. The RS sequence is not a binary but is a bipolar
sequence with only two characters "1 that can easily be mapped into a binary sequence
using proper offsetting and scaling (namely −1 → 0, and 1 → 1). The n’th term of the sequence,
bn , is defined by the following equation:
an ¼
EQ-TARGET;temp:intralink-;e001;116;421
n
X
i¼1
εi · εiþ1 ;
bn ¼ ð−1Þan ;
(1)
in which εi are the digits in the binary expansion of n and bn is þ1 if an is even and −1 if an is
odd. To map this sequence into a binary representation, all −1 are considered as 0 labeling a
missing tooth in the underlying (periodic) physical grating. The length of each term is equal to n.
To grasp the semiperiodic characteristics of the structure in the numerical analysis, the supercell should be considered long enough. The latter has been chosen to have a length of 15.3 μm
corresponding to 32 periods (teeth) of the underlying periodic reference frame. This length conforms to the 5th term of the TM sequence, as well as to the last 32 digits of the 7th term of the FB
sequence and to the 32th term of the RS sequence, yielding three promising realizations of semiperiodic gratings that become subject to further analysis.
3 Numerical Modeling and Simulation Scenarios
The design of the whole thin-film solar cell includes a 400-nm-thick crystalline silicon absorbing
layer, without doped layers,20 with an additional 100-nm Si3 N4 layer on top as an antireflection
coating. The reason for such a choice of the antireflection layer is that it completely covers the
front-grating and hence creates a protective passivation layer that is suitable for further reflection
reduction. The plasmonic back grating consists of a binary Ag nanograting embedded above a
40-nm Ag back layer inside the absorbing layer having a reference grating period of 400 nm, a
tooth height of 180 nm, and duty cycle of 0.4. These parameters are retrieved from the aforementioned evolutionary optimization8 and have been slightly modified (i.e., correspondingly
rounded) for feasibility reasons. As already described, the semiperiodic structure has the
same physical parameters as the periodic one while just omitting the teeth that are in affinity
to the zeroes of the underlying mathematical binary sequence. The excitation is given by a perpendicularly impinging broadband plane wave containing both s- and p-polarization mimicking
an AM1.5 solar illumination. Figure 1 depicts the schematics of the simulated thin-film structure
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Fig. 1 Cross-section of the modeled solar cell.
with periodic double gratings, while the semiperiodic gratings are illustrated in field distribution
profiles (Figs. 5 and 6).
The numerical analysis of the overall solar cell was carried out using the finite-elementsmethod (FEM) based simulation platform COMSOL Multiphysics™ (together with the Wave
Optics Module), where the semiperiodic nanostructured thin-film solar cells are compared to a
similar reference cell having a periodically structured back (and front) grating. The best candidate
for an appropriate thin-film cell topology is chosen through extensive study of the spectral response
of the absorption in the c-Si active layer of the different nanostructured cells while tracking down
the cell with the best figure-of-merit, i.e., the largest spectrally integrated absorption.
In a further stage of the study, a semiperiodic front grating with the same material as the
active layer has been added to the structure.21 The periodicity and duty cycle of the reference
frame for both the front and back gratings are the same; however, the front grating’s tooth height
is one quarter of the back grating height. As already shown in Ref. 21, a constructive coupling
between (semiperiodic) front and back grating is vital for enhancing the efficiency of light trapping inside the active layer, where the shape of the front grating should be correlated to the back
grating design (as has been elucidated in Ref. 22). In the following, two different versions of
front and back grating combinations were proposed. The first one is a semiperiodic front grating
having the same binary sequence as the back grating, and the second one is a front grating with a
bitwise inversion of the back grating’s binary sequence. Hence, in alignment to the periodic
reference grid, the front grating would have a missing tooth, where in the back grating, a
tooth is present. It will be shown that such a semiperiodic double-grating structure (based
on a RS sequence) has a remarkable effect on the optical absorption in the thin active layer
yielding an absorption enhancement up to 110.2% relative to a cell without gratings and
28.3% relative to the same structure with a periodic double grating.
4 Semiperiodic Back Grating
As mentioned in the previous section, a semiperiodic plasmonic back grating is added to the
solar cell that is realized by choosing one of the three different semiperiodic binary sequences.
