High performance asymmetrical push-pull small molecules end-capped with cyanophenyl for solution-processed solar cells

Article abstract of DOI:10.1039/c4cc04020a

Two novel asymmetrical push-pull small molecules have been synthesized successfully, consisting of triphenylamine and diketopyrrolopyrrole as a fundamental dipolar D-π-A structure with ethynylbenzene as the π-bridge. TPATDPPCN end-capped with cyanophenyl exhibits a low optical band gap of 1.65 eV, and an impressive PCE of 5.94% has been achieved. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04020a

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High performance asymmetrical push–pull small
molecules end-capped with cyanophenyl for
solution-processed solar cells†
Cite this: Chem. Commun., 2014,
50, 10251
Received 26th May 2014,
Accepted 15th July 2014
Hang Gao,a Yanqin Li,a Lihui Wang,a Changyan Ji,a Yue Wang,b Wenjing Tian,b
Xichuan Yanga and Lunxiang Yin*a
DOI: 10.1039/c4cc04020a
www.rsc.org/chemcomm
Two novel asymmetrical push–pull small molecules have been architecture can not only easily reduce the energy band gap
synthesized successfully, consisting of triphenylamine and diketo- (Eg), but also effectively control the PV properties of materials.
pyrrolopyrrole as a fundamental dipolar D–p–A structure with In general, the molecular configurations of most SMDs can be
ethynylbenzene as the p-bridge. TPATDPPCN end-capped with classified into two main categories: symmetrical D–A–D and
cyanophenyl exhibits a low optical band gap of 1.65 eV, and an A–D–A systems. In contrast, the solution-processed asymmetric
impressive PCE of 5.94% has been achieved.
dipolar D–A or D–A–A systems are relatively less explored, and
their PV performance lags far behind those of symmetrical
Solution-processed bulk heterojunction (BHJ) organic solar SMD-based OSCs.7 From the study of dye-sensitized solar cells,
cells (OSCs) have experienced rapid development during the we learned that excellent organic dyes with D–A molecular
past two decades because they represent one of the most architectures exhibited remarkably high PCEs of over 10%.8
promising technologies for environmentally friendly solar In addition, important progress has been made on vacuum-
energy conversion.1 So far, fullerene derivatives such as deposited SMD-based OSCs with a PCE of 6.8%.9 Consequently,
PC61BM and PC71BM have been widely used as representative we believe that these asymmetrical push–pull small molecules
n-type acceptor materials for BHJ OSCs.2 Much work has been possess attractive potential for BHJ OSCs. However, such types
done to develop novel p-type donors in order to maximize of molecules generally suffer from an insufficient light-
photovoltaic (PV) efficiency for commercialization. Small mole- absorption range, which may be one of the main impediments
cular donors (SMDs) have attracted considerable attention for to further improvement of their performance.10 Therefore, it is
preparing BHJ OSCs due to the advantages over their polymer highly desired to design new asymmetrical push–pull small
counterparts, including well-defined molecular structures, molecules with narrow Eg for BHJ OSCs.
easier purification and better batch-to-batch reproducibility.3
In this communication, two new asymmetrical push–pull
SMD-based BHJ single cells reported to date exhibit an encoura- small molecules TPATDPP and TPATDPPCN have been
ging power conversion efficiency (PCE) of 9.02%,4 which is designed and synthesized, consisting of triphenylamine (TPA)
comparable with the best PCE of 9.2% for polymers,5 making and diketopyrrolopyrrole (DPP) as a fundamental dipolar D–p–A
solution-processed SMD-based OSCs a fast growing field. How- structure with ethynylbenzene as p-linkage, as shown in Fig. 1(a).
ever, the overall performance of SMD-based BHJ devices is still In these molecules, the TPA unit behaves as the D building
behind those of their polymer counterparts. Hence, solution- block due to its good hole-transporting capability and excellent
processed SMD-based OSCs still require better material design
toward 10–15% efficiency for future applications.
To design ideal SMDs, alternating electron-donating (D) and
electron-accepting (A) units in the conjugated backbone are
commonly applied to improve PV performance,6 because such
a School of Chemistry, Dalian University of Technology, Dalian, China.
E-mail: lxyin@dlut.edu.cn; Fax: +86-411-84986040; Tel: +86-411-84986040
b State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun, China
† Electronic supplementary information (ESI) available: Measurement, character-
Fig. 1 Molecular structures (a) and TGA plots at a heating rate of 10 1C minÀ1
ization and calculation, synthesis and characterization, as well as PV device
under a nitrogen atmosphere (b) of TPATDPP and TPATDPPCN.
fabrication and hole-mobility measurement details. See DOI: 10.1039/c4cc04020a
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electron-donating properties.11 On the other hand, DPP has been
regarded as a promising A group owing to its prominent
electron-deficient properties and strong light-harvesting capa-
city, as well as excellent photochemical stability.12 Moreover,
ethynylbenzene-linkage could not only improve the molecular
planarity but also facilitate the intramolecular charge transfer
(ICT) process because of the coplanar and rigid nature of
CRC.13 In order to further extend the spectral regions, the
cyano-group was introduced into the molecule as an A unit,
which could effectively reduce Eg.14 Accordingly, an asymmetri-
cal D–A–A-type TPATDPPCN end-capped with cyanophenyl (CP)
was synthesized and studied, resulting in a low optical Eg of
1.65 eV in film. Finally, their PV properties were investigated using
Fig. 2 (a) Theoretically calculated energy levels and electronic-density
distribution; (b) normalized UV-vis absorption spectra of TPATDPP and
TPATDPPCN in CHCl3 and in thin film.
