A photoconductive thienothiophene-based covalent organic framework showing charge transfer towards included fullerene

Article abstract of DOI:10.1002/anie.201208514

Filling a honeycomb: The thienothiophene-based covalent organic framework (COF) can be loaded with a complementary semiconductor, such as a fullerene derivative, and electronic interactions are observed (see figure). The novel periodic interpenetrated donor-acceptor system shows the spectroscopic signatures of efficient charge transfer on the nanoscale. Photovoltaic activity was demonstrated upon integrating the COF:fullerene film into a device. Copyright

Full text of DOI:10.1002/anie.201208514

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DOI: 10.1002/anie.201208514
Energy Transfer
A Photoconductive Thienothiophene-Based Covalent Organic
Framework Showing Charge Transfer Towards Included Fullerene**
Mirjam Dogru, Matthias Handloser, Florian Auras, Thomas Kunz, Dana Medina,
Achim Hartschuh, Paul Knochel,* and Thomas Bein*
Organic bulk heterojunctions combining electron donor and
acceptor phases are of great interest for designing organic
photovoltaic devices.[1] While impressive advances have been
achieved with these systems, so far a deterministic control of
their nanoscale morphology has been elusive. It would be
a major breakthrough to be able to create model systems with
periodic, interpenetrating networks of electron donor and
acceptor phases providing maximum control over all struc-
tural and electronic features.
an electron acceptor in organic photovoltaics.[11] Because of
the lack of structural order in the respective bulk hetero-
junctions it is very difficult to assess the impact of molecular
building blocks, bonding motifs, and energy levels on the
microscopic processes involving light-induced exciton forma-
tion, charge separation, and transport in such systems. Hence
ordered charge-transporting networks with a periodicity of
several nanometers are of great interest to understand the
mechanistic details of the light-induced processes and ulti-
mately to obtain design rules for the creation of efficient and
stable organic photovoltaic devices.[12,13]
The new TT-COF was synthesized under solvothermal
conditions by co-condensation of thieno[3,2-b]thiophene-2,5-
diyldiboronic acid (TTBA) and the polyol 2,3,6,7,10,11-
hexahydroxytriphenylene (HHTP; Figure 1a). Reaction
parameters are described in the Supporting Information.
As described in the following, the thienothiophene-based
COF forms stacks in an AA arrangement, as confirmed by N2
sorption and powder X-ray diffraction.
Powder X-ray diffraction (PXRD) confirms the formation
of a highly crystalline COF. Identification of the new structure
was conducted by comparison of structures modeled with MS
Studio (see Figures S1–S5 in the Supporting Information).[14]
Corresponding powder patterns were simulated and com-
pared to the experimentally obtained data. For previous COF
structures different stacking types of the hexagonal planar
sheets were reported.[1] Hence calculations were carried out
simulating an eclipsed AA arrangement and a staggered AB
arrangement. The experimental PXRD pattern for TT-COF
agrees very well with the simulated pattern for an eclipsed
AA arrangement (Figure 1b) with a hexagonal P6m symme-
try. Moreover, unit-cell parameters determined from the
experimental X-ray patterns match very well with those
obtained from the structure simulations (peak broadening
included). FT-IR spectroscopy can confirm the presence of
the newly formed boronate ester functionality. As previously
reported, the attenuation of the OH stretching band resulting
from the ester formation is apparent, and furthermore the
most characteristic modes of the C-B and C-O functionalities
can be assigned to the bands at 1395 cmÀ1 and 1353 cmÀ1 (see
Figure S8 in the Supporting Information).[15]
Herein we report a significant step towards this goal on
the basis of the recently discovered class of crystalline
covalent organic frameworks (COFs) which are created by
condensation of molecular building blocks.[2–5] Specifically,
the stacked layers of two-dimensional COFs permit charge
migration through the framework,[6] and several semicon-
ducting structures[7] with high carrier mobilities[8–10] have been
described. We have created a COF containing stacked
thieno[2,3-b]thiophene-based building blocks serving as elec-
tron donors (TT-COF), with high surface area and a 3 nm
open pore system. This open framework takes up the well-
known fullerene electron acceptor [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM), thus forming a novel structurally
ordered donor–acceptor network. Spectroscopic results dem-
onstrate light-induced charge transfer from the photoconduc-
tive TT-COF donor network to the encapsulated PCBM
phase in the pore system. Moreover, we have created the first
working COF-based photovoltaic device with the above
components. The organization of the molecular building
blocks into a crystalline framework with defined conduction
paths provides a promising model system for ordered and
interpenetrated networks of donors and acceptors at the
nanoscale.
