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M. Shi et al. / Dyes and Pigments 95 (2012) 377e383
Fig. 8. AFM phase images of violanthrone derivative:PCBM blended films.
Fig. 5 presents cyclic voltammograms (CV) of 3e5 films and their
organic semiconductors with the relatively low-lying HOMO
energy levels (wꢀ5.1 eV) and the broad absorption ranges
extended to 800 nm, implying that they can be used as electron
donors in OSCs. Preliminary photovoltaic study discloses that the
violanthrone derivative with the shortest hexyloxy substituent
(compound 5) exhibits the best photovoltaic performance
(PCE ¼ 0.54%) because the shortest linear side chain is favor of the
electrochemical data are summarized in Table 1. From Fig. 5, it is
found that, three violanthrone derivatives exhibit the same electro-
chemical behaviors: two couples of quasi-reversible reduction/
oxidation peaks in the positive potential region, and a couple of
irreversible redox waves in the negative potentialrange are observed,
indicating that all of compounds are typical p-type materials. The
onset oxidation potentials (Eox) are 0.33 V versus Fc/Fcþ for 3, 0.29 V
versus Fc/Fcþ for 4, and 0.28 V versus Fc/Fcþ for 5, from which the
HOMO energy levels of 3, 4, and 5 are estimated as ꢀ5.13, ꢀ5.09, and
ꢀ5.08 eV, respectively, because the absolute energy level of Fc/Fcþ is
4.8 eV below vacuum [15]. Similarly, the LUMO energylevels and of 3,
4, and 5 can be calculated as ꢀ3.34, ꢀ3.28, and ꢀ3.27 eV, respectively,
from theironset reductionpotentials (Ered). Thus, the electrochemical
bandgaps (Eegc) are determined as 1.79,1.81, and 1.81 eV for 3, 4, and 5,
respectively. Based on the above data, it can be predicted that
compounds 3e5 would be potential electron donors in OSCs since
they possess both relatively low-lying HOMO energy levels as well as
low band gaps.
In order to evaluate the photovoltaic properties of compounds
3e5, OSCs were fabricated with the blends of different violanthrone
derivatives and PCBM (1:1, w/w) as the active layers. Fig. 6 shows
IeV curves of the three OSCs in dark and under the illumination of
AM 1.5G solar irradiance (100 mW/cm2). From Fig. 6, their corre-
sponding open circuit voltage (VOC), short circuit current (ISC), fill
factor (FF), and power conversion efficiency (PCE) are obtained and
summarized in Table 2. It is obviously observed that the OSCs based
on three violanthrone derivatives exhibit approximate open circuit
voltages (w0.7 V), which agrees exactly with the HOMO energy
levels of 3e5. However, three devices give different short circuit
currents and follow the sequence: 3 < 4 < 5, which is consistent
intermolecular pep actions in the solid state, providing a relatively
high hole mobility of 3.15 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1. These findings
suggest that the appropriate molecular tailoring of some dyestuffs
is a simple and feasible route to good organic small molecule
photovoltaic materials.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Nos. 50990063, 51073135, 51011130028). We
also would like to gratefully acknowledge support from the
National High Technology Research and Development Program of
China (863 Program) (No. 2011AA050520).
Appendix A. Supporting information
Supplementary data related to this article can be found online at
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pep stacking in 3e5 films,
because the stronger interactions favor the transportation of
pep
charge carriers in OSCs. Therefore, among three devices, the OSC
made from 5:PCBM blend shows the largest ISC (2.14 mA/cm2) and
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4. Conclusion
We synthesize three soluble violanthrone derivatives with
different alkyloxy groups, and find that all compounds are p-type