Prominent short-circuit currents of fluorinated quinoxaline-based copolymer solar cells with a power conversion efficiency of 8.0%

Article abstract of DOI:10.1021/cm302861s

A tailor-made medium-band gap fluorinated quinoxaline-based conjugated polymer of PBDT-TFQ was designed and synthesized as a donor material for bulk-heterojunction (BHJ) solar cells. This polymer is possessed of an intrachain donor-acceptor architecture and exhibits a broad and strong absorption spectrum across the entire UV-vis region. The introduction of F atoms with high electron affinity to the quinoxaline moiety is effective in further lowering both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of PBDT-TFQ to attain higher open-circuit voltage (V_oc). With an optimized blend ratio of PBDT-TFQ:PC₇₁BM (1:1, w/w), a high power conversion efficiency (PCE) of 8.0% was obtained, with a V_oc of 0.76 V, a short-circuit current density (J_sc) of 18.2 mA cm^-2, and a fill factor (FF) of 58.1% under AM 1.5G irradiation. The resulting copolymer reveals an outstanding J_sc value, arising from the higher hole mobility of PBDT-TFQ, together with the better continuous percolation pathways within the polymer blend for efficient exciton dissociation and charge transport.

Full text of DOI:10.1021/cm302861s

Article
pubs.acs.org/cm
Prominent Short-Circuit Currents of Fluorinated Quinoxaline-Based
Copolymer Solar Cells with a Power Conversion Efficiency of 8.0%
,
†,‡
†,‡
†
†
†,‡
Hsieh-Chih Chen,* Ying-Hsiao Chen, Chi-Chang Liu, Yun-Chen Chien, Shang-Wei Chou,
,
†,‡
and Pi-Tai Chou*
†
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, Republic of China
‡
Center of Emerging Material and Advanced Devices, National Taiwan University, Taipei 10617, Taiwan, Republic of China
S Supporting Information
*
ABSTRACT: A tailor-made medium-band gap fluorinated quinoxaline-
based conjugated polymer of PBDT-TFQ was designed and synthesized as
a donor material for bulk-heterojunction (BHJ) solar cells. This polymer is
possessed of an intrachain donor−acceptor architecture and exhibits a
broad and strong absorption spectrum across the entire UV−vis region.
The introduction of F atoms with high electron affinity to the quinoxaline
moiety is effective in further lowering both the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels of PBDT-TFQ to attain higher open-circuit voltage (Voc).
With an optimized blend ratio of PBDT-TFQ:PC BM (1:1, w/w), a high
71
power conversion efficiency (PCE) of 8.0% was obtained, with a V of 0.76
oc
−2
V, a short-circuit current density (J ) of 18.2 mA cm , and a fill factor
sc
(
FF) of 58.1% under AM 1.5G irradiation. The resulting copolymer reveals
an outstanding J value, arising from the higher hole mobility of PBDT-TFQ, together with the better continuous percolation
sc
pathways within the polymer blend for efficient exciton dissociation and charge transport.
KEYWORDS: solar cells, photovoltaic, quinoxaline, semiconductor, bulk heterojunction
INTRODUCTION
surpassing 7% under AM1.5G illumination have recently been
reported.
■
1b,10
In the past few decades, extensive efforts have been directed at
developing solution-processed polymer solar cells (PSCs),
because of their potential for being lightweight, having
Central to the electron donor unit, the benzo[1,2-b:4,5-b′]-
dithiophene (BDT) has emerged as an attractive building block
for BHJ solar cells due to its rigidity, coplanarity by fusing a
benzene with two flanking thiophene units, high hole mobility,
extremely extended π-conjugated structures, and proper side
1
−3
flexibility, and being used in low-cost energy harvesting.
So far, the most efficient architecture of PSCs is upon the
donor−acceptor (D-A) bulk-heterojunction (BHJ) structure
with a three-dimensional interpenetrating network of con-
11
chain patterns for enhanced solubility. Recently, a variety of
compounds in terms of BDT-based copolymer family, which
absorb more light by fine-tuning energy levels of polymers to
better match their band gaps with the solar spectrum, have
achieved substantial PCEs of 4%−7% when copolymerized with
various electron acceptors, including benzothiadiazole
1
−4
jugated polymers blended with soluble fullerene derivatives
5
or nanocrystals. PSC performance has significantly advanced in
the past few years through the design and synthesis of novel
6
conjugated polymers, modified interfacial layers, optimized
7
6a,8
film morphologies, and engineered device architectures. In
addition, increasing the molecular weight of conjugated
polymers has also demonstrated an improvement in PSC
performance via promotion of hole mobility, optical property,
1
0b,c
3a,10a,f
3b,d
(
(
(
BT),
thienothiophene (TT),
diketopyrrolopyrrole
and thienopyrroledione
12a
DPP),1 thiazolothiazole (TTZ),
2b
TPD)
derivatives. Recently, the incorporation of an
1
d,3c,e
electron-withdrawing F atom onto the periphery of A is
attractive, since its small size is expected to minimize the
undesired steric interactions, and its strong electron-affinity
nature is also expected to stabilize the HOMO and LUMO
and film morphology.
