1
Van der Waals Materials for Atomically
-Thin Photovoltaics: Promise
and Outlook
Deep Jariwala
1,2
, Artur R. Davoyan
1,2,3
, Joeson Wong
1
and
Harry A. Atwater
1,2,3,4*
1
Department of Applied Physics and Material Science, California Institute of Technology,
Pasadena, CA-
91125, USA
2
Resnick Sustainability Institute, California Institute of Technology, Pasadena, CA-
91125,
USA
3
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA-
91125, USA
4
Joint Center for Artificial Photosynthesis, Ca
lifornia Institute of Technology, Pasadena,
CA
-91125, USA
Abstract:
Two
-dimensional (2D) semiconductors provide a unique opportunity for
optoelectronics
due to
their layered atomic structure,
electronic
and optical properties.
To
date, a
majority of the application
-oriented
research
in this field
has been focused on field
-
effect electronics as well as photodetectors and light emitting diodes. Here we present a
perspective on the use of
2D semiconductors for
photovoltaic
applications. We discuss
photonic device design
s that
enable light trapping in nanometer
-thick
ness
absorber layers
, and
we
also
outl
ine
schemes for efficient carrier transport and collection. We further provide
theoretical estimates of efficiency indicating
that 2D semiconductors can in
deed be
competitive
with
and complementary
to conventional photovoltaics, based
on
favorable
energy
bandgap, absorption, external radiative efficiency
, along with
recent experimental
demonstrations.
Photonic and electronic design of
2D semiconductor
photov
oltaic
s represents
a new direction for
realizing ultrathin, efficient solar cells with applications ranging from
conventional power generation to
portable
and
ultralight
solar power.
*Corresponding author: haa@caltech.edu
Keywords:
Transition metal dic
halcogenides, heterostructures, light
-trapping, Shockley
-
Quessier, nanophotonics, 2D materials
Since the isolation of graphene as the first free
-standing two
-dimensional (2D) material
(from graphite), the class of layered 2D materials with weak van
der Waals inter
-planar bonding
has expanded significantly. Two
-dimensional materials now span a great diversity of atomic
structure and physical properties. Prominent among these are the semiconductor chalcogenides
of transition and basic metals (Mo, W, G
a, In, Sn, Re etc.)
1-3
, as well as layered allotropes of other
p-block elements of the periodic table such as P, As, Te etc.
4
The availability of atomic layer
thickness samples of stable, passivated, and dangling bond free semiconductor materials ushers
in a new phase in solid state device design and optoelectronics.
1, 5-
8
A notable feature of the metal
chalcogenide 2D semiconductors is the transition from an indirect bandgap in bulk to direct
bandgap (E
g
) in monolayer form, resulting in a high photoluminescence quantum yield (PL QY)
9-
10
in turn corresponding to high radiative efficiency. This combined with the bandgap ranging
from visible to near infrared part of the spectrum (1.1 to 2.0 eV)
1
makes the chalcogenides
2
attractive
as candidates for
photovoltaic applications. Likewise, small bandgap elemental 2D
semiconductors such as black phosphorus and alloys of arsenic and phosphorus are attractive
candidates for thermo
-photovoltaic applications in their few layer to bulk form (E
g
≤ 0.6 eV). In
the ultrathin limit (<5 layers) the increase in bandgap to > 1 eV due to quantum confinement
makes them attractive for conventional photovoltaic applications as well.
11-
13
However, the
current inability to synthesize these materials in a scalable manner wit
h precise control over
thickness combined with their lack of air stability has restricted their investigation to preliminary
photocurrent generating devices from mechanically exfoliated samples.
Owing to the above
-mentioned attributes, 2D semicon
ductors have been used to
demonstrate devices with photovoltaic effects.
