A versatile star-shaped organic semiconductor based on benzodithiophene and diketopyrrolopyrrole

Article abstract of DOI:10.1039/c9tc00905a

We report the synthesis of a new star-shaped π-conjugated oligomer, BDT(DPP)₄, containing a benzodithiophene core and four diketopyrrolopyrrole arms. The thermal, electrochemical and optical properties are characterized and the results complemented by computational studies. The utility of the molecule is demonstrated in both solar cell and field-effect transistor devices. In the former, BDT(DPP)₄ displays low efficiency when used as an acceptor in blends with poly(3-hexylthiophene) but exhibits promising performance as a donor, in blends with either a fullerene or a non-fullerene acceptor. In field-effect transistors BDT(DPP)₄ exhibits typical p-type transistor behavior, which is in accordance with its better donor performance in solar cell devices.

Full text of DOI:10.1039/c9tc00905a

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J. Panidi, A. V. Marsh, W. Huang, M. Daboczi, J. Kim, Z. Fei, T. D. Anthopoulos and M. Heeney, J. Mater.
Chem. C, 2019, DOI: 10.1039/C9TC00905A.
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Journal of Materials Chemistry C
A versatile star-shaped organic semiconductor based on benzodithiopheVnieweArticle Online
DOI: 10.1039/C9TC00905A
and diketopyrrolopyrrole
Qiao He,a Munazza Shahid,a Julianna Panidi,b Adam V. Marsh,a Wentao Huang,b Daboczi,
Matyas,b Ji-Seon Kim,b Zhuping Fei,c Thomas D. Anthopoulosd and Martin Heeney*a
aDepartment of Chemistry and Centre for Plastic Electronics, White City Campus, Imperial
College London, London W12 0BZ, UK. E-mail: m.heeney@imperial.ac.uk
bDepartment of Physics and Centre for Plastic Electronics, South Kensington Campus,
Imperial College London, London SW7 2AZ, UK.
cInstitute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science,
Tianjin University, Tianjin 300072, P. R. China.
dDivision of Physical Sciences and Engineering, King Abdullah University of Science and
Technology, Thuwal 23955-6900, Saudi Arabia.
Abstract
We report the synthesis of a new star-shaped -conjugated oligomer, BDT(DPP)4,
containing a benzodithiophene core and four diketopyrrolopyrrole arms. The thermal,
electrochemical and optical properties are characterized and the results complemented by
computational studies. The utility of the molecule is demonstrated in both solar cell and field-
effect transistor devices. In the former, BDT(DPP)4 displays low efficiency when used as an
acceptor in blends with poly(3-hexylthiophene) but exhibits promising performance as a donor,
in blends with either a fullerene or a non-fullerene acceptor. In field-effect transistors
BDT(DPP)4 exhibits typical p-type transistor behavior, which is in accordance with its better
donor performance in OSCs.
1 Introduction
1

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DOI: 10.1039/C9TC00905A
The development of monodisperse -conjugated oligomers has been an area of significant
interest over recent years.1-5 Such materials can exhibit promising performance in a range of
optoelectronic devices, as well as serving as useful models for the behavior of their polymeric
analogues. In addition as discrete, well-defined molecules they can typically be prepared in
high purity with excellent synthetic reproducibility. Many fascinating linear (one dimensional)
oligomers have been reported, but by combining three or more identical conjugated ‘arms’ onto
a central core, rotationally symmetric molecules can also be prepared with higher
dimensionality.6, 7 Such materials, often described as star-shaped, can exhibit markedly
different properties to those of the conjugated arms alone, with the higher dimensionality
resulting in significant changes to molecular assembly and intermolecular interactions.8 In
addition the close proximity of the core and conjugated arms can result in enhanced
intramolecular interactions.