Moreover, alternating electron D
and A subunits in polymer main chains has been an important
strategy in designing high-performance polymers for PSC
applications, because intramolecular charge transfer (ICT) from
an electron-donating unit to an electron-withdrawing moiety
within the fundamental repeat unit allows facile tuning of band
gaps, frontier molecular orbital energies, and light absorption
14
energy levels. Furthermore, fluorination also causes a decrease
in the band gap in solid state, arising from the fact that the
9
strength. Therefore, remarkably more incident photons can be
Received: September 5, 2012
Revised: December 1, 2012
harvested for efficient charge carrier generation. Based on this
tactic, the power conversion efficiencies (PCEs) of PSCs
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/cm302861s | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
a
Scheme 1. Synthetic Route Adopted for the Preparation of Monomers and Polymers
a
The reagents and conditions: (i) Fe, AcOH, 50 °C, 4 h; (ii) 1,2-bis(3-hexyloxyphenyl)ethane-1,2-dione, AcOH, reflux, overnight; (iii) 2-(tributyl-
stannyl)thiophene, PdCl (PPh ) , toluene, reflux, 24 h; (iv) NBS, DMF, 30 °C, 8 h; and (v) Pd(PPh ) , toluene, reflux, 72 h.
2
3
2
3 4
onset
onset
improvement in planarization of the conjugated backbone
through favorable F-atom-induced intramolecular and/or
intermolecular interactions.
in which Eox and Ered are the onset potentials for the oxidation and
17
reduction processes, respectively. TM-AFM images were taken on a
NanoScope IIIa controller (Veeco Metrology Group/Digital Instru-
ments, Santa Barbara, CA), using a built-in software (version
V6.13R1) to capture images. X-ray diffraction (XRD) pattern was
collected using a Rigaku-TTRAX III diffractometer, operating at an
accelerating voltage of 50 kV and current of 300 mA. A nickel (Ni)-
filtered Cu Kα radiation source (0.154 nm) was used. A scanning rate
10a,b,14
In this regard, we report a straightforward synthetic
methodology to prepare a new D-A alternating copolymer,
namely, poly{4,8-bis(2′-ethylhexyloxy)-benzo[1,2-b:4,5-b′]-
dithiophene-alt-[5,8-bis(5′-thiophen-2′-yl)-6,7-difluoro-2,3-bis-
(
1
3″-hexyloxyphenyl) quinoxaline]} (PBDT-TFQ; see Scheme
−1
of 4° min and a scanning range of ∼1.5°−40° 2θ were carefully
), which turns out to be a very promising candidate for high-
chosen to allow for complete X-ray penetration into the powdery
PBDT-TFQ film. The absorption spectra were obtained on a Hitachi
U-4100 spectrophotometer. Hole mobility was measured according to
performance solar cells. This polymer, designed based on the
intrachain D-A copolymer motif, was expected to exhibit a
broad absorption spectrum. In addition, the introduction of F
atoms with high electron affinity to the quinoxaline moiety is
effective in further lowering both the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) levels of PBDT-TFQ to attain
1
8
a similar method described in the literature, using a diode
configuration of ITO/PEDOT:PSS/polymer: PC BM/Pd. The
SCLC current was measured under dark conditions using a Keithley
Model 2400 source meter.
Solar Cell Device Fabrication and Characterization. Opti-
mized solar cell devices used in this study were prepared by dissolving
PBDT-TFQ and PC BM (purchased from Nano-C) in 1,2-
dichlorobenzene (DCB) in the weight ratios of 1:0.5, 1:1, and 1:2
with a PBDT-TFQ concentration of 10 mg mL . For device
71
higher open-circuit voltage (V ), short-circuit current density
oc
1
b,3a,10a,b,14
71
(
Jsc), and PCE.