In Fig. 2, the spectral response of the absorption for all three different semiperiodic back gratings
is plotted in comparison to the periodic reference structure. As the main differences in the spectral signatures are observed within the range from 600 to 950 nm, only this wavelength range is
depicted. From these spectral characteristics, a general feature of the semiperiodicity already
becomes visible. The spectral responses of all semiperiodic back gratings show a kind of offset
together with some additional spectral peaks that appear most prominently in regions where the
spectral response of the periodic reference grating yields its minima (examples are highlighted in
Fig. 2 with corresponding shadings). Together with the apparent spectral peak broadening, this is
in full accordance with the expected broadband coupling within semiperiodic or randomized
grating structures.
The reason behind this enhancement is that field maxima are more dispersed through the
whole active layer instead of being localized in few distinct hot spots. In some specific spectral
regions, the absorption enhancement is due to the fact that a valley in the absorption spectrum of
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Fig. 2 Simulated spectral response of the active layer’s optical absorption in structures with semiperiodic back gratings compared to the periodic reference structure for (a) a TM back grating, (b) a
FB back grating, and (c) an RS back grating.
the periodic reference structure is changed into a peak due to the breaking of the periodicity,
namely the introduction of corresponding grating defects. A further issue characteristic to the
semiperiodic back grating is the slightly increased volume of the absorbing layer due to the
missing teeth. In order to underpin our reasoning, the electric field distribution in the active
layer for all back grating realizations, namely the periodic, TM, FB, and RS gratings, are
depicted in Fig. 3 for an operating wavelength of 880 nm. At this particular wavelength, a valley
in the absorption spectrum of the periodic structure is complemented by one up to few peaks
introduced by the semiperiodic gratings. This goes along with electric fields being widely distributed over the whole active region.
The results of this analysis are listed in Table 1, showing the spectrally integrated optical
absorption (i.e., the figure-of-merit) of all investigated back grating structures. The figures represent the percentages of absorbed power relative to the total illuminated power. As can be seen,
all three semiperiodic structures outperform the periodic back grating and the RS sequence
seems to show the best performance of all semiperiodic structures, which might be due to
its closest proximity to random structures.
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Fig. 3 Simulated electric field distributions at an operating wavelength of 880 nm for (a) a periodic
reference back grating, (b) a TM back grating, (c) FB back grating, and (d) an RS back
grating.
Table 1 Optical absorption and carrier generation rate for periodic and semiperiodic single and
double grating structures, where the previous ones act as periodic reference case (note that only
“design-A” can act as a meaningful double-grating reference structure with comparable
periodicity).
Periodic
grating
Thue–Morse Fibonacci Rudin–Shapiro
grating
grating
grating
Optical absorption for a single back grating
40.9%
45.1%
46.3%
46.1%
Optical absorption for double gratings of design-A
45.2%
49.8%
48.0%
52.3%
Optical absorption for double gratings of design-B
—
55.7%
54.5%
58.0%
40.7%
50.9%
49.5%
48.8%
Carrier generation rate for double gratings of design-B
5 Adding a Front Grating
In a further step of the study, a semiperiodic front grating is added to the structure. The introduction of the front gratings enables an additional light coupling due to the emergence of additional modes (and resonant states) in the active layer. The resulting interplay between front and
back gratings is likely to further enhance the light coupling to the active layer by light trapping
effects and additional mode coupling due to an extension of the spectral coupling range induced
by the double-grating topology itself. Such a structure is well capable of exciting various modes
such as surface modes.23 It has been shown that any superimposed structure (i.e., a double grating) given by the two neighboring gratings is apt to yield an overall structure with a higher
degree of nonperiodicity.19
We have already mentioned that a front grating with a ¼ tooth height compared to the back
grating is best suited with respect to both the in-coupling and the optical absorption
enhancement.21 The front-grating parameters such as tooth height are chosen based on an extensive optimization leading to an effective coupling between the two gratings. The main question
that remains is how to define the front grating’s semiperiodicity to gain the best possible constructive broadband coupling between the two gratings, where the two grating structures are
expected to be highly correlated. In the following, two different (semiperiodic) double-grating
designs are explored.
