Diketopyrrolopyrrole (DPP) functionalized tetrathienothiophene (TTA) small molecules for organic thin film transistors and photovoltaic cells

Article abstract of DOI:10.1039/c5tc01348h

Two novel π-conjugated small molecules based on the electron-deficient diketopyrrolopyrrole (DPP) and the electron-rich fused tetrathienoacene (TTA) frameworks are synthesized and characterized. As verified in the bandgap compression of these chromophores by electrochemistry and density functional theory (DFT) computation, these DPP-TAA derivatives exhibit substantial conjugation and ideal MO energetics for light absorption. The large fused TTA core and strong intermolecular SS interactions enforce excellent molecular planarity, favoring a close-packed thin film morphologies for efficient charge transport, as indicated by grazing incidence wide angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analysis. Top-gate/bottom-contact thin film transistors based on these systems exhibit hole mobilities approaching 0.1 cm² V^-1 s^-1. Organic photovoltaic cells based on DDPP-TTAR:PC₇₁BM blends achieve power conversion efficiencies (PCE) > 4% by systematic morphology tuning and judicious solvent additive selection.

Full text of DOI:10.1039/c5tc01348h

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Vegiraju, X. Yu, E. F. Manley, M. R. Butler, M. J. Leonardi, P. Guo, W. Zhao, Y. Hu, K. Prabakaran, R. Chang,
M. Ratner, L. Chen, A. Facchetti, M. M. Chen and T. Marks, J. Mater. Chem. C, 2015, DOI:
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ARTICLE
Diketopyrrolopyrrole (DPP) Functionalized
Tetrathienothiophene (TTA) Small Molecules for
Organic Thin Film Transistors and
Photovoltaic Cells
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2014,
Accepted 00th January 2014
a‡
b‡
c
c
Nanjia Zhou, Sureshraju Vegiraju, Xinge Yu, Eric F. Manley, Melanie R.
DOI: 10.1039/x0xx00000x
c
c
a
d
d
Butler, Matthew J. Leonardi, Peijun Guo, Wei Zhao, Yan Hu, Kumaresan
www.rsc.org/
b
a
c
c
Prabakaran, Robert P. H. Chang, Mark A. Ratner, Lin X. Chen, Antonio
c,d
b
a,c
Facchetti, * MingꢀChou Chen, * Tobin J. Marks, *
Two novel πꢀconjugated small molecules based on the electronꢀdeficient
diketopyrrolopyrrole (DPP) and the electronꢀrich fused tetrathienoacene (TTA)
frameworks are synthesized and characterized. As verified in the bandgap
compression of these chromophores by electrochemistry and density functional theory
(
DFT) computation, these DPPꢀTAA derivatives exhibit substantial conjugation and
ideal MO energetics for light absorption. The large fused TTA core and strong
intermolecular S···S interactions enforce excellent molecular planarity, favoring a
closeꢀpacked thin film morphologies for efficient charge transport, as indicated by
grazing incidence wide angle Xꢀray scattering (GIWAXS), atomic force microscopy
(
AFM), and transmission electron microscopy (TEM) analysis. Topꢀgate/bottomꢀ
contact thin film transistors based on these systems exhibit hole mobilities
2
ꢀ1 ꢀ1
approaching 0.1 cm V s . Organic photovoltaic cells based on DDPPꢀ
TTAR:PC BM blends achieve power conversion efficiencies (PCE) > 4% by
7
1
systematic morphology tuning and judicious solvent additive selection.
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Jou5Trnal Name
Introduction
Organic semiconductors are suitable candidates for the highꢀthroughput, largeꢀscale, inexpensive, and
environmentally benign manufacture of diverse optoꢀelectronic devices. Over the past two decades, a
number of polymeric and small molecule materials have been successfully employed in both organic thin
film transistors (OTFTs) and organic solar cells (OSCs), two of the most investigated semiconductor
1
ꢀ5
device classes. Toward the goal of better understanding and commercializing these new materials
technologies, the development of novel organic semiconductors has been a major driver for enhancing
device performance, and remains at the forefront of current R&D.
For the design of small molecule semiconductors, a donorꢀacceptor (DꢀA) strategy is commonly
employed to specifically tune the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energies for broader solar light absorption, for ohmic contact at electrode
interfaces, and to maximize builtꢀin OSC potentials. Among various conjugated building blocks, the
electronꢀdeficient diketopyrrolopyrrole (DPP) moiety has received extensive attention due to its tunable
6
ꢀ15
solubility and relatively high electron affinity for adjusting orbital energetics.
