A low bandgap asymmetrical squaraine for high-performance solution-processed small molecule organic solar cells

Article abstract of DOI:10.1039/c4cc03831b

A novel asymmetrical squaraine ASQ-5 bearing indoline as an end capper exhibits a low bandgap of 1.43 eV and a broad absorption band in the Vis-NIR region of 550-850 nm in thin films, hence renders solution-processed organic solar cells with an impressive J_sc of up to 11.03 mA cm^-2 and an excellent PCE of 4.29%.

Full text of DOI:10.1039/c4cc03831b

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A low bandgap asymmetrical squaraine for
high-performance solution-processed small
molecule organic solar cells†
Cite this: Chem. Commun., 2014,
50, 9346
Received 20th May 2014,
Accepted 21st June 2014
Daobin Yang,‡a Qianqian Yang,‡b Lin Yang,a Qian Luo,a Yao Chen,a Youqin Zhu,b
Yan Huang,*a Zhiyun Lu*a and Suling Zhao*b
DOI: 10.1039/c4cc03831b
www.rsc.org/chemcomm
A novel asymmetrical squaraine ASQ-5 bearing indoline as an end reported an ASQ photovoltaic material ASQC bearing 9-carbazyl as an
capper exhibits a low bandgap of 1.43 eV and a broad absorption end capper, obtaining the highest Voc (1.12 V) of a single BHJ SMOSC
band in the Vis-NIR region of 550–850 nm in thin films, hence based on small molecule donors because of its extremely deep HOMO
renders solution-processed organic solar cells with an impressive level of ꢀ5.46 eV.8 However, this ASQ material (ASQC) exhibited a
Jsc of up to 11.03 mA cmꢀ2 and an excellent PCE of 4.29%.
medium bandgap (1.65 eV), which is attributed to the weak electron-
donating capability of 9-carbazyl and large dihedral angle (481) between
In recent years, small molecular organic solar cells (SMOSC) have the dihydroxyphenyl ring and the 9-carbazyl core; this is one of the
attracted great interest because of their advantages over their polymer most important reasons for limiting an increase of Jsc (7.00 mA cmꢀ2),
counterparts, including a well-defined molecular structure, facile resulting in a relatively low PCE of the corresponding BHJ photovoltaic
material synthesis, definite molecular weight and high purity without device (2.82%). Therefore, we attempt to develop ASQ with a low
batch to batch variations,1 while a bulk-heterojunction (BHJ) device bandgap by introducing a suitable end capper. Indoline, a construc-
structure is demonstrated to be more effective in achieving high tional unit widely used in dye-sensitized solar cell materials,9 but
power conversion efficiency (PCE).2 Among the multifarious kinds of never used in OSC materials as an end capper, is incorporated into
small molecular photovoltaic materials that have been developed, the 4-amino-2,6-dihydroxy phenyl group owing to its much stronger
squaraine dyes have demonstrated strong potential for use in OSC,3 electron-donating capability and smaller steric hindrance than
since they possess high molar extinction coefficients, intense absorp- 9-carbazyl,10 so that the resulting compound ASQ-5 might possess a
tion in Vis-NIR spectral regions, excellent photochemical and photo- low bandgap, resulting in an excellent Jsc and PCE. Additionally,
physical stability;4 so far, the record PCE for solution-processed another compound ASQ-6 with 1,2,3,4-tetrahydroquinoline instead of
squaraine-based BHJ-OSC has been as high as 5.50%.5 It is noted indoline is constructed, 1,2,3,4-tetrahydroquinoline, which exhibits a
that the squaraine dye used in this device has a symmetrical similar electron-donating capability to indoline, while they show
molecular structure (SQ, D–A–D), however, asymmetrical squaraines different steric conformation.11 Thus, the effect of different end
(ASQ, D–A–D0) are more promising candidates relative to SQ because cappers, indoline vs. 1,2,3,4-tetrahydroquinoline, on the optoelectro-
of their much better molecule structural tenability.6
nic properties of ASQ has been investigated too.
Nevertheless, the PCE of ASQ-based BHJ-SMOSC is relatively low
The synthetic routes to the target molecules are illustrated in
(0.20–2.05%), which should be mainly ascribed to their low Voc Scheme 1. All the synthetic and characterization details could be
(0.24–0.69 V) and Jsc (1.40–9.05 mA cmꢀ2).7 Very recently, we have found in ESI.† The benzo[e]indole semi-squarylium compound 3a
condensed with aromatic compound 1b or 2b to obtain the
a Key Laboratory of Green Chemistry and Technology (Ministry of Education),
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China.