14-
16
In most cases these have been proof
-of-
concept devices reporting the basic feasibility of photovoltage generation in such material
systems. At the same time, the field of photovo
ltaics is at an advanced stage, with GW
-scale
commercial production for silicon
-based photovoltaics now a reality and cost of solar power
expected to achieve parity with fossil fuel based power plants by 2020. Therefore, simply the
demonstration of a photo
voltaic effect in novel and emerging semiconductors is no longer by
itself a requisite for sustained interest from the perspective of photovoltaics application. In this
perspective, we argue that 2D semiconductors are indeed promising for photovoltaic
appl
ications and have the potential to not only match but also surpass the performance and
complement the conventional photovoltaic technologies based on Si and GaAs. We present a
detailed and comparative analysis based on optical and electronic properties of 2D
semiconductors and conclude that it is feasible in principle to design photovoltaic devices with
power conversion efficiencies exceeding 25%. We further present strategies for photonic and
electronic device design to maximize light trapping/useful absor
ption and efficient extraction of
photo
-excited carriers, respectively. We also give a brief outlook for the prospects for tandem
integration to enable a ‘2D
-on
-silicon’ dual junction tandem solar cell. Finally, we provide a
roadmap for the advances needed
to achieve high
-performance photovoltaic devices with
nanometer thick absorbers and provide a critical assessment for future research developments
in this area.
Absorption and Photonic Design:
Light absorption in the active layers of a photovoltaic cell is one of the key performance
metrics that dictates device efficiency. For semiconductors, including 2D materials, the
absorption is governed by the electronic band structure and energy
bandgap. There is an inherent
trade
-off between bandgap (and v
oltage) with absorption (and photocurrent). In Fig. 1a, we
summarize the bandgap energies and absorption coefficients for all major photovoltaic materials
investigated to date at the commercial and research scales. As is evident from Fig. 1a and the
discus
sion above, most materials considered for photovoltaic applications have energy bandgaps
close to the optimum value of 1.34 eV.
In modern photovoltaic devices, light trapping techniques are often employed to
maximize incident light absorption and ph
otocurrent generation in the active layer to increase
3
the cell efficiency, which also has the benefit of reducing thickness and thus material use and
device weight.
17-
19
The extent of light trapping in a medium in both the ray optic (bulk)
20
and
nanophoton
ic (sub
-wavelength) regimes
21
may be quantified by the ratio of imaginary and real
parts of dielectric function, i.e., loss tangent. Figures 1b and 1c show spectral dependence of
absorption coefficients and loss tangents in the sulfides and selenides of Mo
and W compared
with Si, GaAs, and the recently emerging hybrid organic
-inorganic perovskites
22-
23
. Owing to the
high refractive indices of the transition metal dichalcogenides (TMDCs), they exhibit significantly
higher absorption per unit thickness as com
pared to Si, GaAs, and even the perovskites. Thus,
TMDCs are ideally suited for highly absorbing ultrathin photovoltaic devices.
Despite the large absorption coefficient values for TMDCs, a free
-standing monolayer only
absorbs ~10 % of the above b
andgap, normally incident photons
9, 24-
25
(Figure 2a). In multilayers
with bulk
-like electronic structure up to ~25 nm thick, the absorption is less than 40% ,
26
and high
surface reflectivity limits absorption. Therefore, light trapping designs and device
architectures
will play a critical role in enabling efficient 2D semiconductor photovoltaics in the ultrathin limit.
Several strategies have been proposed and preliminary demonstrations have been reported
including use of plasmonic metal particles, shells
or resonators to enhance photocurrent and
photoluminescence.
27-
37
More sophisticated and lossless dielectric optical cavities such as
photonic crystals and ring resonators have also been used to enhance absorption, mainly aimed
at emission applications in monolayer samples.
38-
41
For large area photovoltaic applications, light trapping strategies that involve little or no
micro
- or nanofabrication and patterning are desirable to improve performance while minimizing
cost.
42
-43
An interesting strategy towards
this end is the use of thin film interference. Figures 2b
and 2d show two common strategies for lithography
-free absorption enhancement in ultrathin
semiconductor films. In both of these designs, a highly reflective metal (e.g., Au or Ag) is used as
a par
t of an “open cavity”, such that resonant light trapping conditions are met. Hence, even an
atomically thin absorber spaced λ/4 away from a reflector (Figure 2b) enables destructive
interference at the interface, resulting in significant absorption enhance
ment.