By installing proper
alkyl side chains at the DPP nitrogen atoms, the processability of the resulting DPP derivatives can be
1
6
enhanced, rendering them compatible with rollꢀtoꢀroll solution processing. The most noteworthy
2
ꢀ1 ꢀ1
17, 18
examples of DPPꢀbased polymers have high hole mobilities, up to 12ꢀ13.9 cm V s in OTFTs.
structures shown in Scheme 1A), and DPP based polymers combined with fused thiophenes such as
thienothiophene (TT) and dithienothiophene (DTT) as donor units have pꢀtype chargeꢀcarrier mobilities
(
2
ꢀ1 ꢀ1 19
20, 21
as high as 10.5 cm V s
. and power conversion efficiencies up to 6.05ꢀ9.4% in OSCs
(structure
shown in Scheme 1B and 1C).
Despite the success of the aforementioned DPPꢀbased polymers, the development and understanding
of DPPꢀbased small molecules has lagged far behind. Unlike polymer syntheses and properties, which are
2
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typically susceptible to batchꢀtoꢀbatch variations, small molecule syntheses generally provide excellent
3
, 22ꢀ25
reproducibility, purity, and yields, leading for far more uniform materials properties.
For these
Scheme 1. Examples of DDPꢀfused thiophene organic semiconductors.
reasons, solutionꢀprocessable small molecules offer great potential for applications in technologically
relevant organic electronic devices. Furthermore, small molecules can achieve precisely controlled
molecular packing and ordered nanophase morphologies, via either selfꢀassembly or controlled solution
2
6, 27
coating techniques.
Thus, the design and implementation of small molecules also opens new
possibilities for understanding fundamental organic semiconductor structureꢀproperty relationships and
further optimizing device parameters. Nevertheless, to date only relatively low pꢀtype mobilities and
1
0
PCEs have been achieved using DPPꢀbased small molecules. For example, our team first reported
2
8
NDT(TDPP) (Scheme 1) exhibiting OPV PCEs of up to 4%, followed by alkyne functionalized
2
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2
ꢀ1 ꢀ1
29
(
TDPP) -EBT exhibiting a mobility of 0.17 cm V s and a PCE of 1.7%. More recently, Huang et.
2
3
0
14
al. and Lin et al. combined DPP with the widely implemented benzodithiophene (BDT) moiety and
demonstrated OSC PCEs of >5%. While these results are promising, more detailed investigations will be
necessary to better understand and further enhance the TFT and OPV device performance of DPPꢀbased
small molecules.
Motivated by the potential of introducing fused thiophene units into electronic polymers, here we
investigate fused thiophenes systems for the design of high performance small molecule materials. From
our previous results, fused thiophene motifs enhance structural planarity and strong intermolecular SꢁꢁꢁS
3
1ꢀ33
interactions.
These properties promote extensive intramolecular πꢀconjugation and close
3
4, 35
36ꢀ
intermolecular πꢀπ stacking, thereby enhancing charge transport.
Indeed, dithienothiopheneꢀ(DTT)
3
1
9
35
2
ꢀ
and tetrathienoaceneꢀ(TTA) derivatives achieve fieldꢀeffect mobilities as high as 10.2 and 0.30 cm V
ꢀ1
s for pꢀ and nꢀ channel OTFTs, respectively. However, on combining fused thiophenes with DPP units,
DPPꢀendꢀcapped DTT (DTT2DPP) and DTTDPP derivatives yield only modest OTFT mobilities of
2
ꢀ1 ꢀ1 40
41
~
0.02 cm V s , and OSC PCEs approaching ~2.2% . To enhance device performance based on this
design principle, we now integrate an additionally conjugated fused thiophene, TTA (Scheme 1), with
DPPs as acceptor moieties. Such aceneꢀbased chromophores are expected to have more expansive
planarity for πꢀπ stacking and enhanced charge transport characteristics which are crucial for OTFT and
OSC performance.
4
2
In this contribution we report two new semiconductors based on a βꢀalkyl substituted TTA core
4
3
endꢀcapped with two DPP moieties: Nꢀalkylated DPPs and Oꢀalkylated DPPs (referred to as DPP’
herein). Note that DPP’ derivatives are known to have smaller energy gaps and lowerꢀlying HOMO
4
3
energies than the DPP analogues. These characteristics are beneficial for both OTFT and OSC devices.
However to date no DPP’ꢀbased small molecules have been explored for applications in organic
electronics and are thus investigated here for comparison. Solutionꢀprocessed OTFT and OSC devices are
4
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fabricated here based with these two molecules, and exhibit PCEs of up to 4% and hole mobilities
2
ꢀ1 ꢀ1
approaching 0.1 cm V s , rendering these the highest performing fused thiophene OSC chromophores
and some of the highest mobility fused thiophene semiconductors reported to date.
