Exciton Transport in Molecular Semiconductor Crystals for Spin-Optoelectronics Paradigm

Article abstract of DOI:10.1002/chem.202003447

Organic semiconductors with long-range exciton diffusion length are highly desirable for optoelectronics but currently remain rare. Here, the estimated diffusion length of singlet excitons (L_D) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent evaporation was shown to be up to approximately 124 nm. These crystals showed a previously unseen parallelogram morphology with layer-by-layer edge-on molecular stacking, isotropic optical waveguiding, radiation rate and non-radiation rate constants of 0.15 and 0.26 ns^?1 respectively, as well as good field-effect transistor hole mobility and theoretically computed strong electronic couplings as high as 109 meV. Photoresponse experiments revealed that the photoconductivity of DPA crystals is surprisingly not related to the radiative pathway but associated with rapid exciton diffusion to the crystal surface for charge separation and carrier bimolecular recombination. Taken together, DPA was shown to be a promising semiconducting material for a new organic optoelectronics paradigm.

Full text of DOI:10.1002/chem.202003447

Accepted Article
Accepted Article
Title: Exciton Transport in Molecular Semiconductor Crystals for Spin-
Optoelectronics Paradigm
Authors: Weigang Zhu, Yajing Sun, Jie Liu, Shuming Bai, Zhicheng
Zhang, Qiang Shi, Wenping Hu, and Hongbing Fu
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1/2020

Chemistry - A European Journal
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Exciton Transport in Molecular Semiconductor Crystals for Spin-
Optoelectronics Paradigm
,+
+
Weigang Zhu,* Yajing Sun, Jie Liu, Shuming Bai, Zhicheng Zhang, Qiang Shi, Wenping Hu,* and Hongbing Fu*
the perspective of physicists, despite of some efforts on P3HT,[6]
polymers are still too complex to be understood, both in
structure, morphology and device applications. In sharp contrast,
small molecules have many advantages. They have certain
chemical structures, are easy to chemically modify, highly
Abstract: Organic semiconductors with long-range exciton diffusion
length is highly desirable for optoelectronics but currently remain
rare. Here, we show the estimated diffusion length of singlet excitons
D
(L ) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent
evaporation is up to ~124 nm. These crystals show a previously
unseen parallelogram morphology with layer-by-layer edge-on
molecular stacking, isotropic optical waveguiding, radiation rate and
crystalline, as well as show high electronic device metrics (e.g.
ITIC, and Y6).[
such as
hexylphenylvinyl)anthracene,
7-8]
Moreover, small molecule semiconductors
-
1
-1
non-radiation rate constants of 0.15 ns and 0.26 ns respectively,
as well as good field-effect transistor hole mobility and theoretically
computed strong electronic couplings as high as 109 meV.
Photoresponse experiments reveal that the photoconductivity of
DPA crystals is surprisingly not related to the radiative pathway, but
associated with rapid exciton diffusion to crystal surface for charge
separation and carrier bimolecular recombination. Taken together,
this study shows that DPA is a promising semiconducting material
for new organic optoelectronics paradigm.
[
9]
α,ω-bis(biphenylyl)terthiophene,
2-(4-
oligo(p-
[
10]
and
phenylenevinylene)[11] show strong photoluminescence (PL) with
high photoluminescence quantum yield (PLQY) up to 90% in
[12]
solid state. For solution processing, molecular semiconductors
with multi-functions can be directly printed into large-area
devices.[13] Therefore, molecular semiconductors are a new
class of carbon-based materials which are desirable in
optoelectronics.[14] Among many molecular semiconductors,
anthracene and its derivatives are particularly interesting, due to
their unique high charge carrier mobility (µ) and high PLQY
simultaneously. We have been working in this research area
since 2009,[15] and have reported several high-performance
anthracene derivatives and corresponding device applications in
TFT, LED, and light-emitting transistor (LET),[14, 16-17] but the
underlying mechanism for such interesting properties remains
unclear, and the understanding of their excitons behavior is also
still in its fancy.[14]
Organic optoelectronic science and technology have attracted
worldwide interests due to their potentials to offer flexible,
stretchable electronic device products that is complementary to
other traditional commercial semiconductor techniques,
therefore efficiently create social and economic benefits. This
novel interdisciplinary is located at the interface between
chemistry, physics, chemical engineering, and materials science
and engineering. Organic optoelectronics involves π-conjugated
polymer and small molecule as the core materials via chemical
synthesis, towards targeted applications including optical data
storage, optical switch,[1] laser,[2] light-emitting diode (LED),[3]
Photo/electrically-generated
exciton
in
molecular
thin film transistor (TFT),[4] and solar cell,[5] etc. From the
semiconductors is known to play the key role in their resulted
optoelectronic and catalysis properties, one of the specific
perspective of chemists, polymers are different from batch-to-
batch synthesis, difficult to scale-up produce in industry (e.g.