44
-45
Another
approach is to place an ultrathin absorbing layer in a direct contact with the back reflector
46
-
47
.
This strategy has worked well for TMDC devices with Au and Ag back reflectors, resulting in record
broadband absorption (> 90%)
26
and quantum efficiency (>70%) values
48
in < 15 nm thick active
layer devices. Note that back reflectors are widely employed in conventional thin
-film
photovoltaic devices, where absorption enhancement is due to multi
-pass light interactions
within the sem
iconductor. We also note that the light trapping may be further enhanced by the
use of nanostructured resonators coupled to thin film absorbers,
49-
50
as is shown schematically
in Figures 2c and 2e.
Overall, due to the large values n and α for TM
DCs, trapping nearly 100% of the incident
light may be achieved for few
-nm thick active layers. However, enabling broadband, nearly
perfect absorption in sub-
nm thick monolayers is more challenging and less scalable, as currently
-
identified light trapping
approaches are likely require fabrication of nanoscopic resonators or
4
antennas on top of or etched into the monolayer.
51-
53
Thus an open challenge at present for
atomically thin photovoltaics is the photonic design of nanostructures that retain the elec
tronic
structure and photonic properties of monolayer 2D materials, while also exhibiting optical cross
sections equivalent to multilayer/bulk samples.
Carrier Collection and Electronic design:
While photon absorption represents one limit to photovoltaic efficiency, the collection of
photo
-excited charge carriers dictates the practical limitations on the maximum attainable
current. Carrier collection is often quantified in terms of the probabilit
y of carrier collection per
absorbed photon, often termed the internal photocarrier collection efficiency (IPCE)
or internal
quantum
efficiency (IQE), and is sensitive to several factors, including device dimension and
design, contacts and material quality
. Frequently also reported is the external quantum efficiency
(EQE) or probability of carrier collection per incident photon, which can asymptotically approach
the IQE for perfect absorption. In reality, the device design and material quality plays a key r
ole
in determining the IQE, whereas EQE depends on these factors as well as photonic design.
A 2D semiconductor absorber can be integrated into a photovoltaic device in several
possible ways. Figure 3 details the schematic designs of some possible approac
hes. To separate
the photo
-excited carriers, one either requires a built
-in potential within the absorber layer, often
provided by a pn junction, or alternatively a uniformly doped absorber layer cladded by carrier
selective contacts.
54
This pn junction ca
n either have an in
-plane configuration spanning adjacent
regions of different composition or doping in the covalently bonded 2D plane, or an out
-of-plane
configuration featuring vertically stacked van der Waals
-bonded layers perpendicular to the 2D
plane.
In each case, the device structure can consist of a homojunction
55-
57
or a heterojunction
58-
61
design. Each of these concepts has advantages and limitations. As an example, for vertically
stacked pn junctions, 2D semiconductors are uniquely positioned to achieve high QE
37, 48, 62
owing
to their atomic
-scale thicknesses, ensuring < 10 nm excited carrier transit distances (Figure 3 b).
Similarly, in
-plane collection devices are well suited for forming junctions via substitutional,
58-
59
chemical,
55
thickness
variation
63
-
65
or electrostatic doping
66
-
69
, as in Figure 3 a, that can enable
large open circuit voltages. Vertically
-stacked junctions may also be more suitable for multilayer
thick absorbers while lateral junctions may be more suitable for monolayer ab
sorbers to
maximize shunt resistance and avoid electrical shorts in the device. Likewise, it is also likely to be
easier to integrate 2D semiconductor photovoltaics as the component sub
-cells of a tandem
photovoltaic structure integrated with or on convent
ional
Si
70-
71
, thin film CIGS, CdTe, or GaAs
72
photovoltaics, or even organic semiconductors
5, 73
-74
, where the 2D semiconductor forms a van
der Waals vertically stacked device. By contrast, a lateral junction would require in
-plane
integration of dissimilar materials. For lateral junctions, the absorber layer crystalline quality and
minority diffusion length are critical, since carriers must be transported in
-plane before reaching
the contacts.