a simple spin-coating device. Among them, the TPATDPPCN- distribution are shown in Fig. 2(a). The electronic density of the
based device showed an impressive PCE of 5.94%, which is the highest occupied molecular orbital (HOMO) is distributed on the
highest efficiency reported to date for solution-processed D–A–A- entire molecule whereas that of the lowest unoccupied molecular
type small molecule OSCs. The results indicate the great potential orbital (LUMO) is located on A units, showing an apparent ICT
of such asymmetrical push–pull SMDs in creating high perfor- process existing in the push–pull molecules. In addition, both the
mance solution-processed OSCs.
HOMO and the LUMO of TPATDPPCN are lower than those of
The synthesis of TPATDPP and TPATDPPCN is described in TPATDPP, which are attributed to the introduction of the CP unit.
Scheme 1. Starting from TPA, compound 1 was obtained by Moreover, the end-capped CP provoked a greater decrease of the
Pd-catalyzed Sonogashira reaction.11b,12c,15 By Miyaura boryla- LUMO, resulting in a large decrease of Eg from 2.22 eV of
tion of compound 1 using Pd(PPh3)2Cl2 as a catalyst, borate TPATDPP to 2.04 eV of TPATDPPCN.
ester 2 was obtained in a yield of 80%. Compound 4 was
The UV-vis absorption spectra of two SMDs in chloroform
prepared by Suzuki coupling of bromide 3 in a high yield of and in thin film are depicted in Fig. 2(b), and the corre-
94%. Bromide 5 was obtained by the NBS bromination of 4 sponding optical data are summarized in Table 1. Both mole-
under mild conditions with a high yield of 98%. Finally, cules exhibit a strong absorption in the long-wavelength region
TPATDPP and TPATDPPCN were facilely synthesized by due to the ICT transition. It is worth mentioning that
Pd-assisted Suzuki coupling in high yields of 89% and 92%, TPATDPPCN exhibits a higher molar extinction coefficient (e)
respectively. Detailed synthetic procedures and characteriza- than that of TPATDPP (see Fig. S10 in ESI†). The solution of
tion are given in the ESI.† Their thermal stability was studied TPATDPP shows an absorption peak at 582 nm, and the
by thermogravimetric-analysis (TGA, see Fig. 1(b)). Their onset spectrum of its film is broadened and red-shifted by 37 nm
decomposition temperatures with 5% weight-loss are 397 1C due to solid-state packing. In comparison with TPATDPP,
and 408 1C, respectively, which demonstrate the exceptional spectra of TPATDPPCN both in solution and in film are greatly
thermal stability for their long-term PV applications.
broadened and red-shifted to 614 and 671 nm, respectively,
The density functional theory (DFT) calculations were carried probably due to the stronger ICT transition and the stronger
out to preliminarily investigate the influence of molecular struc- intermolecular interaction in the solid-state. As a result, the
ture on their electronic properties (the calculation method is given optical Eg of TPATDPP and TPATDPPCN corresponding to their
in the ESI†). The calculated energy levels and electronic-density absorption onsets is 1.80 eV and 1.65 eV, respectively.
The cyclic voltammogram (CV) measurements in dichloro-
methane were carried out to evaluate approximately the energy
levels of the two molecules.12c,16 The CV curves are shown in
Fig. 3(a), and the pertinent electrochemical data are summar-
ized in Table 1 (experimental details and calculation are given
in the ESI†). TPATDPP exhibits the onset reduction potential at
À1.37 V, corresponding to the reduction of the DPP fragment.
On the other hand, two reduction waves are observed in the
cathodic potential regime for TPATDPPCN. The first onset
reduction potential at À1.27 V can be ascribed to the reduction
of the DPP fragment and the second one is assigned to the
reduction of the CP group. In contrast to the reduction beha-
vior, the oxidation potentials of the two molecules are relatively
insensitive, and the first onset oxidation potentials are
observed at 0.53 V and 0.50 V for TPATDPP and TPATDPPCN,
respectively, corresponding to the oxidation of the TPA. As a
Scheme 1 Synthetic route for TPATDPP and TPATDPPCN.