Given the fascinating properties that such star-shaped molecules can exhibit, it is
unsurprisingly that they have been explored for utilization as an active component in a range
of devices such as organic light emitting diodes, field-effect transistors (OFETs) and organic
solar cells (OSCs). In the area of OSCs, star-shaped materials have been utilized both as the
donor or the acceptor component of the blend. The utilization of star-shaped materials with a
twisted 3D geometry has been effective as a method to reduce intermolecular interactions and
suppress the formation of large domains within the blend.9-12 Such large domains are
detrimental to performance, since they reduce interfacial area and exciton harvesting.13-15 It
should be noted that although the introduction of intramolecular twisting facilitates smaller
domain sizes, it may have detrimental effects on intermolecular π-π stacking and thus charge
transport ability of the acceptors in the blends. For example Yan et al. reported a series of
perylene diimide (PDI) tetramers and demonstrated that decreasing intramolecular twisting
2

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could lead to higher electron mobilities, fill factors (FF) and device performance, which is a
DOI: 10.1039/C9TC00905A
critical concern for star shaped molecules.15, 16
Given the importance of intramolecular twisting on charge carrier mobility and device
performance, we were interested to investigate the use of benzo[1,2-b:4,5-b']dithiophene (BDT)
as the central core of a star-shaped molecule. BDT possesses a large and rigid conjugated
structure, which could potentially benefit charge transport due to enhanced delocalization and
coplanar π-π stacking. Furthermore conjugated ‘arms’ can be attached to the BDT core via both
the 2,6-thienyl positions and the 4,8-phenyl positions. These two positions differ in their steric
environments, so different degrees of twisting could be expected for substituents in either
position, which may offer interesting opportunities to tune performance. Two-dimensional
conjugated polymers based on the BDT unit have achieved great success for application as
donor materials in OSCs, because of their deep lying highest occupied molecular orbital
(HOMO) level and high hole mobility. However, to the best of our knowledge, there have been
only two BDT based star-shaped materials reported so far, with no reports of their application
as donors in OSCs or as the semiconductor in an OFET device.11, 17
In terms of conjugated arms, we focused on thiophene flanked diketopyrrolopyrrole (DPP)
units. DPP has been extensively utilized in the development of both electron and hole
transporting materials, due to its high backbone planarity and strong intermolecular π-π
stacking.18 It has also has an intense absorption in the visible range, which is interesting for
potential solar cell applications.19 There have been previous reports of star-shaped materials
containing DPP arms, with have been utilized as either the donor or acceptor materials in solar
cell blends, with efficiencies around 5%.9, 20-23 Taking these considerations into account, herein
we report the synthesis of a new star-shaped molecule based on BDT as a core and DPP as
arms, BDT(DPP)4, and explore its potential application in OFET and OSC devices.
3

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2 Experimental section
DOI: 10.1039/C9TC00905A
2.1. Molecular modeling
Computational details were presented as follows: Density functional theory calculations
were performed using Gaussian G09 rev.d01 and GaussView 5.0.9 visualization software24,
using the B3LYP functional25, 26. Geometry optimizations were performed with full relaxation
of all atoms in gas phase without solvent effects. Vibration frequency calculation was
performed to check that the stable structures had no imaginary frequency.
2.2. Measurements and characterization
GC-MS was carried out on an Agilent GC 7890A and MS 5975C. Matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was performed on a
1
13
Bruker ultrafleXtreme MALDI-TOF analyzer. The H and C NMR spectra were measured
on a Bruker AVANCE 400MHz spectrometer and the chemical shifts were given in parts per
million. Elemental analysis was performed in a Thermo Scientific (Calro Erba) elemental
analyzer, configured for the percentage of C, H, and N. Preparative recycling GPC was run at
room temperature using chloroform as eluent. The system consisted of a high pressure liquid
chromatography apparatus (JAI LaboACE LC-5060 series) equipped with a pump (P-LA60,
flow rate 10 ml/min), a UV detector (UV-VIS4ch LA, λ = 210nm, 254nm, 330nm, 400nm) and
two columns (Jaigel 2HR and 2.5 HR, inner diameter 20 mm× length 600 mm each). UV-vis
spectra were recorded on a UV-1601 Shimadzu UV-vis spectrometer. Electrochemical
measurements were carried out under nitrogen with a deoxygenated solution of tetra-n-
butylammonium hexafluorophosphate (0.1 M) in CH3CN using a computer-controlled
CHI660C electrochemical workstation, a glassy-carbon working electrode coated with samples,
a platinum-wire auxiliary electrode, and Ag/AgCl as a reference electrode. Potentials were
referenced to ferrocenium/ferrocene (FeCp2+/0) couple by using ferrocene as an internal
standard. Thermogravimetric analysis (TGA) measurements were performed on a Shimadzu
4

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Journal of Materials Chemistry C
thermogravimetric analyzer (model DTG-60) under a nitrogen flow at a heating rDaOteI: 1o0f.1013V09ie/Cw°9ACTrtCic0le0O9n0li5nAe
min−1. Differential scanning calorimetry (DSC) measurements were performed using a Mettler
differential scanning calorimeter (DSC822e) under nitrogen at a scanning speed of 10 °C min−1.