A high PCE of up to 8.0% has been
achieved with a blend ratio of 1:1 PBDT-TFQ:PC BM.
−1
71
fabrication, solution was stirred for more than 12 h to ensure complete
dissolution. Patterned indium tin oxide (ITO)-coated glass substrates
EXPERIMENTAL SECTION
Measurement and Characterization. Detailed experimental
procedures for the synthesis and characterization of the PBDT-TFQ
copolymer are given in the Supporting Information. H, C, and
NMR spectra were recorded on a Varian Mercury 400. Mass spectra
were obtained on a Finnigan LCQ mass spectrometer. Chemical shifts
■
(
Kintec Company, 15 Ω/□) were successively cleaned by ultra-
sonication in 1% neutral detergent in water, followed by deionized
1
13
19
water, then acetone, and finally ethanol for 20 min each. The
F
substrates were then dried and O plasma cleaned immediately prior to
2
the deposition of a 40-nm-thick layer of aqueous PEDOT:PSS PH500
(
H.C. Starck) solution, as verified by a Filmetrics Model F10-RT-UV
system. Deposition of the PEDOT:PSS layer was followed by baking at
40 °C for 20 min. Substrates were subsequently transferred to an
inert N -filled glovebox (<0.1 ppm O and H O), and the active layer
(
δ), quoted in parts per million (ppm), and coupling constants (J)
were recorded in Hertz (Hz). All NMR spectra were recorded in
deuterated chloroform or 1,1,2,2-tetrachloroethane (CDCl3 or
C D Cl ) and solvent containing 0.003% TMS as an internal
1
2
2
2
2
2
4
was spin-coated. The wet film was slowly dried in a covered Petri dish
for a certain time and subsequently annealed at 140 °C for 10 min in
reference. The molecular weight and molecular weight distribution
of the synthesized polymers were measured with a Waters GPC
Breeze system) using tetrahydofuran (THF) as an eluent at 35 °C.
The apparatus was equipped with two Waters Styragel columns (HR3
and HR4E), a refractive index detector (Waters 2414), and a dual-
wavelength absorbance detector (Waters 2487). Polystyrene standards
the glovebox. The thicknesses of the PBDT-TFQ:PC BM composite
(
71
films were adjusted to ∼90−160 nm by controlling the spin-coating
rates with two steps. The coated substrates were then transferred to a
−6
thermal evaporator and evacuated to ≤1 × 10 Torr before a 30-nm-
thick calcium layer, followed by a 100-nm aluminum electrode layer
(
Waters) were used for calibration. Cyclic voltammetry (CV) was
2
were deposited. The active area of each device was 0.04 cm with a
performed on an Autolab PGSTAT 30 Electrochemical Workstation
with a three-electrode system in a solution of 0.1 M tetrabutylammo-
nium perchlorate (Bu NClO ) in deoxygenated acetonitrile (CH CN)
length:width ratio of 1:1. The fabricated device was encapsulated in a
nitrogen-filled glovebox with UV epoxy and cover glass. The J−V
curves were measured with a Newport−Oriel (Sol3A Class AAA Solar
4
4
3
−1
at a scan rate of 50 mV s . The polymer films were coated on working
electrodes of the ITO sheet by solvent casting. A platinum wire was
used as the counter electrode, and a silver wire was used as a quasi-
−2
Simulators) AM 1.5G light source operating at 100 mW cm , and
independently cross-checked using a 300-W AM 1.5G source
−2
reference electrode. The potential of the as-prepared polymer was
operating at 100 mW cm for verification. The light intensity was
determined by a monosilicon detector (with KG-5 visible color filter)
calibrated by the National Renewable Energy Laboratory (NREL) to
minimize spectral mismatch. A 0.20 cm × 0.20 cm shadow mask was
used to cover the active area for avoiding interference from both
scattering light and adjacent current leakage. The IPCE spectra were
measured using a lock-in amplifier with a current preamplifier under
short-circuit conditions. The devices were illuminated by mono-
chromatic light from a xenon lamp passing through a monochromator
with a typical intensity of 30 μW. A calibrated monosilicon diode with
known spectral response was used as a reference.
