Improved photovoltaic performance of star-shaped molecules with a triphenylamine core by tuning the substituted position of the carbazolyl unit at the terminal

Article abstract of DOI:10.1039/c5ta00166h

Two star-shaped molecules of TPA(BT-2Cz)₃ and TPA(BT-3Cz)₃ with a triphenyl-amine (TPA) core were designed and synthesized, in which 2- and 3-carbazolyl units (2Cz and 3Cz) are attached to the benzothiadiazole (BT) arms as planar ter

Full text of DOI:10.1039/c5ta00166h

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DOI: 10.1039/C5TA00166H
Improved photovoltaic performance of star-shaped
molecules with triphenylamine core by tuning substituted
position of carbazolyl at terminal
Pei Zhou,a Dongfeng Dang,a Manjun Xiao,a,b Qiong Wang,a Juan Zhong,a Hua Tan,a
Yong Pei,a RenqiangYang,b* Weiguo Zhua*
a Department of Chemistry, Key Lab of EnvironmentꢀFriendly Chemistry and
Application in the Ministry of Education, Xiangtan University, Xiangtan 411105, P. R.
China
bQingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, P. R. China
Corresponding author: Prof. Weiguo Zhu; Renqiang Yang
Tel: +86ꢀ731ꢀ58293377
Fax: +86ꢀ731ꢀ58292251
Eꢀmail addresses: zhuwg18@126.com

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Abstract:
Two starꢀshaped molecules of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 with a triphenylꢀ
amine (TPA) core were designed and synthesized, in which 2ꢀ and 3ꢀcarbazolyl units
(2Cz and 3Cz) are attached to the benzothiadiazole (BT) arms as planar terminal
groups, respectively. Although both molecules show a similar broad UVꢀVis
absorption covering from 300 nm to 560 nm, different photovoltaic performances
were observed owing to the varied linked positions of terminal carbazolyl units. In
contrast to TPA(BTꢀ2Cz)3ꢀbased solar cells, solutionꢀprocessable organic solar cells
(OSCs) based on TPA(BTꢀ3Cz)3 exhibited better photovoltaic performance with the
maximum power conversion efficiency up to 3.94% and an enhanced shortꢀcircuit
current of 8.80 mA cmꢀ2. This work further demonstrates that tuning the substituted
position of terminal carbazolyl group can significantly improve the photovoltaic
performance for its resulting small molecules.
Keywords:Organic solar cells, Starꢀshaped molecules, Photovoltaic performance,
Carbazolyl, Triphenylamine.
2

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1. Introduction
Solution processable organic solar cells (OSCs) exhibit great potential in large area
applications for their advantages of low cost, light weight and capability in fabricating
flexible devices.1ꢀ4 Among them, bulk heteroꢀjunction (BHJ) OSCs using small
molecules (SMs) as donor materials are under intense investigation because these
SMs have wellꢀdefined molecular structures, no end group contaminants, and good
batchꢀtoꢀbatch reproducibility.3ꢀ5 Currently, significant progress has been made for
these SMs based organic solar cells (SMꢀOSCs) with the achievement of power
conversion efficiency (PCE) up to 6%ꢀ10%.6ꢀ10 However, in contrast to their polymer
counterparts, SMꢀOSCs still show an inferior photovoltaic performance and some key
parameters, such as shortꢀcircuit current density (Jsc) and fill factor (FF), have not
been improved well.11 Therefore, developing novel SMs with broad and strong
absorption to sunlight, matched energy levels and high carrier mobility is still
needed.1ꢀ4 One facile strategy is to construct the DꢀAꢀtype SMs using the proper
electronꢀrich moiety (D) and electronꢀdeficient moiety (A) forming the
intraꢀmolecular charge transfer (ICT).12,13 On the other hand, the starꢀshaped
molecules usually exhibited much stronger absorption and higher holes mobility than
their linear counterparts, which could lead to an enhanced Jsc and PCE.14 For instance,
Li et al reported a series of the triphenylamine (TPA)ꢀbased SMs with different spatial
configurations and found these SMs with threeꢀarmed configuration (tꢀTPAꢀBT)
displayed not only a much broader and stronger absorption, but also a better
photovoltaic performance than those biꢀarmed and linear molecules.15 Based on the
above consideration, the starꢀshaped DꢀAꢀtype conjugated SMs are paid more and
more attention currently.16
As known, TPA and its derivatives exhibited high holes mobility and good solution
processability benefiting from their threeꢀdimensional (3ꢀD) spatial structure, and is
widely used as a donor unit to construct the DꢀA type molecules for the applications
in OSCs.17ꢀ21 2,1,3ꢀBenzothiadiazole(BT) unit has been demonstrated to be a
promising type of acceptor building block due to its good electronꢀwithdrawing
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nature.22ꢀ25 Some researchers developed a type of starꢀshaped molecules containing
both TPA and BT building blocks22ꢀ24 and found that the increased photovoltaic
performances were obtained by tuning terminal groups for these TPAꢀBTꢀbased
molecules. However, satisfactory result has not been made yet.