Results and Discussion
1
. Synthesis
A generalizable synthetic approach to molecules 1 and 2 is presented in Schemes 2 and 3. DPP
functionalized TTAs (1) and (2) are synthesized via Stille coupling, as shown in Scheme 2. Alkylated
4
2
tetrathienothiophene (TTAR; 3), prepared according to literature procedures, is first diꢀdeprotonated
with nꢀBuLi, and then diꢀalkylstannylated to generate distannyl tetrathienothiophene (4), which can be
recrystallized from ether and obtained in 67% yield. The monoꢀbrominated Nꢀalkyl and the Oꢀalkyl DPPꢀ
2
8
43
based aryl bromides 5 and 6 are prepared in ~50% yields according to previously published
procedures (Scheme 3). These units are then coupled with 4 to give the DPP endꢀcapped DDPP-TTAR
(1) and DDPP’-TTAR (2) in ~ 66% and 62% yield, respectively. The new molecules were then purified
by chromatography and recrystallization from CH Cl to give materials suitable for used in OTFT and
2
2
OSC devices. To the best of our knowledge this newly synthesized DDPP’ acceptor moiety is employed
Scheme 2. Synthesis of semiconductors DDPPꢀTTAR (1) and DDPP’ꢀTTAR (2).
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Scheme 3. Synthesis of semiconductors DPPꢀBr (5) and DPP’ꢀBr (6).
for the first time in small molecules for organic electronics objectives. All new compounds are
characterized by conventional chemical and physical methodologies. Full experimental details can be
found in the ESI.
2
. Thermal, Optical, and Electrochemical Properties of DDPP-TTARs (1 and 2)
For the new molecular semiconductors 1 and 2, differential scanning calorimetry (DSC) data reveal
sharp endotherms at 222°C and 128°C, respectively, while thermogravimetric analysis (TGA) plots
demonstrate negligible weight loss (~5%) until ca. 375°C and 294°C, respectively, as summarized in
Table 1. Thus, under the same conditions, a higher melting point as well as greater thermal stability is
observed for 1 versus isomer 2.
6
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Figure 1. Optical spectra of monoꢀbrominated DPPs and DDPP-TTAR (1) and DDPP’-TTAR (2) in oꢀ
C H Cl solution.
6
4
2
Molecules 1 and 2 both show visible light absorption over a broad range, making them excellent
candidates for OSCs. The optical absorption spectrum of 2 in oꢀC H Cl (Figure 1) is significantly redꢀ
6
4
2
shifted versus that of 1 (Eopt, gap = 1.69 eV), indicating greater πꢀelectron delocalization and a compressed
optical band gap (Eopt, gap = 1.66 eV; Table 1), in good agreement with the band gap values obtained from
electrochemical measurements (via infra). The stability of these two compounds with respect to
photooxidation was also investigated by monitoring the decay of the optical absorption maxima in aerated
oꢀC H Cl solutions exposed to a white fluorescent lamp light at room temperature. No decomposition is
6
4
2
observed after several days of continuous illumination.
Differential pulse voltammograms (DPVs; see Figure S1) of 1 and 2 were next recorded in oꢀC H Cl
2
6
4
o
at 25 C, and data are summarized in Table 1. The DPV of 1 exhibits an oxidation peak at +0.92 V and a
reduction peak at ꢀ1.02 V, whereas those of 2 are located at lower potentials (E = +0.86 V and E = ꢀ
ox
red
0
5
.91 V). The estimated HOMO (ꢀ5.12 eV) and LUMO (ꢀ3.17 eV) of 1 thus lie below than those of 2 (ꢀ
.06 eV and ꢀ3.28 eV, Figure 2b, Table 1), corresponding to a smaller band gap for the latter. HOMO
energies (EHOMO) were estimated using the conventional equation, EHOMO = ꢀ(4.20 + E ); assuming
ox
ferrocene/ferrocenium oxidation at ꢀ4.8 eV. The lowꢀlying HOMO energies for the two DDPP-TTARs
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suggest that the two molecules should have good air stability versus pentacene and other previously
28,30
reported fused thiophenes employing DTTs or TTAs.
Furthermore, the HOMO energies of the
present TTA compounds have ideal alignment favoring hole injection from contacting Ag electrodes in pꢀ
type TFTs.
Table 1. Thermal, optical, and electrochemical properties of semiconductors 1 and 2.