+
corrected by the standard reference of Fc/Fc in CH CN (0.45 V vs
3
+
Ag/Ag electrode), where the absolute energy level of ferrocene is
−
4.8 eV below the vacuum level. The HOMO and LUMO energy
levels of the polymer were calculated according to the equation
onset
EHOMO = − (Eox + 4.35) eV
and
onset
ELUMO = − (Ered + 4.35) eV
B
dx.doi.org/10.1021/cm302861s | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
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−1
Figure 1. (a) Cyclic voltammogram of PBDT-TFQ in the thin film at a scan rate of 50 mV s . (b) UV−vis absorption spectra of PBDT-TFQ in
1,2-dichlorobenzene solution and as a film.
RESULTS AND DISCUSSION
strong absorption spectral feature across the entire UV−vis
region, with two absorption bands located at 401 and 553 nm.
The shorter wavelength peak is assigned to the localized π−π*
transitions, whereas the lower energy band corresponds to the
ICT between D and A moieties. In film, the absorption
becomes broader and is significantly red-shifted with three
peaks at ∼421, 583, and 612 nm, respectively. The large red-
■
On the basis of the D-A concept, the medium electron-
donating BDT moiety and strong electron-accepting TFQ unit
were utilized to synthesize the medium-band gap PBDT-TFQ
copolymer. The synthesis routes of the monomers and
corresponding polymer are illustrated in Scheme 1. The
copolymer PBDT-TFQ was obtained by a Stille cross-coupling
polymerization of 2BrTFQ with BDT using Pd(PPh ) as a
catalyst via thermal heating, and was in good yield, as shown in
the Supporting Information. PBDT-TFQ exhibits an excellent
film-casting property, as well as great solubility in common
halogenated solvents. The weight-average molecular weights
shift is observed in the absorption maxima (λ ) relative to
max
3
4
dilute solution as a result of a more planar polymer chain
structure and/or the degree of interchain π-stacking in the solid
film. Such ordered packing of polymer chains is beneficial for
improving the charge carrier mobility of resulting films.
Furthermore, the absorption edge (λedge) of PBDT-TFQ film
−
1
(
M ) of PBDT-TFQ copolymers are 37 and 71 kg mol , with
w
opt
is 718 nm, from which the Eg value for PBDT-TFQ, deduced
polydispersity indexes (PDIs) of 3.27 and 1.54, respectively, as
determined by gel permeation chromatography (GPC) using
THF as an eluent calibrated with polystyrene standards.
Thermal stability of the polymer was confirmed by
thermogravimetric analysis (TGA) at a temperature ramp rate
from the onset of absorption in the solid state, is determined to
be 1.73 eV (Table 1).
Table 1. Optical and Electrochemical Properties of PBDT-
TFQ
of 10 °C min− under N (see Figure S12 in the Supporting
1
2
Information). The onset decomposition temperature of PBDT-
TFQ at 5% weight loss is 333 °C, indicating high thermal
stability of the as-prepared polymer.
λedge
[nm]
λ
max [nm]
HOMO LUMO EgEC
Egopt
a
a
b
c
The HOMO and LUMO energy levels of PBDT-TFQ were
calculated by cyclic voltammogram (CV) from the onset
potential of the first oxidation and reduction peaks shown in
Figure 1a. The corresponding HOMO and LUMO energy
levels of PBDT-TFQ were determined to be −5.52 and −3.30
eV, respectively. PBDT-TFQ exhibits a deeply low-lying
HOMO energy level, which is in an ideal range to ensure
better air stability and can be anticipated to approach high Voc,
in accordance with the linear correlation of the difference
between the HOMO energy level of a donor polymer and the
polymer solution
film
film
[eV]
[eV]
[eV]
[eV]
PBDT-
TFQ
401,
553
421,
583,
6
718
−5.52
−3.30
2.22
1.73
12
a
HOMO and LUMO levels were estimated from the onset of the
oxidation and reduction peaks of cyclic voltammogram. Electrochem-
ical band gap calculated from cyclic voltammogram. Optical band gap
estimated from the onset of UV−vis spectrum of the film.
b
c
BHJ solar cells based on the PBDT-TFQ polymer were
fabricated in a typical configuration of ITO/PEDOT:PSS/
polymer:PC BM (1:0.5, 1:1, and 1:2, w/w)/Ca/Al and tested.
PSCs were optimized empirically by scrutinizing variations in
the molecular weight of the polymer, the polymer concen-
trations, the blend ratios of the polymer to PC BM, the choice
of solvent, the processing additives, the spin speed, and the
thermal treatment. More than 200 devices have been fabricated
in total, and the optimized blend ratio was 1:1. The resulting Jsc,
13
LUMO energy level of an acceptor. Indeed, the introduction
of electron-deficient F atoms can further lower both the
HOMO and LUMO energy levels, and the result agrees well
7
1
1
b,3a,10a,b,14
with the previous reports.