The junction design and junction type also dictates the configuration of contacts required
for carrier collection. For vertically stacked junctions, one transparent, low
-absorptive loss
5
contact is essential for efficient optical absorption. Graphene has emerged as one alternative
75-
76
; however,
the sheet resistance of graphene still remains comparatively higher than for
transparent conductive oxides. Metal contacts are attractive alternatives, particularly for lateral
junction devices but metals typically result in loss of active area due to shadowing effect
s.
Nonetheless, with appropriate photonic design one can achieve effectively transparent contacts
composed of metallic structures.
77
Carrier selective contacts are also highly desirable for ultrathin
2D absorber layers, where the excited
-carrier transit di
stances are much less than the
characteristic carrier diffusion lengths, enabling device design without a built
-in potential or
electric field to separate carriers within a nearly intrinsic absorber layer. To date, little knowledge
or effort has been devot
ed to the design, optimization or demonstration of carrier selective
contacts for 2D TMDC based photovoltaics, and this is an opportunity for further research.
Progress, Challenges
, and Outlook:
While stable semiconducting TMDCs have only been is
olated and studied since 2011,
scientific progress has been rapid and extensive. However, a majority of the scientific progress
has been achieved using mechanically exfoliated 2D semiconductor layers which have allowed
small prototype devices to be realize
d, but this synthesis method is not scalable to areas of
relevance for large scale photovoltaics. Significant effort has also been devoted to large area
synthesis of 2D semiconductors via chemical vapor deposition (CVD).
78-
79
However, it is only
recently t
hat the community has begun to develop an understanding of the issues pertaining
growth, defects, and material quality using this method.
80-
81
Therefore, even though numerous
results have been published demonstrating proof of concept photovoltaic devices, no systematic
attempts have been made to address the fundamental issues that underlie development of
efficient photovoltaics, i.e. optical absorption, carrier collection, and open
-circuit voltage. While
several initial concepts for light management have be
en proposed for atomically thin
semiconductors, including the concepts noted above, few approaches have immediate promise
for integration into functional devices, and even fewer have the potential cost
-effectiveness and
scalability. One promising approach is the use of few-
layer thickness TMDCs directly placed on
reflective metal substrates as highlighted in Figure 2d above. This approach avoids any
micro/nanofabrication requirements for enhancing absorption. Further, the 2D TMDC absorber
can be directly gr
own on the metallic substrates over large areas, suggesting a potentially
scalable fabrication approach.
To further assess the viability of 2D semiconductor photovoltaics, it is worth evaluating
them i) in the context of commercial, mass
-produced
single junction photovoltaic technologies
and ii) to consider 2D semiconductor photovoltaics relative to detailed balance efficiency limits.
82
Figure 4a is a modified detailed balance model comparison of the maximum efficiency for a single
junction photovoltaic cells as a function of the absorber layer bandgap, for different values of
external radiative efficiency (ERE). In modified detailed balance models, ERE describes the
fraction of total recombination current that results in radiative emission that ultimately escapes
from a photovoltaic cell, and is assumed to have values ranging from much less than unity up to
6
unity. External radiative efficiency is a function of several parameters, including intrinsic
parameters such as material quality and electronic band structure, as well as extrinsic factors
such as electronic and photonic design. Similarly, internal radiative efficiency (IRE) represent
s
intrinsic material parameters and describes the fraction of recombination that is radiative
internally
within a photovoltaic device
– a closely related concept to the figure
-of-merit known
as photoluminescence quantum yield used for light emitters. In th
e asymptotic limit of perfect
device design, the maximum ERE achievable is bounded by the IRE. Photovoltaic
cells that reach
the thermodynamic detailed balance efficiency limit for their bandgaps must have EREs
approaching unity, but this is difficult to a
chieve in practice.