Atomic force microscope (AFM) images were obtained with Agilent 5500 atomic force
microscope system operated in the tapping mode. Samples were prepared following the same
procedures for fabrication of solar cells. X-ray diffraction (XRD) measurements were
performed on drop-cast films using a Panalytical X’Pert MRD diffractometer equipped with a
nickel-filtered Cu Kα1 beam and X’Celerator detector using a 40 mA current and 40 kV
accelerating voltage. Photoluminescence (PL) spectra were carried out for the neat and blend
films (spin-coated on a glass substrate) by using a FLS1000 Photoluminescence Spectrometer
(Edinburgh Instruments) with an XE2 continuous xenon lamp. Given their film PL intensities,
for P3HT:BDT(DPP)4 and its neat films, an excitation wavelength of 520 nm (3 nm excitation
bandwidth (EBW) and 2 nm emission bandwidth (EMBW)) were used. For BDT(DPP)4:C8-
ITIC and its neat films, an excitation wavelength of 600 nm (3 nm EBW, 1nm EMBW) were
used. For BDT(DPP)4:PC71BM and its neat films, an excitation wavelength of 520 nm (8nm
EBW, 5nm EMBW) were used.
2.3. OPV Fabrication and Characterization
Organic photovoltaic devices were fabricated using regioregular P3HT (Mn = 51k, PDI =
1.76, regioregularity (RR) 97%) as the donor, or C8-ITIC and PC71BM as acceptor materials,
blended with BDT(DPP)4 respectively. Devices were fabricated onto pre-patterned ITO
covered glass substrates with the device architecture ITO/ZnO/Active layer/MoO3/Ag. After
sequential cleaning of the ITO with the detergent (Decon 90), acetone and isopropyl alcohol, a
zinc acetate dihydrate precursor solution (219.5 mg in the mixture of 2 mL 2-methoxyethanol
and 60 μL monoethanolamine) was spin coated and was annealed at 200 °C for 50 mins.
P3HT:BDT(DPP)4 blend solutions were dissolved in chlorobenzene, or BDT(DPP)4:C8-ITIC
5

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and BDT(DPP)4:PC71BM blend solutions were dissolved in chloroform with a concentration
DOI: 10.1039/C9TC00905A
of 20 mg ml−1 and were stirred overnight. The blend solutions were preheated and then spin
coated onto the ZnO coated ITO substrate and the thicknesses of the active layers were ~80-
100 nm. The active layers were used as-spun or annealed at 130 °C for 10 min. To complete
the devices, MoO3 (ca. 10 nm) and Ag (ca. 80 nm) were evaporated sequentially under high
vacuum (ca. 10−5 Pa) under shadow masks to fabricate devices with an area of 5 mm2. Current-
voltage characteristics were measured with a Keithley 236 source/measure unit under AM 1.5
solar illumination (Oriel 300 W solar simulator) at an intensity of 100 mW cm−2. All electrical
measurements of OPVs were executed in the inert Nitrogen purged devices chamber. External
quantum efficiency (EQE) was measured by a 100 W tungsten halogen lamp (Bentham IL1
with Bentham 605 stabilized current power supply) coupled to a monochromator with
computer-controlled stepper motor (Bentham M300, 300 mm focal length, slit width 3.7 nm,
1800 lines/m grating) The photon flux of light incident on the samples was calibrated using a
UV-enhanced silicon photodiode. A 590 nm long-pass glass filter was inserted into the beam
at illumination wavelengths longer than 620 nm to remove light from second-order diffraction.