D
parameters, exciton diffusion length (L ), is used to
[
5]
quantitatively determine specific applications. For organic solar
cells, the photoactive layer materials generally show L of 10-20
D
nm,[18] and this allows excitons to separate into free charge
NF300 , and N2200), and generally show low crystallinity. From
efficiently within microscopic phase segregated domains of bulk-
heterojunction films.[19] For some single crystals exhibiting
[
a] W. Zhu,[+] J. Liu, S. Bai, Prof. Q. Shi, Prof. W. Hu, Prof. H. Fu
Institute of Chemistry, Chinese Academy of Science (ICCAS), Beijing
National Laboratory for Molecular Sciences (BNLMS)
Beijing, 100190, China
singlet fission, L
D
of triplets can surprisingly reach several
micrometres (µms) for good photoconductivity performance.[
20]
E-mail: zhuweigang@iccas.ac.cn (W.Z.); hongbing.fu@iccas.ac.cn
D
On the other hand, the experimentally determined L values of
organic semiconductors are largely varied using different
(H.F.)
methods (such as PL mapping,[
21-23]
PL quenching, and
W. Zhu, J. Liu, S. Bai
University of Chinese Academy of Sciences (UCAS)
Beijing, 100049, China
photoconductivity) in literature,[24-25] and the correlations
between L and device parameters are still lacking, which is
D
Dr. Y. Sun,[+] Prof. Z. Zhang, Prof. W. Hu
highly interested within optoelectronics research community.
Herein, by using 2,6-diphenyl anthracene (DPA) as prototype,
Department of Chemistry, School of Science, Tianjin Key Laboratory of
Molecular Optoelectronic Sciences (TJ-MOS), Tianjin University (TJU)
Tianjin, 300072, China
D
we report that the estimated L of singlet excitons in DPA
E-mail: huwp@tju.edu.cn (W.H.)
] These two authors contributed equally to this work.
crystals is up to 124 nm. The DPA single crystals are grown by
tetrahydrofuran (THF) solvent evaporation, and show a new
parallelogram crystal morphology with edge-on molecular
orientation relative to substrate. Steady and time-resolved PL
spectroscopy measurements reveal that DPA crystals show
isotropic optical waveguiding property, PL lifetime (τ) of 2.45 ns,
[+
[
b] This work was supported by the National Natural Science Foundation
of China (No. 20925309, 21190034, 21073200, 21073079, J1103307),
the National Basic Research Program of China (973) 2011CB808402,
2012CB933102, the Specialized Research Fund for the Doctoral
Program of Higher Education (SRFDP. 20110211130001) and the
Fundamental Research Funds for the Central Universities and 111
Project. Weigang would like to thank his parents for support over years
on the occasion of their near 70 years’ birthday, and wish them all the
best, happiness, joy, and health.
-1
f
radiation rate constant (k ) of 0.15 ns , and non-radiation rate
-1
constant (knr) of 0.26 ns , respectively. Field-effect transistor
FET) measurements show that DPA crystals have good hole
(
) of ~2 cm V s-1 in ambient, which is supported by
2
-1
mobility (µ
h
Supporting information for this article is available from the author or via
a link at the end of the document.