83
Direct bandgap materials such as
GaAs can exhibit EREs in the range of 1% < ERE < 20%, as compared to the typically <1% ERE
achievable in an indirect bandgap material such as Si. Notably, organic
–inorganic hybrid
perovskites are direc
t bandgap materials that have the potential for external radiative
efficiencies comparable to those for the highest
-quality direct bandgap semiconductors. In the
2D materials literature, ERE is not a commonly reported parameter and instead PLQY is generally
reported. By assuming the PLQY to be approximately equal to the IRE, and therefore the
maximum achievable ERE as the PLQY, we estimate the efficiency limits of TMDC
-based
photovoltaic devices as shown in Figure 4a. Monolayer materials with direct bandgaps have
recently been shown to exhibit
much higher
PLQY ( ~ 95% experimentally achieved)
10
and thus
also consequently ERE, compared to their indirect bandgap multilayer counterparts. Monolayer
2D semiconductors have relatively larger bandgap values (1.6
-2.1 eV) and large exciton binding
energies (0.6
-0.9 eV)
84
-
86
due quantum carrier confinement. Large exciton binding energies are
a priori
a disadvantage for high photovoltaic efficiency, and thus ‘exciton management’ is likely
to be an important aspect for 2D semiconductor photovoltaics. For single
-absorber devices,
maximum attainable power conversion efficiencies in monolayer absorber devices are
comparable to those for devices with multilayer absorber layers, which have more optimal
bandgap values (1.1
-1.3
eV) albeit with low PLQYs(
~
10
-4
–10
-2
) and therefore low ERE due to their
indirect bandgap nature. This suggests that although monolayer TMDCs are exciting for
photovoltaic power due to their direct bandgaps, even the highest quality monolayer materials
wi
th PLQY
~
1 would only result in an overall detailed balance power conversion efficiency
between 26
-27% in single
-junction devices, which can also be achieved with multilayer TMDCs
which have PLQYs values that are 2
-3 orders of magnitude lower. The above p
oint is especially
relevant, since for multilayer (10
-15 nm) TMDCs broadband, angle insensitive light
-trapping,
efficient carrier collection and device fabrication are relatively straightforward and have been
experimentally achieved to a large extent, in c
ontrast to the situation for monolayer absorber
layers. However, the bandgaps of monolayer TMDCs are in the range that would be nearly ideal
for top cell structures in a two
-junction tandem device together with e.g., a Si bottom cell device
(Figure 4b). M
onolayer photovoltaics might also be interesting for narrow
-band light harvesting
for colored and semi
-transparent photovoltaics in architectural and indoor applications
87
, and
also applications where light weight or portability is highly desirable.
7
To date, power conversion efficiencies in ultrathin 2D semiconductor photovoltaic
devices have remained below 5 %, as shown in Figure 4 c. The vast majority of reports of 2D
semiconductor photovoltaic device demonstrations have used monolayer absorber
layers.
However, there are very few quantitative reports of power conversion efficiency under 1 sun
AM1.5 or monochromatic illumination, spectral dependence of EQE and absorption in the active
layers of the device. This lack of information makes it very ch
allenging to compare literature
reports and complicates the assessment of quantitative performance estimates for a reported
photovoltaic device. The plots in Figure 4 c show a nearly linear dependence of power conversion
efficiency on the external quantum efficiency for bandgap values ranging from 1.1
-2 eV and ERE
values ranging from 1 to 10
-4
. A key observation from this plot is that one can attain greater than
20% power conversion efficiencies, even with bulk
-like TMDC absorber layers, provided that the
absorption and EQE are nearly perfect, enabled by appropriate photonic and electronic design.
Literature values for high EQE devices nonetheless still show less than 5% power
conversion efficiencies. The quantity limiting further efficiency improvement is
the open circuit
voltage (V
OC
), whose importance as a key parameter has been largely overlooked, and thus paths
to voltage improvement remain largely uninvestigated. Despite recent reports of high absorber
material radiative efficiencies, an overwhelming majority of the reported V
OC
values for TMDC
and other 2D semiconductors based photovoltaic devices are < 0.5 V
1, 5, 14-
15
with record values
of only ~0.8
-0.9 V in split gated, in
-plane homojunction devices
66, 68
. This implies a bandgap
-V
oc
offset (W
oc
= E
g
– V
oc
) > 0.8 V for most 2D semiconductor photovoltaic structures reported to
date. A number of these reports have been for devices exhibiting a photovoltaic effect dominated
by the Schottky barrier between the semiconductor and a metal or graphene contac
t.
