Efficient small molecule photovoltaic donor based on 2,3-diphenyl-substituted quinoxaline core for solution-processed organic solar cells

Article abstract of DOI:10.1039/c7ra01859b

A novel D-π-A-π-D-type organic small molecule (OSM) named (TPACN)₂Qx was designed and synthesized for solution-processible organic solar cells (OSCs), which contained 2,3-diphenyl-substituted quinoxaline (Qx) as the central acceptor (A) unit, t

Full text of DOI:10.1039/c7ra01859b

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Efficient small molecule photovoltaic donor based
on 2,3-diphenyl-substituted quinoxaline core for
solution-processed organic solar cells†
Cite this: RSC Adv., 2017, 7, 23779
Bao Xie,‡a Sheng Bi,‡b Rui Wu,‡a Lunxiang Yin,a Changyan Ji,ac Zhengjiang Caia
a
*
and Yanqin Li
A novel D–p–A–p–D-type organic small molecule (OSM) named (TPACN)2Qx was designed and
synthesized for solution-processible organic solar cells (OSCs), which contained 2,3-diphenyl-
substituted quinoxaline (Qx) as the central acceptor (A) unit, triphenylamine (TPA) as terminal electron-
donor (D) moieties, and acrylonitrile as the p-linkage segment. With an effective electron-withdrawing
property and relatively stable quinoid geometry of Qx, (TPACN)2Qx exhibits a narrow band-gap of
1.88 eV and strong and broad absorption at 300–700 nm. As a consequence, an excellent power
conversion efficiency (PCE) of 6.25% was achieved based on the (TPACN)2Qx:PC61BM blend using
a simple spin-coating process in a solution, which is the highest efficiency achieved to date for Qx-core
based solution-processed OSM photovoltaic (PV) devices. The impressive result demonstrates that OSMs
employing quinoxaline derivative as an electrophilic unit can compete with their polymer counterparts.
Received 14th February 2017
Accepted 17th April 2017
DOI: 10.1039/c7ra01859b
rsc.li/rsc-advances
encouraging PCE of 11.30% has been achieved based on OSM
donor materials,5 which is comparable to the PV performance of
the best polymers.6 However, the overall performance of OSMs,
to date, is still behind those of their polymer counterparts.
Therefore, BHJ OSCs based on OSMs still require intensive
investigation for the design of materials.
Introduction
OSCs have emerged as a competitive alternative to inorganic
solar cells for their prominent advantages of light weight, low
cost and exibility in large-scale fabrication.1 Over the past two
decades, solution-processed bulk heterojunction (BHJ) photo-
voltaic (PV) devices based on a blend of electron donors and
acceptors have attracted much attention and extensive investi-
gation due to their higher internal quantum efficiency and the
easier fabrication technology.2 Nowadays, most research efforts
have been focused on donor materials because fullerene
derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM), have proven to be ideal acceptor materials and have
already been commercialized.3 Among the OSC donor mate-
rials, OSM materials have shown numerous promising advan-
tages over conjugated polymers in terms of well-dened
structure, easier purication, better batch-to-batch reproduc-
ibility and high charge carrier mobility.4 In particular, an
To design ideal OSM donor materials, the following issues
should be considered:7 (I) possessing broad and strong
absorption via reducing the band-gap (Eg) of molecules, which
results in an increased short-circuit current density (Jsc); (II)
lowering the highest occupied molecular orbit (HOMO) of
materials, which correlates with the improved open-circuit
voltage (Voc); (III) possessing better solubility, miscibility and
lm-forming characteristics to improve the morphology of the
active layer and increase the ll factor (FF) of the PV device. To
address these issues, developing new D–A conjugated molecules
has become an effective strategy as the Eg and energy levels of
D–A-type OSMs can be effectively tuned through chemical
structure modications.8 In our previous study, we found that
OSMs with alternating D–A conjugated conguration could
possess both lower HOMO level and narrow band-gap by tuning
their building blocks.9 We have reported a D–p–A–p–D-type
OSM donor material, BDCTBT,10 in which benzothiadiazole
(BT) as the A unit and TPA as the D unit were connected by a p-
bridge containing an acrylonitrile segment, and the chemical
structure of BDCTBT is shown in Fig. 1. In practical photovoltaic
testing, the devices based on BDCTBT as donor and PC61BM as
acceptor exhibited a PCE of 3.85%,10 and aer subsequent
device optimization, a relatively high PCE of 4.84% was obtai-
ned.9b Our research demonstrates that D–A–D-type molecules
aSchool of Chemistry, Dalian University of Technology, Linggong Road 2, Dalian, P. R.
China. E-mail: liyanqin@dlut.edu.cn; Fax: +86-411-84986040; Tel: +86-411-
84986040
bSchool of Mechanical Engineering, Dalian University of Technology, Linggong Road 2,
Dalian, P. R. China
cHunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, Hunan
University of Humanities, Science and Technology, Lou'di, Hunan 417000, P. R. China
† Electronic supplementary information (ESI) available: Synthetic procedures,
NMR spectra, computational electronic transitions of the synthesized
compounds and AFM images of blend lms. See DOI: 10.1039/c7ra01859b
‡ The authors have equal contribution to the manuscript.
