Enhanced open circuit voltage of small molecule acceptors containing angular-shaped indacenodithiophene units for P3HT-based organic solar cells

Article abstract of DOI:10.1039/c8tc04608e

Angular indaceno[2,1-b:6,5-b′]dithiophene (a-IDT) as an analogue of linear-IDT (l-IDT), in which the central phenyl ring was linked to the β-position of the thiophene ring but fused on its α-position, was designed and exploited via intramolecular annulati

Full text of DOI:10.1039/c8tc04608e

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Materials Chemistry C
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PAPER
Enhanced open circuit voltage of small molecule
acceptors containing angular-shaped
indacenodithiophene units for P3HT-based
organic solar cells†
Cite this: J. Mater. Chem. C, 2018,
6, 12347
cd
Hongyan Huang,‡ab Bo Xiao,
‡
Chengting Huang,a Jing Zhang,a Shuli Liu,a
Nina Fu,a Baomin Zhao,*a Tianshi Qin,b Erjun Zhou *c and Wei Huang
*
ae
Angular indaceno[2,1-b:6,5-b0]dithiophene (a-IDT) as an analogue of linear-IDT (l-IDT), in which the
central phenyl ring was linked to the b-position of the thiophene ring but fused on its a-position, was
designed and exploited via intramolecular annulation. Two A–D–A type small molecule acceptors (SMAs)
l-IDTBTRh and a-IDTBTRh, with l-IDT and a-IDT as the central cores, benzothiadiazole (BT) as the
p-bridge acceptor segments and 3-ethylrhodanine as the end-capping groups, were synthesized and
employed for P3HT-based fullerene free organic solar cells. The geometric shape of the l-IDT and a-IDT
subunits plays a pivotal role in governing the optoelectronic properties, the charge mobility, the
morphology and the photovoltaic performance of the resulting acceptors. Preliminarily, the device
based on P3HT/l-IDTBTRh (1 : 08, w/w) displayed a circuit current density (Jsc) of 8.81 mA cmꢀ2, an
open-circuit voltage (Voc) of 0.86 V and a fill factor (FF) of 71.0%, delivering a decent PCE of 5.38%,
which is associated with the lower bandgap, the better charge mobility and the morphology. Despite the
inferior photovoltaic performance (2.53%), solar cells based on BHJ blends of a-IDTBTRh and P3HT
offer a remarkably higher Voc of 0.92 V as a result of the high-lying LUMO energy level (ꢀ3.59 eV),
representing one of the highest values among the reported IDT-based A–D–A type SMAs for solar
cells containing P3HT. The results suggested that introducing an a-IDT segment as the central core into
A–D–A type SMAs can effectively increase the LUMO energy level of SMAs, offering an efficient strategy
to design non-fullerene small molecule acceptors with high Voc based-P3HT organic solar cells.
Received 12th September 2018,
Accepted 22nd October 2018
DOI: 10.1039/c8tc04608e
rsc.li/materials-c
donors and fullerene derivatives as the acceptors have
attracted widespread attention because of their low cost,
Introduction
lightweight and mechanical flexibility.1–6 With the great
endeavors devoted to the molecular engineering of the donor
and acceptor materials, assisted by limited success in deriv-
atizing the fullerene derivatives, the power conversion effi-
ciency (PCE) of BHJ OSCs has exceeded 11%.7–10 Fullerene
derivatives have been prevailing acceptor materials in the
development of BHJ OSCs owing to their isotropic electron
transporting ability and high electron mobility. Nevertheless,
fullerene derivatives suffer from intrinsic drawbacks, such
as poor solubility and weak visible light absorption, high
production cost and difficulty in energy level tuning, which
makes it difficult to enhance the photovoltaic performance of
BHJ OSCs.11,12 Benefiting from the attraction of well-defined
molecular structures, broad absorption and tunable energy
levels, small molecule acceptors (SMAs) have become promising
alternatives to fullerene derivatives.13–18 PCEs over 17% have
been realized for BHJ OSCs based on SMAs, surpassing their
fullerene-based counterparts.19
Over the past few decades, the state-of-the-art bulk hetero-
junction (BHJ) organic solar cells (OSCs) containing polymer
a Key Laboratory for Organic Electronics and Information Displays, Jiangsu Key
Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National
Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University
of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.