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single-crystal-structure based computed large electronic
couplings. It will be seen that these allow good photoconductivity
response of DPA crystals, but very slow current decay in the
photo-switch experiment. Unexpectedly, the photoconductivity is
not related to the radiative process, but is associated with the
dynamics that laser-induced singlet excitons rapidly diffuse to
crystal surface for charge separation and carrier bimolecular
method, the nucleation and growth process of DPA crystal
nucleus under THF evaporation is rapid, thus leading to the
formation of a metastable parallelogram morphology. DPA
crystals show strong blue PL at CIE coordinate of (0.15, 0.10)
(Figure S3) with measured PLQY (Ф
f
) of 0.37 (Figure 1B),
agreeing well with our previous data of powder.[
14]
Transmission electron microscopy (TEM), selected area
electron diffraction (SAED), and X-ray diffraction (XRD)
experiments were performed to determine the single crystal
structure. Typical TEM image (Figure 1C, S4) and
corresponding SAED pattern (Figure 1D) on individual DPA
crystal reveal its single crystalline nature. The measured d-
space of 7.81 Å and 6.59 Å in nice electron diffraction patterns
are indexed to (010) and (001), respectively, compared with our
previously deposited cif structure: a = 17.973 Å, b = 7.352 Å, c =
D
recombination (BR). The L together with device mechanisms
and “structure-mobility-exciton-property” correlations show that
DPA has the potential to open up a new spin-optoelectronics
paradigm.
°
°
°
6
.245 Å, α = 90.00 , β = 90.646 , γ = 90.00 (CCDC no. 1044209,
Figure S5).[ Note that interestingly, these results are in accord
well with the Materials Studio software simulations. Typical XRD
curve of DPA crystals in-situ grown on clean glass substrate
26]
(
Figure 1E) shows very strong (h00) peaks, indicating that DPA
molecules are “edge-on” orientated relative to substrate with in-
plane intermolecular π-electronic couplings and they are layer-
by-layer stacked to form ~100 nm to several μm thick crystals,
as measured by Atomic Force Microscopy (Figure S6).
Importantly, this unique molecular packing of DPA crystal is
perfect for in-plane charge transport in the FET configuration,
which should yield good charge carrier mobility (vide infra).
Photophysical parameters of DPA were firstly characterized.
DPA monomer in THF shows well-structured absorption and PL
spectra, but interestingly 0-0 absorption peak of monomer
shows lower oscillator strength (f) compared with 0-1 and 0-2
absorption. The Ф
diphenyl anthracene. For the 0-0 PL peak of crystals (Figure
A), it shows lower f due to significant self-absorption of a large
f
of monomer in THF is 0.66 referred to 9,10-
2
quantity of crystals in the experiments. The PL decay lifetime (τ)
of monomer and crystal determined on a streak camera are 4.29
ns and 2.45 ns (Figure 2B), respectively. To further quantify the
radiation pathway of S
1 0
to S (Figure 4B), the radiation rate
constant (k
f
) is calculated to be 0.15 ns-1 (Table 1) by using the
equation:[
27]
k
f
f
= Ф / τ
This value is mild among the reported values from single
[
28]
crystals of organic materials. The corresponding non-radiation
-1
[25, 28]
rate constant (knr) is 0.26 ns , by using the equation:
= k / (k
Ф
f
f
f
+ knr)
To better understand the photophysical properties, the electronic
structures and vibrational properties of ground state and excited
state in solid DPA were obtained by first principles calculation.
Considering the aggregation effects, we adopt the hybrid
Quantum Mechanics/Molecular Mechanics (QM/MM) method,
selecting one central DPA molecule as the high-precision layer
with (TD) DFT calculation. And for the surrounding molecules,
we use the low-precision molecular dynamics with Universal
Force Field (UFF) force field (Figure S7). After optimizing
Figure 1. DPA crystal and structure. (A) Optical and (B) PL images of DPA
crystals; (C) TEM, (D) SAED images, and (E) XRD pattern of crystal.