Measurement duration for a given wavelength was sufficient to ensure the current had
stabilized.
2.4. OFET Fabrication and Characterization
Top-gate, bottom-contact (TG-BC) organic field effect transistors were fabricated in order
to extract the charge carrier mobility of the material. Glass substrates were sonicated in Decon
90 for 10 minutes, followed by sonication in acetone and isopropanol. Source-drain contacts
were formed by thermal evaporation of 40 nm Au followed by 10 nm of FeCl3 in high vacuum
(10−6 mbar). FeCl3 was used to modify the work function of the evaporated Au. Films were
deposited from 10 mg/ml solutions, that were heated and stirred at 50 °C (chloroform) or
100 °C (chlorobenzene) to ensure BDT(DPP)4 was fully dissolved before spin coating. Spin
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coating was carried out at 2000 rpm for 60 s, followed by thermal annealing at 100 °C for 15
DOI: 10.1039/C9TC00905A
min. The fluoropolymer dielectric, CYTOP, (900 nm) was spin cast onto the organic
semiconductor layer and annealed at 100 °C before aluminium gate electrodes were deposited
by thermal evaporation. OFETs were fabricated and tested in a nitrogen atmosphere (glovebox).
Electrical measurements were carried out using an Agilent B2902A semiconductor parameter
analyser. Charge carrier mobility was extracted from the slope of the ID1/2 vs VG and the value
was then applied to the following equations:
∂ID,l
L
for the linear mobility:
μl =
(
)
WCiVD ∂VG
2
ID,s
∂
2L
for the saturation mobility:
μs =
(
)
WCi ∂VG
3 Results & Discussion
C4H9
C2H5
O
C4H9
S
N
C2H5
O
O
N
S
B
S
O
S
C4H9
C2H5
O
N
S
O
C2H5
O
S
S
N
N
C2H5
Pd2(dba)3, P(MeOPh)3
Br
O
S
O
S
C4H9
N
S
C4H9
B
B
+
C4H
S
9C2H5
C4H9
S
O
O
THF, K2CO3 aq.
43.9%
C2H5
O
O
C2H5
N
S
S
O
S
S
N
C4H9
O
O
S
B
N
C2H5
O
N
O
S
C4H9
DPP-Br
C2H5
C4H9
BDT-Bpin4
BDT(DPP)4
Scheme 1 Synthetic route to compound BDT(DPP)4.
Synthesis. The BDT core was initially modified to include two thienyl groups in the 4,8-
positions. This was then tetra-borylated using an Ir catalyst to afforded 4,8-diyl(bis(thiophene-
5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (BDT-Bpin4) following the reported
route.11 BDT(DPP)4 was then prepared by the fourfold Suzuki coupling reaction between
(BDT-Bpin4) and 3-(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP-Br)27 using Pd2(dba)3/ P(MeOPh)3 as the
catalyst (Scheme 1). The full synthetic routes to all intermediates are shown in Scheme S1. The
crude BDT(DPP)4 product from the cross-coupling was initially purified by column
7

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chromatography to remove excess DPP-Br and catalyst residues. Although tDhOeI: 1r0e.1s0u3Vl9ite/iCwn9AgTrtCic0le0O9n0li5nAe
product appeared to have good purity by 1H NMR, further purification by recycling preparative
gel permeation chromatography (GPC) removed a small impurity, which was identified by
MALDI-TOF MS as the trimer, BDT(DPP)3. The final product BDT(DPP)4 was fully
characterized by MALDI-TOF MS, 1H NMR, 13C NMR, and elemental analysis. BDT(DPP)4
was readily soluble in common organic solvents such as dichloromethane, chloroform and
chlorobenzene at room temperature, owing to the eight solubilizing 2-ethylhexyl sidechains.