E-mail: iambmzhao@njupt.edu.cn, wei-huang@njtech.edu.cn
b Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials
(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing
211816, China
c CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for
Excellence in Nanoscience, National Center for Nanoscience and Technology,
Beijing 100190, China. E-mail: zhouej@nanoctr.cn
d University of Chinese Academy of Sciences, Beijing 100049, P. R. China
e Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical
University (NPU), Xi’an 710072, China
† Electronic supplementary information (ESI) available: TGA spectra, NMR
spectra, and device optimization. See DOI: 10.1039/c8tc04608e
‡ H. Y. Huang and B. Xiao contributed equally to this work.
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To date, SMAs with acceptor–donor–acceptor (A–D–A) architec- voltage (Voc) (around 0.6 V) for the P3HT/PCBM system. Therefore,
tures, where the donor unit (D) as the central core can be combined the exploitation of novel A–D–A type SMAs having high-lying LUMO
with a wide variety of electron-withdrawing groups (A), have been the energy level for improving Voc in P3HT-based BHJ OSCs is vital. For
preferred acceptors for high-performance BHJ OSCs.13–15 The penta- the first time, Zhan’s group reported A–D–A type SMAs containing
cyclic indaceno[2,1-b:6,5-b0]dithiophene (IDT) unit, as a thiophene- IDT as the central donor core and successfully applied them to
fused heteroarene, with two thiophene rings fused with the central P3HT-based BHJ OSCs.24,25 Very recently, Zhou’s group has made
phenyl ring into linear pentacyclic aromatics, represents one of the significant achievements for improving Voc in P3HT-based BHJ
most prospective motifs for constructing SMAs due to its rigidity and OSCs,34–36,40 more notably, they have developed the small molecule
planarity.17,20–25 Additionally, the peripheral substituents on the IDT acceptor BTA2 containing IDT and realized a double Voc of 1.22 V
backbones can not only ensure high solution processability of IDT- compared to that of the PC61BM-based control device (about 0.6 V)
based acceptors but also suppress their aggregation in solid states. when blending with the donor material of P3HT.35
Sharing these chemical structural features, a series of A–D–A type
On account of the aforementioned considerations, it is of great
SMAs that utilize IDT units as the core and different electron- interest to design new A–D–A SMAs containing IDT units for
withdrawing units as end-capping groups were developed recently. improving Voc in P3HT-based BHJ OSCs. We have focused on the
Considerable efforts have been devoted to modifying the mole- design and synthesis of the materials for OFETs and BHJ OSCs.54–60
cular structures through side chain engineering,26,27 backbone And we have recently exploited three A–D–A SMAs including N-
conjugation regulation,28–32 heteroatom substitution,33 p-bridge annulated perylene diimide, and the non-fullerene BHJ OSCs could
acceptor modulation,34–40 and choosing end-capping acceptor achieve a high Voc of 1.14 V.60 Inspired by our previous work, herein,
segments.38,41,42 However, less attention has been paid to the a-IDT as an analogue of l-IDT, in which the central phenyl ring was
geometric shape of the IDT unit as well as its A–D–A type SMAs. linked to the b-position of the thiophene ring but fused on its a-
Note that naphthodithiophene (NDT), another thiophene-fused position, was designed and developed via intramolecular annulation
heteroarene, can be categorized into linear-shaped NDT (l-NDT) for the first time (Fig. 1). Two A–D–A type SMAs l-IDTBTRh and a-
and angular-shaped NDT (a-NDT) depending on the geometry of the IDTBTRh (Fig. 1), with l-IDT and a-IDT as the central cores,
fused thiophenes and a wide variety of NDT-based organic semi- benzothiadiazole (BT) as the p-bridge acceptor moiety and 3-
conductor materials have been developed for organic field-effect ethylrhodanine as the end-capping group, were synthesized and
transistor (OFET) and OSC applications.43–48 Besides, A–D–A type employed for P3HT-based non-fullerene organic solar cells. The
SMAs based on a-NDT have been reported and successfully applied effects of the geometric shapes of the l-IDT and a-IDT subunits on
to ternary blend OSCs.49 Similarly, the IDT unit should also possess the thermal, optical, electrochemical and film-forming properties,
angular and linear configurations of angular-IDT (a-IDT) and linear- the charge mobility and the photovoltaic performance of the result-
IDT (l-IDT), where either the a- or b-positions of the thiophene units ing acceptors were investigated. A preliminary photovoltaic study of
are attached to the central phenyl moiety.