Recently, our group reported another starꢀshaped molecule containing the TPA
central unit and diketopyrrolopyrrole (DPP) arms, and found that introducing planar
terminal groups of phenanthrenes (PN) can enhance photovoltaic performances for
this TPAꢀDPPꢀbased molecule in OSCs significantly.18 Based on this achievement, we
try to design novel starꢀshaped TPAꢀBTꢀbased molecules by introducing new planar
terminal group in order to further improve their photovoltaic performance. As
carbazolyl (Cz) unit has a planar molecular structure, good chemical and
environmental stability,26ꢀ28 furthermore, the topology, absorption, energy level and
holes mobility of its SMs was found to be tuned easily by altering the carbazolyl
coupling position in SMs.29ꢀ31 Therefore, in this paper, we utilized TPA as core, BT as
three arms and different substituted carbazolyl as terminal groups to design two
starꢀshape molecules of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3, which bear 2ꢀcarbazolyl
(2Cz) and 3ꢀcarbazolyl (3Cz) terminal groups, respectively. Similar thermal stability
and UVꢀVis absorption profile from 300 nm to 560 nm were observed for both SMs.
However, compared to their counterparts, TPA(BTꢀ2Cz)3 showed lower HOMO
energy level and TPA(BTꢀ3Cz)3 displayed higher charge carrier mobility. Significant
effect of the substituted positions of carbazolyl on photovoltaic performance of its
SMs was further observed in the device with a structure of ITO/PEDOT: PSS/Active
Layer/Ca/Al. Better and improved photovoltaic performance was exhibited in the
TPA(BTꢀ3Cz)3ꢀbased devices, in which the maximum PCE and Jsc values were 3.94%
and 8.80 mA/cm2, respectively. Our results further demonstrated that introducing
terminal carbazolyl groups and tuning their linked positions could significantly
improve the photovoltaic performance for its SMs in OSCs.
2. Experimental section
2.1.Materials
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All reagents and chemicals were purchased from commercial sources (Aldrich,
Acros, TCI) and used without further purification unless stated otherwise. Tetraꢀ
hydrofuran (THF) was distilled over sodium and benzophenone under an inert nitroꢀ
gen atmosphere. The detailed syntheses of 2ꢀ(7ꢀbromobenzo[c][2,1,3]thiadiazolꢀ 4ꢀyl)
ꢀ9ꢀoctylcarbazole(BrꢀBTꢀ2Cz), 3ꢀ(7ꢀbromobenzo[c][2,1,3]thiadiazolꢀ4ꢀyl)ꢀ9ꢀoctylcarꢀ
bazole (BrꢀBTꢀ3Cz) and tris(4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)phenyl)
amine (TPAꢀ3B) were outlined in Supporting Information.
2.2. Synthesis
2.2.1 Synthesis of TPA(BTꢀ3Cz)3
To a solution ofcompounds BrꢀBTꢀ3Cz(280 mg, 0.56 mmol) and TPAꢀ3B(70 mg,
0.11 mmol) in toluene (50 mL), anhydrous ethanol (10 mL) and potassium carbonate
aqueous solution (2 M, 2 mL) was added tetrakis(triphenylphosphine)palladium [Pdꢀ
(PPh3)4] (25 mg) under nitrogen atmosphere. The mixture was stirred at 80 oC for 24 h.
The reaction was then quenched with water at ambient temperature and the mixture
was extracted with dichloromethane (DCM). The combined organic layer was dried
over anhydrous magnesium sulfate (MgSO4)and distilled to remove off the solvent.