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out with the standard Schlenk technique unless otherwise
indicated. Toluene and tetrahydrofuran (THF) were pretreated
over sodium/benzophenone under nitrogen atmosphere before
reaction.
Photovoltaic device fabrication
The BHJ device was fabricated via solution-processable spin-
coating process with a typical architecture of ITO/PEDOT:PSS/
donor:PC61BM/Al. The ITO-glass substrates were cleaned in an
ultrasonic bath in deionized water, methanol, acetone, toluene
and isopropanol for 5 min, successively. Aer being dried with
high pressure nitrogen gas-ow, a thin layer of poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT : PSS)
was spin-coated onto the pretreated substrate at 4000 rpm sꢀ1 for
60 s. Aer being baked at 120 ꢁC for 15 min, all prepared
substrates were transferred into the nitrogen-lled glovebox.
Then, the blended solution of (TPACN)2Qx and PC61BM in
Fig. 1 Chemical structures of (TPACN)2Qx and BDCTBT.
with a p-bridge containing an acrylonitrile segment are
a successful conguration for OSM donor material. To date, we
have found that the PCEs of solution-processed BHJ devices chloroform with various weight ratios was spin-coated on the top
based on D–A–D-type OSM with BT as the electrophilic core of the prepared substrate at 1500 rpm sꢀ1 for 60 s. Finally, an
have never exceeded 5%.8c,9b,11 Based on above design concepts, aluminium layer as the electrode was deposited on top of the
we synthesized a new D–A–D-type OSM named (TPACN)2Qx as active layer by thermal evaporation under vacuum (ca. 10ꢀ4 Pa),
shown in Fig. 1, in which the central A unit of BDCTBT was yielding six individual cells with 5 mm2 effective area for each.
The hole-only devices using high work-function metal gold (Au)
as the electrode for hole mobility measurement were fabricated
with a structure of ITO/PEDOT:PSS/donor:PC61BM/Au.
replaced by a strong electron-withdrawing unit, which was 2,3-
diphenyl-substituted quinoxaline (Qx).
Quinoxaline, as one signicant class of high performance
pigments, is widely considered as a promising building block
for OSCs due to the strong electron-withdrawing property of two
imine nitrogens and relatively stable quinoid geometry.12 Qui-
noxaline derivatives can be easily structurally modied, and
their electronic properties can be changed with various
substituents. Among a wide variety of quinoxaline derivatives,
Qx has shown to be an easily synthesized excellent electron-
withdrawing unit for the synthesis of narrow band gap conju-
gated polymers used in high efficiency polymer solar cells
(PSCs).13 Thus far, PSCs based on Qx have achieved promising
PCEs over 8% with careful device optimization.14 In addition,
important progress has also been made on Qx-based dye
sensitized solar cells (DSSCs) with a PCE near 10%.15 Although
Qx shows outstanding PV performance in PSCs and DSSCs, it
seems to be hysteretic in OSM solar cells.16 Consequently, we
believe that Qx-based OSMs possess tremendous development
potential for BHJ-OSCs.
Accordingly, (TPACN)2Qx was designed and synthesized
successfully, and its photoelectric properties were fully investi-
gated. Remarkably, an excellent PCE of 6.25% has been achieved
for BHJ devices based on (TPACN)2Qx as the donor and PC61BM
as the acceptor, which is the highest efficiency reported to date
for solution processed Qx-based OSM BHJ-OSCs. This study
clearly indicates that Qx-based OSMs possess great potential in
obtaining high performance for solution-processed PV devices.
Measurements and characterization
1H-NMR and 13C-NMR spectra were recorded by Bruker
AVANCE II 400 MHz spectrometer with CDCl3 as solvent and
tetramethylsilane (TMS) as internal standard substance. High
resolution mass spectra (HRMS) were recorded with a MALDI
Micro MX spectrometer. The UV-Vis absorption spectra of the
material in chloroform solution and in the lm state were taken
on an Agilent Cary 5000 spectrophotometer. Cyclic voltammetry
(CV) experiments were conducted using a CHI610D electro-
chemical workstation from CH Instruments, Inc., and it was
performed in 0.1 M anhydrous Bu4NBF4/CH2Cl2 solution at
a scan rate of 100 mV sꢀ1 under nitrogen atmosphere, using the
glass-carbon electrode, Ag/Ag+ electrode (Ag in 0.1 M AgNO3
solution of MeCN) and platinum wire electrode as the working
electrode, reference electrode and counter electrode, respec-
tively. In addition, the ferrocene–ferrocenium (Fc/Fc+) couple
was selected as the internal standard. Electrochemical band
gaps (ECgV) and the pertinent energy levels (HOMOCV and
LUMOCV) could be achieved according to the following empir-
ferrocene
ical equations:17 HOMOCV/LUMOCV ¼ ꢀ[Eox/red ꢀ E
