Ambipolar organic semiconductors with cascades of energy levels for generating long-lived charge separated states: A donor-acceptor1-acceptor2 architectural triarylamine dye

Article abstract of DOI:10.1039/c4tc00860j

A donor-acceptor1-acceptor2 architectural 4-styryltriphenyl amine-based organic semiconductor was synthesized for solar cell applications. Sequential electron transfers together with effective hole transfer lead to a charge separated state lifetime of 650 ns, therefore boosting the short circuit current and efficiency of single layer organic photovoltaic cells. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00860j

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Ambipolar organic semiconductors with cascades
of energy levels for generating long-lived charge
separated states: a donor–acceptor1–acceptor2
architectural triarylamine dye†
Cite this: J. Mater. Chem. C, 2014, 2,
5466
Received 28th April 2014
Accepted 16th May 2014
Tianyang Wang,a Krishanthi C. Weerasinghe,b Dongzhi Liu,a Wei Li,a Xilong Yan,a
DOI: 10.1039/c4tc00860j
a
b
*
*
Xueqin Zhou and Lichang Wang
www.rsc.org/MaterialsC
A donor–acceptor1–acceptor2 architectural 4-styryltriphenyl amine- transfer can be suppressed by the cascade design of energy
based organic semiconductor was synthesized for solar cell applica- levels of sequential modules. Multi-donor designed systems
tions. Sequential electron transfers together with effective hole (D1–D2–A) have been reported with cascades of energy levels
transfer lead to a charge separated state lifetime of 650 ns, therefore and therefore long-lived charge separated states, of which the
boosting the short circuit current and efficiency of single layer organic electron migration rates among donors were too low to generate
photovoltaic cells.
effective photocurrents in solar cells.16–23 Although the efficiency
of dye-sensitized solar cells exceeds 12% by the multi-acceptor
design with a D–A sensitizer and titanium oxide (TiO2) as the
second acceptor,24 the limited electron transfer in the interface
between the sensitizer and TiO2 is still an issue and the short-
lived charge separated states in such systems also present one
of the major challenges in attaining highly efficient solar cells.
Herein, we report a metal-free and donor–acceptor1–acceptor2
(D–A1–A2) architectural organic semiconductor, 4,40-dimethyl-400-
(4-(4-chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)-
1,3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-AEAQ),
of which long-lived charge separated states were generated due
to cascades of electronic energy states and appropriate electron
transfer rates inspire potential applications in photovoltaic
cells. The design of MTPA-TRC-AEAQ is shown in Scheme 1.
Efficient conversion of sunlight requires an efficient photoin-
duced charge transfer and separation that are capable of gener-
ating long-lived charge separated states, therefore producing
applicable photocurrents in solar cells or driving multi-electron
chemistry of fuel synthesis.1–6 Both electron donor and acceptor
modules in a donor–acceptor (D–A) organic sensitizers determine
the driving force of charge transfers,4–11 while the distance,
spatial orientation, and exibility between the donor and the
acceptor signicantly inuence the efficiency and the rate of
photoinduced charge transfers.6–9 These parameters, governed by
electronic coupling, reorganization energy, and attenuation
factor, are key considerations in the optimization of efficient
charge separation as well as in the elongation of the lifetime of
the corresponding radical ion pairs.10–12 A key advancement in the
eld is the synthesis of an organic sensitizer beyond utilizing p-
conjugated linkers as they not only favor the throughbond elec-
tron migration from the donor to the acceptor, but also enhance
back transfer, resulting in faster charge recombination.
Inspired by natural photosynthetic processes, where the
formation of cascades of short-range photoinduced energy
transfer and multistep electron transfer occurs, researchers
have developed a large variety of supramolecular systems in the
combination of various donors and acceptors.4–23 Back electron
aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
China. E-mail: zhouxueqin@tju.edu.cn
bDepartment of Chemistry and Biochemistry, Southern Illinois University, 1245 Lincoln
Drive, Carbondale, IL 62901, USA. E-mail: lwang@chem.siu.edu
† Electronic supplementary information (ESI) available: Experimental and
computational details with complete spectroscopic and structural analyses. See
DOI: 10.1039/c4tc00860j
Scheme 1 Design of MTPA-TRC-AEAQ with a cascade of electron
transfers.
5466 | J. Mater. Chem. C, 2014, 2, 5466–5470
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Donor module MTPA has been previously found with excel- perturbation to the D–A1 module. The characteristic band of
lent electron-donating and hole-migrating abilities and its MTPA-TRC-AEAQ at 500 nm is in between AEAQ (503 nm) and
HOMO–LUMO transition involves intramolecular charge AEAQt (497 nm), an indication of the electronic and steric
transfer (ICT) from the triphenylamine moiety to the phenylene effects between the MTPA-TRC and AEAQ modules. However,
moiety.25 Triazine (TRC) is a typical electron-accepting module these effects do not change the nature of various transitions as
and would be able to improve the electron-injection and elec- evidenced by similar shapes of the corresponding peaks.