E-mail: huangyan@scu.edu.cn, luzhiyun@scu.edu.cn
b Key Laboratory of Luminescence and Optical Information (Ministry of Education),
Institute of Optoelectronics Technology, Beijing Jiaotong University,
Beijing 100044, P. R. China. E-mail: slzhao@bjtu.edu.cn
† Electronic supplementary information (ESI) available: Synthetic procedure,
crystal data and packing, CV curves, charge carrier mobility calculation method,
photovoltaic device data and AFM images. CCDC 986049 and 986050. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc03831b
Scheme 1 Synthetic routes to the target molecules.
‡ The first two authors contributed equally to this work.
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The UV-vis absorption spectra of the two compounds in solution
and thin films are shown in Fig. 2, and data are summarized in
Table 2. In solution, the maximum absorption peak of ASQ-6 was
671 nm, which was slightly blue-shifted 7 nm compared with that of
ASQ-5 (678 nm), which is due to the existence of a remarkably
twisted conformation (j2 = 56.51). Additionally, both of them exhibit
a considerably high molar extinction coefficient (log e 4 5.00). In
comparison with their absorption spectra in solution, the ICT
absorption bands of ASQ-5 and ASQ-6 in films were significantly
red-shifted, extending to 725 and 713 nm, respectively. It is note-
worthy that absorption bands of the two compounds in solution
display an identical full width half maxima (FWHM) of 39 nm, while
the thin film absorption band of ASQ-5 is broader than that of ASQ-6
(FWHM, 166 nm vs. 146 nm), which should be attributed to its more
coplanar conformation than that of ASQ-6, this broadening is
important for OSC since it leads to improved spectral overlap with
the solar irradiance spectrum. Determined by the UV-vis absorption
onset of the films, the optical bandgap of ASQ-5 is lower than ASQ-6
(1.43 eV vs. 1.49 eV), which is the lowest bandgap based on
photovoltaic materials of asymmetrical squaraine dyes.14
According to cyclic voltammetry measurement (Table 2 and Fig. S2,
ESI†), both the compounds exhibit a similar reversible one-electron
oxidation wave at 0.29 V; the HOMO energy levels of the two
compounds are calculated to be ꢀ5.09 eV, which are mainly attributed
to the similar electron-donating capability of indoline and 1,2,3,4-
tetrahydroquinoline. The LUMO energy levels of ASQ-5 and ASQ-6 are
calculated to be ꢀ3.66 and ꢀ3.60 eV, respectively, which are deduced
from the HOMO levels and the corresponding optical bandgaps.15
According to the experimental and calculation results, the hole
mobility of the as-prepared ASQ-5 and ASQ-6 neat films are 2.00 ꢂ
10ꢀ5 cm2 Vꢀ1 sꢀ1 and 1.80 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 (Fig. S3, ESI†),
respectively, while the hole mobility of the donor–acceptor blend-
ing films are reduced to 1.71 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and 1.36 ꢂ
10ꢀ6 cm2 Vꢀ1 sꢀ1 (Fig. S4, ESI†), respectively. The difference in
hole mobility observed for ASQ-5 and ASQ-6 can most likely be
ascribed to differences in molecule packing. ASQ-5 has a shorter
intermolecular distance than that of ASQ-6 (3.38 Å vs. 3.84 Å),
which should result in slightly higher carrier mobility.
Fig. 1 ORTEP diagrams for ASQ-5 (left) and ASQ-6 (right).
Table 1 Conjugated backbone conformation and packing of the ASQ
determined from single crystal structures (j: dihedral angle)
Compound
j1 (1)
j2 (1)
Distance (Å)
ASQ-5
ASQ-6
1.3
3.9
3.8
56.5
3.38
3.84
asymmetrical squaraine ASQ-5 or ASQ-6 with a high yield of
nearly 80%, which possess an excellent solubility in common
organic solvents, such as chloroform and 1,2-dichlorobenzene
(440 mg mLꢀ1). Additionally, high quality films of the two
compounds could be achieved through spin-coating, suggesting
that they are very suitable for solution-processing.
The molecular structures of the two compounds (vide Fig. 1,
Table S1, ESI†) are determined by X-ray diffraction on single crystals.