ꢀ
E
d
(eV)
DPV
gap
b
DSC
TGA
UVꢀVis λ
b
LUMO Eox HOMO UV
max
Cmpd.
o
o
Ered (V)
b
a
T ( C) ( C; 5%)
m
(nm)
(eV)
(V)
(eV)
c
c
c
c
1
2
222
375
294
615
643
ꢀ1.02
ꢀ0.91
ꢀ3.17
ꢀ3.28
0.92
0.86
ꢀ5.12
ꢀ5.06
1.69
1.66
1.95
1.77
128
a
b
+
In oꢀC H Cl . by DPV in oꢀC H Cl at 25 °C. All potentials referenced to an Fc /Fc internal standard
6
4
2
6
4
2
c
(
at +0.6 V). Ered = reductive potential; E = oxidative potential. Using HOMO= ꢀ(4.2+ E ); LUMO= ꢀ
ox
ox
d
(
4.2+ E ). Energy gap estimated from optical absorption.
red
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Figure 2. Energetic alignment of molecules 1 and 2. a) DPVꢀderived HOMO and LUMO energy levels;
b) DFTꢀderived molecular orbital contours.
Electronic structure calculations were performed at the B3LYP/6ꢀ31G** level of density functional
theory (DFT; Figure 2b). Both molecules have similar ground state geometries characterized by
relatively coplanar backbones, with the HOMO level electron density clearly localized on the DPP donor
and the TTA moieties. The computed HOMO energies are ꢀ4.79 eV and ꢀ4.90 eV for 1 and 2,
respectively, and the excited state (LUMO) energies for 1 and 2 are ꢀ2.80 eV and ꢀ2.99 eV, respectively,
yielding optical gaps of 1.99 and 1.91 eV for 1 and 2, respectively. That 2 has lower computed
HOMO/LUMO energies as well as a compressed band gap versus 1, agrees well with the experimental
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4
3
optical absorption and DPV data. Timeꢀdependent DFT (TDDFT) computations show that the principal
optical transitions of 1 and 2 exhibit similar oscillator strengths, (2.67 and 2.64, respectively), in good
ꢀ1
ꢀ1
agreement with the experimental extinction coefficients for 1 and 2 (82,900 and 125,000 M cm ,
respectively).
3
. Film morphology and charge transport properties
Grazing incidence Xꢀray diffraction (GIXRD) and AFM were employed to characterize the film
morphologies of the two TTAR materials, and were performed on thin films prepared by spinꢀcoating
chloroform (CF) solutions of 1 and 2 onto Si/SiO substrates (Figure 3). Furthermore, to identify the
2
effects of postꢀannealing on molecular ordering, which can provide guidelines for OTFT fabrication,
GIXRD and AFM data were also collected on films processed at various postꢀannealing temperatures.
The pristine films fabricated at room temperature show relatively modest crystallinity for both 1 and 2
(Figures 3a, b). The
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Figure 3. Film morphology for semiconductors 1 and 2 as a function of postꢀdeposition thermal
annealing. a, b) GIXRD and c ,d) AFM topographical images of 1 and 2 after annealing at the indicated
temperatures.
The firstꢀorder diffraction peaks (100) are located at 2θ = 4.2˚ and 4.5˚, corresponding to dꢀspacings of
2
1.0 and 19.6 Å, for 1 and 2, respectively. However, film crystallinity for both molecules is dramatically
enhanced upon thermal annealing. For 1, the GIXRD patterns exhibit increasing intensities of the (100)
diffraction peaks with increasing annealing temperatures, showing secondꢀorder reflections for films
annealed above 110 ˚C (at 2θ = 9.3˚). Enhanced crystallinity on annealing can also be inferred from the
AFM images (Figure 3c). Interestingly for 2, the crystallinity gradually increases with increasing
temperature up to ~110 ˚C, but then falls for temperatures above 150 ˚C. This result agrees well with the
AFM topographical images, with the film annealed at 150 ˚C exhibiting significantly smaller grains than
those annealed at 110 ᵒC.
