Topological considerations for the design of molecular donors with multiple absorbing units

Article abstract of DOI:10.1021/ja501711m

The molecule AT1, with two weakly conjugated chromophores, was designed, synthesized, and examined within the context of its film forming tendencies. While the addition of the second chromophore to the central core enables broadening of the absorption spectrum, this change is mostly apparent in films that are grown slowly. Grazing incidence X-ray scattering (GIWAXS) analysis indicates that these spectral characteristics correspond to an increase in solid state ordering. This information, in combination with differential scanning calorimetry, suggests that the overall molecular shape provides a kinetic barrier to crystallization. As a result, one finds the absence of molecular order when AT1 is combined with PC₇₁BM in solution-cast blends. These findings highlight the importance of molecular topology when designing molecular components for solar cell devices.

Full text of DOI:10.1021/ja501711m

Communication
pubs.acs.org/JACS
Topological Considerations for the Design of Molecular Donors with
Multiple Absorbing Units
Lai Fan Lai,†,‡ John A. Love,† Alexander Sharenko,† Jessica E. Coughlin,†,∥ Vinay Gupta,†,§ Sergei Tretiak,∥
Thuc-Quyen Nguyen,†,# Wai-Yeung Wong,‡ and Guillermo C. Bazan*,†,⊥
†
Center for Polymers and Organic Solids, University of California, Santa Barbara, California 93106, United States
‡ Institute of Molecular Functional Materials and Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist
University, Waterloo Road, Hong Kong, P.R. China
§
Organic and Hybrid Solar Cell Group, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Dehli, 110012, India
∥Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico
87545, United States
⊥
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, Saudi Arabia
#
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
S
* Supporting Information
ABSTRACT: The molecule AT1, with two weakly
conjugated chromophores, was designed, synthesized,
and examined within the context of its film forming
tendencies. While the addition of the second chromophore
to the central core enables broadening of the absorption
spectrum, this change is mostly apparent in films that are
grown slowly. Grazing incidence X-ray scattering (GI-
WAXS) analysis indicates that these spectral characteristics
correspond to an increase in solid state ordering. This
information, in combination with differential scanning
calorimetry, suggests that the overall molecular shape
provides a kinetic barrier to crystallization. As a result, one
finds the absence of molecular order when AT1 is
combined with PC71BM in solution-cast blends. These
findings highlight the importance of molecular topology
when designing molecular components for solar cell
devices.
Figure 1. Compounds p-DTS(FBTTh2)2 and AT1.
tures must either tune frontier energy levels to increase open
circuit voltages (VOC) or increase the number of absorbed
photons and thereby the short-circuit current density (JSC).
Broad absorption may be achieved by coordinating different
chromophores within a single molecule. This strategy has been
successfully applied to oligothiophenes, with dye units as end
caps.10−12 In these examples, the dye units extend the
conjugation length, and therefore adjust the electronic structure
of the entire molecule. In this contribution we examine a
different design strategy, which is based on integrating
independent absorbing units at two ends of a symmetric
core. Figure 1 shows the specific molecule AT1. The central
chromophore C1 (red) is linked on both sides with
chromophore C2 (blue). Although C1 and the two C2 units
appear on paper to be conjugated, it was anticipated that they
would absorb nearly independently as a result of the molecular
topology, namely the twist in the conjugated backbone from
steric interference between the hexyl side chains on the internal
thiophene units. It also seemed reasonable that the internal
mprovements in materials design, processing techniques,
and device architectures have led to improvements in the
I
power conversion efficiencies (PCE) of organic photovoltaic
(OPV) devices.1 Significant effort on blends comprised of
narrow band gap polymer donors with fullerene acceptors in a
bulk heterojunction (BHJ) architecture has led to PCE values
on the order of 10%.2 However, recent successes with
molecular donors have led to renewed efforts to understand
and optimize this class of materials because of their
monodisperse nature and ease of purification relative to their
polymeric counterparts.3,4
A molecular design that has garnered significant attention
and led to high PCEs couples electron-rich donor (D) units
and electron-deficient acceptor (A) units.5,6 An example of this
class of molecules is p-DTS(FBTTh2)2, shown in Figure 1,
which has achieved a PCE of 9%.7−9 As the internal quantum
efficiency of molecular devices approaches unity, further
advancements in molecular OPVs will likely require new
strategies in molecular design. Such novel chemical architec-
Received: February 18, 2014
Published: March 24, 2014
© 2014 American Chemical Society
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core of AT1 would have the lowest energy transition and would
determine the highest occupied molecular orbital (HOMO)
energy level, due to the extended conjugation length and the
presence of the electron-rich dithieno(3,2-b;2′,3′-d)silole
(DTS) unit.