In the two compounds, the squarate cores and phenyl moieties both
show similar quasi-coplanar conformations (vide Table 1, j1 o 41) due
to the existence of strong hydrogen bonding interactions (OꢁꢁꢁH–O).
The p-system of ASQ-5 has nearly planar conformation as there is only a
small twist (j2 = 3.81) between the dihydroxyphenyl ring and
the indoline core, leading to more extended conjugation systems of
ASQ-5,12 while ASQ-6 displays a remarkably twisted conformation with
j2 of 56.51. The much larger j2 in ASQ-6 is mainly attributed to its
steric hindrance originating from H1 and H2. According to the packing
diagrams of ASQ-5 (shown in Fig. S1, ESI†), the distance between two
neighboring molecules is much shorter than that of ASQ-6 (3.38 Å vs.
3.84 Å), the shorter p–p distance results from the much smaller twist
(j2) of ASQ-5, indicating the presence of significant p–p interaction,
which may facilitate the charge carrier transportation.13
To evaluate the photovoltaic performance of the two compounds,
devices with a structure of glass/ITO/MoO3 (80 Å)/ASQ:PC71BM
(800 Å)/LiF (8 Å)/Al (1000 Å) have been fabricated. OSC devices were
optimized with respect to donor–acceptor blend ratios and thermo-
annealing, and the representative photovoltaic data of the devices are
summarized in Tables S2 and S3 (ESI†), and current density–voltage
( J–V) curves are shown in Fig. S5 and S6 (ESI†). The optimized blend
ratio of ASQ : PC71BM is 1 : 5, the photoactive layer annealing tem-
perature is 70 1C for 20 min, and the corresponding J–V curves and
key data of the devices are given in Fig. 3 and Table 3.
Fig. 2 Absorption spectra of ASQ in solution (left) and thin films (right).
Table 2 Optical and electrochemical properties of the ASQ
Absorption lmax (nm)
Solution (log e)
FWHM (nm)
Solution
onset
ox
Compound
Film
Film
Eogpt (eV)
E
(V)
HOMO (eV)
LUMO (eV)
ASQ-5
ASQ-6
678 (5.37)
671 (5.36)
725, 657
713, 648
39
39
166
146
1.43
1.49
0.29
0.29
ꢀ5.09
ꢀ5.09
ꢀ3.66
ꢀ3.60
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BHJ-SMOSC shows an impressive Jsc of 11.03 mA cmꢀ2 and an
excellent PCE of 4.29%, while that based on ASQ-6 exhibits a
Jsc of 9.50 mA cmꢀ2 and a PCE of 3.66%. These preliminary studies
demonstrate that ASQ-5 is a perspective electron donor candidate,
and the indoline-modification strategy may pave a new way for
achieving photovoltaic devices with greatly improved Jsc and PCE.
We acknowledge the financial support for this work from
the National Natural Science Foundation of China (project No.
21190031 and 21372168) and New Century Excellent Talents in
University (grant No. NCET-10-0220). We are grateful to the
Analytical & Testing Center of Sichuan University for providing
single crystal X-ray diffraction data for the objective molecules.
Fig. 3 J–V curves of photovoltaic devices (left) and EQE curves (right).
Table 3 Photovoltaic performances of the ASQ
Active layer (w/w)
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE (%)
ASQ-5 : PC71BM = 1 : 5
ASQ-5 : PC71BM = 1 : 5a
ASQ-6 : PC71BM = 1 : 5
ASQ-6 : PC71BM = 1 : 5a
0.82
0.81
0.83
0.82
10.29
11.03
8.97
0.45
0.48
0.46
0.47
3.80
4.29
3.42
3.66
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´
¨
In conclusion, a novel asymmetrical squaraine ASQ-5 bearing
indoline as an end capper with a compromised low bandgap of
1.43 eV with a broad absorption band and a HOMO energy level of
ꢀ5.09 eV has been synthesized. Compared with ASQ-6 bearing 1,2,3,4-
tetrahydroquinoline as an end capper, ASQ-5 exhibits a lower band- 16 (a) A. J. Heeger, Adv. Mater., 2014, 26, 10; (b) V. D. Mihailetchi,
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gap, broader absorption band, much shorter intermolecular distance,
and higher carrier mobility, which are attributed to its more
planar conformation. Therefore, solution-processed ASQ-5-based
Funct. Mater., 2004, 14, 1005.
9348 | Chem. Commun., 2014, 50, 9346--9348
This journal is ©The Royal Society of Chemistry 2014