DPA crystals were grown by a solvent evaporation method.
In a typical experiment, material was completely dissolved in
THF, and the solution was dropped onto substrate, crystals then
appeared after THF was slowly evaporated. DPA crystals show
thin and large size up to hundreds of μms (Figure 1A, S1), and
a new parallelogram shape different from that obtained from
completely, we calculate the vertical (Ever) and adiabatic
physical vapour transport (PVT) method[14]
.
This crystal
excitation energy (Ead) of DPA molecule, Ever = 3.02 eV and Ead
= 2.85 eV, respectively, as shown in Figure 2E. By analyzing
the properties of excited states, we find the transition process
with the maximum oscillator strength f happens in the 0-0
transition, f0-0 = 0.18, while interestingly f0-1 = 0.002. With the
morphology difference can be explained by Materials Studio
software simulation (Figure S2). The computed result shows
that DPA has a stable hexagon morphology but a potential
metastable parallelogram crystal shape. Compared with PVT
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help of thermal vibration correlation function, the calculated
emission spectrum is shown in Figure 2F and the 0-0 emission
peak is around 450 nm, which are consistent well with the
was thermally evaporated (Figure S10). The typical transfer and
output curves of DPA crystal FET device are shown in Figure
h
3B,C, respectively. The μ calculated in the saturation region is
2.26 cm V s-1, and the maximal mobility obtained is 2.41 cm2
2
-1
experimental results from DPA crystals. The calculated radiative
transition rate k
r
is 0.053 ns- and the internal conversion
1
V-1 s-1 at ambient conditions. The average μ
h
-1
value collected
-
1
from 10 separated devices is 1.92  0.38 cm V s-1, much lower
than the result grown from PVT method, which is known to
provide very high single crystal quality with thin thickness.[
2
transition rate kIC is 0.0148 ns , much lower the experimental
values (Table 1). In contrast, the calculated Φ is 0.78, higher
f
31]
than the experiment value of 0.37. Overall, the photophysical
computations rationalize the strong PL property of DPA.
However, the threshold voltage (V
with our previous observation from PVT grown crystal based
FETs.[14] The unusual large V
reflects intrinsic different charge
T
) herein is > -20 V, consistent
T
transport property of DPA from other common organic
semiconductors in FET geometry, thus DPA is a special class of
material that deserves further studies of charge transport
physics.
Figure 3. FET carrier mobility and electronic couplings. (A) Typical optical
image of a DPA crystal FET; (B) Typical transfer and (C) corresponding output
characteristics of a DPA crystal FET; (D) Molecular packing in DPA crystal.
Figure 2. Photophysical properties and computation. (A) Diffuse absorption
spectrum of DPA powder and PL spectrum of DPA crystals; (B) PL decay of
monomer and crystals; (C) Typical PL image of an individual DPA crystal and
Table 1. Optoelectronic properties of DPA single crystals.
(
D) corresponding PL spectra under 408 nm laser excitation; (E) The transition
[
a]
τ
ns)
k
f
-
k
nr
-1
μ
h
L
D
(nm)
process scheme for DPA; (F) The calculated fluorescence spectrum.
Ф
f
c
1
(cm2 V-1 s )
-1
(
(ns )
(ns )
Moreover,
micro-area
photoluminescence
(µ-PL)
0
.37
2.45
0.15
0.26
1.92 ± 0.38
123.63
0.57
measurements[27] were performed to spatially investigate PL
propagation and optical waveguiding properties in individual
DPA crystal (Figure 2C, S8). Under the excitation of a λ = 408
nm laser, the generated PL propagates and out-couples along
four directions to crystal edges with similar optical loss, leaving
similar PL intensity at the crystal edges (Figure 2D). This
[
a] Average with one standard deviation (1σ) is taken over 10 separate
devices.