+
1/2
4.8] eV and ECgV ¼ LUMOCV ꢀ HOMOCV, where Eox was the
measured onset oxidation potential and Ered was the measured
ferrocene
1/2
onset reduction potential relative to Ag/Ag+, and E
¼
0.05 eV versus Ag/Ag+. Thermogravimetric analysis (TGA) was
carried out on a TA Instruments Q50 thermogravimetric
analyzer at a heating rate of 10 ꢁC minꢀ1 under nitrogen
atmosphere. Differential scanning calorimetry (DSC) was per-
Experimental
Reagents and materials
formed unꢀd1er nitrogen atmosphere at a heating/cooling rate of
ꢁ
All reagents were purchased commercially and used without 10 C min with a TA Instruments Q20 differential scanning
further purication unless specied. All reactions were carried calorimeter. Atomic force microscopy (AFM) measurement was
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conducted with a DI Nanoscope Dimension 3100 atomic force d 8.10 (s, 2H), 7.78 (d, J ¼ 8.0 Hz, 10H), 7.66 (m, 8H), 7.48–7.39
microscope. The current density–voltage (J–V) characteristics of (m, 10H), 7.32 (t, J ¼ 8.0 Hz, 8H), 7.17–7.11 (m, 12H), 7.07 (d, J ¼
PV devices were measured with a computer-controlled Keithley 8.0 Hz, 4H). 13C-NMR (100 MHz, CDCl3): d 151.67, 149.99,
2400 Source Measure Unit under simulated AM 1.5 G illumi- 146.66, 146.63, 146.06, 140.90, 138.60, 137.14, 134.89, 133.84,
nation with an intensity of 100 mW cmꢀ2, calibrated with 130.95, 130.73, 130.61, 129.60, 129.25, 128.27, 127.48, 126.47,
a standard silicon solar cell. The incident-photons-to-current 126.37, 126.08, 125.89, 125.77, 124.43, 123.33, 120.86, 118.69,
efficiency (IPCE) measurement was recorded on a SM-25 107.19. MALDI-TOF HRMS: 1187.3934 [M+] (calcd for
photoelectric conversion analyzer. The hole-mobility (mh)
measurement of the material was performed with the Keithley
2400 Source Measure Unit system in dark. The mh data can be
received by tting the J–V curves in a double logarithmic scale
with a space-charge-limited current (SCLC) model according to
the Mott–Gurney law:18 J ¼ (9/8) 30 3r mh(V2/L3), where J is current
density, 30 is the permittivity of free space, 3r is the relative
permittivity and assumed as approximately 3.0, mh is the hole
mobility value, V is the effective voltage, and L is the thickness of
the active layer.
C82H55N6S2: 1187.3930).
Results and discussion
Material characterization and thermal properties
The chemical structure of the newly synthesized compound
(TPACN)2Qx was conrmed by 1H-NMR, 13C-NMR spectroscopy
and MALDI-TOF HRMS, as shown in the Experimental section.
These results are consistent with the proposed structure and
molecular weight. Furthermore, (TPACN)2Qx possesses excel-
lent solubility in common organic solvents such as chloroform,
dichlorobenzene, and toluene, which meets the requirement for
solution-processable PV device fabrication.
Synthetic procedures
The synthetic process of (TPACN)2Qx is described in Scheme 1.
The quinoxaline-based acceptor (compound A) was synthe-
sized in six steps starting from commercially available 1,2-
benzenediamine following the procedures reported in liter-
ature.10,12b,19 Starting from the commercialized triphenylamine
as original material, a solution of POCl3 and N,N-dime-
thylformamide (DMF) was dropped carefully via Vilsmeier
reaction to obtain compound 1. Knoevenagel condensation
reaction of 4-bromophenyl acetonitrile and 4-(diphenylamino)
benzaldehyde was carried out using NaOH as a base in EtOH,
resulting in compound 2 in a yield of 91%. The donor unit
(compound D) was synthesized by Miyauraborylation of
compound 2 and bis(pinacolato)diborane in a yield of 81%.
Finally, (TPACN)2Qx was prepared by Pd(PPh3)4-assisted Suzuki
coupling reaction of compound D and compound A in a yield of
90%. Detailed synthetic procedures and characterization are
given in ESI.† Mp: 290–293 ꢁC. 1H-NMR (400 MHz, CDCl3):
The thermal stability properties of the material were investi-
gated rst by means of thermogravimetric analysis (TGA). The 5%
weight loss temperature (Td) of (TPACN)2Qx was determined from
TGA curves, as shown in Fig. 2(a). It exhibits particularly high
ꢁ
ꢁ
thermal stability with a Td greater than 494 C, which is 151 C
higher than Td of the reference compound BDCTBT as plotted in
the inset of Fig. 2(a). In addition, the thermal properties of
(TPACN)2Qx were investigated by differential scanning calorim-
etry (DSC) analysis. Fig. 2(b) shows the DSC curves of (TPACN)2Qx
ꢀ1
ꢁ
at a heating/cooling rate of 10 C min under a nitrogen ow.
When (TPACN)2Qx was heated, the endothermic peak due to the
melting temperature was observed at 319 ꢁC. Cooling process
exhibited an opposite exothermic peak due to crystallization at ca.