The thermal properties of BDT(DPP)4 were investigated by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). BDT(DPP)4 shows good thermal stability
with decomposition temperatures (5% weight loss) of over 400 °C in a nitrogen atmosphere
(Fig. 1a). Unlike the thermal behavior of previously reported BDT11 or DPP9 based star shaped
materials which were amorphous, the DSC of BDT(DPP)4 exhibited a relatively sharp melting
exotherm at 265 °C with a corresponding endotherm at 183 °C upon cooling, suggestive of a
crystalline material (Fig. 1b and Fig. S1). Upon second heating a cold crystallization occurred
around 195 °C, followed by an identical melting exotherm at 265 °C. The reproducibility of
the peaks upon thermal cycling further attests to the good thermal stability of the material.
(a)
(b)
100
90
Cooling
80
Heating
70
60
100
200
300
400
500
600
50
100 150 200 250 300
Temperature (°C)
Temperature (°C)
Fig. 1 (a) TGA and (b) DSC curves of BDT(DPP)4 at a heating/cooling speed of 10 °C min−1
under N2.
DFT Calculation. To investigate the structural and electronic properties of BDT(DPP)4, the
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ground state geometry and frontier molecular orbitals were calculated using density functional
DOI: 10.1039/C9TC00905A
theory (DFT) at the B3LYP/6-31G* level (Fig. 2), with all alkyl chains replaced by methyl
group for simplicity.28 The calculations suggest that the planarity of the arms of BDT(DPP)4 is
strongly directional, with a near-planar configuration along the molecular axis where the DPP
is linked to the 2,6-thienyl positions of BDT, and a more twisted confirmation when linked to
the 4,8-phenyl positions of the BDT with a twist angle around 52° (Fig. 2 (a, b)). This
approximately planar long axis, with a twisted cross-axis is distinctively different to the twisted
geometries of reported star-shaped materials. The distributions of the frontier molecular orbital
are exhibited in Fig. 2 (c, d). The HOMO of BDT(DPP)4 delocalizes throughout the entire
molecule despite the large twist in the perpendicular arms, whilst its LUMO is located on the
planarised axis. The energy levels of the HOMO and LUMO orbitals are estimated to be -4.97
eV and -3.09 eV respectively, suggesting that the molecule will be predominately a p-type
semiconductor.
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DOI: 10.1039/C9TC00905A
51.6°
52.2°
Fig. 2 (a, b) Optimized geometry and (c, d) frontier molecular orbitals of BDT(DPP)4.
Electronic and Optical Properties. As shown in Fig. 3a, the UV-Vis absorption of
BDT(DPP)4 solution in chloroform (10−6 M) exhibits two peaks, a weak absorption around 400
nm and a strong absorption in 500−650 nm region with a maximum extinction coefficient of
2.78 × 106 M−1 cm−1 at 590 nm. Upon film formation the max of low energy peak is red-shifted
to 622 nm, with the formation of a pronounced longer wavelength shoulder around 690 nm.
The red shift is indicative of extended conjugation in the solid state, likely due to enhanced
planarity caused by intermolecular interactions of BDT(DPP)4 in the film. The appearance of
the longer wavelength shoulder is further indicative of such intermolecular interactions, and
has previously been attributed to molecular aggregation.29 The optical band gap of the
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DOI: 10.1039/C9TC00905A
BDT(DPP)4 film is 1.69 eV, as estimated from the absorption edge (733 nm).
The electrochemical properties were investigated as a thin film by cyclic voltammetry (Fig.
3b). The LUMO and HOMO energy levels of BDT(DPP)4 were determined to be −3.33 and
−5.36 eV respectively, from the onset of the oxidation and reduction peaks relative to reference
energy level of ferrocene/ferrocenium (−4.8 eV). The electrochemical band gap is in reasonable
agreement with that calculated by DFT, although the absolute experimental values are slightly
deeper than those predicted, in agreement with other studies.
(a)
(b)
1.0
BDT(DPP) sol.
4
BDT(DPP) film
4
0.8
0.6
0.4
0.2
0.0
0/+
FeCp
2
400
500
600
700
800
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
Wavelength (nm)
Potential (V)
Fig. 3 (a) UV-vis absorption spectra of BDT(DPP)4 in chloroform and as spin-coated film, and
(b) cyclic voltammogram of BDT(DPP)4 as drop-cast film in acetonitrile solution containing
tetrabutylammonium hexafluorophosphate (0.10 M) with a scanning rate of 0.1 V s−1 at room
temperature.