10252 | Chem. Commun., 2014, 50, 10251--10254
result, the less-negative reduction potential and the slightly
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Table 1 Photophysical, electrochemical, hole mobility and PV performance data for TAPTDPP and TPATDPPCN
sol
film
l
l
Eogpt
eV
HOMOCV
eV
LUMOCV
eV
ECgV
eV
mh
RS
RSh
Voc
V
Jsc
PCE
(%)
max
max
Compound
nm
nm
cm2 VÀ1 sÀ1
O cm2
O cm2
mA cmÀ2
FF
TAPTDPP
TPATDPPCN
582
614
619
671
1.80
1.65
À5.28
À5.25
À3.38
À3.48
1.90
1.77
7.49 Â 10À5
56.00
11.40
156
251
0.97
0.93
7.07
14.86
0.30
0.43
2.06
5.94
1.51 Â 10À4
Fig. 4 (a) J–V curves of BHJ devices with a configuration of ITO/PEDOT:PSS/
SMDs:PC61BM/Al under an illumination of AM 1.5 G at 100 mW cmÀ2. Inset:
device structure; (b) J–V curves of the hole-only devices with a structure
of ITO/PEDOT:PSS/SMDs:PC61BM/Au in the dark. Inset: the J–V curves in
a double logarithmic scale and the solid lines are fits of the data points to a
SCLC model.
Fig. 3 (a) CV curves of compounds in 0.1 M Bu4NBF4/CH2Cl2 at a scan
rate of 100 mV sÀ1; (b) energy levels of the components for the PV device.
less-positive oxidation potential of TPATDPPCN relative to
TPATDPP can be rationally ascribed to the presence of the
electron-withdrawing CP block, resulting in a relatively low Eg
of 1.77 eV. The energy level diagram of the components for the
PV device is schematically depicted in Fig. 3(b). Both TPATDPP
and TPATDPPCN in principle can behave as donors in BHJ OSCs,
and the fairly low-lying HOMO would ensure a relatively large
open-circuit voltage (Voc). Notably, the narrow Eg of TPATDPPCN
would result in an increased short-circuit current density (Jsc).
To investigate the PV properties of two SMDs, BHJ devices
based on the SMDs:PC61BM active layer with an area of 0.05 cm2
were fabricated and characterized. The related current density–
voltage (J–V) curves of the devices are described in Fig. 4(a) and
the PV parameters are summarized in Table 1. The device based
on TPATDPP with a lower HOMO level gave a relatively high Voc
of 0.97 V, a Jsc of 7.07 mA cmÀ2, a fill factor (FF) of 0.30 and a PCE
of 2.06%. Remarkably, the TPATDPPCN based device exhibits
exhibits about 2 times higher mh than that of TPATDPP, which is
consistent with its high PV performance. Further experimental
proof of better PV performance for TPATDPPCN is supported by its
higher crystallinity (XRD patterns in Fig. S12, ESI†) and its better
bicontinuous interpenetrating network (AFM images in Fig. S13,
ESI†). In addition, the resistance changes are shown in Table 1.
In contrast, the optimized D–A–A-typed TPATDPPCN reduces the
series resistance (RS) from 56 O cm2 to 11.40 O cm2 and increases
the shunt resistance (RSh) from 156 O cm2 to 251 O cm2, which
leads to the increase of FF, Jsc and PCE. Further experiments on
device optimization are underway in our laboratory.
In conclusion, two novel asymmetrical push–pull SMDs,
TPATDPP and TPATDPPCN, have been synthesized and applied
in the investigation of solution-processed BHJ OSCs. The
dipolar D–A–A structural design end-capped with the CP unit
enables TPATDPPCN to exhibit a narrow Eg, resulting in a high
Jsc of 14.86 mA cmÀ2 with a high PCE of 5.94%. The efficiency is
among the highest ever reported for solution-processed D–A–A-
type small molecule-based BHJ OSCs. The high performance is
primarily attributed to the narrow Eg and the reasonably low-
lying HOMO level as well as the high hole-mobility. The results
gave an important guide for developing novel asymmetrical
push–pull small molecule PV materials.
higher performance, with a Voc of 0.93 V, a Jsc of 14.86 mA cmÀ2
,
a FF of 0.43 and a PCE of 5.94%, in agreement with its higher
photon-to-current response (see Fig. S11 in the ESI†). The high
PV performance of the TPATDPPCN-based device can be attri-
buted to both the narrow Eg induced by the CP group and the
better carrier transporting capacity as evidenced by hole mobility
(mh) measurements. Moreover, TPATDPPCN shows a stronger
intermolecular interaction as evidenced by a more red-shifted
absorption in film, contributing to its carrier transporting proper-
ties. Bear in mind that the charge mobility of the active layer
might be closely related to the charge stability, and the film
morphology as well as the intermolecular interaction. In order
to investigate the mh of the synthesized SMDs, J–V characteristics
of the hole-only devices were measured in the dark (details are
given in the ESI†), as depicted in Fig. 4(b). By fitting the J–V curves
in a double logarithmic scale to a space-charge-limited current
(SCLC) model, as shown in the inset of Fig. 4(b), the mh can be
derived. The mh values of TPATDPP and TPATDPPCN are 7.49 Â
10À5 and 1.51 Â 10À4 cm2 VÀ1 sÀ1, respectively, and the data are
summarized in Table 1. It is interesting to note that TPATDPPCN
This work was supported by the NSFC (No. 21102013,
51173065), the Dalian Committee of Science and Technology
(2011J21DW001), the SRF for ROCS-SEM (No. 201001438,
201001439), the Fundamental Funds for the Central Universities
(DUT11LK20) and the RFDP (No. 20090041120017).
Notes and references
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