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To investigate the charge transport properties of 1 and 2, bottomꢀgate topꢀcontact (BGTC) OTFTs
were first fabricated using heavily nꢀdoped Si as the gate, 300 nm SiO as the dielectric, and spinꢀcoated
2
organic semiconductor layers. The films were examined at room temperature after annealing at 70, 110,
or 150 ˚C. Finally, gold (Au) source/drain (s/d) electrodes with a channel width/length of 1000/50 ꢀm
were evaporated on top of the organic layer, and the completed TFTs were characterized in ambient air
(Figure 4). All devices show pꢀtype transistor behavior and no ambipolar characteristics, with the relevant
FET metrics summarized in Table 2. The TFTs based on the unannealed 1 films yield an average hole
ꢀ
4
2
ꢀ1 ꢀ1
mobility (µ ) of 3.8×10 cm V s , ~2.4x greater than those of molecule 2. With thermal annealing at
h
ꢀ4
2
ꢀ1 ꢀ1
various temperatures, the µ of 1 is found to significantly increase to 7.7×10 cm V s at 70 ˚C, and
h
ꢀ2
2
ꢀ1 ꢀ1
further reach 1.2×10 cm V s for annealing at 110 ˚C. However, annealing 1 films at higher
ꢀ3
2
ꢀ1 ꢀ1
temperatures decreases the hole mobility, with 4.5×10 cm V s measured for 150 ˚C annealing. In
contrast, the TFT performance of semiconductor 2 decreases upon postꢀdeposition thermal annealing,
which is likely the result of grain boundary formation. These transport results are consistent with the
AFM images shown in Figure 3d, where higher annealing temperatures result in small “island” formation,
thus compromising the connectivity required for efficient charge transport.
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Figure 4. OTFT characteristics of semiconductors 1 and 2. a) Transfer plots of 1 unannealed 1 films yield
ꢀ4
2
ꢀ1 ꢀ1
an average hole mobility (µ ) of 3.8×10 cm V s , ~2.4 times greater than that of semiconductor 2. With
h
ꢀ4
2
ꢀ
thermal annealing at various temperatures, the µ of 1 is found to substantially increase to 7.7×10 cm V
h
1
ꢀ1
ꢀ2
2
ꢀ1 ꢀ1
s at 70 ˚C, and increase to 1.2×10 cm V s at 110 ˚C; b) Transfer plots of 1 and 2 in TGBC device
configurations, after annealing at 110 ˚C.
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Table 2. TFT performance metrics (hole/electron mobilities, I /I ratios, and threshold voltages, V ) for
on off
t
small molecule films in bottomꢀgate/topꢀcontact (BGTC) devices upon postꢀdeposition annealing at the
indicated temperatures. Average values for five devices are shown.
µh
Semiꢀ
conductor
Annealing
temperature (˚C)
On/off
Vt (V)
2
ꢀ1 ꢀ1
(cm V s )
ꢀ4
ꢀ4
ꢀ2
ꢀ2
ꢀ3
ꢀ3
ꢀ4
ꢀ4
ꢀ2
ꢀ5
3
RT
3.8×10
7.7×10
1.2×10
9.1×10
4.5×10
1.4×10
6.4×10
1.7×10
1.8×10
5.3×10
10
ꢀ9.5
4
7
1
1
1
0
10
ꢀ18
1
5
10
10
ꢀ4.9
5
10 (BCTG)*
50
10
ꢀ26.4
ꢀ8.7
4
10
3
RT
10
ꢀ11.2
ꢀ9.7
3
7
1
1
1
0
10
2
3
10
10
ꢀ6.4
3
10 (BCTG)*
50
10
ꢀ15.6
ꢀ10.5
2
10
*
Bottom contact, top gate device geometry
Next, the topꢀgate/bottomꢀcontact (TGBC) TFT architectures were adopted to further enhance the
OTFT performance. Thus, Si/SiO wafers with patterned Au s/d electrodes were utilized as substrates,
2
and both 1 and 2 solutions were spinꢀcoated on these and postꢀannealed at various temperatures.
2
Subsequently, a Cytop layer with a unit capacitance of 4.9 nF/cm was spunꢀcast as the top gate dielectric
4
4
layer. Finally, Au contacts were evaporated onto the dielectric as the gate electrodes. Here, optimum
results are achieved by measuring 1 and 2 in vacuum and after annealing at 110 °C. TFTs based on
1
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2
ꢀ1 ꢀ1
semiconductor 1 yield a hole mobility of ~ 0.1 cm V s , whereas those base on 2 yield a µ value up to
h
ꢀ2
2
ꢀ1 ꢀ1
~
2×10 cm V s . Clearly Cytop improves device characteristics, likely via reduction of the interfacial
traps arising from the suboptimal film morphology, particularly for semiconductor 2. Despite having
favorable energetics, the fact that semiconductor 2 exhibits overall lower TFT performance can be
explained by a combination of relatively poor intermolecular packing and macroscopically suboptimal
morphology (Figure 3).
4
. Bulk-Heterojunction Film Properties and OSC Fabrication
It is wellꢀknown that the nanophase morphology of bulkꢀheterojunction OSC donorꢀacceptor blends
22, 45, 46
can be strongly affected by the processing conditions.
Straightforward outꢀofꢀplane GIXRD can
provide insightful information regarding molecular crystallinity and packing, thus directly yielding
guidelines for the fabrication conditions to be used for OSCs. Specifically, here we investigate the effects
of 1,8ꢀdiiodooctane (DIO) as a solvent additive, which can play a significant role in modifying the
3
2, 47
crystallinity of small molecule blends and DꢀA phase separation.