tron-transportation abilities of its conjugated derivatives.26,27 As
As shown in Fig. 1b, the uorescence maximum from the
such, it was chosen as the rst acceptor. An anthraquinone MTPA module of MTPA-TRC-AEAQ is at 464 nm, similar to that
derivative (AEAQ) is another common acceptor and has been of MTPA-TRC, but is red-shied from that of compound MTPA.
reported with a lower LUMO than triaryl modules.18,19,25 There- The uorescence intensity of MTPA is substantially quenched in
fore, it was employed as the second acceptor. Our computa- MTPA-TRC and further quenched (ꢀ98%) in MTPA-TRC-AEAQ
tional results of MTPA, TRC, and AEAQ together with bonded upon the excitation at 380 nm. Upon excitation at 490 nm, the
molecules AEAQt, MTPA-TRC and MTPA-TRC-AEAQ (the opti- uorescence emission from the AEAQ module in MTPA-TRC-
mized structures and calculated orbital contours can be found AEAQ shown in Fig. 1c splits into three peaks (535, 579, and
in the ESI, Fig. S1–S4†) further conrmed the cascades of elec- 610 nm) with the maximum at 535 nm, which is consistent with
tronic energy levels, thus indicating that the designed organic the uorescence characteristics of AEAQ, but is quenched by
molecule can thermodynamically facilitate electron transfers.
Upon synthesis of MTPA-TRC-AEAQ (the synthesis details
about 46%.
To understand the quenching mechanism, we carried out
can be found in the ESI†), we rst examined photophysical the time-resolved uorescence experiments. The emission life-
properties of this D–A1–A2 molecule together with four other times from these experiments are provided in Table 1. While a
species, MTPA, AEAQ, AEAQt, and the simple D–A molecule single exponential decay was observed for the MTPA compound,
MTPA-TRC, using the absorption and uorescence measure- two-exponential decays were found for both MTPA-TRC and
ments. As shown in Fig. 1a, when the triazine module was MPTA-TRC-AEAQ.
bonded to MTPA, the absorption maximum of MTPA-TRC
The two-exponential decays of the uorescence spectra were
became 387 nm, which was red-shied from 376 nm of MTPA. oen explained by an electron transfer from LUMO+1 to LUMO
This is an indication of an enhanced ICT character, which was for various D–A dyes.4,5 We expect that electron transfer is also
conrmed by the electron density reduction for the overlapping responsible for the uorescence quenching of MTPA-TRC based
region of HOMO and LUMO. When a second acceptor AEAQ was on our computational studies, which has been veried by
attached to MTPA-TRC to form MTPA-TRC-AEAQ, the absorp- electrochemical determination (see Fig. S6†). As illustrated in
tion maximum of this new compound is at 386 nm, which is Fig. 2, the determined HOMO at MTPA and LUMO at TRC
very close to that of MTPA-TRC, illustrating insignicant conrm a typical D–A chromophoric structure for MTPA-TRC.
Accordingly, the fast decay component (0.29 ns) of the uores-
cence from MTPA-TRC was assigned to the electron transfer
from the MTPA module to the TRC module, whereas the slow
process (1.73 ns) was attributed to the solvation relaxation of the
excited states.
The results from the time resolved emission spectra of
MTPA-TRC-AEAQ at 460 nm in Table 1 reveal that the uores-
cence from the MTPA module also decays biexponentially with
lifetimes of 1.61 and 0.70 ns, respectively. For MTPA-TRC-AEAQ,
the orbital level of the TRC module could not be evaluated
directly from the electrochemical determination of MTPA-TRC-
AEAQ (Fig. S7†). Then the TRC orbital level in MTPA-TRC-EA
was employed given the effect to the substitution of the second
Table 1 The emission lifetimes of MTPA, MTPA-TRC, AEAQt and
MTPA-TRC-AEAQ in toluene by fitting transient fluorescence spectra
shown in Fig. S5† with exponential decay equations
Emission lifetime, s/ns
lex ¼ 366 nm,
lem ¼ 460 nm
lex ¼ 366 nm,
lem ¼ 700 nm
lex ¼ 457 nm,
lem ¼ 700 nm
Compounds
MTPA
MTPA-TRC
AEAQt
1.80
1.73, 0.29
—
—
—
—
—
—
Fig. 1 Absorption (a) and fluorescence emission spectra excited at
380 nm (b) and 490 nm (c) of MTPA-TRC-AEAQ (black), MTPA-
TRC (blue), MTPA (green), AEAQ (red) and AEAQt (magenta) in toluene
(5 ꢁ 10ꢂ6 mol Lꢂ1).
4.90, 0.47
4.84, 0.31
MTPA-TRC-AEAQ
1.61, 0.70
4.85, 0.30
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chlorine at TRC by an ethylamine group. As shown in Fig. 2,
MTPA-TRC-AEAQ has been conrmed experimentally with a
typical D–A1–A2 chromophoric structure and MTPA, TRC and
AEAQ modules play the roles of D, A1 and A2, respectively.