The most prominent hole-conducting material used in
organic solar cells is poly(3-hexylthiophene) (P3HT), a thio-
phene-containing polymer with high charge-carrier mobili-
ties. The soluble fullerene derivative PCBM is often used as
[*] Dr. M. Dogru, M. Handloser, F. Auras, Dr. T. Kunz, Dr. D. Medina,
Prof. Dr. A. Hartschuh, Prof. Dr. P. Knochel, Prof. Dr. T. Bein
Department of Chemistry and Center for NanoScience (CeNS),
Ludwig-Maximilians-Universitꢀt Munich (LMU)
Butenandtstraße 5–13 (E), 81377 Munich (Germany)
E-mail: knoch@cup.uni-muenchen.de
The 11B MAS NMR spectrum (see Figure S9 in the
Supporting Information) shows a trigonal-planar boron
atom with a chemical shift of d = 21 ppm, which can be
distinguished from the starting material (TTBA: d = 15 ppm).
Transmission electron microscopy (TEM) images show the
nanoscale morphology of the crystals. A slightly tilted side
view shows the long ordered channels with distinct pore sizes
(see Figure S12 in the Supporting Information). A top view
bein@lmu.de
Homepage: http://www.bein.cup.uni-muenchen.de
[**] The authors gratefully acknowledge funding from the NIM excel-
lence cluster (DFG). We thank Bastian Rꢁhle for 3D graphics and
Markus Dçblinger for transmission electron microscopy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208514.
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Figure 2. a) TEM micrograph (top view) along the c-axis showing the
hexagonal structure of TT-COF. b) Nitrogen sorption isotherm of
*
degassed TT-COF measured at 77 K ( : adsorption *: desorption)
~
~
and TT-COF:PCBM ( : adsorption : desorption).
Figure 1. a) Reaction scheme for the co-condensation of TT-COF.
b) Comparison of the experimentally observed PXRD pattern (top) with
the simulated patterns using the module Reflex of MS Studio in an AA
arrangement (middle) and AB arrangement (bottom).
they are in good agreement (Table 1). In crystalline materials
these values can be predicted using geometric methods.[16,17]
Based on these results we can conclude that the pores of TT-
COF are not blocked by solvent molecules or by oligomeric
fragments.
along the c-axis shows hexagonal structures with a pore-to-
pore distance of around 3 nm (Figure 2a). The morphology
identified by TEM can be ascertained by scanning electron
microscopy (SEM), which shows small crystals with sizes of
about 100 nm that grow into larger arrangements (see Fig-
ure S13 in the Supporting Information). Based on peak
broadening of the PXRD patterns we obtain an average
domain size of about 30 nm.
The porosity of TT-COF was confirmed with N2 sorption
measurements at 77 K. The obtained type IV isotherm is
characteristic for mesoporous materials (Figure 2b), and
shows a very well-defined jump in the uptake as a result of
the highly crystalline mesoporous pore system. The very high
Brunauer-Emmett-Teller (BET) surface area was calculated
to be 1810 m2 gÀ1 and the pore volume is 1.19 cm3 gÀ1.
Evaluation of the isotherm using the NLDFT-based kernel
gives a pore size of 3 nm (see Figure S7 in the Supporting
Information). The experimentally obtained surface area and
pore volumes from the sorption data were compared to
theoretical values obtained with the Connolly method and
Table 1: Theoretical and experimental values for surface area and pore
volume of TT-COF.