Theoretical computations were performed to understand the
charge carrier transport properties. The DPA molecules pack in
a herringbone fashion similar to most anthracene derivants, so
we define the electronic couplings of charge transfer accordingly,
“
isotropic” optical waveguiding property of DPA is consistent with
our and other groups’ observations on organic molecular
crystals.[27, 29] Note that both of the PL propagation directions are
1 2
as shown in Figure 3D. The coupling values T , T , P of DPA
crystals are computed to be 109, 109, and 18 meV (Table S1),
respectively. Although the computation methods are slightly
almost parallel to crystal edges and not related to the molecular
packing structure. The collected PL intensity and optical
waveguiding property is not influenced by applied voltage bias
even up to 100 V (Figure S9), indicating exciton behavior is not
affected by electric field, and this would be very interesting in
other electronic device operations, such as solar cell and LET.
Charge carrier mobility of DPA crystals was studied by using
a bottom-gate top-contact FET configuration (Figure 3A). Gold
different, the T
of pentacene (79, 45 meV)
1 2
and T are surprisingly much higher than those
[
32]
[33]
and hexacene (88, 60 meV),
suggesting that DPA is a potential high-mobility semiconductor,
while the reorganization energy (λ) of DPA is 143 meV, much
[
32]
higher than that of pentacene (95 meV),
and Hexacene (79
meV).[ This may compromise the final charge carrier mobility
33]
of DPA single crystals (Table 1).
(
Au) was selected as the source (S) and drain (D) electrode
The L
D
plays the central role in organic optoelectronic
material. FET devices were fabricated using the home-
developed “copper grid mask” method,[30] and gold electrode
devices, however, there is limited study on LD of organic
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semiconductor crystals.[20,
25,
34-35]
The L
intensity of only tens of nW (Figure 4C). This is understandable
D
here is roughly
calculated using the equation previously used for perovskite
semiconductors which we believe is not perfect for organics but
indeed a best-case method:[36]
D
in terms of L vs crystal thickness, which allows photogenerated
free charge carriers quickly move to electrodes. Surprisingly, the
collected µ-PL intensity does not change when measuring
photoconductivity (Figure S16), contrary to the previous
photoconductivity experiments on rubrene crystal where triplet
excitons are involved.[20] In DPA crystals, the radiation pathway
is not related to the free charge carrier photogeneration, which
eliminates the possibility that DPA crystal shows singlet exciton
Where μ is exciton mobility (here we assume as FET hole
mobility, note that the hole mobility can be much lower than the
fission. Therefore, the non-radiation processes from 1
to 1
S
1
S
0
mobility of singlet excitons, so the real L
exciton lifetime (here is PL decay lifetime), k
constant, and T is the sample temperature. Using this method
and equation, the obtained L s are estimated values but these
D
would be higher), τ is
(
4
k
nr) play the key role in the measured photoconductivity (Figure
B, 2B).
In a typical photo-switch experiment, with a voltage bias of
0 V and laser excitation intensity of 0.547 uW, DPA crystal
B
is Boltzmann’s
D
5
do not change the final understandings on the material.
Unexpectedly, using the values we measured from FET and
device shows rapid current increase to “on” state, while drop to a
stable state very slowly (tens of seconds) with a current decay,
but the overall current on/off ratio remains significant (Figure
streak camera, the L
D
of DPA crystal is 123.63 nm. By applying
the mobility value of 34 cm V s-1 we obtained previously,[14]
2
-1
L
D
4D). This slow current decay suggests that large numbers of
is calculated to be as high as 464 nm, among the good values of
singlet exciton in organic semiconductors,[25, 37] but much shorter
free charge carriers are existed after the laser excitation
immediately, but also suffer significant recombination due to the
high carrier density. To further quantify the hole-electron
than those of perovskite bulk crystals (L
exciton transport with high µ and strong PL property of DPA,
D
>10 µm).[36] The singlet
h
recombination, the photocurrent
excitation intensities (Iirrs) is measured, using the equation:
I
ph under different laser
which is limited presented in the organic electronic material
family,[21, 34] should provide an ideal platform for advanced
optoelectronics, such as efficient solar electricity, LET, spintronic
device, and even electrically-pumped lasing.