The synthesis of AT1 is shown in Scheme S1, and the full
synthetic and characterization details are provided in the
Supporting Information (SI). Thin film cyclic voltammetry
(CV) measurements show oxidation and reduction waves from
which we estimate that the HOMO and lowest unoccupied
molecular orbital (LUMO) energies are −5.29 and −3.14 eV,
respectively, corresponding to a band gap of 2.1 eV (Figure S6).
These data indicate that the frontier molecular orbitals of AT1
are very similar to those of p-DTS(FBTTh2)2, suggesting that
the levels are primarily determined by the C1 core.8
determined spectrum (Figure S17).15 Natural transition orbitals
show that the transition around 480 nm is mainly from C2
groups and the transition at 590 nm arises from C1 almost
exclusively (Table S3); see dashed lines in Figure 2a. Therefore,
the twist angle of 57° effectively breaks electronic communi-
cation between C1 and C2. The dependence of the absorption
spectrum as a function of twist angle is provided in the SI.
Figure 2a also shows that the absorption of AT1 films
exhibits a large red shift compared to solution. The features are
dependent on the evaporation rate of the CB. When spin-cast
for 60 s and allowed to dry under nitrogen, λmax = 620 nm. A
broad, ill-defined shoulder peak appears at low energies that
shifts λonset to 775 nm. A significantly different absorption is
observed if the film is allowed to dry slowly in a closed Petri
dish containing CB vapor. These conditions lead to shifts of
λmax to 760 nm and of λonset to 825 nm (band gap = ∼1.5 eV).
The sharpness of these features suggests increased molecular
order16 relative to the as-cast samples and highlights the
influence of processing history.
Figure 2a shows that the absorption spectrum of AT1 in
chlorobenzene (CB) exhibits an onset of absorption (λonset) at
Differential scanning calorimetry (DSC) measurements of
AT1 were performed at different heating rates (Figure 2b and
Figures S9−S10). At a rate of 5 °C/min, a cold crystallization is
observed at 67 °C during the heating cycle, followed by a
melting transition at 87 °C. No recrystallization peak is
observed upon cooling. At the slower rate of 1 °C/min, a
distinct melting temperature of 85 °C is observed with no
indication of cold crystallization. However, upon cooling at the
slower rate, the emergence of an exothermic peak at 56 °C was
observed, identified as crystallization from the melt. Both the
UV−visible absorption and DSC data therefore indicate that
AT1 has a substantial resistance to crystallization.
Figure 2. (a) Absorption spectra of AT1 in solution and films at
different drying rates. Dotted vertical lines represent calculated
transitions. Also shown is the spectrum from a 60:40 AT1:PC71BM
blend film cast from CB and DIO. (b) DSC scans at two different
speeds.
Grazing incidence wide-angle X-ray scattering (GIWAXS)
was used to probe changes in AT1 ordering; see Figure 4. The
664 nm, corresponding to an optical band gap of 1.9 eV, and a
broad absorption maximum (λmax) at ∼575 nm. These features
are similar to those of p-DTS(FBTTh2)2; however, in AT1 a
second absorption band is discernible as a shoulder at ∼475
nm. This second band is reasonably attributed to absorption of
the C2 chromophores, effectively broadening the range of
absorption, compared with C1 alone.
Density functional theory (DFT) calculations were employed
using CAM-B3LYP/6-31G(d,p).13,14 The minimum energy
structure of AT1 was calculated to have a dihedral twist angle of
57° between the two internal thiophenes; see Figure 3. Time-
dependent DFT studies were completed using a dielectric
similar to CB to determine excitation energies, and the
calculated spectrum matches well with the experimentally
Figure 4. GIWAXS profiles of as-cast films of AT1 spin coated from
CB solution for 10 s: (a) slow drying; (b) fast drying.
GIWAXS pattern of the fast-dried film exhibits two isotropic
peaks at approximately q = 0.37 and 1.77 Å−1, corresponding to
d-spacings of 17.0 and 3.5 Å, respectively. We assign the higher
q reflection as the π−π stacking peak and the lower q reflection
as an “alkyl chain stacking peak”, or arising from π-stacked units
separated by alkyl side chains.17−19 These spacings are
consistent with structurally similar molecules6,20,21 and
common conjugated polymers.22−24 The slow-dried film
exhibits the same reflections, as well as additional reflections
at q = 0.74, 1.09, and 1.46 Å−1. These additional reflections are
attributed to higher order alkyl chain stacking peaks. The π−π
stacking peak in the slow-dried film is highly anisotropic,
appearing primarily in the in-plane direction. This suggests
AT1 preferentially π-stacks in the plane of the substrate, though
a more specific understanding of molecular orientation is not
possible at this stage, as attempts to grow a suitable single
crystal were unsuccessful. The presence of higher order
Figure 3. Top-down and side-on view of the lowest energy
conformation of AT1.
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reflections and a preferred orientation of crystallites indicate
that AT1 films are able to achieve a greater degree of order
when evolved more slowly.