The residue was purified by column chromatography on silica gel using DCM and
petroleum ether (PE) (V/V= 1:1) as the eluent to give an orange solid (78 mg, yield
1
48.15%). H NMR (400 MHz, CDCl3), δ (ppm): 8.72 (s, 3H), 8.22 (d, J = 15.5 Hz,
3H), 8.15 (d, J = 8.4 Hz, 3H), 8.04 (d, J = 7.6 Hz, 6H), 7.92 (d, 3H), 7.88 (d, J = 7.1
Hz, 3H), 7.59 (d, J = 7.3 Hz, 3H), 7.56–7.39 (m, 15H), 4.38 (t, J = 5.7 Hz, 6H),
2.24–1.69 (m, 6H), 1.49–1.13 (m, 30H), 0.98–0.80 (br, 9H). MALDIꢀMS (MSD, m/z)
for [M]+ C96H90N10S3, 1478.65; found, 1478.85.13C NMR (100 MHz, CDCl3), δ
(ppm): 154.67, 154.23, 147.32, 140.97, 140.49, 133.94, 132.40, 131.68, 130.25,
128.33, 127.89, 127.85, 127.17, 125.92, 124.50, 123.29, 123.12, 121.33, 120.67,
119.11, 108.95, 108.80, 77.43, 77.11, 76.79, 43.30, 31.89, 29.78, 29.48, 29.27, 29.11,
27.41, 22.69, 14.17. Elemental analysis for C96H90N10S3: Calcd C, 77.94; H, 6.10; N,
9.47. Found C, 77.86; H, 6.21; N, 9.28.
2.2.2 Synthesis of TPA(BTꢀ2Cz)3
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Compound TPA(BTꢀ2Cz)3 was synthesized according to the synthetic procedure of
TPA(BTꢀ3Cz)3 as an orange solid with a yield of 50.60%.1H NMR (400 MHz,
CDCl3), δ (ppm): 8.26 (d, J = 7.9 Hz, 1H), 8.17 (d, J = 7.5 Hz, 1H), 8.11 (s, 1H), 8.04
(d, J = 7.5 Hz, 2H), 7.95 (d, J = 6.9 Hz, 1H), 7.88 (d, J = 6.9 Hz, 1H), 7.82 (d, J = 8.0
Hz, 1H), 7.48 (dd, J = 15.8, 7.7 Hz, 4H), 7.28 (d, J = 10.3 Hz, 1H), 4.53–4.27 (br, 6H),
2.06–1.80 (br, 6H), 1.49–1.02 (m, 30H), 0.92–0.76 (br, 9H).MALDIꢀMS (m/z):
1478.85. MALDIꢀMS (MSD, m/z) for [M]+ C96H90N10S3, 1478.65; found,
1478.73.13C NMR (100 MHz, CDCl3), δ (ppm): 154.58, 154.24, 147.41, 141.24,
140.73, 134.86, 133.95, 132.35, 132.33, 130.31, 128.48, 127.69, 125.95, 124.53,
122.99, 122.69, 120.63, 120.44, 120.18, 119.00, 109.95, 108.84, 77.40, 77.08, 76.76,
43.23, 31.87, 29.45, 29.25, 29.10, 27.42, 22.66, 14.12. Elemental analysis for
C96H90N10S3: Calcd C, 77.94; H, 6.10; N, 9.47. Found C, 77.69; H, 6.28; N, 9.34.
2.3. Characterization and measurement
All 1H NMR and 13C NMR spectra were recorded on a Bruker AVꢀ400 using CDCl3
as solvent. MALDIꢀTOF mass spectrometric measurements were performed on
BrukerBifiex III MALDI TOF. The theoretical study was performed using the density
functional theory (DFT), as approximated by the B3LYP employing the 6ꢀ31G**
basis set in Gaussian09. UVꢀVis spectra were measured on a PerkinꢀElmer Lamada 25
spectrometer. Thermogravimetric analyses (TGA) were performed on a Netzsch TG
209 analyzer and differential scanning calorimeter (DSC) was measured on a TA
o
Q200 Instrument under nitrogen atmosphere at a heating rate of 20 C minꢀ1. Cyclic
voltammograms (CV) were performed with a threeꢀelectrode electrochemical cell in a
0.1 M tetra(nꢀbutyl)ammonium hexafluorophosphate (TBAPF6) solutionwith a scan
100 mV sꢀ1 at room temperature under argon atmosphere. A platinum wire and
Ag/AgCl (0.1 M) were used as counter electrode and reference electrode, respectively.