P3HT/SMA blend films was conducted for exploring their potential
On the other hand, many D–A type copolymers have been applications in non-fullerene BHJ OSCs.
selected as donor materials for high-performance non-fullerene
BHJ OSCs.18–20,50 Compared to these copolymers, the simplest
homopolymer of poly(3-hexylthiophene) (P3HT) can be easily
synthesized at low cost, demonstrating that P3HT has become
Results and discussion
Synthesis and thermal properties
the most representative and promising donor polymer for BHJ OSC
applications.51–53 However, P3HT possesses a high-lying HOMO The synthetic routes and details of the small molecule acceptors
energy level (around ꢀ5.00 eV), resulting in a low open-circuit are shown in Scheme 1 and in the ESI.† Compounds 2 and 3
Fig. 1 Chemical structures of l-IDT, a-IDT and l-IDTBTRh, a-IDTBTRh.
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Scheme 1 Synthetic routes to l-IDTBTRh and a-IDTBTRh.
were synthesized using the Suzuki reaction between diethyl as the solvent.62 Finally, the target small acceptor molecules
2,5-dibromoterephthalate (1) and 2-thiophenylboric acid, l-IDTBTRh and a-IDTBTRh were obtained by the Knoevenagel
3-thiophenylboric acid with a yield of 73% and 64%, respec- condensation of compounds l-IDTBT and a-IDTBT with 3-ethyl-
tively. In order to improve the solubility of the conjugated rhodanine, respectively. The chemical structures of the
polymers and suppress the aggregation, alkyl side chains that intermediates and the target small acceptor molecules were
incorporated p-(octyloxy)phenyl derivatives were introduced. confirmed via 1H NMR and 13C NMR spectroscopy and MALDI-
l-IDT and a-IDT were synthesized by following the reported TOF MS (Fig. S3–S10, ESI†). Both the small acceptor mole-
procedure through the addition of p-(octyloxy)phenyl Grignard cules exhibited excellent solubility at room temperature in
reagent into a solution of compounds 2 and 3 containing the common organic solvents, such as tetrahydrofuran (THF),
ester to give the corresponding alcohol and then cyclized chloroform (CF) and chlorobenzene (CB). Additionally, the
subsequently through the acid-mediated reaction.61 By treating thermal properties of these small acceptor molecules were
the compounds l-IDT and a-IDT with n-butyllithium (n-BuLi), evaluated by thermogravimetric analysis (TGA). As shown in
followed by adding trimethyltin chloride, l-IDTSn and a-IDTSn Fig. S1 (ESI†) and Table 1, the 5% weight loss temperature is
were obtained via rotary evaporation and directly used for the 376 and 389 1C for l-IDTBTRh and a-IDTBTRh, respectively,
next step without any treatment. l-IDTBT and a-IDTBT as the indicating that the two small acceptor molecules exhibited
important intermediates were synthesized using the Stille excellent stability and meet the requirements of fabricating
coupling reaction with Pd(PPh3)4 as the catalyst and toluene organic solar cell devices.