Therefore the fast decay component is due to the electron
transfer from MTPA to TRC followed by consecutive electron
transfer to the AEAQ module. The longer lifetime of the fast
component in MTPA-TRC-AEAQ than that in MTPA-TRC is due
to a slightly higher TRC LUMO in MTPA-TRC-AEAQ with respect
to MTPA-TRC, shown in Fig. 2, which reduces the driving force
for the electron to transfer from MTPA LUMO and thus slows
the process. The lifetime of the slow component, which is
attributed to the solvation relaxation of the MTPA singlet, is
slightly shorter than that of MTPA-TRC. This is due to the
intramolecular photoinduced Forster energy transfer from the
excited MTPA module to the AEAQ module, which was further
conrmed by the appearance of a weak characteristic peak from
the AEAQ module that emerged concurrently at about 579 nm in
the MTPA-TRC-AEAQ spectrum (Fig. 1b), whereas little absorp-
tion and uorescence were detected for either AEAQt or AEAQ.
Similar biexponential decay kinetics of the uorescence were
also found at 700 nm from the AEAQ module; the lifetime of fast
component (0.31 ns) is shorter than the corresponding value of
AEAQt (0.47 ns). Additionally, the lower HOMO of the AEAQ
module than that of the MTPA module allows hole transfer from
the excited AEAQ to MTPA, making MTPA-TRC-AEAQ
ambipolar.
Fig. 3 Nanosecond transient absorption spectra (a) and kinetics at
600 nm (b) of MTPA-TRC (1 ꢁ 10ꢂ5 mol Lꢂ1) and the nanosecond
transient absorption spectra (c) and kinetics at 440 nm (d) of MTPA-
TRC-AEAQ (1 ꢁ 10ꢂ5 mol Lꢂ1) in nitrogen following excitation at
410 nm, 8 ns laser pulses. The solid lines are the fitting curves by the
single-order exponential decay equation and the blue line in (d) was
obtained in air. The solvent is toluene.
To determine the lifetime of the charge separated state in
compounds MTPA-TRC and MTPA-TRC-AEAQ, we performed Furthermore, it could also indicate that the decay ratio between
nanosecond transient absorption measurements. Fig. 3a shows electron transfer and solvation relaxation is decreased slightly
the characteristics of MTPA+ absorption in the MTPA-TRC in MTPA-TRC-AEAQ. Similar spectra obtained in the presence of
compound in the range 440–700 nm with the negative DOD oxygen conrm the formation of the charge separated state
corresponding to the bleach of ground states. Fig. 3b shows the MTPA+-TRC-AEAQꢂ, of which the lifetime was evaluated to be
formation of charge separated state MTPA+-TRCꢂ with an esti- 650 ns. The transient absorption difference spectra excited at
mated lifetime of 80 ns.
500 nm (Fig. S8†) also exhibit the absorption features of AEAQꢂ
When the second acceptor, AEAQ, was attached to MTPA- and MTPA+ above 420 nm. All these illustrate a hole transfer
TRC, signicantly different transient absorption spectra and from the excited AEAQ module to the MTPA module. The life-
kinetics were observed. The signicant different features in time of the resulted charge separated state MTPA+-TRC-AEAQꢂ
Fig. 3c with respect to Fig. 3a are due to the addition of AEAQꢂ due to hole transfer was found to be around 680 ns.
absorption at 340 nm and 440 nm and the bleaching of the
The eightfold elongation of the lifetime in MTPA-TRC-AEAQ
AEAQ ground state at 500 nm, which were also observed over MTPA-TRC is determined in toluene. These lifetimes
previously for the AEAQ system.28,29 The slightly weaker MTPA+ change with the solvent and will differ in the solid state. We
signal in MTPA-TRC-AEAQ is due to the overlap of the MTPA+ expect some changes in the lifetimes for both materials when
absorption with the bleaching of the AEAQ ground state. they are assembled into solar cell devices, but still expect a
signicantly longer charge separation in MTPA-TRC-AEAQ. To
illustrate the promise of MTPA-TRC-AEAQ due to its superior
charge separation property, we assembled single layer organic
photovoltaic cells (SLOPV) using both materials and conducted
preliminary tests on these devices. As shown in Fig. 4, the dye
was placed between ITO and Ag electrodes with a thickness of
about 250 nm by spin coating it from tetrahydrofuran. The
photovoltaic characteristics under AM 1.5 G illumination
(100 mW cmꢂ2) are given in Table 2 (the J–V curves of SLOPV can
be found in Fig. S9†). The maximum incident photon-to-elec-
tron conversion efficiency was found to be 46% at 410 nm for
Fig. 2 Illustration of orbital energy levels of MTPA-TRC and MTPA-
MTPA-TRC-AEAQ devices (Fig. 4) and the photovoltaic charac-
teristics of MTPA-TRC-AEAQ are higher in SLOPV than reported
TRC-AEAQ determined electrochemically. The red and blue lines
indicate the observed electron and hole transfers, respectively.
5468 | J. Mater. Chem. C, 2014, 2, 5466–5470
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This work was supported by National Nature Sciences Founda-
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