37, 62, 88
Given that monolayer TMDC bandgaps lie generally in the range of 1.6
-2.1 eV, whereas multilayer
bandgaps range from 1.1
-1.3 eV, there is still significant room to improve V
OC
. For the case of
monolayer absorbers, the large exciton binding ener
gy due to the extreme 2D nature of carrier
confinement, poses a challenge for the charge separation from bound excitons after absorption.
To address the issue of charge separation and transport, it is useful to draw insights from
concepts found in the lite
rature for other excitonic devices, such as organic and dye sensitized
solar cells.
89-
93
To separate bound excitons, one either needs a junction in the active layer or
carrier-
selective contacts with built
-in potential that exceeds the exciton binding ener
gy. The
large binding energy will nonetheless result in a voltage penalty.
94-
95
Strategies that may enable
the voltage penalty and exciton binding energy to be reduced include increasing the carrier
concentration
96
or adding cladding layers with high static dielectric constants.
86, 97
Achieving high
V
OC
therefore remains a critical hurdle towards achieving high efficiency photovoltaic devices
from atomically thin semiconductor absorber layers. Key strategies to address the low V
OC
include
achieving control over doping and band alignment for pn homojunctions and heterojunctions, in
addition to optimizing the band alignments, bandgaps and conductivity of materials and
interfaces used to form carrier selective contacts. Progress will likely require a systematic
interdisciplinary effort combining concepts from chemistry, physics, and materials science to
achieve this goal.
8
In the future, atomically thin materials will continue to garner attention for ultrathin and
ultralight weight photovoltaics. However, no new photovoltaic technology likely to have a large
impact unless it offers attributes than are either superior to those for existing Si photovoltaics or
which can be usefully combined with Si photovoltaics, in order to widen the adoption of
photovoltaic
s by improving efficiency and lowering cost. Therefore, achieving high power
conversion efficiencies should remain a prime objective for atomically thin photovoltaics in 2D
materials. Notable however, is that the bandgaps for monolayer TMDCs are almost ide
ally suited
for high
-efficiency photovoltaics in a two
-junction tandem photovoltaic design featuring a Si
bottom cell
as seen in
Fig
ure 4 b,
especially
considering the near
-unity
PLQY
values recently
observed.
In addition to
tandem design
s combining 2D sem
iconductors with Si photovoltaics
, the
inherently
lightweight and flexible nature of atomically thin absorbers may enable use of these
materials in mobile and portable power applications as well as building integrated photovoltaic
applications where design
of semitransparent photovoltaics is desirable. Also of interest are
applications requiring high radiation hardness, such as space
-based photovoltaics. The van der
Waals bonded layered structure of TMDCs and 2D materials may facilitate easier integration with
other substrates and active layers.
Aside from practical, high
-efficiency photovoltaic applications, TMDCs and other 2D
semiconductors such as layered hybrid organic
-inorganic perovskites
98-
99
are also of considerable
fundamental scientific interest in light
-matter interactions and energy conversion. The ability to
create photovoltaic devices where photo
-excited carrier transit distances are comparable to
tunneling and hot carrier diffusion lengths presents opportunities to investigate and develop
fundamentally novel mechanisms for energy conversion involving electromagnetic radiation. In
particular, the presence of highly stable excitons at room temperature in free standing, oxide
free, van der Waals layers represents a distinctive class of materials that possesses the radiative
efficiency attributes of both direct gap inorganic semiconductors (GaAs/AlGaAs, GaN/InGaN
quantum wells) and organic quantum confined semiconductors (small aromatic molecules,
semiconducting polymers and carbon nanotubes). Further, the ability to electrostatically and
dynamically tune the environment around these quantum
-confined semiconductors, in order to
influence their optical properties, presents new avenues for photonics and optoelectronics.