Table 1 Optical and electrochemical properties of l-IDTBTRh and a-IDTBTRh
Solutionb
Filmc
e (Mꢀ1 cmꢀ1
)
lmax (nm)
ledge (nm)
Eogpt d (eV)
E
(V)/HOMO (eV)
E
(V)/LUMO (eV)
onset
a
ox
onset
red
Compound
Td (1C)
lmax (nm)
l-IDTBTRh
a-IDTBTRh
376
389
618
548
9 ꢁ 104
667
570
751
653
1.65
1.89
1.02/ꢀ5.42
1.23/ꢀ5.63
ꢀ0.72/ꢀ3.68
ꢀ0.81/ꢀ3.59
5.3 ꢁ 104
a
b
c
The 5% weight-loss temperatures under a nitrogen atmosphere. Measured in dilute dichloromethane solution. Cast from dichloromethane
d
solution. Bandgap estimated from the onset wavelength (ledge) of the optical absorption: Eogpt = 1240/ledge
.
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Fig. 2 (a) Absorption spectra of l-IDTBTRh and a-IDTBTRh in solution, (b) absorption spectra of l-IDTBTRh, a-IDTBTRh and P3HT in thin films,
(c) absorption spectra of the blend films P3HT : l-IDTBTRh (1 : 0.8, w/w) and P3HT : a-IDTBTRh (0.8 : 1, w/w), and (d) cyclic voltammograms of l-IDTBTRh
and a-IDTBTRh recorded from solution at a scan rate of 50 mV sꢀ1
.
Optical properties
the absorption of P3HT as the donor material. In particular, the
blend films of l-IDTBTRh/P3HT displayed stronger and broader
absorption than those of a-IDTBTRh/P3HT, benefitting from
absorbing more photons and generating high photocurrent.
Fig. 2 shows the absorption spectra of the small molecule
acceptors l-IDTBTRh and a-IDTBTRh, respectively, where the
solution spectrum was measured in dilute dichlorobenzene
and the thin films were prepared by spin-coating the dichloro-
benzene solution on the quartz glass substrate. The related
Electrochemical properties
optical data including the maximum absorption peak (lmax), It is well known that the matched energy levels between the donor
the absorption edge (ledge), the molar absorption coefficient (e) and acceptor materials are very important for high-performance
and the optical band gap (Eogpt) are summarized in Table 1. As organic solar cells. Cyclic voltammetry (CV) with Ag/AgCl as a
shown in Fig. 2a and b, for the two small molecule acceptors, reference electrode and the Fc/Fc+ couple as an internal reference
the absorption spectra were mainly dependent on the geo- was executed to determine the energy levels of the two small
metric shapes of the l-IDT and a-IDT subunits. In dilute molecule acceptors. The CV curves are shown in Fig. 2d and the
solution, the molar extinction coefficients of l-IDTBTRh and related electrochemical data are summarized in Table 1. The
a-IDTBTRh were 9 ꢁ 104 Mꢀ1 cmꢀ1 and 5.3 ꢁ 104 Mꢀ1 cmꢀ1
,
HOMO and LUMO levels were calculated from the onset poten-
ox
red
and the maximum absorption peaks in the long wavelength tials (E
) and reduction potentials (E ), respectively. The
onset onset
regions which were located at 618 nm for l-IDTBTRh and HOMO/LUMO energy levels were ꢀ5.42/ꢀ3.68 eV for IDTBTRh
548 nm for a-IDTBTRh, respectively, correspondingly red- and ꢀ5.63/ꢀ3.59 eV for a-IDTBTRh, respectively, corresponding
shifted to 667 nm and 570 nm for their spectra in thin films, with those of wide-bandgap polymer donors such as P3HT to
which may be attributed to the stronger intermolecular inter- guarantee efficient exciton dissociation. Their LUMO levels of the
actions formed in the thin films. Moreover, the spectrum of two small molecule acceptors are higher than those of PC61BM
l-IDTBTRh showed a significantly red-shifted absorption com- (B3.9 eV), and thus a higher Voc, especially a-IDTBTRh should be
pared to that of a-IDTBTRh, suggesting that the l-IDT moiety expected in P3HT-based BHJ OSCs. The results demonstrate that
may induce more effective conjugation than the a-IDT segment the geometric shape of the l-IDT and a-IDT subunits also exerts
and thus stronger p–p stacking existed in l-IDTBTRh. Optical notable effects on the energy level for the resulting acceptors and
bandgaps (Eogpt) deduced from the absorption edges of the thin a-IDT may be a good choice to design A–D–A type SMAs with an
film spectra were determined to be 1.65 eV for l-IDTBTRh and upshifted LUMO energy level for increasing Voc in P3HT-based
1.89 eV for a-IDTBTRh. The absorption spectra of the blend BHJ OSCs. The electrochemical energy gaps (Eegc) were estimated
films are shown in Fig. 2c, and the spectral features of the two to be 1.74 eV for l-IDTBTRh and 2.04 eV for a-IDTBTRh, which are
small molecule acceptors exhibited some complementary with larger than their corresponding Eogpt in the thin films.