The thin films on the Pt disk, formed by dropꢀcasting the molecular chloroform
solution (analytical reagent, 10 mg/mL) in the disk, was used as the working electrode.
Surface morphologies were recorded by atomic force microscopy (AFM) on a
VeecoꢀDI Multimode NSꢀ3D apparatus in a trapping mode under normal air condition
at room temperature.
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2.4. Fabrication and characterization of organic solar cells
The photovoltaic cells were made with a traditional sandwich structure through the
following steps. Firstly, the indium tin oxide (ITO)ꢀcoated glass substrates were
cleaned by a series of ultrasonic treatments in acetone, following by deionized water,
then 2ꢀpropanol each for 10 min. The substrates were dried under a stream of nitrogen
and subjected to the treatment of Ar/O2 plasma for 5 min. Secondly, a filtered aqueous
solution of poly(3,4ꢀethylenedioxyꢀthiophene)ꢀpoly(styrenesulfonate) (PEDOT:PSS;
Bayer AG) was spunꢀcast onto the cleaned ITO surface at 4000 rpm for 30 s and then
o
baked at 150 C for 30 min to form a PEDOT: PSS thin film with a thickness of 30
nm. Thirdly, a blend solution of materials and [6,6]ꢀphenylꢀCꢀ71ꢀbutyric acid methyl
ester (PC71BM) in chlorobenzene (CB), filtered through a 0.45 µm poly(tetrafluoroꢀ
ethylene) filter, was spun cast at 2000 rpm for 30 s onto the PEDOT: PSS layer. The
substrates were dried under N2 atmosphere at room temperature and then annealed at
o
150 C for 15 min in a nitrogenꢀfilled glove box. The devices were completed after
thermal deposition of a 10 nm calcium and a 100 nm aluminum film as the cathode at
a pressure of 6×10ꢀ4 Pa. The active area was 0.1 cm2 for each cell. The thicknesses of
the spunꢀcast films were recorded by a profilometer (AlphaꢀStep 200; Tencor
Instruments).
Current densityꢀvoltage (JꢀV) characteristics of devices were measured by a comꢀ
puterꢀcontrolled Keithley 2602 source measurement unit in the dark under an AM 1.5
G irradiation at an intensity of 100 mW cmꢀ2 (Oriel 91160, 300 W). PCE was detected
under a monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator
equipped with Oriel 70613NS QTH lamp) and the calibration of the incident light was
performed with a singleꢀcrystalline silicon diode. External quantum efficiency (EQE)
was measured with a silicon photodiode and a computerꢀcontrolled light sourceꢀ
monochromatorꢀlockꢀin system. All device measurements were carried out in air at
room temperature.
3. Results and discussion
3.1. Synthesis and thermal stability
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The synthetic routes of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3are outlined in Scheme 1.
As the key precursors of both starꢀtype molecular arms, 2ꢀ(7ꢀbromobenzo[c][2,1,3]
thiadiazolꢀ4ꢀyl)ꢀ9ꢀoctyl carbazole (BrꢀBTꢀ2Cz) and 3ꢀ(7ꢀbromobenzo[c][2,1,3]thiaꢀ
diazolꢀ4ꢀyl)ꢀ9ꢀoctylcarbazole (BrꢀBTꢀ3Cz) were prepared through the common
Suzuki coupling reaction using Pd(PPh3)4 as the catalyst (See Supporting Information).
Similarly, TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 were easily obtained through Suzuki
coupling reaction in a moderate yield of 50%. Two target SMs were confirmed well
1
with H NMR, 13C NMR, TOFꢀMS and elemental analysis. Like the other starꢀtype
molecules, both TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 are readily dissolved in the
common organic solvents, such as DCM, chloroform (CHCl3) and chlorobenzene (CB)
at the room temperature.
The thermal properties of both molecules were evaluated with thermogravimetric
analysis (TGA) and differential scanning calorimeter (DSC). The recorded TGA and
DSC curves are depicted in Fig S1 (a) and (b), respectively (See Supporting Inforꢀ
mation). Their corresponding data are summarized in Table 1. The thermal decomꢀ
o
o
position temperatures of 469 C and 463 C are observed for TPA(BTꢀ2Cz)3 and TPA
ꢀ(BTꢀ3Cz)3 with a 5% weight loss, respectively, implying that both molecules have
good thermal properties. On the other hand, no distinct endothermic and exothermic
peaks are exhibited for TPA(BTꢀ3Cz)3 during the heating and cooling processes.