227 C, and the corresponding enthalpy was 78.8 kJ molꢀ1. The
ꢁ
larger crystallization enthalpy indicates that it has a tendency to
crystallize to keep its stability. In should be noted that the crys-
tallization of the materials in the active layer is benecial to
charge separation and transport in PV applications.20
Theoretical calculations
The computational analysis can be used to screen a variety of
OSMs and highlight some promising chemical structures to
Fig. 2 (a) TGA curve of (TPACN)2Qx at a heating rate of 10 ꢁC minꢀ1
under a nitrogen atmosphere. Inset: TGA analysis of BDCTBT as
a contrast. (b) DSC curves of (TPACN)2Qx at a heating/cooling rate of
10 ꢁC minꢀ1 under a nitrogen flow.
Scheme 1 Synthetic routes of (TPACN)2Qx.
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pursue synthetically, which is also desirable for investigation of electronic properties of materials. Herein, a theoretically effec-
the theoretical optical and electronic properties of new mate- tive intramolecular charge transfer (ICT) process between
rials.21 In this study, we preliminarily predict the molecular terminal TPA and central Qx via the p-linker could be predicted.
geometry and investigate the theoretical optical and electronic
The computational energy levels and molecular orbital
properties of (TPACN)2Qx with density functional theory (DFT) depictions are plotted in Fig. 3(b), with detailed electronic
and time dependent-density functional theory (TD-DFT) using transition data included in Table S1 of ESI†. The electronic
Gaussian 09 soware with the Becke's three-parameter densities of the HOMO are highly distributed on the entire
gradient-corrected functional (B3LYP) with a polarized 6- molecular backbone without a signicantly biased distribution,
31G(d) basis set.22 The summary of the computational results is whereas those of lowest unoccupied molecular orbital (LUMO)
represented in Fig. 3, with the calculated data of electronic are predominantly localized at the central electrophilic core
transitions included in the ESI†. The optimized geometry of (Qx), revealing an apparent ICT process from the electron-
(TPACN)2Qx and calculated dihedral angles are shown in donating segment to the electron-withdrawing unit. Further-
Fig. 3(a). According to the optimized molecular geometry, the more, the electronic densities of HOMO-1 are predominantly
torsion angles between the Qx and thiophene units are ca. 14ꢁ, located on their terminal segments with a greater proportion.
resulting in the molecular backbone being slightly distorted This feature reveals that the terminal TPA group could probably
from planarity, and this distortion is a result of steric demand of play an important role in stabilizing the separated hole and
the molecular structure. As expected, the torsion angles thus improve the hole transporting capacity.23 The calculated
between the acrylonitrile linker and terminal TPA unit are only energy levels of the HOMODFT and LUMODFT are ꢀ4.72 eV and
ca. 4ꢁ, which depict a considerably coplanar structure. As we ꢀ2.21 eV, respectively. Despite the error between theory and
know, the coplanar properties of the molecular backbone could experimental results, the computational analysis has proven to
probably have a pronounced inuence on the optical and be desirable for early design of new materials.24
The theoretical optical absorption spectrum of (TPACN)2Qx
with the related electronic transitions is shown in Fig. 3(c).
Accordingly, two primary absorption bands were observed. The
long wavelength absorption band resulted from the rst singlet
excited state, which could be attributed to the electron transi-
tion from HOMO to LUMO. The short one primarily resulted
from the sixth singlet excited state, which is a result of the
electron transition from HOMO to LUMO+2. Despite the
discrepancy between simulation and experiment caused by
solvent effects and intermolecular interactions and so on, the
computational results could provide important reference for the
experimental data.
Optical properties
The optical properties of (TPACN)2Qx were studied using UV-Vis
absorption measurement. The absorption proles in chloro-
form solution and in the lm state are shown in Fig. 4 with the
corresponding optical data summarized in Table 1. This
compound exhibits two primary absorption peaks in a broad
absorption range covering 300–600 nm in solution and 300–
700 nm in lm. Moreover, the quantitative measurement of
absorption in solution (inset of Fig. 4) shows that this
compound possesses strong absorption capacity with a high
molar extinction coefficient (3) of 1.08 ꢂ 105 and 9.08 ꢂ 104 Mꢀ1
cmꢀ1 at 433 and 482 nm, respectively. In comparison with the
ICT-induced maximum absorption peak in the long wavelength
(lsol) of (TPACN)2Qx, the spectrum in the lm state is greatly
broadened, and the ICT-induced absorption peak (llm) is red-
shied 25 nm, indicating a strong intermolecular p–p inter-
action in the solid state. Accordingly, the optical band gap
(Eogpt) of (TPACN)2Qx estimated with its absorption onset is
1.94 eV, which is smaller than Eogpt of the reference compound
BDCTBT as listed in Table 1. This result is in accordance with its
Fig. 3 Theoretical computational analysis for (TPACN)2Qx (a) opti-
mized ground-state geometry and dihedral angles; (b) calculated
experimental band gap via the electrochemical method.
Therefore, (TPACN)2Qx obtained smaller value of
energy levels and molecular orbital depictions; and (c) predicted
absorption spectrum and electronic transitions.
a
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Fig. 5 (a) Cyclic voltammogram of (TPACN)2Qx in 0.1 M Bu4NBF4/
CH2Cl2 solution at a scan rate of 100 mV sꢀ1; (b) energy-level diagram
of the components for (TPACN)2Qx-based photovoltaic device.