5.1 Design-A
The first design is a front grating having the same binary sequence as the back grating (termed as
design-A), which means that the front grating is a copy of the back grating with a reduced tooth
height and consists of c-Si, the same material as the active layer. The spectral responses of the
optical absorption for all semiperiodic sequences—i.e., TM, FB, and RS in comparison to the
periodic case—are depicted in Fig. 4(a). There is an exemplary wavelength region of interest
highlighted in blue. This area is around 780 nm, displaying a peak in the spectral response of the
periodic reference structure. In the case of the semiperiodic double gratings, this peak region is
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Fig. 4 Simulated spectral responses of the active layer’s optical absorption for the four semiperiodic sequences in the double-grating design according to (a) the design-A and (b) the design-B.
not only broadened but also complemented by multiple emerging peaks with even higher amplitudes. To analyze this effect, the electric field distribution is displayed in Fig. 5, showing snapshots at an operating wavelength of 780 nm of all semiperiodic double gratings conforming to
the design-A including the periodic reference double grating. In the periodic reference structure,
a regular pattern of field maxima occurs mostly in the vicinity of the top and bottom grating teeth.
Through adding defects to the system—namely in the case of semiperiodic double gratings—
these fields undergo irregular patterns, which tend to be better distributed and hence, more volume filling within the absorbing area (where the latter is slightly increased in all semiperiodic
designs). A characteristic feature of the semiperiodic grating design extracted from Fig. 5 is that
the front grating teeth act more like individual diffusers with respect to the impinging sunlight,
where the metallic back grating teeth behave somehow like concentrators. The overall effect,
Fig. 5 Design-A: Simulated electric field distributions at an operating wavelength of 780 nm for
different semiperiodic sequences: (a) the periodic reference double grating, (b) TM double grating,
(c) FB double grating, and (d) RS double grating.
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which has to be pictured at each operating wavelength and thus to be quantified over the whole
wavelength range, is an increased spectrally integrated absorption of light in the active region,
which is also quantified in Table 1.
5.2 Design-B
The second design includes a front grating with an inverted semiperiodic binary sequence compared to the sequence of the back grating (termed as design-B). The absorption spectra of all
double-grating realizations conforming to design-B are plotted in Fig. 4(b), showing a better filling
effect (with multiple peak formation) at wavelength ranges, where the absorption spectrum of
periodic reference structure exhibits broad minima (valleys). While inspecting Fig. 6, one observes
that light diffusion at the front teeth seems more effective due to the increased “propagation volume” beneath each tooth, which is a characteristic feature of the inverted front sequence increasing
the probability for multiple interference/resonance pattern formation in the active layer. In designB, the effect of dispersed light through the active layer is further pronounced over the overall
spectral range leading to an improvement of the spectrally integrated absorption. This can be
extrapolated in Fig. 6 from a spectral snapshot of the electric field distribution within all semiperiodic designs at a wavelength of 880 nm. A final confirmation is given in Table 1, which shows
design-B with a RS sequence superior to the other semiperiodic realizations.
It should be mentioned that each double grating slightly alters the absorptive volume.
However, calculation of normalized absorption per volume shows that the enhancements are
mainly due to the excited modes instead of increase or decrease of the absorptive volume.
To illustrate reasons for the effectiveness of the RS design, we will refer to a more comprehensive picture of our optimization scenario, as depicted in Fig. 7, showing the absorption averaged over the active layer thickness as a function of grating position and operating wavelength,
respectively. This 2-D map stands for the full display of the absorption, which should yield a
suitable distribution of hot spots in order to maximize its value (and the figure-of-merit) when
integrated over both dimensions of this map. It is worth mentioning that such kind of design
strategy significantly deviates from a classical grating design, where the emphasis is put on
specific grating shapes apt for commensurable wavelength coupling.
What can be inferred with respect to the success of the RS sequence is the presence of prominent stripes located between corresponding defects and present at virtually all frequencies, which
cluster all the absorption hotspots necessary to outperform the rival semiperiodic structures. The
presence of spatial hotspots is complemented by the emergence of additional resonances in the
spectral response. This overall picture perfectly mimics what is at stage, namely the optimization
of the trade-off between strong and broadband light coupling to funnel maximal light intensity
into the thin active layer.
The current study is based on a fixed active layer thickness, however, simulations for either
thinner or thicker material clarifies that the effectiveness and superiority of semiperiodic structure does not depend on the active material thickness.
To see if carrier generation rate increases in line with absorption enhancement, this parameter
is calculated for design-B. With the assumption that each absorbed photon creates a pair
Fig. 6 Design-B: Simulated electric field distributions at an operating wavelength of 880 nm for
different semiperiodic sequences: (a) the periodic reference double grating, (b) TM double grating,
(c) FB double grating, and (d) RS double grating.