However, an endothermic peak at 246 °C and exothermic peak at approximately
208 °C are observed for TPA(BTꢀ2Cz)3, indicating that different crystalline properties
were obtained by altering the linking position of the terminal carbazolyl.
3.2 Theoretical calculations
The groundꢀstate geometry of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 was fully optimiꢀ
zed by density functional theory (DFT) at the B3LYP/6ꢀ31G** level and an ethyl
group was used instead of octyl group to make the calculation easier. The minimumꢀ
energy conformations and electronꢀstateꢀdensity distributions are represented in Fig. 1.
The dihedral angles between phenyl of TPA and BT, as well as between BT and Cz for
TPA(BTꢀ3Cz)3 are calculated to be 34.71o and 34.83o, respectively, which are much
8

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smaller than those corresponding dihedral angles in TPA(BTꢀ2Cz)3, indicating that
TPA(BTꢀ3Cz)3 has more planar molecular structure than TPA(BTꢀ2Cz)3. Therefore,
TPA(BTꢀ3Cz)3 is expected to have better πꢀπ stacking than TPA(BTꢀ2Cz)3 and exhibit
the increased Jsc and PCE values.32 In addition, the electron density of HOMO of
TPA(BTꢀ3Cz)3 is delocalized on the whole molecule and such a delocalization is
much less for TPA(BTꢀ2Cz)3. As a result, TPA(BTꢀ3Cz)3 could have a slightly
increasing HOMO energy level in comparison with TPA(BTꢀ2Cz)3. On the other hand,
the electron density of LUMO for both molecules is mainly localized on the BT units,
which is consistent with the reported literatures.25,33
3.3 Optical properties and electrochemical properties
The UVꢀVis absorption spectra of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 in chloroꢀ
form solution and solid films are shown in Fig. 2 and their corresponding absorption
data are listed in Table 1. By employing carbazole as the terminal, broad absorption
spectra from 300 nm to 570 nm with two obvious absorption peaksare observed. The
highꢀlying region at about 320 nm corresponds to the πꢀπ* transition and the lowꢀ
lying region from 400 to 550 nm is assigned to the intraꢀmolecular charge transfer
(ICT) interaction between electronꢀrich moiety (Cz and TPA) and electronꢀdeficient
moiety (BT).32,34 Compared to the absorption profiles in solution, those ones in thin
films exhibited significant redꢀshift up to 24~30 nm, which is probably caused by
intermolecular πꢀπ stacking interaction. The similar absorption spectra for both SMs
in solutions and thin films imply that the substituted position of carbazole has little
effect on the absorption spectra.
Fig. 3 shows the cyclic voltammetry (CV) curves of TPA(BTꢀ2Cz)3 and TPA(BTꢀ
3Cz)3 and their correlate data are also listed in Table 1. The potentials are internally
calibrated using the ferrocene/ferrocenium of the (Fc/Fc+) redox couple (4.8 eV below
the vacuum level). The HOMO and LUMO energy levels (EHOMO and ELUMO) are
calculated to be ꢀ5.25 eV/ ꢀ3.24 eV for TPA(BTꢀ2Cz)3 and ꢀ5.18 eV/ ꢀ3.15 eV for
TPA(BTꢀ3Cz)3 from the onset oxidation potential (Eox) and the onset reduction
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potential (Ered) according to the referenced equations.35 It is noticed that TPA(BTꢀ
2Cz)3 have a deeper HOMO energy level, which is consistent with our DFT
calculation result using the B3LYP/6ꢀ31G** model. The lower EHOMO value should be
available for TPA(BTꢀ2Cz)3 to exhibit higher Voc in OSCs.