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groups on the l-IDT moiety exhibited a dihedral angle about 1171
to the backbone plane for l-IDTBTRh, similarly to a-IDTBTRh
(about 1191), which is conducive to the formation of the inter-
penetrating networks and the phase separation when blending
with P3HT as the donor material. Clearly, a similar HOMO energy
distribution was observed for l-IDTBTRh and a-IDTBTRh, and
mainly located along the whole molecule skeletons. However, the
LUMO energy distributions of the two small molecules were
obviously different. For a-IDTBTRh, the electron density of the
LUMO orbital was mainly positioned on the BT unit, the linked
double bond and the terminal acceptor, whereas that of l-IDTBTRh
was localized along the whole molecule skeleton, indicating that
the geometric shape of the l-IDT and a-IDT subunits has an impact
on the LUMO energy distribution of the resulting small molecule
acceptors. The calculated HOMO/LUMO energy levels were ꢀ5.18/
ꢀ3.25 eV for l-IDTBTRh and ꢀ5.27/ꢀ3.15 eV for a-IDTBTRh. The
limited electron density delocalization of the LUMO energy could
be responsible for the slightly higher LUMO energies of a-IDTBTRh
than l-IDTBTRh. The trend in the variation of the energy levels
agreed well with those obtained from the CV measurements.
Fig. 3 Simulated molecular geometries obtained by DFT calculations for
simplified molecules of l-IDTBTRh and a-IDTBTRh.
Theoretical calculations
Photovoltaic properties and mobility
Density functional theory (DFT) calculations were carried out To evaluate the photovoltaic properties of l-IDTBTRh and
using the Gaussian 09 program at the B3LYP/6-31G level of a-IDTBTRh as the acceptor materials, the BHJ OSC devices with a
theory to investigate the molecular geometry and molecular conventional configuration of ITO/PEDOT:PSS/P3HT : l-IDTBTRh
frontier orbitals for l-IDTBTRh and a-IDTBTRh. All alkyl side or a-IDTBTRh/Ca/Al using P3HT as the donor material were
chains were replaced by methyl groups to save the time required fabricated (Fig. S2, ESI†). The J–V curves and the relevant para-
for calculations. As shown in Fig. 3, the two small molecules meters such as short circuit current density ( Jsc), Voc and fill factor
l-IDTBTRh and a-IDTBTRh adopt nearly flat backbone configura- (FF) are shown in Fig. 4a and Table 2. More detailed device
tions, thus facilitating the charge transport. The octyloxyphenyl parameters are provided in the ESI† (Tables S1–S4). The optimal
Fig. 4 (a) J–V curves based on P3HT : l-IDTBTRh (1 : 0.8, w/w) and P3HT : a-IDTBTRh (0.8 : 1, w/w) as the active layers upon thermal annealing treatment.
(b) The EQE profiles and the corresponding integrated Jsc of the organic solar cells in structures of ITO/PEDOT:PSS/P3HT : l-IDTBTRh (1 : 0.8, w/w) or
P3HT : a-IDTBTRh (0.8 : 1, w/w)/Ca/Al. (c) The J–V curves of the hole-only devices. (d) The J–V curves of the electron-only devices.
