Reticulated heterojunctions for photovoltaic devices

Article abstract of DOI:10.1002/anie.201004055

An organic semiconductor device is formed by the self-assembly on a transparent electrode surface. The donor (see picture; dibenzotetrathienocoronene, yellow layer) deposits as supramolecular cables, and the acceptor (C₆₀, orange) subsequently infiltrates this network. This network provides a donor-acceptor interface that is interwoven at the nanoscale. When incorporated into a solar cell, the active layer provides large increases in power conversion efficiencies.

Full text of DOI:10.1002/anie.201004055

Angewandte
Chemie
DOI: 10.1002/anie.201004055
Molecular Electronics
Reticulated Heterojunctions for Photovoltaic Devices**
Alon A. Gorodetsky, Chien-Yang Chiu, Theanne Schiros, Matteo Palma, Marshall Cox,
Zhang Jia, Wesley Sattler, Ioannis Kymissis, Michael Steigerwald, and Colin Nuckolls*
Herein we present a molecular self-assembly process on the
surface of transparent electrodes that yields a new type of
organic semiconductor device structure: the donor deposits as
supramolecular cables, and the acceptor subsequently infil-
trates this network. This process results in a
donor–acceptor interface that is interwoven at
the nanoscale. When incorporated into photo-
voltaic devices, such nanostructured films provide
large increases in power conversion efficiency.
DBTTC; Figure 1a).[14] 6-DBTTC stacks into hole-transport-
ing, columnar superstructures (Figure 1b), and on polymer
coated indium–tin oxide (ITO) electrodes, these columns
merge into a supramolecular three-dimensional network of
To date, the most efficacious materials for
organic photovoltaics are formed from the phase-
segregated mixtures of semiconducting polymers
and derivatized fullerenes—the bulk heterojunc-
tion (BHJ).[1–5] Phase segregation in the BHJ
creates random networks of donor–acceptor
domains, and these numerous domains yield an
increased interfacial area, which in turn affords an
increased photovoltaic response.[1–5] Efficiencies as
Figure 1. a) Molecular structure of 6-DBTTC. b) Illustration of the assembly of
6-DBTTC into hole-transporting columns.
high as 4% have been reported for single-cell
BHJs with small molecules as the electron
donors;[6–12] higher efficiencies have been reported
for tandem cells.[13] Improving the structure and size of the
donor and acceptor domains within the BHJ remains an
unmet challenge.
cables. This network functions as a scaffold for the molecular
recognition and directed assembly of C60, thereby forming a
nanostructured p–n bulk heterojunction. When incorporated
into a solar cell (Figure 2a), the nanostructured active layer
provides a three- to fourfold increase in the power conversion
efficiency.
We address this challenge with a new type of organic
semiconductor, termed dibenzotetrathienocoronene (6-
[*] Dr. A. A. Gorodetsky, C.-Y. Chiu, W. Sattler, Dr. M. Steigerwald,
Prof. C. Nuckolls
Department of Chemistry and
The Center of Electron Transport in Molecular Nanostructures
Columbia University, New York, NY 10027 (USA)
Fax: (+1)212-932-1289
E-mail: cn37@columbia.edu
Homepage: nuckolls.chem.columbia.edu
Dr. T. Schiros, M. Cox, Z. Jia, Prof. I. Kymissis
Energy Frontier Research Center
Columbia University, New York, NY 10027 (USA)
Figure 2. a) Illustration of the OPV device architecture with a nano-
structured active layer. b) Typical current density–voltage (J–V) curves
for a 6-DBTTC device with (blue) and without illumination (red) at
100 mWcmÀ2
.
Dr. M. Palma
Department of Applied Physics and Applied Mathematics
Columbia University, New York, NY 10027 (USA)
The molecular design of 6-DBTTC incorporates two well-
known organic semiconductor motifs: the contorted hexa-
benzocoronenes (HBCs)[15–21] and the anthradithiophenes.[8,22]
Contorted HBCs consist of three fused interpenetrating
pentacene subunits that form a doubly concave shape due
to steric interactions at the periphery of the molecule.[15]
These molecules self-organize to form columnar, hole-trans-
porting nanostructures in liquid-crystalline phases,[15]
cables,[16] and self-assembled monolayers,[17] with concomitant
field effect mobilities of up to about 1 cm2 VÀ1 sÀ1.[23] We have
[**] This work was supported by the Office of Naval Research under
Award Numbers N00014-09-01-0250 and N00014-09-1-1117, by the
National Science Foundation under Award Number CHE-0936923,
and by the Center for Re-Defining Photovoltaic Efficiency Through
Molecule Scale Control, an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001085. The National
Science Foundation (CHE-0619638) is thanked for the acquisition of
an X-ray diffractometer.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004055.