Pore size
[nm]
Surface area
Pore volume
[m2 gÀ1
]
[cm3 gÀ1
]
TT-COF
Connolly
TT-COF:PCBM
3.0
3.0
2.4
1810
1810
700
1.19
1.17
0.49
All these results demonstrate that TT-COF is a well-
defined mesoporous crystalline material with highly organ-
ized sheets resulting in high surface areas. Semiconducting
polymers based on the thieno[2,3-b]thiophene building block
are known and these polymers exhibit high charge-carrier
mobilities and are stable under ambient conditions.[18] More-
over, photoinduced charge transfer from polymerized thie-
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nothiophene derivatives to fullerene-based acceptor mole-
cules is known.[19] This prompted us to study the electronic
interactions between TT-COF and PCBM. The large pores of
TT-COF allow for the uptake of the fullerene molecules
(because of attractive van der Waals interactions between the
host and the guest molecules) and thus for the investigation of
the resulting host–guest interactions (Figure 3).
Once the electron acceptor PCBM is loaded into the TT-COF,
the emission band at l = 487 nm is completely quenched. In
the literature, very efficient light-induced electron transfer on
the order of femtoseconds has been reported for
PCBM:P3HT blends.[19] In the PCBM:TT-COF system, the
photoinduced charges can dissociate at the nanostructured
interface between TT-COF and PCBM. In bulk heterojunc-
tions, short diffusion pathways on the order of 10–20 nm are
necessary to reach the electron acceptor PCBM within the
lifetime of the photoinduced charges.[22,23] Given the ordered
nanochannels of the donor material TT-COF, having about
3 nm channel diameter and thin molecular walls, such short
diffusion pathways are clearly available in the interpenetrat-
ing system PCBM:TT-COF. Therefore the observation of PL-
quenching confirms the interpenetrated structure of TT-COF
and PCBM.
Time-resolved photoluminescence (PL) was studied to
learn more about the light-induced interactions between the
TT-COF host and PCBM. The PL transients and their
respective fit-functions of PCBM and TT-COF loaded with
PCBM are shown in Figure 4b. In the bulk, PCBM shows bi-
exponential PL decay on the nanosecond time scale, while
TT-COF decays tri-exponentially with an average lifetime of
117 ps (Table 2).
Figure 3. Schematic representation of the host–guest complex of TT-
COF and a PCBM molecule to scale (C grey, O red, B green, and
S yellow. For clarity only one PCBM molecule is shown, whereas in the
experiments described, the COF channels are loaded with the PCBM
phase.
Table 2: PL-lifetimes of the decay components of PCBM, PCBM:TT-COF
and TT-COF.
Figure 2b shows a significant reduction of the absorbed
amount of nitrogen after loading TT-COF with PCBM. The
BET surface area of pure TT-COF decreases from 1800 m2 gÀ1
to 700 m2 gÀ1 for TT-COF:PCBM. Furthermore, the average
pore size distribution shows a broad range of pores up to 3 nm
(see Figure S7 in the Supporting Information).
Average
lifetime
(intensity
weighted)
Lifetime
exp.1
Lifetime
exp.2
Lifetime PL decay
exp.3
TT-COF
PCBM
117 ps
191 ps
(38%)
1406 ps
(76%)
1070 ps
(45%)
54 ps
13 ps
(48%)
Tri-exp.
Bi-exp.
Tri-exp.
(14%)
794 ps
(24%)
46 ps
The pore volume decreases significantly from 1.19 cm3 gÀ1
to 0.49 cm3 gÀ1 (see Table 1). Furthermore, we evaluated the
amount of PCBM adsorbed in the TT-COF using UV/Vis
spectroscopy (see Figure S17 in the Supporting Information).
From a 16 mgmLÀ1 PCBM solution in chlorobenzene 1.7 mg
PCBM are adsorbed into 5 mg COF. Normalizing the amount
adsorbed obtained by UV/Vis measurements to 1 g of TT-
COF would give an uptake of 340 mg of PCBM. Thus the pore
volume of TT-COF:PCBM should be corrected by the
amount of PCBM (a factor of 1.34), and this results in
a pore volume of 0.66 cm3 gÀ1 of TT-COF. Therefore 0.53 cm3
of the volume is displaced by PCBM. The calculated density
for 0.53 cm3 PCBM is 0.64 gcmÀ3, half of the bulk crystal
density (1.3 gcmÀ3), thus indicating that PCBM is not densely
packed in the 3 nm pores of TT-COF.