3.4 Photovoltaic performance
To probe the photovoltaic properties of both starꢀshaped molecules, single bulk
heterojunction organic solar cells were fabricated by solution process with a structure
of ITO/PEDOT: PSS/Active Layer/Ca/Al under an optimal spinꢀcoating rate of 2000
rpm. As demonstrated, the blend ratios (w/w) in photoactive layer could affect the
photovoltaic performance of OSCs seriously,36 therefore, the blend ratios between
SMs to PC71BM were optimized primarily here, as shown in Fig. S2. The optimized
blend ratio of 1:4 was obtained for both moleculesꢀbased solar cells. Fig. 4 displays
the current densityꢀvoltage (JꢀV) characteristics of TPA(BTꢀ2Cz)3 and
TPA(BTꢀ3Cz)3ꢀbased OSCs in the optimized conditions under illumination of AM 1.5
G, 100 mW mꢀ2. Their corresponding photovoltaic data are listed in Table 2. An
increased PCE of 3.94 % with a Jsc of 8.80 mA cmꢀ2 and a FF of 46.4% is observed in
the TPA(BTꢀ3Cz)3ꢀbased solar cells under annealing at 110 oC, while the
TPA(BTꢀ2Cz)3ꢀbased cells exhibited the maximum PCE of 3.11 % with a Jsc of 7.69
mA cmꢀ2 and a FF of 40.4%. It is obvious that TPA(BTꢀ3Cz)3 exhibited better
photovoltaic performance except the slightly low Voc value in contrast to
TPA(BTꢀ2Cz)3 in the OSCs. Therefore, our results here indicated that tuning the
substituted position of carbazolyl at the molecular terminal could improve the
photovoltaic performance for its SMs. Furthermore, appending the planar carbazolyl
terminal group is also an efficient method to achieve the highꢀperformance
photovoltaic molecules.
Fig. 5 shows the external quantum efficiency curves (EQE) of the devices at the
optimized conditions. The spectral response across the wavelength range of 325ꢀ675
nm is clearly observed for both devices. However, while TPA(BTꢀ2Cz)3ꢀbased device
is replaced with TPA(BTꢀ3Cz)3ꢀbased device, the EQE is significantly increased from
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a maximum value of 60% to 75%. It indicates that the photoelectron conversion
process is more efficient in the TPA(BTꢀ3Cz)3ꢀbased device than the
TPA(BTꢀ2Cz)3ꢀbased device. Higher EQE value is available for TPA(BTꢀ3Cz)3 to
present better photovoltaic performance in the OSCs.
In order to further study why TPA(BTꢀ3Cz)3ꢀbased solar cells exhibited better
performance, the surface morphologies of the TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3
blending films with PC71BM under the optimized conditions were recorded by atomic
force microscopy (AFM) and are illustrated in Fig. 6. The rootꢀmeanꢀsquare (RMS)
roughness of 0.4 and 0.6 nm are exhibited for the TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3
blending films, respectively. It implies that both blending films show fairly homoꢀ
geneous surface. However, nanoscopic fibrousphase separation is obviously observed
in the TPA(BTꢀ3Cz)3 based blending film. This ordered fibrous morphology should be
one of the reasons for the significantly improved Jsc and FF because the interꢀfibrillar
regions contributes to high exciton dissociation and charge transport efficiency.37
3.5 Holes mobility
The holes mobility of the blend films was measured by the space charge limited
current (SCLC) method in the holeꢀonly devices with a structure of ITO/PEDOT:
PSS/Active Layer/Au, which were approximated by the MottꢀGurney equation:38
9 ε0 εr ꢁe V 2
J =
8 d 3
Here J is the current density, εr is the molecular dielectric constant, ε0 is the
freeꢀspace permittivity (8.85×10ꢀ12 F/m), d is the thickness of blend films (96 nm and
85 nm for TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3, respectively),V= VapplꢀVbi, Vappl is the
applied potential, and Vbi is the builtꢀin potential which results from the difference of
the work function between the anode and the cathode. Fig. 7 displays the J1/2ꢀV curves
of the holeꢀonly TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3ꢀbased devices. The corresponding
holes mobility are listed in Table 2. The holes mobilities up to 7.9×10ꢀ5 and 9.0×10ꢀ5
cm2V−1s−1 are exhibited in the holeꢀonly TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3ꢀbased
devices, respectively. The observed higher hole mobility forTPA(BTꢀ3Cz)3 is also
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available for the FF improvement in the TPA(BTꢀ 3Cz)3ꢀbased cells.
4. Conclusion
In conclusion, two TPAꢀcored starꢀshaped molecules of TPA(BTꢀ2Cz)3 and TPAꢀ
(BTꢀ3Cz)3 were obtained with 2ꢀ and 3ꢀcarbazolyl groups at terminals, respectively.