Acknowledgements
:
This work
is part of the “Light
-Material Interactions in Energy Conversion” Energy
Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under award no. DE
-SC0001293. D.J. and A.R.D., acknowledge additi
onal support from
the Space Solar Power project and the Resnick Sustainability Institute Graduate and Postdoctoral
Fellowships. A.R.D. also acknowledges support in part from the Kavli Nanoscience Institute
Postdoctoral
Fellowship. J.W. acknowledges support
from the National Science Foundation Graduate Research
Fellowship under grant no. 1144469.
All the authors acknowledge support of the
Space Solar Power
Initiative
at Caltech
funded by the Northrop Grumman Corporation.
9
Figures:
Figure 1: Semiconductor a
bsorption
figures of merit for photovoltaic applications
:
a. Comparison of
energy bandgaps (eV) and absorption coefficients (cm
-1
) for a variety of semiconductor materials used for
commercial as well as research
-scale photovoltaics. The TMDC
s (both bulk and monolayers) of Mo and W
have some of the highest absorption coefficients among known materials. b. Spectral absorption
coefficient for selected photovoltaic materials, including Si and GaAs, as well the newly emerging methyl
ammonium lead iodide perovskites (MAPbI
3
) alongside the TMDCs. c. Loss tangent for the same materials
in (b).
10
Figure 2: Possible light t
rapping
configurations for enhancing sunlight absorption
:
a. A freestanding
TMDC monolayer absorbs only a fraction of the incident
sunlight (~10%), necessitating the use of light
trapping techniques to increase the absorption. b. Monolayer absorber in a Salisbury screen-
like
configuration where the spacer thickness is ~λ/4 and the reflector is a low loss metal such as Ag, Au, or
Al.
c. Schematic of a TMDC monolayer coupled with resonators/antennas to enhance light absorption. d.
Schematic of ultrathin, multilayer van der Waals absorber directly placed on a smooth reflective metal,
where absorption is due to thin film interference. e. Resonantly absorbing nanometer scale
antennas/resonators etched into a multilayer van der Waals material.
11
Figure 3. Carrier collection schemes for Van der Waals materials and structures
:
a. Schematic in
-plane
junction concepts for photovoltaic devices. Heterojunctions can be formed between two TMDC layers, as
well as within the same TMDC material in which the thickness varies, since the bandgap is a thickness
-
dependent parameter in the ult
rathin limit. Homojunctions can be created by electrostatic or modulation
doping as well as substitutional doping. b. Schematic diagrams for out
-of-plane junction concepts. The
contacts between active layers are primarily van der Waals in nature. Heterojunctions can be formed by
integrating two or more disparate TMDCs with different doping types and concentration. Van der Waals
material heterostructures can also be integrated with conventional photovoltaic materials such as Si or
III -V materials to make tan
dem cells. Graphene can effectively serve as transparent top contact material.
Out-
of-plane homojunctions can only be formed with substitutionally-
doped layers stacked on top of one
another.
12
Figure 4
. Photovoltaic efficiency analysis
: comparing TMD
Cs with established photovoltaic technologies
a. photovoltaic efficiency of a single junction cell using a modified Shockley
-Queisser detailed balance
model that assumes a non-
unity external radiative efficiency (ERE) of the semiconductor absorber layer.
Some of the 2D material absorbers have been included, based on known or estimated values of ERE from
PLQY reported or achieved in literature. b. Detailed balance power conversion efficiency estimates for a
tandem cell structure with monocrystalline Si as the bottom cell. The plot colors correspond to varying
ERE values
, as depicted in a. The materials parameters are
based on known ERE values
100
or record device
performance.
101-
103
c.
Plot of efficiency as a function of external quantum efficiency (EQE) for two different
values of ERE (1 and 10
-4
) for materials with bandgaps ~1.1 eV and 2 eV corresponding to upper and lower
bounds in available TMDC bandgaps. The relevant 2D materials based devices have been appropriately
mapped onto the plot based on literature reports.
48, 59, 61, 64, 66, 69
13
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