Fig.
4 Normalized UV-Vis absorption spectra of (TPACN)2Qx in
chloroform solution and in the film state. Inset: quantitative
measurement of absorption spectra in chloroform solution at
a concentration of 1.0 ꢂ 10ꢀ5 mol Lꢀ1
.
Eogpt compared to that of BDCTBT, which could probably
contribute to an efficient short-circuit current.
Electrochemical properties
To investigate the electrochemical property of (TPACN)2Qx,
cyclic voltammetry (CV) measurement was carried out in 0.1 M
Bu4NBF4/CH2Cl2 solution under the protection of nitrogen (the
experimental details are described in the Experimental section).
The cyclic voltammogram of (TPACN)2Qx and corresponding
electrochemical energy-level diagram are depicted in Fig. 5,
with the pertinent data summarized in Table 1. The onset
oxidation potential (Eox) at 0.4 V is clearly observed in the anode
potential region as a result of the oxidation of the TPA unit. On
the other hand, the onset reduction potentials (Ered) at ꢀ1.48 V
are easily obtained in the cathodic potential region due to the
reduction of Qx as the electron-withdrawing core. The electro-
chemical energy levels (HOMOCV and LUMOCV) and band gap
(ECgV) could be calculated from Eox and Ered (the calculation
methods are recorded in the Experimental section). Accord-
ingly, the HOMOCV of (TPACN)2Qx is ꢀ5.15 eV, and the LUMOCV
is ꢀ3.27 eV, resulting in a narrow ECgV of 1.88 eV. Therefore,
(TPACN)2Qx obtained a smaller value of ECgV relative to that of
BDCTBT, which could be rationally ascribed to the strong
electron-withdrawing effect of Qx with electrophilic character-
istics. This result is in good agreement with the spectral data.
Herein, the improvement of Jsc of the PV device will be expected
as a result of the decreased energy gap of (TPACN)2Qx relative to
that of BDCTBT. The electrochemical energy level of
(TPACN)2Qx together with PC61BM was plotted in Fig. 5(b).
These results clearly indicate that this compound in principle
can behave as a donor in the BHJ PV device with PC61BM as the
acceptor material.
Fig.
6 (a) J–V curves of the PV devices based on a blend of
(TPACN)2Qx and PC61BM with varied D/A weight ratios (1 : 1, 1 : 2 and
1 : 3) under an illumination of AM 1.5 G (100 mW cmꢀ2); and (b) IPCE
spectra of the related devices.
Photovoltaic properties
To fully investigate the PV characteristics of (TPACN)2Qx, the
BHJ devices with a conventional structure of ITO/PEDOT:PSS/
(TPACN)2Qx : PC61BM/Al were fabricated by
a
solution-
processed spin-coating technique. The optimization of
(TPACN)2Qx and PC61BM (D/A) weight ratios was studied in
particular; herein, the blend of (TPACN)2Qx and PC61BM with
varied weight ratios (1 : 1, 1 : 2 and 1 : 3) was performed. The J–V
curves of the devices with varied D/A weight ratios are shown in
Fig. 6(a), and the relevant PV parameters are listed in Table 2.
The initial device was fabricated from the blend solution of
(TPACN)2Qx and PC61BM in a 1 : 1 (w/w) ratio, and it showed
a PCE of 3.53% with JSC of 8.23 mA cmꢀ2, VOC of 0.94 V and FF of
0.46. Aer the D/A composition ratio was increased to 1 : 2 (w/w),
the PCE greatly increased to 6.25% with a high JSC of 14.08 mA
cmꢀ2, VOC of 0.95 V and FF of 0.47. A 5.09% drop in efficiency
was observed on increasing the fullerene loading at the D/A
composition ratio of 1 : 3 (w/w), with a decreased JSC of 12.86
mA cmꢀ2, VOC of 0.92 V and FF of 0.43. Remarkably, the device
based on (TPACN)2Qx : PC61BM composition in a 1 : 2 weight
Table 1 The optical and electrochemical data of (TPACN)2Qx
Compound
lsol (nm)/3 (Mꢀ1 cmꢀ1
)
llm (nm)
Eogpt (eV)
Eox (V)/HOMOCV (eV)
0.40/ꢀ5.15
Ered (V)/LUMOCV (eV)
ꢀ1.48/ꢀ3.27
ECgV (eV)
(TPACN)2Qx
433/108 338, 482/90 762
507
1.94 (2.05)a
1.88 (1.99)a
a
Data of the reference compound BDCTBT.10
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Table 2 The hole mobility data and PV parameters of devices based on blend of (TPACN)2Qx and PC61BM with varied D/A weight ratios
(TPACN)2Qx : PC61BM
(w/w)
mh (cm2 V ꢀ1 sꢀ1
)
Rs (U cm2)
Rsh (kU cm2)
JSC (mA cmꢀ2
)
VOC (V)
FF
PCE (%)
1 : 1
1 : 2
1 : 3
5.19 ꢂ 10ꢀ5
1.29 ꢂ 10ꢀ4
8.66 ꢂ 10ꢀ5
25
13
20
0.26
0.32
0.30
8.23
14.08
12.86
0.94
0.95
0.92
0.46
0.47
0.43
3.53
6.25
5.09
ratio showed the optimal PV performance. This value is the ratio of 1 : 2 showed relatively high IPCE response, in line with
highest efficiency reported so far for a solution-processed OSM its best PV performance.