OPV Properties. To evaluate the photovoltaic properties of BDT(DPP)4, inverted solar cell
devices were fabricated with a configuration of ITO/ZnO/active layer/MoO3/Ag. As shown in
Fig. S2 and Fig. 4b, the film absorption profile and energy levels of BDT(DPP)4 match with
those of regioregular poly(3-hexylthiophene) (P3HT), acceptor PC71BM, and our previously
reported acceptor C8-ITIC30 (Fig. 4a), with sufficient energetic offsets to potentially act as
either the donor or acceptor component. However, utilizing BDT(DPP)4 as an acceptor in
blends with P3HT as the donor (blend ratio of 1:1) did not make efficient solar cells. Despite
exhibiting a decent voltage (0.74V), a very low PCE of 0.47% was obtained (Table 1 and Fig.
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4c), primarily due to the low fill factor (FF) and photocurrent. Attempts to improve the
DOI: 10.1039/C9TC00905A
performance by either thermal annealing at 130 °C, solvent annealing or the use of processing
additives (Table S1) were unsuccessful. Furthermore, the utilization of a different P3HT batch
with higher molecule weight (Mn = 68 KDa, PDI = 1.65 and RR = 0.97) did not improve
performance.
By applying BDT(DPP)4 as a donor material, more promising PCEs of 3.9% and 2.5%
were achieved when blending with C8-ITIC and PC71BM respectively. Both devices gave
reasonable photocurrent, 10.1 mA cm−2 and 8.54 mA cm−2 respectively, but the efficiency was
limited by relatively low fill factors, which might result from non-optimal blend morphology.
However the use of solvent additive diiodooctane or annealing did not result in improved
performance (Table S1). The external quantum efficiency (EQE) spectra of the
P3HT:BDT(DPP)4, BDT(DPP)4:PC71BM and BDT(DPP)4:C8-ITIC blended films are shown
in Fig. 4d. The BDT(DPP)4:C8-ITIC device exhibits higher external quantum efficiencies
(EQE = 44%) than the P3HT: BDT(DPP)4 (EQE = 7%) and BDT(DPP)4:PC71BM devices
(EQE = 37%) in agreement with the higher device performance. Notably clear photocurrent
generation can be observed from absorption of BDT(DPP)4 in the PC71BM blend. The highest
performing BDT(DPP)4:C8-ITIC device shows a broader EQE spectra than the P3HT:
BDT(DPP)4 and BDT(DPP)4:PC71BM devices, primarily due to the small band gap of the C8-
ITIC acceptor. The Jsc value calculated from EQE of encapsulated BDT(DPP)4:C8-ITIC
device is 9.6 mA cm−2, which matches well with Jsc values measured in encapsulated solar cell
devices in <5% deviation.
The surface morphologies of the active blend layers were probed by atomic force
microscopy (AFM) and the height and phase images of blended films are shown in Fig. S3.
Both the P3HT:BDT(DPP)4 and BDT(DPP)4: PC71BM blends appear relatively smooth with
root-mean-square (RMS) roughness of 1.2 nm and 0.8 nm respectively, whilst BDT(DPP)4:C8-
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ITIC exhibits rougher with a RMS roughness of 3.6 nm. The higher surface roughness probably
DOI: 10.1039/C9TC00905A
relates to the high crystallinity of C8-ITIC compared to PC71BM or P3HT. The C8-ITIC blend
also shows the presence of what appear to be large crystallites or aggregates on the surface.
Such large scale phase segregation would reduce exciton diffusion/separation efficiencies in
their blended films and may be a reason for the low fill factor.
The influence of blending on microstructure was further investigated using out-of-plane
X-ray diffraction and photoluminescence quenching (PLQ) measurements. Films coated under
identical condition to those used for PV devices were used for PLQ measurements, but such
film gave very weak diffraction in our XRD set-up, so thicker drop cast films were utilized.