Atomic force microscopy reveals that slow versus fast drying
leads to considerably different surface topographic features; see
Figure S13. Specifically, coarser and blockier features are
observed for the slow dried film, compared to a fiber-like
morphology when the film dries more quickly.
topology; see Figure 3. This “awkward” structure is reminiscent
of spiro-type and tetrahedral multichromophore systems that
are resistant to crystallization and often provide amorphous
thin films.29,30 A combination of GIWAXS and absorption
spectroscopy shows that it is possible to find growth conditions
through slow solvent evaporation that yield ordered films of
pure AT1. However, the introduction of PC71BM exacerbates
the kinetic barriers for AT1 crystallization from solution. This
absence of crystallization likely impedes achieving the BHJ
morphology necessary for achieving high PCE. Looking
forward, the work highlights the need for developing new
processing strategies that allow different film growth profiles of
BHJ blends and for design strategies that allow incorporation of
multiple absorbing units within a planar molecular topology.
Charge transport was examined by using a hole-only diode
structure in which a film of AT1 was sandwiched between an
ITO/poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate)
(PEDOT:PSS) bottom electrode and an evaporated Au top
contact. Devices containing slow dried films were all electrically
shorted, presumably because of the inhomogeneous surface
features. Diodes fabricated with the quickly dried films were fit
with the space charge limited current model described by the
Mott−Gurney law (SI) to give a hole mobility of μp = 2.5 ×
10−4 cm2/(Vs), comparable with other BHJ donor materi-
als.25,26
Using solution casting conditions which have been successful
for structurally similar molecules8,27 (35 mg/mL chloroben-
zene, 50:50 AT1:PC71BM), the initial examination of solar cells
utilized the device architecture of glass/ITO(∼150 nm)/
PEDOT:PSS(∼35 nm)/BHJ(∼80 nm)/Al(∼100 nm). Blends
were cast from pure solvent, as well as with small amounts of
the solvent additive diiodooctane (DIO); see Figure S11. As-
cast from CB, AT1:PC71BM solar cells show negligible
performance (PCE = 0.3%, VOC = 0.59 V, JSC = 1.78 mA/
cm2, FF = 26%). Addition of 0.4% DIO by volume to the
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental information, solar cell fabrication, thin film
characterization, and full computational report are available.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bazan@chem.ucsb.edu.
Notes
The authors declare no competing financial interest.
casting solution enhances the PCE to 1.3% (VOC = 0.75 V, JSC
=
ACKNOWLEDGMENTS
■
6.39 mA/cm2, FF = 27%). This increase in VOC begins to
approach what would be expected empirically based on the
energy levels of AT1.8
Financial support for the synthesis of the molecules was
provided by the Office of Naval Research (N00014-11-1-0284).
V.G. was supported by the Indo-US Science and Technology
Forum (IUSSTF), Award No. Indo-US Research Fellowship/
2012-2013/26-2012. A.S. would like to acknowledge support
from a National Science Foundation Graduate Research
Fellowship. J.L. is supported by the Center for Energy Efficient
Materials, an Energy Frontier Research Center funded by the
Office of Basic Energy Sciences of the U.S. Department of
Energy. W.-Y.W. thanks the University Grants Committee of
HKSAR, China (Project No. [AoE/P-03/08]), Hong Kong
Research Grants Council (HKBU202410) and Hong Kong
Baptist University for financial support. Portions of this
research were carried out at the Stanford Synchrotron
Radiation Light Source user facility operated by Stanford
University on behalf of the U.S. Department of Energy, Office
of Basic Energy Sciences. We also acknowledge support of the
Los Alamos National Laboratory (LANL) Directed Research
and Development program. LANL is operated by Los Alamos
National Security, LLC, for the National Nuclear Security
Administration of the U.S. Department of Energy under
Contract DE-AC52-06NA25396.
Crystallization of the donor within the BHJ blend has proven
paramount to providing effective molecular OPVs.21,27,28 We
therefore examined the order within any AT1 phase in the BHJ
blend films first by examining the absorption profiles of
AT1:PC71BM films obtained from a number of conditions
(Figure S11). Despite different solvent, additive, and thermal
annealing conditions, the absorption of the blends exhibit no
evidence of the vibronic structure related to AT1 crystal-
lization; see the green trace in Figure 2a for a representative
example. GIWAXS spectra for blend films with and without
DIO, provided in Figure S12, appear almost identical. Although
there is some scattering in the region where one would expect
alkyl chain stacking reflections from AT1, there are no clear
features. There is also no apparent indication of any π−π
stacking peak. It is evident that the incorporation of PC71BM
prevents AT1 from overcoming the kinetic barrier to
crystallization resulting in a very poorly organized BHJ blend.
Additionally, blends of AT1 with two other common acceptor
molecules, a perylenediimide and vinazene derivative, showed
no evidence of vibronic structure in the absorption traces either
(Figure S14) indicating that difficulties in crystallization are
specific to the structure of the donor material.
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