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Table 2 Photovoltaic performances of the organic solar cells based on P3HT/l-IDTBTRh or a-IDTBTRh under the illumination of AM 1.5 G, 100 mW cmꢀ2
mh
me
Blend
D/A (w/w)
Jsc (mA cmꢀ2
)
Voc (V)
FF (%)
PCE (%)
(cm2 Vꢀ1 sꢀ1
)
(cm2 Vꢀ1 sꢀ1
)
P3HT:l-IDTBTRha
P3HT:a-IDTBTRhb
1 : 0.8
0.8 : 1
8.81 (8.84)c
5.00 (4.87)c
0.86
0.92
71.0
55.0
5.38
2.53
2.32 ꢁ 10ꢀ6
1.15 ꢁ 10ꢀ5
1.46 ꢁ 10ꢀ6
8.84 ꢁ 10ꢀ6
a
b
c
Thermal annealing at 140 1C for 10 min. Thermal annealing at 120 1C for 10 min. Integrated from EQE data.
conditions for the device fabrication were carefully screened by The surface morphology of the blend films was explored
choosing the D/A ratio, the spin-coating speed, the solvent, the by atomic force microscopy (AFM). As shown in Fig. 5, the
thermal annealing temperature and solvent additives. The root-mean-square (RMS) roughness values are 2.94 nm for
optimal D/A weight ratios were found to be 1 : 0.8 for P3HT/ P3HT/l-IDTBTRh and 1.34 nm for P3HT/a-IDTBTRh, respectively.
l-IDTBTRh and 0.8 : 1 for P3HT/a-IDTBTRh, and the optimal As usual, too large or small phase separation in the blend film is
thermal annealing temperatures were 140 1C and 120 1C for not conducive to the charge separation and transport, resulting in
P3HT/l-IDTBTRh and P3HT/a-IDTBTRh, respectively. As shown the reduction of Jsc and FF. Therefore, the lower RMS value of the
in Fig. 4a and Table 2, the P3HT/l-IDTBTRh based device blend film P3HT/a-IDTBTRh may be responsible for the lower Jsc,
displayed a decent PCE of 5.38% with a Jsc of 8.81 mA cmꢀ2
a Voc of 0.86 V, and a remarkable FF of 71.0%, which may stem
,
FF and photovoltaic performance.
For insights into the charge transport properties of the two
from the strong and broad light-harvesting range as well as the small molecule acceptors, the charge mobilities of the optimized
balanced charge carrier transport. With respect to P3HT/a- P3HT/l-IDTBTRh and P3HT/a-IDTBTRh blend films were mea-
IDTBTRh,a Jsc of 5.00 mA cmꢀ2, a Voc of 0.92 V, a FF of 55.0% sured by the space charge limited current (SCLC) method. The
and an inferior PCE of 2.53% were delivered. The angular- hole mobility (mh) and electron mobility (me) were acquired from
or linear-shape of the IDT subunits results in different photo- the device configuration of ITO/PEDOT:PSS/active layer/Au and
voltaic performances of the small molecule acceptors. It should ITO/titanium(diisopropoxide)bis(2, 4-pentanedionate), 75% in
be noted that these devices yielded a higher Voc (40.82 V) than isopropanol liquid (TIPD)/active layer/Al, respectively. As shown
that of PC61BM-based control devices (B0.6 V), especially for in Fig. 4c and d, the as-cast blend films of P3HT and a-IDTBTRh
P3HT/a-IDTBTRh, a Voc of 0.92 V was obtained, which is one of exhibited a mh of 1.46 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and a me of 8.84 ꢁ
the highest values among the reported IDT based A–D–A type 10ꢀ6 cmꢀ2 Vꢀ1 ꢀ1, whereas the blend films of P3HT/l-IDTBTRh
s
SMAs. Generally, solvent additives, such as 1,8-diiodooctane showed a slightly higher mh of 2.32 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and me of
(DIO), diphenyl ether (DPE), and 1-chloronaphthalene (CN), were 1.15 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1. Additionally, a more balanced me/mh
employed to optimize the morphology and thus improve the photo- (4.96) of the P3HT/l-IDTBTRh blend film is achieved than that of
voltaic performance.63 Disappointingly, as shown in Table S4 the P3HT/a-IDTBTRh blend film (6.05). The results indicate that
(ESI†), these solvent additives could not improve the photo- P3HT/l-IDTBTRh possesses a higher and more balanced charge
voltaic performance of P3HT/l-IDTBTRh and P3HT/a-IDTBTRh. mobility, which could partly explain the enhanced Jsc, FF and
photovoltaic performance in OSCs compared to P3HT/a-IDTBTRh.