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found that the contorted HBCs form a shape-complementary
complex with n-type acceptors, such as C60 and C70, yielding an
intimate, self-assembled donor–acceptor interface.[21] In pho-
tovoltaic devices, this interface results in high open-circuit
voltages and an average power conversion efficiency of about
0.55%.[21] The present study capitalizes on the desirable
properties of the HBCs and combines them with the proper-
ties of the anthradithiophene class of molecules that have
been used to great effect in organic electronics.[8,22]
We have tested many HBC derivatives in OPV devices,
but the 6-DBTTC is vastly superior.[24,25] The generalized
architecture of the photovoltaic device is illustrated in
Figure 2a. To fabricate the devices, we used clean glass
substrates patterned with the ITO anode, which were in turn
coated with about 40 nm of PEDOT:PSS (poly(3,4-ethyl-
enedioxythiophene) poly(styrenesulfonate)). The 6-DBTTC
was then spincast onto the PEDOT:PSS layer to a total
thickness of about 90 nm (including the PEDOT:PSS layer),
and the substrate was annealed at 1508C. The active layer was
completed by thermal evaporation of a circa 40 nm thick film
of C60. Subsequently, an aluminum cathode was deposited
through a shadow mask to furnish the final device.
Figure 3. Fluorescence microscopy images of films from 6-DBTTC that
were a) unannealed, b) annealed at 1008C, and c) annealed at 1508C.
The corresponding noncontact AFM images and cross-sectional pro-
files (indicated in blue) of films from 6-DBTTC for conditions (a)–(c)
are shown in (d)–(f), respectively.
The current density–voltage curves exhibit almost ideal
diode behavior (Figure 2b). The short-circuit current density
(Jsc) is 6.7 mAcmÀ2, the open-circuit voltage (Voc) is 0.60 V,
and the fill factor (FF) is 0.47. These parameters yield a power
conversion efficiency (PCE) of about 1.9%.[26] The corre-
sponding external quantum efficiency (EQE) spectrum
resembles both the 6-DBTTC and C60 thin-film spectra[27]
and reaches values of ꢀ 65% (Supporting Information,
Figure S1).[26] These are excellent values for small molecule
BHJ organic photovoltaics.[6–12]
The 6-DBTTC molecule provides a three- to fourfold
increase in efficiency compared to the HBC devices we have
previously prepared.[21] We can compare the electronic
properties of 6-DBTTC to HBC to help elucidate why 6-
DBTTC yields more efficient devices. The 6-DBTTC highest
occupied molecular orbital (HOMO) is 5.1 eV below the
vacuum level and lowest unoccupied molecular orbital
(LUMO) is 2.3 eV below vacuum, yielding a bandgap of
2.8 eV (Supporting Information, Figure S2).[28] These values
are similar to those found for HBC,[21] and as expected 6-
DBTTC acts as an electron donor. The solution absorption
spectrum for 6-DBTTC is similar to that of the parent HBC
(Supporting Information, Figure S3). DFT calculations reveal
that the electronic structure of 6-DBTTC is dominated by the
radialene-like resonance structure (Figure 1a) that character-
izes the parent HBC.[18] The important conclusion is that the
physical and electronic structure of 6-DBTTC closely resem-
ble that of HBC.
pronounced for films annealed at 1508C (Figure 3c,f), and
they typically possess widths of hundreds of nanometers and
heights of 10 to 30 nm. These cables are uniformly distributed
over the entire electrode surface, with no apparent preferred
direction of alignment. It is also important to note that the
cross-sectional views of the fibers indicate that they are non-
planar and possess further corrugation at the nanoscale.
To determine how important this self-assembly process is
to the performance of the devices, we also tested control
devices with 6-DBTTC donor layers that were either not
annealed (Supporting Information, Figure S4 and S5) or were
vapor-deposited rather than spin-cast (Supporting Informa-
tion, Figure S6). Such devices display a drop in the current
density and up to a threefold reduction in the PCE. This is
similar to the efficiencies seen in the previously reported
HBC-based OPV devices that do not form cables.[21]
How does 6-DBTTC pack within these cables? To answer
this, we grew large (0.43 ꢀ 0.36 ꢀ 0.26 mm3) single crystals of 6-
DBTTC and measured the X-ray diffraction. Although the
aromatic core could be observed, it could not be fully refined
due to the low intensity (signal-to-noise ratio) of the
diffraction data, thus precluding the precise determination
of the atomic positions. Nevertheless, it is evident that the
molecules pack in columnar stack inside a triclinic unit cell.[29]
To further elucidate the structure of the 6-DBTTC cables
grown at the electrode surface, we utilized synchrotron-based
grazing incidence X-ray diffraction (GIXD), as shown in
Figure 4.[30] The two-dimensional pattern (Supporting Infor-
mation, Figure S7) indicates strong fiber texture with diffrac-
tion intensity confined to the lateral (Qr or in-plane) and
vertical (Qz or out-of-plane) directions, respectively, with
significant differences in peak spacing along Qr and Qz. The
Qr pattern is dominated by the [100] and [010] reflections; a
peak corresponding to diffraction from the [001] plane also
appears, albeit with weaker intensity. However, the intensity
The unique feature of 6-DBTTC is a heat-induced self-
assembly process on the surface of the electrode
(PEDOT:PSS-coated ITO). Figure 3 shows fluorescence and
atomic force microscopy images of spin-cast films before and
after annealing. Pristine unheated films are flat and feature-
less with a root-mean-square roughness of about 1 nm; few
structural features are evident (Figure 3a,d). However, cable-
like structures begin to emerge in films heated to 1008C
(Figure 3b,e). These structures become more numerous and
¯
ratio of these peaks is inverted in Qz, where the [011] peak is
dominant and the other reflections observed along Qr are
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likely free of the bottlenecks and dead-ends that inherently
accompany typical BHJs.[2–5] Consequently, photovoltaic
devices with this active layer feature high power conversion
efficiencies. The simple device processing conditions also
indicate that 6-DBTTC is promising partner with longer
wavelength absorbers in small molecule tandem cells.[13]
Furthermore, the efficiencies reported here are likely a
lower limit for the potential of the 6-DBTTC motif, the
UV/Vis transitions of which have a limited overlap with the
solar emission spectrum.