Figure S4 shows the diffraction patterns of P3HT:BDT(DPP)4, BDT(DPP)4:C8-ITIC and
BDT(DPP)4:PC71BM blend films as well as their respective neat BDT(DPP)4, P3HT, C8-ITIC
and PC71BM films. Photoluminescence spectra are shown in figure S5. Similar to previous
reports,31 the neat P3HT films exhibited a lamellar diffraction peak at 2Φ =5.5°, with weaker
second and third order peaks at 11° and 16.5°. BDT(DPP)4 exhibited a weak peak at 5.1°. Upon
blending, the P3HT:BDT(DPP)4 film showed a clear diffraction peak corresponding to P3HT,
although the second and third order peaks were no longer visible. Upon blending, the
P3HT:BDT(DPP)4 film showed a clear diffraction peak corresponding to P3HT, although the
second and third order peaks were no longer visible. PLQ measurements using 520 nm
excitation demonstrated 90% quenching efficiency upon blending. Whilst indicating that
effective photo-induced charge transfer between P3HT and BDT(DPP)4 can occur, the
relatively low quenching efficiency suggests that optimum intermixing of the two materials has
not occurred, accounting for the low photocurrent and device performance.
For blends of BDT(DPP)4:C8-ITIC, the XRD shows clear peaks corresponding to both the
C8-ITIC and the BDT(DPP)4, in accordance with the aggregates observed by AFM. Almost
complete photoluminescence quenching (<99%) was also observed upon excitation at 600 nm,
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indicating effective exciton transport and separation, and likely accounting for the improved
DOI: 10.1039/C9TC00905A
performance of this blend. In contrast, the neat PC71BM and blend BDT(DPP)4:PC71BM films
did not show any diffraction peaks, suggestive of a relatively amorphous film in agreement
with the AFM. The PLQ measurements were complicated due to the weak fluorescence of both
BDT(DPP)4 and PC71BM, so the bandwidth of excitation and emission slits were adjusted to
enlarge the signals. The BDT(DPP)4:PC71BM blend film showed ~88% fluorescence
quenching compared to BDT(DPP)4 neat film, again suggestive of non-optimal intermixing.
Overall it is clear that BDT(DPP)4 behaves better as a donor than an acceptor, probably
due to the relatively high lying LUMO level. As a donor, its performance compares well to
other star-shaped DPP containing materials, which have reported efficiencies up to 4.6% using
polymeric acceptors.19 Furthermore improvements in performance may be possible by tuning
of the DPP sidechains or blending with other less crystalline, non-fullerene acceptors.
C4H9
C2H5
C4H9
C2H5
O
S
N
(a)
N
S
O
C6H13
n
N
S
S
O
O
S
S
N
C2H5
C2H5
C4
H
S
S
9 C4H9
S
C4H9C4H9
C2H5
O
C2H5
O
N
S
S
P3HT
N
O
S
N
C2H5
N
S
O
C2H5
C4H9
C4H9
BDT(DPP)4
O
NC
C8H17
C8H17
O
CN
S
O
S
S
O
S
NC
C8H17
C8H17
CN
C8-ITIC
PC71BM
(c)
(d)
50
40
30
20
10
0
P3TH:BDT(DPP)4
BDT(DPP)4:PC71BM
BDT(DPP)4:C8-ITIC
4
0
-4
P3HT:BDT(DPP)4
BDT(DPP)4:PC71BM
BDT(DPP)4:C8-ITIC
-8
-12
-0.3
0.0
0.3
0.6
0.9
300 400 500 600 700 800 900
Voltage (V)
Wavelength (nm)
Fig. 4 (a) chemical structures and (b) Energy levels of P3HT, BDT(DPP)4, C8-ITIC and
PC71BM, (c) J–V curves and (d) EQE spectra of OSC devices based on P3HT:BDT(DPP)4,
BDT(DPP)4:PC71BM and BDT(DPP)4:C8-ITIC.
Table 1 The J−V characteristics of OSC devices based on BDT(DPP)4.