The trend of the mobilities of the two blend films is
consistent well with their PCE values.
The external quantum efficiency (EQE) plots of l-IDTBTRh
and a-IDTBTRh based on the optimized BHJ OSCs are shown in
Fig. 4b. The EQE curve of the OSCs based on a-IDTBTRh/P3HT
(0.8 : 1, w/w) covered a broad wavelength range from 300 to
650 nm with the maximum EQE value of 42% at ca. 520 nm,
while the EQE curve of the organic solar cell based on l-IDTBTRh/
P3HT (1 : 0.8, w/w) exhibited a higher EQE value of 51% at 522 nm
and a broader wavelength range. The higher EQE values of the
organic solar cell based on l-IDTBTRh agree with the higher Jsc
of the device mentioned above. The Jsc values calculated from
integration of the EQE spectra with the AM 1.5 G reference
spectrum are 8.84 mA cmꢀ2 for l-IDTBTRh and 4.87 mA cmꢀ2
for a-IDTBTRh, which is in accord with those obtained from
J–V measurements.
Conclusions
Fig. 5 AFM height (a and b) and phase (c and d) images of blend films
In summary, we have successfully developed angular-shaped
a-IDT as the analogue of l-IDT via intramolecular annulation,
upon thermal treatment: (a and c) P3HT : l-IDTBTRh (1 : 0.8, w/w); (b and d)
P3HT : a-IDTBTRh (0.8 : 1, w/w).
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in which the central phenyl ring was linked to the b-position of
the thiophene ring but fused on its a-position. Two A–D–A type
SMAs l-IDTBTRh and a-IDTBTRh with l-IDT and a-IDT as the
central cores were also synthesized and employed for P3HT-
based organic solar cells. The geometric shape of the l-IDT and
a-IDT subunits plays a pivotal role in governing the optoelec-
tronic properties, the charge mobility, the morphology and the
photovoltaic performance of the resulting acceptors. Both small
molecule acceptors possessed excellent thermal stability and
solubility in common organic solvents and adopted nearly flat
backbone configurations. Compared to a-IDTBTRh, l-IDTBTRh
exhibited a lower bandgap and better charge mobility and
morphology. By blending with P3HT, BHJ OSCs delivered a
decent PCE of 5.38% with a higher Jsc of 8.81 mA cmꢀ2 and a FF
of 71.0% for l-IDTBTRh. Although displaying an inferior photo-
voltaic performance (2.53%), solar cells based on BHJ blends of
a-IDTBTRh and P3HT offer a remarkably high Voc of 0.92 V as a
result of the upshifted LUMO energy level (ꢀ3.59 eV), which is
one of the highest values among the reported IDT based A–D–A
type SMAs. Our work demonstrates that a-IDT is an effective
building block to construct promising A–D–A SMAs with high-
lying LUMO energy levels, providing a strategy to increase Voc in
P3HT-based organic solar cells.
¨
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was financially supported by the National Key
Research and Development Program of China (2017YFB0404501),
the National Basic Research Program of China-Fundamental
Studies of Perovskite Solar Cells (2015CB932200), the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD) and the Program for Changjiang Scholars
and Innovative Research Team in University (IRT-15R37), and the
NJUPT Culturing Project (NY214080, NY214087 and NY218056).
25 H. Bai, P. Cheng, Y. Wang, L. Ma, Y. Li, D. Zhu and X. Zhan,
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