Received: July 2, 2010
Revised: July 29, 2010
Published online: September 16, 2010
Keywords: molecular electronics · organic materials ·
photovoltaics · self-assembly · semiconductors
Figure 4. GIXD intensity integrated along Qz (out-of-plane) and Qr (in-
plane) of thermally annealed (1508C) films of 6-DBTTC on ITO. The
simulated pattern based on the diffraction of the 6-DBTTC crystal is
also shown.
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DBTTC crystal, though the [011] peak of the film appears at a
¯
slightly lower value of Q.[31] The good agreement between the
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represented by pi-stacked cables, whilst the difference
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[13] Efficiencies of about 7% have been reported for more complex
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[14] A detailed description of the synthesis of 6-DBTTC can be
found in the Supporting Information.
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The next step in the device preparation is the thermal
evaporation of C60 onto these cables. The 6-DBTTC thin-film
fluorescence emission is now quenched, indicating complete
coverage with C60 (Supporting Information, Figure S8). The
cables can also no longer be directly visualized by AFM and
the films are relatively flat. The lack of contrast in the phase
images indicates that the topmost layer consists of a single
material (Supporting Information, Figure S9). Moreover, a
sulfur 2p (S2p) signal from the 6-DBTTC is observed with X-
ray photoelectron spectroscopy for C60-covered 6-DBTTC
films[21,32,33] (Supporting Information, Figure S10). Given that
the inelastic mean free path of electrons under our exper-
imental conditions (315 eV kinetic energy) is only about
10 nm,[34] the detection of a significant number of S2p
photoelectrons supports interpenetration of the two organic
layers. It is important to note that we can directly visualize the
fibers beneath the C60 film by increasing the exposure time
and gain in the fluorescence microscope (Supporting Infor-
mation, Figure S8). This data in conjunction with our other
physical measurements shows that we have the formation of a
nanostructured active layer, in which the donor and acceptor
components are interdigitated as in Figure 2a. Overall, it
appears that the 6-DBBTC cables act as a template for the
organization of the fullerene.
Controlled annealing of solution-processed 6-DBTTC
thin films yields a crystalline donor layer, which consists of
a supramolecularly assembled, three-dimensional network of
cables. This network possesses a large effective interfacial
surface area and further guides the subsequent assembly of
fullerenes. The resulting interdigitated architecture is most
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[22] The incorporation of fused and pendant thiophenes in linear
acenes has been shown to improve molecular stability, facilitate
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[23] HBC films have shown one-dimensional photoconductivity
along the aligned molecular stacks; see Refs [18] and [19].
[24] Devices with donor layers from alkyl-substituted HBC deriva-
tives exhibited power conversion efficiencies similar to those of
devices from the unsubstituted HBC in Ref. [21].
[29] Unit cell: a = 19.6, b = 20.2, c = 21.7 ꢄ; a = 112.4, b = 93.5, g =
101. 98.
[30] The GIXD patterns for 6-DBTTC on ITO and PEDOT:PSS
coated ITO are very similar, with some peak broadening
observed on the polymer coated surface.
[31] The critical angle for the 6-DBTTC film and ITO substrate are
about 0.18 and 0.218, respectively, so for incident angles (a) 0.1 <
a < 0.28, the X-ray light will penetrate into the film but be 100%
reflected from the film-substrate interface. The intensity to the
¯
right of the [011] peak position in Qz is due to scattering from the
¯
[011] peak centered around the reflected rather than the incident
beam, and as such, disappears for a > 0.218. The offset between
¯
the “reflection-mode” [011] peak and the primary (transmission
¯
mode) [011] peak matches the shift with a expected from the
(2p/l)sin2q relationship.
[25] See the Supporting Information and Ref. [21] for a more
detailed description of the device fabrication.
[26] The external quantum efficiency and power conversion effi-
ciency were calculated as described in: A. Moliton, J. M. Nunzi,
Polym. Int. 2006, 55, 583.
[27] The thin-film 6-DBTTC spectrum is broadened and red-shifted,
thereby tailing beyond 700 nm into the near infrared. Both the
band broadening and red-shift are expected to aid efficient light
harvesting; see Ref. [10].
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[34] C. Martin, E. T. Arakawa, T. A. Callcott, J. C. Ashley, J. Electron
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[28] The corresponding HOMO and LUMO values were deduced
from electrochemical studies in solution as described in: A. J.
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