G Model
CCLET 4333 No. of Pages 4
W. Liu et al. / Chinese Chemical Letters xxx (2017) xxx–xxx
3
the Eopt
spectrum of the pure ITCT and ITIC films (Fig. 3a), decreases from
.59 eV to 1.55 eV. The LUMO and HOMO energy levels, measured
g
, calculated from the absorption onset of the absorption
1
with cyclic voltammetry (CV) (Fig. S1b), are ꢁ4.02 eV and ꢁ5.65 eV
for ITCT and ꢁ3.97 eV and ꢁ5.66 eV for ITIC (Fig. 3b and Table S1).
The electrochemical LUMO-HOMO band gap reduces from ITIC to
ITCT.
The solar cells were fabricated with a wide band gap (WBG)
conjugated polymer of PBDB-T poly[(2,6-(4,8-bis(5-(2-ethylhexyl)
0
thiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene)-co-(1,3-di(5-thio-
0
phene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c ]dithiophene-
4
Fig. 4. The J-V curves (c) and EQE spectra (d) of the optimal solar cells with ITCT and
ITIC as the acceptor, respectively.
,8-dione)] as the donor material, and using the 1:1 (w/w) blended
N719:PrC60MAI as the binary cathode buffer layer (CBL) [37]. Here,
0
N719 is di-tetrabutylammonium-cis-bis(isothiocyanato)bis(2,2 -
decreases, suggesting recombination losses of the mobile charges.
The larger decrease in the ITIC photocurrent suggests more severe
recombination in the ITIC cell. To understand the recombination of
mobile charges, we conducted the incident light intensity (P)
dependence of J – V characteristics. Fig. 5b and Fig. S3 show the log
0
bipyridyl-4,4 -dicarboxylato) ruthenium(II) and PrC60MAI is C60
,
N,N,N-trimethyl-1-(2,3,4-tris(2-(2-methoxyethoxy)ethoxy)phe-
nyl)dimethanaminium monoadduct iodide salt [38]). The device
structure was ITO/PEDOT:PSS/active layer/CBL/Al. Here, PEDOT:
PSS was poly (3,4-ethylenedioxythiophene):poly (styrene sulfo-
nate). The device performance of ITCT was free of DIO (1,8-
diiodooctane): no obvious changes in the device performance were
observed as the DIO contents were varied between 0% and 1%
J
sc ꢁ log P plots and Voc ꢁ logP plots, which were obtained
a
according to the equations Jsc / P and Voc / (nkT/q)ln(P) with k, T,
and q are the Boltzmann constant, temperature in Kelvin, and the
elementary charge, respectively. The fitting
.940 and the fitting n value was 1.40 kT/q and 1.32 kT/q for the
ITCT and ITIC binary devices, respectively. The values close to
.000 means that the monomolecular mechanism dominates the
a value was 0.980 and
(
Table S2 in Supporting information). Table 1 shows the
0
photovoltaic data from the optimized device and Fig. 4a gives
a
the relative current-density ꢁ voltage (J ꢁ V) curve. An average Jsc
1
2
of 17.88 mA/cm and FF of 0.723 were obtained in the binary ITCT
recombination and the deviation from 1.000 suggests the
involvement of the bimolecular mechanism [37,41,42]. The more
severe bimolecular recombination in the ITIC device associates
with the lower FF value than the ITCT cell.
We again conducted fluorescence spectra to see the differences
in the donor ꢁ acceptor charge separation efficiency under the
open-circuit condition. Fig. 5c shows the fluorescence spectra of
the pristine films of PBDB-T, ITCT and ITIC as well as the binary
blend films. Under excitation of 630 nm, a fluorescent band was
observed from the PBDB-T film and the maximum intensity around
cell. In the case of the ITIC binary cell, these values were 16.92 mA/
2
cm and 0.655, respectively. Both cells have equal energy loss
opt
opt
values (Eloss = 0.7 eV, Eloss = E
g
ꢁ eVoc with E
g
is the BHJ film’s
opt
optical band gap). The smaller the ITCT E
g
is and the smaller the
ITCT cell Voc is. The maximum PCE was 11.27% for the ITCT devices.