PV-device using Qx as the electrophilic core. It should be noted
To deeply understand the optoelectronic properties of
that high efficiency might be closely related to a more optimized (TPACN)2Qx and have an insight into its PV performance, the hole-
morphology of the active layer. The better PV performance of the only devices were fabricated and measured to obtain the hole-
device with D/A ratio of 1 : 2 (w/w) was further supported by mobility (mh) with the space charge limitation of current (SCLC)
atomic force microscopy (AFM) analysis of the blended method (the details are described in the Experimental section).18
(TPACN)2Qx : PC61BM active layer (Fig. S14 in ESI†). Although all The mh is oen regarded as one of the most important physical
the blend lms with various D/A composition ratios (1 : 1, 1 : 2, parameters for the design of PV donors, and it can be achieved in
1 : 3, w/w) correspond to the favourable bicontinuous inter- the dark with the structure of ITO/PEDOT:PSS/donor:PC61BM/Au.
penetrating networks, the blend with a D/A ratio of 1 : 2 (w/w) As a result, the J–V characteristics of the devices at different weight
records the lowest surface roughness of 0.50 nm and the best ratios (1 : 1, 1 : 2 and 1 : 3) are depicted in Fig. 7. Accordingly, the
interpenetrating network due to its smooth and homogeneous tting plots of the data points in the SCLC regime are depicted in
morphology, in agreement with its best PV performance. More- the inset of Fig. 7, and the mh data can be derived in the region
over, the series resistance (Rs) of the device with D/A ratio of 1 : 2 with a slope of 2 as shown in Table 2.25 The pertinent mh values are
ꢀ1
was reduced from 25 U cmꢀ2 to 13 U cmꢀ2, and the shunt 5.19 ꢂ 10ꢀ5, 1.29 ꢂ 10ꢀ4 and 8.66 ꢂ 10ꢀ5 cm2 V
sꢀ1 for D/A
resistance (Rsh) was increased from 0.26 KU cmꢀ2 to 0.32 KU ratios of 1 : 1, 1 : 2 and 1 : 3, respectively, which keeps a consis-
cmꢀ2, which contributes to the slightly improved FF and PCE tent trend with their JSC and PCE values as expected. In contrast,
values. The good performance of the (TPACN)2Qx-based device the device based on a D/A ratio of 1 : 2 possesses the optimal hole
demonstrates that Qx is a type of effective central-acceptor transporting capacity, in agreement with the greatest mh value
building block applied in OSM-PV materials.
among them. The results would probably give some valuable hints
Further experiment of PV characteristics was carried out with on the PV performance of devices and provide an important guide
incident photon-to-current efficiency (IPCE) measurement, as for developing new donor materials.
shown in Fig. 6(b). All devices based on (TPACN)2Qx with
various D/A composition ratios demonstrated signicant
photon-to-current responses in the range of 300–700 nm with
Conclusions
the maximum IPCE of 96%, 79%, and 58% at around 350 nm,
91%, 79%, and 62% at around 410 nm and 62%, 60%, and 43%
at around 500 nm for D/A ratios of 1 : 2, 1 : 3 and 1 : 1,
respectively, which were in accordance with the trend of their
PCE values. In contrast, the device based on a D/A composition
In summary, a novel D–p–A–p–D-type OSM donor material,
(TPACN)2Qx, containing Qx as the electrophilic core, was
designed and synthesized for solution-processable OSCs. The
excellent PCE of 6.25% has been achieved based on an opti-
mized D/A ratio of 1 : 2 using a simple spin-coating process
from solution, which is the highest efficiency so far for solution-
processed Qx-core based OSM BHJ-OSCs. The good PV perfor-
mance is mainly attributed to the narrow Eg, lower HOMO
energy level and high mh value caused by the effective electron-
withdrawing property, relatively stable quinoid form of Qx and
high hole-mobility capacity of (TPACN)2Qx-based devices. The
impressive result demonstrates that OSM donors employing
a quinoxaline derivative as the electrophilic unit can compete
with their polymer counterparts. This study clearly indicates
that Qx-based OSMs possess great potential for obtaining high
performance solution-processed PV devices. Further device
optimization is underway in our laboratory.
Acknowledgements
Fig.
7 J–V curves of the hole-only devices with different D/A
composition ratios in dark. Inset: the J–V curves at double logarithmic
scale, and the solid lines are fitting plots of the data points by a SCLC
model.
This research was nancially supported by the NSFC (No.
21102013, 21471024 and 21101020), the SRF from ROCS-SEM
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(No. 201001438 and 201001439) and the Fundamental Funds 13 (a) M. L. Keshtov, S. A. Kuklin, N. A. Radychev, I. E. Ostapov,
for the Central Universities (No. DUT16ZD205, DUT11LK20,
DUT15ZD118 and DUT15LK20).