1206 ps
PCBM:TT-
COF
18 ps
(12%)
(43%)
When included in the channels of TT-COF, the PCBM
contribution can be modeled by a single exponential decay
component of 1070 ps, which is close to the intensity weighted
average lifetime of 1206 ps of the bulk material. Importantly,
the decay of TT-COF with a lifetime below 50 ps (46 and
18 ps) is substantially faster than the average decay in the
bulk, thus indicating an efficient additional relaxation channel
in agreement with the quenching observed in the PL-spectra
(see Figure 4a).[24]
The absorption spectra obtained for thin films of pure TT-
COF, TT-COF:PCBM, and PCBM (see Figure S18 in the
Supporting Information) confirm the existence of the small
thienothiophene and triphenylene units in the framework.
The notable optical density over the full visible spectrum is
partially caused by light scattering.[20] The fluorescence
emission spectrum of TT-COF shows that the framework
exhibits a blue emission upon excitation at l = 380 nm
(Figure 4a). Blue-emissive polymers based on boronate
ester condensation have been described by Lavigne et al.,
and recently the blue-emissive TP-COF has been reported.[15]
To further explore the capabilities of the TT-COF:PCBM
sytem, we incorporated a COF film of about d = 200 nm
thickness into a ITO/TT-COF:PCBM/Al device structure.
Spincoating a rather concentrated PCBM solution on the
COF film not only leads to filling of the COF pores, but also
produces an overlayer, which ensures that the aluminium top
contact does not short-circuit the cell. Under simulated
AM1.5G full sun illumination we measured an open-circuit
voltage of 622 mV,
a short-circuit current density of
0.213 mAcmÀ2, and 40% fill factor, thus giving rise to
a power conversion efficiency of 0.053% (Figure 4c). Illumi-
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nation density-dependent I–V measurements (see Fig-
ure S15 in the Supporting Information) prove that the
current scales almost linearly with the light intensity. The
external quantum efficiency of the device (see Figure S16 in
the Supporting Information), that is, the number of
extracted electrons per number of incident photons, reveals
that the majority of the photocurrent is generated from
wavelengths below l = 530 nm, which is in line with the
absorption spectrum of the COF film. A maximum EQE of
3.4% is observed at l = 405 nm. The tail at longer wave-
lengths exhibits the characteristic feature of PCBM above
l = 700 nm, which indicates that both interpenetrating
networks are photoelectrically active.
Structural disorder of the donor and the acceptor
phases can limit charge-carrier mobility and the charge
separation in organic bulk heterojunctions and hence can
decrease the performance of organic solar cells. The
creation of a crystalline semiconducting framework such
as a hole transporter whose pores can be filled with the
respective acceptor might ultimately overcome these
limitations. The novel thienothiophene-based TT-COF
described herein exhibits a significant photoresponse.
Furthermore, the TT-COF structure is thermally stable
and can be handled under ambient conditions. To increase
the efficiency of organic solar cells it would be desirable to
overcome the disadvantage of the very short exciton
diffusion lengths in organic polymers like PCBM, P3HT,
or MDMO-PPV. The novel interpenetrated TT-
COF:PCBM structure presented herein (where the dis-
tance between the donor network and the acceptor phase is
only a few nm) supports efficient charge transfer within the
lifetime of the excitons. First studies of a COF-based
photovoltaic device with an electron donor in the walls and
the complementary electron acceptor in ordered periodic
channels show that light-induced charge-separation and
collection is clearly feasible with these systems. Hence we
propose that interpenetrated networks of crystalline porous
semiconducting COFs with electron acceptor phases such
as PCBM present promising model architectures for the
understanding and further development of efficient photo-
voltaic devices.
Received: October 22, 2012
Published online: February 4, 2013
Keywords: boron · covalent organic frameworks ·
.
energy transfer · fullerenes · photoconductivity
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