To identify the effects of DIO on the
molecular packing of the new semiconductors, GIXRD measurements were first performed on neat films
of 1 and 2, spinꢀcast from chloroform (CF) solutions with varying amounts of DIO as the processing
additive. The GIXRD measurements (Figure 5) indicate that DIO addition introduces a higher degree of
ordering for both 1 and 2, identified by the higher intensities of the Bragg reflections.
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Figure 5. GIXRD patterns of 1 and 2 films prepared in CF with different vol% of DIO as the processing
solvents. a) semiconductor 1; b) semiconductor 2.
Because of the promising 1 and 2 HOMO and LUMO energetics and favorable OTFT charge
transport properties, OSCs were next fabricated using PC BM as the acceptor with an inverted cell
7
1
architecture: ITO/ZnO/active layer/MoO /Ag (Figure 6, with data in Table 3). Furthermore, based on the
3
GIXRD results, DIO was used to optimize the BHJ blend film morphology for OSC performance. For
blends based on 1, a modest PCE of 0.59% is obtained for films processed without DIO. This result
agrees well with the poor morphology identified by TEM and PL measurements (see details below and
Figure S3). With increasing amounts of DIO loading, the PCE gradually increases to 4.02% at 1.0 vol%
DIO, mainly due to a significant increase in J . This behavior can also be readily understood by the
sc
enhanced crystallinity of 1, benefiting from the appropriate domain size for exciton separation and
efficient charge transport (see more below). However, higher DIO loadings result in decreased PCE
metrics (Table S1), suggesting that ideal small molecule crystallinity is obtained using ~1.0 vol% DIO as
1
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the processing additive. Interestingly, 2 films without DIO show even poorer OSC performance than to 1
Figure S2,
(
Figure 6. Inverted OSC device performance: (a) illuminated J–V characteristics of 1:PC BM (1:1) with
71
varying DIO vol%. (b) External quantum efficiency (EQE) spectra of bestꢀperforming OSCs.
Tables 3 and S1). Similar to the trends observed for 1, the addition of DIO up to 0.5ꢀ1.0 vol% gradually
increases the PCE from 0.36% to 0.48%, which is mainly attributed to higher FFs. However, a slightly
lower J is also observed. The device parameters achieved using these and several other fabrication
sc
variants are summarized in Table S1. Noticeably, all OPVs based on 2 show lower V than blends based
oc
on 1, consistent with their HOMO energies. (Figure 2)
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Table 3. Summary of organic solar cell performance metrics
2
a
Semiconductor DIO vol% Voc (V)
J (mA/cm )
FF (%)
40.9
40.1
53.5
32.7
48.2
51.4
PCE (%) (average PCE)
0.59 (0.49)
sc
0
0.745
0.732
0.722
0.520
0.597
0.597
1.93
7.25
10.4
2.13
1.67
1.58
1
2
0.5
2.13 (2.05)
1.0
4.02 (3.98)
0
0.36 (0.34)
0.5
.0
0.48 (0.42)
1
0.48 (0.43)
a
Average PCE obtained from at least eight devices.
To investigate the efficiency of photoinduced charge transfer in the OSC active layer,
photoluminescence (PL) measurements were first performed on neat 1 and 2 films, as well as their small
molecule:PC BM blends prepared using several different solvents (Figure S3). Interestingly, the neat
71
films exhibit a strong PL signal for 1, but no detectable PL for 2. This indicates that 2 undergoes
radiationless energy transfer processes responsible for energy loss after photon absorption. In principle,
PL quenching can be used to qualitatively diagnose the active layer morphology of the films made with
fullerene derivatives, with a higher PL quenching efficiency suggesting more intimate DꢀA mixing and
4
smaller domain sizes. Indeed, the blend films based on 1:PC BM processed without DIO exhibit more
71
intense PL intensity signals versus the neat 1 films, indicating a suboptimal morphology where domain
sizes may be larger than the typical exciton diffusion lengths of 5ꢀ20 nm. However, with 0.5 vol% and 1.0
1
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vol% DIO as the processing additive, the PL quenching efficiencies are significantly increased,
suggesting an improved morphology for exciton dissociation.