A. Y. Nikolaev, I. O. Konstantinov, M. M. Krayushkin,
E. N. Koukaras, A. Sharma and G. D. Sharma, RSC Adv.,
2016, 6, 71232–71244; (b) C. Lee, Y. Li, W. Lee, Y. Lee,
J. Choi, T. Kim, C. Wang, E. D. Gomez, H. Y. Woo and
B. J. Kim, Macromolecules, 2016, 49, 5051–5058; (c) Z. Lu,
C.-h. Li, C. Du, X. Gong and Z.-s. Bo, Chin. J. Polym. Sci.,
2013, 31, 901–911; (d) R. Singh, G. Pagona,
V. G. Gregoriou, N. Tagmatarchis, D. Toliopoulos, Y. Han,
Z. Fei, A. Katsouras, A. Avgeropoulos, T. D. Anthopoulos,
M. Heeney, P. E. Keivanidis and C. L. Chochos, Polym.
Chem., 2015, 6, 3098–3109; (e) E. Wang, L. Hou, Z. Wang,
Notes and references
1 (a) P. Cheng and X. Zhan, Chem. Soc. Rev., 2016, 45, 2544–
2582; (b) G. J. Hedley, A. Ruseckas and I. D. Samuel, Chem.
Rev., 2017, 117, 796–837; (c) K. Leo, Nat. Rev. Mater., 2016,
1, 16056, DOI: 16010.11038/natrevmats.12016.16056.
2 (a) A. W. Hains, Z. Liang, M. A. Woodhouse and B. A. Gregg,
Chem. Rev., 2010, 110, 6689–6735; (b) H. Kang, G. Kim,
J. Kim, S. Kwon, H. Kim and K. Lee, Adv. Mater., 2016, 28,
7821–7861.
¨
¨
Z. Ma, S. Hellstrom, W. Zhuang, F. Zhang, O. Inganas and
M. R. Andersson, Macromolecules, 2011, 44, 2067–2073; (f)
Y. Zhang, J. Zou, H.-L. Yip, K.-S. Chen, D. F. Zeigler, Y. Sun
and A. K. Y. Jen, Chem. Mater., 2011, 23, 2289–2291.
3 (a) J. C. Hummelen, B. W. Knight, F. LePeq and F. Wudl, J.
Org. Chem., 1995, 60, 532–538; (b) M. M. Wienk,
J. M. Kroon, W. J. Verhees, J. Knol, J. C. Hummelen, 14 (a) H.-C. Chen, Y.-H. Chen, C.-C. Liu, Y.-C. Chien, S.-W. Chou
P. A. van Hal and R. A. Janssen, Angew. Chem., Int. Ed.,
2003, 42, 3371–3375; (c) B. Kan, M. Li, Q. Zhang, F. Liu,
X. Wan, Y. Wang, W. Ni, G. Long, X. Yang, H. Feng, Y. Zuo,
M. Zhang, F. Huang, Y. Cao, T. P. Russell and Y. Chen, J.
Am. Chem. Soc., 2015, 137, 3886–3893; (d) M. Li, K. Gao,
and P.-T. Chou, Chem. Mater., 2012, 24, 4766–4772; (b)
D. Liu, W. Zhao, S. Zhang, L. Ye, Z. Zheng, Y. Cui, Y. Chen
and J. Hou, Macromolecules, 2015, 48, 5172–5178; (c)
J. Yuan, L. Qiu, Z. Zhang, Y. Li, Y. He, L. Jiang and Y. Zou,
Chem. Commun., 2016, 52, 6881–6884.
X. Wan, Q. Zhang, B. Kan, R. Xia, F. Liu, X. Yang, H. Feng, 15 (a) X. Li, Y. Zhou, J. Chen, J. Yang, Z. Zheng, W. Wu, J. Hua
W. Ni, Y. Wang, J. Peng, H. Zhang, Z. Liang, H.-L. Yip,
X. Peng, Y. Cao and Y. Chen, Nat. Photonics, 2016, 11, 85–
90; (e) Y. Liu, L. A. Renna, M. Bag, Z. A. Page, P. Kim,
J. Choi, T. Emrick, D. Venkataraman and T. P. Russell, ACS
Appl. Mater. Interfaces, 2016, 8, 7070–7076.
4 (a) Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245–
4272; (b) O. Ostroverkhova, Chem. Rev., 2016, 116, 13279–
13412; (c) B. Walker, C. Kim and T.-Q. Nguyen, Chem.
Mater., 2011, 23, 470–482.
and H. Tian, Chem. Commun., 2015, 51, 10349–10352; (b)
K. Pei, Y. Wu, A. Islam, S. Zhu, L. Han, Z. Geng and
W. Zhu, J. Phys. Chem. C, 2014, 118, 16552–16561; (c)
K. Pei, Y. Wu, H. Li, Z. Geng, H. Tian and W. H. Zhu, ACS
Appl. Mater. Interfaces, 2015, 7, 5296–5304; (d) J. Yang,
P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi,
P. Comte, K. Pei, T. W. Holcombe, M. K. Nazeeruddin,
¨
J. Hua, S. M. Zakeeruddin, H. Tian and M. Gratzel, J. Am.
Chem. Soc., 2014, 136, 5722–5730.