The EQE spectrum is given in Fig. 4b, which covers a wide
wavelength region of 360 ꢁ 800 nm. The integrated Jsc values are
given in Table 1, which agree well with that Jsc obtained from the J
ꢁ
V measurements. The larger Jsc in the ITCT than the ITIC device is
mainly contributed from the higher EQE in the wavelength region
6
75 nm was 320 a.u./100 nm. Upon excitation with the 700 nm
light, the maximum fluorescent intensity was 117 a.u./100 nm at
75 nm and 31 a.u./100 nm at 812 nm for the ITCT and IT-T-IC pure
of 600 ꢁ 850 nm, e.g., the absorption region of ITCT/ITIC region.
opt
This indicates that the larger absorptivity and the smaller E
g
of
7
ITCT than ITIC associates with the larger Jsc. The electron mobilities
films, respectively. After the PBDB-T and ITCT (ITIC) was blended,
the fluorescence of both the polymer and acceptor was signifi-
cantly quenched and the fluorescence intensity was below 5 a.u./
(
(
3
e
m ) (Fig. S2), measured with the space-charge-limited current
ꢁ
4
2
ꢁ1 ꢁ1
SCLC) method, were calculated to be 5.1 ꢀ10 cm
ꢂ
V
ꢂ
s
and
for the binary ITCT and ITIC solar cell
) were of 6.1 and
s , respectively. Comparable electron and hole
ꢁ
4
2
ꢁ1 ꢁ1
.8 ꢀ 10 cm
ꢂ
V
ꢂ
s
1
00 nm, approaching to the detection limit of the fluorescence
blends, respectively, and the hole mobilities (m
h
5
mobilities are obtained in both the ITCT and ITIC binary blends.
Fig. 5a shows the photocurrent generation and mobile charge
drift under driving by the internal voltage. Here, the photocurrent
instrument. The fluorescence data indicate that the donor-acceptor
charge separation is effective in both the ITCTand ITIC blends. Fig. 6
shows transmission electron microscopy (TEM) images of the ITCT
and ITIC solar cell blends. The fine film-morphologies are
consistent well with the fluorescence quenching data observed
from both ITCT and ITIC binary blends.
In summary, we have demonstrated that introduction of a
polarizable thiophene on the electron-accepting unit, cyclopenta
[
can effectively increase the quinoidal character on the resulting
LBG SMA molecule (ITCT), which reduces the optical band gap and
enhances the near IR absorptivity, compared to the known ITIC.
When blended with the wide band gap polymer donor, PBDB-T, the
ITCT binary device shows a larger Jsc and a higher FF than the
ꢁ
5
2
ꢁ1 ꢁ1
.1 ꢀ10 cm
ꢂ
V
ꢂ
(J
ph) is the current difference between the illumination and dark,
ph = Jlight ꢁ Jdark, and the internal voltage (Vin) is the voltage
difference between the applied and build-in voltage, Vin = VBI
app. At the high Vin regime, all of the photogenerated excitons are
J
ꢁ
V
c]thiophen-4-one-5-methylene-6-(1,1-dicyanomethylene) (CT),
separated by the high applied field and are all drifted to the right
electrode. In this case the photocurrent is mainly determined by
the exciton generation ability of the binary blend [39,40]. The
larger photocurrent in the high Vin range, for example, >1 V agrees
opt
well with the larger absorptivity and the smaller E
g
of ITCT than
ITIC. With the decrease in the Vin, the scale of the photocurrent
Table 1
2
Summary of the photovoltaic data of the ITIC and ITCT binary devices. All the data were obtained under illumination of AM 1.5G, 100 mW/cm light source.
a
a
sc (mA/cm2)
cacl. (mA/cm2)
b
FF a
a
Active layer
V
oc (V)
J
J
PCEave (%)
PCEmax (%)
PBDB-T:ITIC 1:1
PBDB-T:ITCT 1:1
0.897 ꢃ 0.007
0.850 ꢃ 0.007
16.92 ꢃ 0.33
16.24
17.09
0.655 ꢃ 0.010
0.723 ꢃ 0.009
9.94 ꢃ 0.34
10.28
11.27
17.88 ꢃ 0.30
10.99 ꢃ 0.28
a
Average values from 10 devices.
Calculated from the EQE spectrum from 360 nm to 900 nm.
b
Liu, Wenxu