Grazing incidence wide angle Xꢀray scattering (GIWAXS) spectra were next acquired to examine the
microstructural details of the OSC active layers. The neat films of both 1 and 2 exhibit similar GIWAXS
patterns, with a single lamellar Bragg feature (Figures 7a, d). The paucity of reflections indicates similar
dꢀspacings and domain sizes for 1 and 2 films. However, upon the addition of PC BM, the ordering of
71
the small molecules in these blends diverges substantially for 1 and 2, resulting in drastically different
film morphologies. In the case of 1ꢀbased films, the lamellar reflection is retained upon PC BM addition
71
(Figures 7b, g), and when the blend film is processed with 1 vol% DIO it develops significantly more
crystallinity, exhibiting both n = 2 and n = 3 order reflections for the lamellar stacking (Figure 7c, g). In
contrast, semiconductor 2 appears to lose all order upon blending with PC BM (Figure 7e, h). When
71
processed with DIO, a single lamellar Bragg peak does appear, but not the n = 2 and n = 3 order
reflections (Figures 7f, h). Additionally, the DIOꢀprocessed blend films of 2 exhibit a reduced intensity
for the PC BM reflection, likely attributable to extensive interpenetration of 2 and PC BM, suggestive
7
1
71
of creating blend films with a predominantly mixed phase.
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Figure 7. Twoꢀdimensional (2D) grazing incidence wideꢀangle Xꢀray scattering (GIWAXS) images of 1
and 2: a, d) neat film processed with CF; b, e) in blend film with PC BM (1:1 w/w) processed with CF;
71
c, f) in blend film processed with CF:DIO = 99%:1%, v/v. g, h) outꢀofꢀplane linecuts of 2D GIWAXS for
1
and 2, respectively.
Transmission electron microscopy (TEM) images and AFM surface tomography images for blend
films with PC BM are shown in Figure 8. Without DIO, 1:PC BM film evidence relatively largeꢀscale
71
71
2
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phase separation on the order of 20ꢀ50 nm, which likely indicates a limited interface between 1 and
PC BM. However, when 0.5 vol% DIO is added, the blend film morphology changes, revealing
71
semicrystalline nanofibrillar structures that are wellꢀblended with PC BM. Further addition of DIO up to
7
1
1
vol% induces higher crystallinity in the 1ꢀbased films, which is consistent with the enhanced J metrics.
sc
In marked contrast, the TEM images of the 2:PC BM films do not exhibit significant electron density
71
contrast nor obvious ordering, implying the absence of discrete small molecule and PC BM domains,
7
1
Figure 8. TEM and tappingꢀmode AFM (inset) images for blend films used for OSC fabrication. a, b, c)
:PC BM = 1:1, processed in CF, CF:DIO (99.5%:0.5%, v/v) and CF:DIO(99%:1%, v/v) as processing
1
7
1
solvents, respectively. d, e, f) 2:PC BM = 1:1, processed in CF, CF:DIO (99.5%:0.5%, v/v) and
7
1
CF:DIO(99%:1%, v/v) as processing solvents, respectively. TEM scan area: 0.5 ꢂm × 0.5 ꢂm. AFM scan
area: 5 ꢂm × 5 ꢂm. CF = CHCl3.
but rather intimate D + A mixing. In terms of OSC performance, this microstructure is likely to increase
2
7
both geminate and nonꢀgeminate recombination losses. This result also agrees well with the GIWAXS
measurements, where the bulkꢀheterojunction mixture of small molecule 2:PC BM shows predominantly
71
miscible, amorphous characteristics.
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To investigate the charge transport characteristics more closely related to the photodiode, space
charge limited current (SCLC) measurements were performed on the neat semiconductor films, as well as
on the blend films processed with various DIO loadings (Figure 9 and Table S2). In the holeꢀonly diodes,
the current density (J) as a function of applied electric field (E) in the space charge limited regime is
4
8
given by eq. 1, where by fitting the JꢀE curve it is possible to extract the zeroꢀfield mobility, µ . Here γ
0
is a fieldꢀdependent coefficient, while ε and L are the semiconductor permittivity (taken as 3ε ) and
s
0
thickness, respectively.
ꢂ ꢄꢅ
ꢈ
ꢀ
ꢁ ꢃ ꢆ ꢇ ꢉ expꢋꢌ√ꢇꢍ
(1)
ꢊ
Figure 9. Typical current densityꢀelectric field semilogarithmic plots for holeꢀonly devices for neat 1 and
films, and their blend films with PC BM (50:50 wt%). Measured data are shown as symbols, while
2
71
solid lines are best fits to the space charge limited conduction (SCLC) model. Mobilities are extracted
from the fitting.
Single diode holeꢀonly and electronꢀonly devices were fabricated using MoO /Au contacts. For neat 1
3
ꢀ2
2
ꢀ1 ꢀ1
ꢀ3
2
ꢀ1 ꢀ1
and 2 films, the SCLC mobilities are as high as 2.5×10 cm V s and 4.9×10 cm V s , respectively.