5 D. Deng, Y. Zhang, J. Zhang, Z. Wang, L. Zhu, J. Fang, B. Xia, 16 (a) D. Kekuda, J.-S. Huang, M. Velusamy, J. T. Lin and
Z. Wang, K. Lu, W. Ma and Z. Wei, Nat. Commun., 2016, 7,
13740, DOI: 13710.11038/ncomms13740.
C.-W. Chu, Sol. Energy Mater. Sol. Cells, 2010, 94, 1767–
¨
1771; (b) W. Li, D. Wang, S. Wang, W. Ma, S. Hedstrom,
6 Y. Yang, Z. G. Zhang, H. Bin, S. Chen, L. Gao, L. Xue, C. Yang
and Y. Li, J. Am. Chem. Soc., 2016, 138, 15011–15018.
7 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci.,
2010, 3, 1427–1436.
D. I. James, X. Xu, P. Persson, S. Fabiano, M. Berggren,
€
O. Inganas, F. Huang and E. Wang, ACS Appl. Mater.
Interfaces, 2015, 7, 27106–27114; (c) J. A. Mikroyannidis,
A. N. Kabanakis, A. Kumar, S. S. Sharma, Y. K. Vijay and
G. D. Sharma, Langmuir, 2010, 26, 12909–12916.
8 (a) H. Gao, Y. Li, L. Wang, C. Ji, Y. Wang, W. Tian, X. Yang
and L. Yin, Chem. Commun., 2014, 50, 10251–10254; (b) 17 (a) Z. Li, Y. Zhang, A. L. Holt, B. P. Kolasa, J. G. Wehner,
C. Ji, L. Yin, K. Li, L. Wang, X. Jiang, Y. Sun and Y. Li, RSC
Adv., 2015, 5, 31606–31614; (c) L. Wang, L. Yin, C. Ji and
Y. Li, Dyes Pigm., 2015, 118, 37–44.
A. Hampp, G. C. Bazan, T.-Q. Nguyen and D. E. Morse,
New J. Chem., 2011, 35, 1327–1334; (b) C.-Y. Yu, C.-P. Chen,
S.-H. Chan, G.-W. Hwang and C. Ting, Chem. Mater., 2009,
21, 3262–3269.
9 (a) C. Ji, L. Yin, L. Wang, T. Jia, S. Meng, Y. Sun and Y. Li, J.
Mater. Chem. C, 2014, 2, 4019–4026; (b) L. Wang, L. Yin, C. Ji, 18 (a) P. W. M. Blom, M. J. M. de Jong and M. G. van Munster,
Y. Zhang, H. Gao and Y. Li, Org. Electron., 2014, 15, 1138–
1148.
10 S. Zeng, L. Yin, C. Ji, X. Jiang, K. Li, Y. Li and Y. Wang, Chem.
Commun., 2012, 48, 10627–10629.
Phys. Rev. B: Condens. Matter, 1997, 55, R656–R659; (b) Z. Li,
Q. Dong, Y. Li, B. Xu, M. Deng, J. Pei, J. Zhang, F. Chen,
S. Wen, Y. Gao and W. Tian, J. Mater. Chem., 2011, 21,
2159–2168.
11 S. Paek, N. Cho, K. Song, M.-J. Jun, J. K. Lee and J. Ko, J. Phys. 19 Q. Peng, J. Xu and W. Zheng, J. Polym. Sci., Part A: Polym.
Chem. C, 2012, 116, 23205–23213.
Chem., 2009, 47, 3399–3408.
12 (a) D. W. Chang, S.-J. Ko, J. Y. Kim, L. Dai and J.-B. Baek, 20 (a) Y. Chen, C. Li, P. Zhang, Y. Li, X. Yang, L. Chen and Y. Tu,
Synth. Met., 2012, 162, 1169–1176; (b) Y. Lee, Y. M. Nam
and W. H. Jo, J. Mater. Chem., 2011, 21, 8583–8590; (c)
H. J. Song, T. H. Lee, M. H. Han, J. Y. Lee and D. K. Moon,
Polymer, 2013, 54, 1072–1079.
Org. Electron., 2013, 14, 1424–1434; (b) F. Liang, J. Lu, J. Ding,
R. Movileanu and Y. Tao, Macromolecules, 2009, 42, 6107–
6114.
This journal is © The Royal Society of Chemistry 2017
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RSC Advances
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21 Y. Cui, P. Li, C. Song and H. Zhang, J. Phys. Chem. C, 2016, 23 S. So, H. Choi, C. Kim, N. Cho, H. M. Ko, J. K. Lee and J. Ko,
120, 28939–28950. Sol. Energy Mater. Sol. Cells, 2011, 95, 3433–3441.
22 R. Chen, Y. Wang, T. Chen, H. Li, C. Zheng, K. Yuan, 24 Q. Liu, Y. Liu, Y. Wang, L. Ai, X. Ouyang, L. Han and Z. Ge,
Z. Wang, Y. Tao, C. Zheng and W. Huang, J. Phys. Chem. B,
2015, 119, 583–591.
New J. Chem., 2013, 37, 3627–3633.
25 L. Wang, L. Yin, L. Wang, B. Xie, C. Ji and Y. Li, Dyes Pigm.,
2017, 140, 203–211.
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