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Fig. 7 The comprehensive display of the active layer’s optical absorption for an RS sequences in
the double-grating design-B as a function of both, the longitudinal grating position and the operation wavelength (weighted by the AM1.5 spectrum). (a) The overall absorption map.
(b) Integrating the map over all grating positions yields the absorption’s spectral response.
(c) Integrating over all wavelengths results in the absorption’s longitudinal distribution. The apparent discontinuity at wavelengths below 650 nm is due to enhanced material absorption.
(electron–hole), the carrier generation rate can be calculated using the following equation, and
results are shown in Table 1.
Z !
2"
~
AM1.5ε 0 0 jEðωÞj
GR ¼
dω;
(2)
2ℏ
EQ-TARGET;temp:intralink-;e002;116;337
6 Conclusion
We have shown that the introduction of semiperiodic back and front gratings following deterministic binary sequences, such as, e.g., TM, FB, or RS sequences are superior to any compared
periodic grating concept and, therefore, well suited to provide an effective photon management
in thin-film solar cells that fosters light trapping in the active region to enhance the solar cell’s
efficiency. The reasoning behind the semiperiodic grating concept lies in the extended spectral
bandwidth of the involved light coupling mechanisms, which is essential for the intended broadband optical absorption.
The best performing design (in terms of the largest absorption and an enhanced carrier generation rate) is a double grating with an RS type of back grating and a front grating that is based
on an inverted version of the back grating sequence (i.e., design-B). As the figure-of-merit to be
maximized consists in the spectrally integrated absorption in the active material volume over the
essential part of the solar spectrum, both the design procedure and the intelligibility of the
achieved design solution are less straightforward than to be hoped for, leaving us in a situation
where most explanations and subsequent conclusions have to be drawn virtually on a phenomenological basis that may be conceptualized according to the absorption map shown in Fig. 7.
Hence, the best performing RS double grating (design-B) displays all convincing attributes such
as a slightly larger active layer volume and the capability to foster irregular but widespread
interference and resonance patterns due to the more pronounced scattering and diffusion
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capabilities of the front grating topology, together with a spectral signature of the absorption that
supports a larger peak density compared to the periodic reference grating. In conclusion, the
deterministic approach toward random grating structures using semiperiodic sequences as an
intermediate stage has proven successful for our broadband designs and thus worth extending
further to multiscale sequences based on correspondingly quantized versions of, e.g., Weierstrass
functions or Perlin noise yielding promising parameterized versions of “digitized roughness,” apt
for functional interfaces in broadband optics.
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21. M. Jalali, H. Nadgaran, and D. Erni, “Implementing double grating as a photon management
technique in thin film solar cells,” Unpublished work (2016).
22. P. Spinelli et al., “Plasmonic light trapping in thin-film Si solar cells,” J. Opt. 14(2), 024002
(2012).
23. M. Faryad and A. Lakhtakia, “Excitation of multiple surface-plasmon-polariton waves using
a compound surface-relief grating,” J. Nanophotonics 6(1), 061701 (2012).
Journal of Nanophotonics
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Vol. 10(3)
Jalali, Nadgaran, and Erni: Semiperiodicity versus periodicity for ultra broadband optical absorption. . .
Mandana Jalali has been a PhD student at Shiraz University since 2011 in optics and lasers in
collaboration with the University of Duisburg-Essen. She is the author of 14 conference papers
and has four paper manuscripts ready to be submitted along with two submitted papers. Her
current research interests include plasmonics, photovoltaics, and photon management.
Hamid Nadgaran is a full professor at Shiraz University, Iran. He received his PhD in physics
from Manchester University, England, in 1992. He has coauthored over 60 publications and 1
book. His current research interests include nanophotonics, plasmonics, optical metamaterials,
and lasers.
Daniel Erni is a full professor at the University of Duisburg-Essen, Germany. He received his
two degrees in electrical engineering from HSR Rapperswil and ETH Zurich in 1986 and 1990,
respectively, and his PhD degree in laser physics from ETH Zurich in 1996. He has coauthored
over 300 publications. His current research interests include optical interconnects, nanophotonics, plasmonics, optical and electromagnetic metamaterials, RF and biomedical engineering,
computational electromagnetics, multiscale multiphysics modeling, and numerical structural
optimization.
Journal of Nanophotonics
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Vol. 10(3)