These SCLC motilities are comparable to those achieved in optimized OTFTs, corroborating the
excellent, nearly omnidirectional charge transport characteristics of both materials. For the blend films
ꢀ4
2
ꢀ1 ꢀ1
ꢀ3
2
ꢀ1 ꢀ1
with PC BM added, the SCLC mobilities for 1 and 2 fall to 1.6×10 cm V s , and 2.3×10 cm V s ,
71
2
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respectively. Note that the addition of DIO in the processing solvent introduces opposite trends in SCLC
mobilities for the BHJ films of 1 and 2. Specifically, DIO greatly increases the SCLC mobility for
–
2
2
ꢀ1 ꢀ1
1
:PC BM blend films up to ꢁ = 1.1×10 cm V s with 1.0 vol% DIO, which is only slightly less than
71 h
that observed for the neat film of 1. This indicates that the greater crystallinity induced by DIO can
promote vertical percolative pathways for blend films of 1. This result is in excellent agreement with the
increased OPV FFs for blend films of 1 processed with DIO. However, we also note that further FF
enhancements may be precluded by limited carrier lifetimes, as a result of currently suboptimal
4
morphology. Interestingly, DIO negatively influences the SCLC mobilities for 2:PC BM blend films,
71
–
5
2
ꢀ1 ꢀ1
with ꢁ as low as 5.0 ± 0.1 × 10 cm V s measured upon using 1.0 vol% DIO. Such a steep reduction
h
in mobility is consistent with the reduced J metrics and poor blend film morphologies observed by the
sc
GIWAXS and TEM measurements discussed above.
Conclusions
In this study two novel DPPꢀbased small molecules employing a central electronꢀrich fused
tetrathienoacene (TTA) core, DDPP-TTAR and DDPP’-TTAR, were designed, synthesized, and
characterized. The expansive TTA cores provide excellent molecular coplanarity and close πꢀstacking,
thus enhancing intermolecular interactions and affording favorable macroscopic morphologies for charge
transport. Benefitting from the electronꢀdeficiency of the DPP cores, both materials have small optical
band gaps and suitable HOMO energies for hole injection in OTFTs. These properties yield sizable OTFT
2
ꢀ1 ꢀ1
mobilities in the range 0.02ꢀ0.09 cm V s . Furthermore, XRD and GIWAXS measurements indicate that
the spinꢀcoated thin film morphologies of both molecules are strongly susceptible to the use of DIO as a
solvent additive. Specifically, small quantities of DIO of up to 1 vol% significantly enhance the
crystallinity of DDPP-TTAR films. When incorporated into inverted OSC active layers, good PCEs of
4
.0% are achieved, which are among the highest yet reported for fused thiophene based small molecules.
These results indicate the TTA cores are promising donor moieties for OTFT and OSC materials
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development. However, the DDPP’-TTAR isomer, despite good OTFT mobility, is found to
predominantly form miscible mixtures with PC BM, thus resulting in relatively poor phase separation
71
morphologies, hence poor PCEs. This study of structureꢀproperty relationships in TTAꢀbased small
molecules identifies the importance of fineꢀtuning the molecular stacking and thin film morphologies by
selecting compatible DꢀA moieties and processing solvents – a key ingredient for achieving high
performance OTFTs and OSCs.
Acknowledgements
This research was supported as part of the ANSER Center, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award
Number DEꢀSC0001059, by AFOSR (FA9550ꢀ08ꢀ1ꢀ0331), and by Polyera Corp. We thank the NSFꢀ
MRSEC program, through the Northwestern University Materials Research Science and Engineering
Center, for characterization facilities (DMRꢀ1121262), and the Institute for Sustainability and Energy at
Northwestern (ISEN), for partial equipment funding. Financial assistance for this research was also
provided by the National Science Council, Taiwan, Republic of China (Grant Numbers NSC102ꢀ2113ꢀMꢀ
0
08ꢀ004 and NSC102ꢀ2923ꢀMꢀ008ꢀ004ꢀMY2)
Notes and references
a
Department of Materials Science and Engineering and Argonne Northwestern Solar Energy Research
Center (ANSER), Northwestern University, 2145, Sheridan Road, Evanston, Illinois, 60208 (USA).
b
Department of Chemistry, National Central University, ChungꢀLi, Taiwan, ROC.
c
Department of Chemistry and the Materials Research Center, the ArgonneꢀNorthwestern Solar Energy
Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois, 60208, USA.
2
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5TC01348H
d
‡
Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA.
N. Zhou and S. Vegiraju contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary information
available should be included here]. See DOI: 10.1039/b000000x/
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