14

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Blend
w/w
1:1
JSC/mA cm−2 VOC/V
FF
PCE (%)
DOI: 10.1039/C9TC00905A
a,b
P3HT:BDT(DPP)4
1.5
0.74
0.75
0.86
0.42
0.39
0.45
0.47 (0.45)d
BDT(DPP)4:PC71BMa
BDT(DPP)4:C8-ITICc
1:2
8.54
10.1
2.5 (1.9)d
3.9 (3.6)d
1:1
a Preheated at 100 °C in chlorobenzene
b Annealed at 130 °C for 10 min
c Preheated at 50 °C in chloroform
d Average PCE values were obtained from over 18 devices.
OFET Properties. The charge transport properties of BDT(DPP)4 were evaluated using
organic field effect transistors (OFETs) in a top-gate, bottom contact device configuration (Fig
5a). Thin films of BDT(DPP)4 were deposited by spin coating from either chloroform or
chlorobenzene, followed by annealing at 150 °C. These two solvents were selected as they have
different boiling points and it was found that chloroform resulted in slightly improved device
performance. Initial devices were fabricated using the as-deposited Au electrodes, in order to
probe for possible ambipolar behavior. However, no electron transport could be observed,
likely due to the high lying LUMO level. With untreated electrodes there was also clear
evidence of contact resistance for hole injection. Therefore a thin layer of amorphous iron (III)
chloride was deposited on top of the Au electrodes by thermal evaporation to improve injection,
following the reported protocol.32 Here we note that the semiconductor film was annealed at
100 °C prior to Cytop deposition, which might potentially lead to some diffusion of the FeCl3.
However, such uncontrollable dopant diffusion would be expected to result in high off-
currents,33 which were not observed in this case. The typical transfer and output curves for
such devices are shown in Fig. 5 (b,c) (for films from chloroform) and Fig. S6 (a,b) (for films
from chlorobenzene). The linear and saturation mobilities are 0.017 and 0.022 cm2 V−1 s−1,
respectively. This is over one order of magnitude higher than other star shaped materials with
DPP arms based on using pyrene or tris(thienyl)benzene cores.34 The promising performance
15

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DOI: 10.1039/C9TC00905A
is likely related to the tendency of molecule to aggregate in the solid phase.
10-5
-1.2
(b)
(a)
(c)
-140
V = - 30 V
D
10-6
10-7
V = - 140 V
D
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
V = - 14 V
G
0
10-8
10-9
10-10
10-11
-120 -90 -60 -30
0
-120-90 -60 -30 0
V (V)
D
V (V)
G
Fig. 5 (a) The top-gate, bottom-contact (TG-BC) transistor architecture employed in this work.
(b) Typical transfer characteristics at VD = -30 V and VD = -140 V. (c) Output characteristics
measured for the same transistor. Transistor channel length (L) and width (W) were 30 μm and
1 mm, respectively.
4 Conclusions
In conclusion, we have designed and synthesized a novel star shaped oligomer,
BDT(DPP)4, based on BDT as the core with DPP as arms. Small amounts of a trimeric impurity
could not be removed by conventional chromatography but were successfully separated by
recycling preparative GPC. The optical, thermal and electrochemical properties of BDT(DPP)4
suggest that it undergoes aggregation in the solid state. DFT calculations support this
observation, suggesting one molecular axis is nearly coplanar. BDT(DPP)4 is demonstrated to
be a versatile semiconducting material, demonstrating promising performance in both organic
field effect transistors and organic solar cells. In the latter application it exhibits superior
performance when utilized as a donor rather than an acceptor. Device efficiencies up to 3.9%
were found when blending with a crystalline non-fullerene acceptor derivative.
Acknowledgements
We thank the China Scholarship Council (CSC) via the CSC Imperial Scholarship, the
16

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Daphne Jackson Trust (supported by RSC and EPSRC), the Royal Society and the Wolfson
DOI: 10.1039/C9TC00905A
Foundation (for Royal Society Wolfson Fellowship) and EPSRC (EP/L016702/1) for financial
support.
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Table of Contents
A novel star-shaped -conjugated oligomer containing a benzodithiophene core and four
diketopyrrolopyrrole arms is reported