Cyano-substitution on the end-capping group: Facile access toward asymmetrical squaraine showing strong dipole-dipole interactions as a high performance small molecular organic solar cells material

Article abstract of DOI:10.1039/c5ta03971a

A novel asymmetrical squaraine derivative bearing a cyano-substituted indoline end-capping group, namely ASQ-5-CN, was designed and synthesized. In comparison with the noncyano-substituted ASQ-5, ASQ-5-CN showed an analogous absorption band-gap in the thi

Full text of DOI:10.1039/c5ta03971a

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DOI: 10.1039/C5TA03971A
Cyano-Substitution on the End-Capping Group: A Facile Access toward
Asymmetrical Squaraine Showing Strong Dipole-Dipole Interactions as
High Performance Small Molecular Organic Solar Cells Material †
Daobin Yang,a,b Yan Jiao,a Lin Yang,a Yao Chen,a Satoshi Mizoi,b Yan Huang,*a Xuemei
Pu,a Zhiyun Lu,a Hisahiro Sasabe*b and Junji Kido*b
a
Key Laboratory of Green Chemistry and Technology (Ministry of Education), College of
Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China. E-mail:
huangyan@scu.edu.cn
b
Department of Organic Device Engineering, Research Center for Organic Electronics,
Yamagata University, Yonezawa 992-8510, Japan. E-mail: h-sasabe@yz.yamagata-u.ac.jp;
kid@yz.yamagata-u.ac.jp
† Electronic supplementary information (ESI) available: electron density distribution and
absorption spectra of the active layers.
Abstract: A novel asymmetrical squaraine derivative bearing cyano-substituted indoline
end-capping group, namely ASQ-5-CN, was designed and synthesized. In comparison with
the noncyano-substituted ASQ-5, ASQ-5-CN shown an analogous absorption band-gap in
thin solid film state, but a 0.11 eV lowered HOMO energy level, which led to a higher Voc.
Density functional theory calculation results revealed that the dipole moment of ASQ-5-CN
was 2 times larger than that of ASQ-5. Hence the stronger dipole-dipole interactions of
ASQ-5-CN might trigger more intense intermolecular packing in ASQ-5-CN, which should
account for the higher hole-mobility of ASQ-5-CN than that of ASQ-5 (4.00 ×10-5 vs. 1.67
×10-5 cm2 V-1 s-1). Accordingly, solution-processed bulk-heterojunction small molecular
organic solar cell using ASQ-5-CN as electron donor exhibited much higher PCE (5.24%)
than that of the reference compound ASQ-5-based device (4.22%) due to its simultaneously
enhanced Voc (0.92 vs. 0.82 V), Jsc (11.38 vs. 10.94 mA cm-2) and FF (0.50 vs. 0.47).
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Additionally, the PCE of the ASQ-5-CN-based device could be improved to be as high as
6.11% when measured at 80 °C, which is the record PCE among the hitherto reported
squaraine-based solution-processed bulk-heterojunction organic solar cells.
1. Introduction
Owing to the well-defined molecular structure and molecular weight, high purity and good
batch to batch reproducibility of small molecular compounds, solution-processed bulk-
heterojunction small molecular organic solar cells (BHJ-SMOSC) have been demonstrated
to be a competitive alternative to their polymeric counterparts recently.[1-7] Among the
numerous kinds of small molecular OSC materials, squaraines dyes have drawn much
attention because they not only possess high molar extinction coefficients, but also show
intense and broad absorption in Vis-NIR spectral regions.[8-20] Nevertheless, although many
efforts have been devoted to the exploitation of novel squaraines for OSC applications, the
performance of photovoltaic squaraines is still unsatisfactory. For example, the record power
conversion efficiency (PCE) of solution-processed squaraine-based BHJ-OSC is only 5.50%,
note that the squaraine compound used just possesses a symmetrical molecular structure
with a D-A-D skeleton.[21]
In comparison with symmetrical squaraines (SSQ), asymmetrical ones (ASQ) bearing a
D-A-D′ rather than D-A-D molecular skeleton should be more promising due to their better
structural tunability.[22,23] Consequently, recently, we have devoted to the development of
novel ASQ materials for BHJ-OSC applications, in which 1,1,2-trimethyl-1H-benzo[e]
indole acts as the D subunit, squaric acid core acts as the A subunit, and
2,6-dihydroxyphenyl group acts as the D′ subunit.[24-27] Through delicate modulation on the
end-capping group of the D′ segment, such as diisobutylamino, 9-carbazyl, diarylamino,
tetrahydroquinoline and indoline, a series of ASQ materials were achieved, and one of them
bearing an indoline end-capping subunit (named ASQ-5, molecular structure shown in Fig.1)
displayed the highest photovoltaic performance (PCE=4.29%). These encouraging results
indicate that ASQ derivatives could act as perspective OSC materials, and indoline unit
could serve as quite promising end-capping unit to construct high performance photovoltaic
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ASQ materials. However, the performance of ASQ-5-based BHJ-SMOSC was still inferior
to that of the device using a SSQ compound as photovoltaic material,[21] which should be
ascribed to its relatively low open-circuit voltage (Voc, 0.81 V) stemming from the relatively
high HOMO energy level of ASQ-5 (-5.09 eV).[25] Therefore, we aimed at the exploitation
of ASQ-5 derivatives with lower HOMO energy level, so that enhanced Voc could be
expected.
Owing to the strong electron-withdrawing nature and linear geometry of cyano groups,
cyano-substitution was a well-adopted strategy with regard to the lowering of HOMO
energy levels of OSC electron donor materials.[28,29] Moreover, according to Cha’s report,
the cyano-modified conjugated polymer shown higher crystallinity than its unmodified
counterpart, resulting in OSC with higher fill factor (FF) and PCE.[30] However, most of the
current studies have been focused on the cyano-substitution on the vinylic linkage of the
molecular skeleton of photovoltaic materials and the dicyanovinyl-substituted compounds,
[31-36] yet never concern has been paid on the effect of cyano-substitution on the end-capping
groups in OSC. Consequently, a new asymmetrical squaraine (named ASQ-5-CN, molecular
structure shown in Fig.1) bearing a cyano-substitution on the indoline end-capping unit of
ASQ-5 was designed and synthesized, and the substitution effect of the cyano group on the
end-capping segment of ASQ compounds has been investigated.
Fig. 1 Molecular structures of ASQ-5 and ASQ-5-CN.
2. Experimental section
2.1 Instruments and characterization
1H NMR and 13C NMR spectra were measured using a Bruker Avance AV ΙΙ-400 MHz
spectrometer, and the chemical shifts were recorded in units of ppm with TMS as the
3