[38]
I
ph = Iillume – Idark
where Iillume is the current under laser excitation, and Idark is the
current detected under dark. Using the equation:[
38]
I
ph ~ (Iirr)c
the photocapture coefficient (c) can be extracted from the ln(Iph
)
~
ln(Iirr) curve. The typical ln(Iph) ~ ln(Iirr) curve of DPA crystal
device exhibits linear relation (Figure 4E), and the c is fitted to
be 0.57, which suggests BR mechanism at the surface of DPA
crystal. This is consistent with L
long-lived excitons rapidly diffuse across the layer-by-layer
edge-on” stacked DPA molecules to the surface of crystal, and
separate into free charge carrier, transport, and recombination
for conductivity (Figure 1E), although for this effect no long L is
D
and crystal thickness, and
“
D
required. Overall, the crystal surface plays an important role in
charge separation and recombination for photoconductivity,
similar to our previous observations on long L
D
materials.[38]
In conclusion, single crystals of DPA have been grown by a
simple solution evaporation method. DPA crystals show a
previously unseen parallelogram morphology, isotropic optical
waveguiding property, radiation rate and non-radiation rate
constants of 0.15 ns and 0.26 ns-1 respectively, good in-plane
-1
-1
s-1, estimated L
u
h
of ~2 cm2
V
D
of 124 nm, unique slow
Figure 4. Photoconductivity measurements. (A) Schematic diagram of μ-PL
and photoconductivity integrated platform; (B) Photophysics model of DPA
crystal; (C) Typical I-V characteristics of DPA crystal based devices under
different Iirrs of λ = 408 nm excitation laser; (D) Corresponding on/off switch
curve; (E) Typical dependence of ln(Iph) on ln(Iirr) of DPA crystal device.
photoconductivity switching, and photocapture coefficient of 0.57.
The L and good µ are consistent with the photocurrent decay
and crystal surface BR mechanism in the photoconductivity
experiments. Computed strong electronic couplings and
D
h
relatively large reorganization energy explain the good µ
ambient conditions, and interestingly the simulated PL spectrum
and strong Φ agree well with the experimental observations.
The and “molecular packing-mobility-exciton-property”
relations here together show that DPA is potential
h
in
With these understandings on L
D
of DPA crystals, we
f
therefore performed photoconductivity measurements. DPA
single crystals were grown by solvent evaporation, and devices
were in-situ fabricated using the same procedure as FET.
Photoresponse measurements were conducted on a home-
developed optoelectronic characterization platform[30] that allows
to simultaneously probe electronic and photonic signals of
samples at micro-nanoscale (Figure 4A). Indeed DPA crystal
shows strong photoresponse to the excitation laser (λ = 408 nm),
and significant enhancement of current is determined under the
L
D
a
semiconductor to open up a new optoelectronics paradigm.
Further related research, underway in our Laboratory, are
printing
flexible,
stretchable,
large-area
carbon-based
semiconducting material devices and circuits, developing
ultrafast time-resolved electron paramagnetic resonance (tr-EPR)
spectroscopy to probe unseen high spin state and dynamics, as
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202003447
COMMUNICATION
COMMUNICATION
Weigang Zhu,*,+ Yajing Sun, Jie Liu,
Shuming Bai, Zhicheng Zhang, Qiang
Shi, Wenping Hu,* Hongbing Fu*
+
Crystal Matter: We report estimated
singlet exciton diffusion length of 124
nm in 2,6-diphenyl anthracene
crystals for advanced optoelectronics.
The crystals show parallelogram
morphology, non-radiation rate
Page No. – Page No.
-1
constant of 0.26 ns , good field-effect
transistor hole mobility, and strong
electronic couplings. These allow
excitons rapidly diffuse to crystal
surface for charge separation and
bimolecular recombination in
Exciton Transport in Molecular
Semiconductor Crystals for Spin-
Optoelectronics Paradigm
photoconductivity experiments.
This article is protected by copyright. All rights reserved.