Both SMs exhibited good thermal stability, broad spectral response region covering
from 325 to 570 nm. Significantly increased photovoltaic performance was observed
in the solutionꢀprocessable organic solar cells based on TPA(BTꢀ3Cz)3 instead of
TPA(BTꢀ2Cz)3. The maximum PCE of 3.94% with Jsc of 8.80 mA cmꢀ2, FF of 46.42%
and Voc of 0.97 V was obtained in the TPA(BTꢀ3Cz)3ꢀbased cell. Our results indicated
that the structures and substituted positions of planar carbazolyl at terminals play a
significant role in improving photovoltaic performance for its SMs in OSCs.
Acknowledgements
Thanks to the Financial Supports from the Major Program for Cultivation of the
National Natural Science Foundation of China (91233112), the National Natural
Science Foundation of China (21172187, 51403178, 51273168, 21202139), the
Innovation Group and Xiangtan Joint Project of Hunan Natural Science Foundation
(12JJ7002 and 12JJ8001), the Open Project for National Key Laboratory of Lumineꢀ
scent Materials and Devices (2014ꢀskllmdꢀ10), Research Foundation of Education
Bureau of Hunan Province (13A102,14C1099) and the Hunan Postgraduate Science
Foundation for Innovation (CX2014B257, CX2013B268).
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the online version
at http://dx.doi.org/xxx/xxx.
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Captions of Figures
Scheme 1. Synthetic routes of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3.
Fig. 1. The optimized geometries and electronꢀstateꢀdensity distributions
ofTPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 using DFT method at the B3LYP/6ꢀ31G** level.
Fig. 2. Normalized UV–Vis absorption spectra of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3
in CHCl3 solution and film state.
Fig. 3. Cyclic voltammograms of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3 film on Pt
electrode.
Fig. 4. Current densityꢀvoltage characteristics of the TPA(BTꢀ2Cz)3 and TPA(BTꢀ
3Cz)3 based OSCs in the optimized conditions under the illumination of AM 1.5 G
100 mW/cm2.
Fig. 5. External quantum efficiency (EQE) curves of the TPA(BTꢀ2Cz)3 and TPAꢀ
(BTꢀ3Cz)3 basedOSCs devices under the optimized conditions.
Fig. 6. AFM topographies of the blend films for TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3
with PC71BM under the optimized condition.
Fig. 7. J1/2ꢀV curves ofTPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3ꢀbased devices for the
measurement of the hole mobility under the optimized conditions.
Table 1. Physicochemical properties of TPA(BTꢀ2Cz)3 and TPA(BTꢀ3Cz)3.
Table 2. Photovoltaic performance of OSCs based on TPA(BTꢀ2Cz)3 and
TPA(BTꢀ3Cz)3 under the illumination of AM 1.5 G 100 mW/cm2 and the holes
mobility of both molecules measured by SCLC.
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Scheme 1:
Fig. 1:
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Fig. 2:
Fig. 3:
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Fig. 4:
Fig. 5:
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Fig. 6:
Fig. 7:
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Table 1:
[a]
[b]
opt
λabs
(nm)
λabs
(nm)
Eg
EHOMO
(eV)
ELUMO
(eV)
Td
Compounds
Egcv(eV)
(eV)
2.15
2.20
(oC)
469
463
TPA(BTꢀ2Cz)3 322,460 334,490
ꢀ5.25
ꢀ3.24
ꢀ3.15
2.01
2.03
TPA(BTꢀ3Cz)3 320,465 333,489
ꢀ5.18
[a] measured in CHCl3 solution; [b] measured in the neat film.
Table 2:
Voc
Jsc
FF PCE Holes mobility
Active layer [a] Blend ratios [b]
(V) (mA/cm2) (%) (%)
(cm2vꢀ1mꢀ1)
7.9×10ꢀ5
TPA(BTꢀ2Cz)3
TPA(BTꢀ3Cz)3
1:4
1:4
1.01
0.97
7.69
8.80
40.40 3.11
46.42 3.94
9.0×10ꢀ5
[a] Blending with PC71BM; [b]Annealed at 110 oC;
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Table of Contents
Significantly enhanced photovoltaic performance with PCE up to 3.94% was achieved
by tuning the substituted positions of carbazolyl at terminal.