Therefore, the relatively
low-lying HOMO level and small band gap, along with good
solubility, favorable geometry, and promising charge carrier
transportation, could act as an effective donor component for
BHJ solar cells (vide infra). Considering the LUMO level of
71
2
b
PC BM is located at −4.02 eV, the offset between donor
V , FF, and PCE values were determined from the J−V curves,
71
oc
polymer and acceptor PC BM should provide an enough
and all of the relevant solar cell parameters are summarized in
Table 2. It is worth noting that PBDT-TFQ-based devices did
not perform optimally when spin-casted from pristine (DCB)
solvent and thus required the use of a processing additive. 1,8-
7
1
driving force for efficient exciton dissociation, ensuring
1a
energetically favorable electron transfer. As shown in Figure
b, PBDT-TFQ in dilute DCB solution exhibits a broad and
1
C
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Table 2. Representative Photovoltaic Performance for PBDT-TFQ:PC BM Devices under AM 1.5G Solar Illumination
7
1
Jsc [mA cm−2]
FF [%]
PCEave [%]
b
thickness [nm]
c
device
D/A ratio
solvent
DCB
Voc [V]
D1
D2
D3
D4
1:0.5
1:0.5
1:1
0.86 ± 0.01
0.81 ± 0.01
0.82 ± 0.01
0.78 ± 0.01
0.76 ± 0.01
0.83 ± 0.02
0.80 ± 0.01
10.3 ± 0.5
14.2 ± 0.2
12.3 ± 0.8
15.5 ± 0.6
17.9 ± 0.3
7.91 ± 0.8
12.9 ± 0.7
46.8 ± 0.3
53.7 ± 0.9
49.1 ± 0.2
56.8 ± 0.7
57.6 ± 0.5
45.1 ± 0.3
54.5 ± 0.3
4.2 ± 0.2 (4.4)
6.2 ± 0.2 (6.4)
4.9 ± 0.4 (5.3)
6.9 ± 0.3 (7.2)
7.8 ± 0.2 (8.0)
3.0 ± 0.4 (3.4)
5.6 ± 0.4 (6.0)
96
90
DCB + 3% DIO
DCB
100
93
1:1
DCB + 3% DIO
DCB + 3% DIO
DCB
a
D5
1:1
106
123
112
D6
D7
1:2
1:2
DCB + 3% DIO
a
b
High-molecular-weight PBDT-TFQ. Only the optimized recipes were considered for the estimation of the average PCE; data have been averaged
c
over 10 devices; the performance of the best device is given in parentheses. Thickness of the active layers.
Diiodooctane (DIO) was used as the additive, because of its
high boiling point (168 °C) and ability to solvate the fullerenes,
leading to a comprehensive impact on J by providing more
sc
optimal morphology for facilitating charge carrier trans-
7a,15
portation.
However, it was found that the addition of
DIO to the active layers tends to drop the V of the devices to
oc
some extent. This reduction can be attributed to the lowering
of charge-separated and charge-transfer-state energies upon
16
additive addition. Overall, devices with 3% DIO additive
exhibit better PCEs than that of the devices without any
additive (see Figure S14 in the Supporting Information). In the
PBDT-TFQ-based devices, the 1:1 blends outperformed the
1:0.5 and 1:2 blends. Aside from the blend ratios, however, they
all showed high performance, with average PCEs above 5.6%
under AM 1.5G illumination, which facilitates the preparation
of high-performance solar cells. With a weight ratio of 1:1
PBDT-TFQ:PC BM, a high V of 0.78 V was observed. This,
7
1
oc
−2
combined with a high J of 15.5 mA cm , and FF = 56.8%,
sc
results in an average PCE of 6.9% under AM 1.5G irradiation.
Moreover, the device was further upgraded to 7.8% (average),
along with the highest PCE of 8.0% (V = 0.76 V, J = 18.2
oc
sc
−2
mA cm , and FF = 58.1%) by utilization of a high-molecular-
weight PBDT-TFQ (71 kg mol ), and it showed substantial
−1
improvement in J and FF, in comparison to that made with a
sc
−1
moderate molecular weight of PBDT-TFQ (37 kg mol ) at
the same blend ratio, as depicted in Figure 2a. Moreover, the
corresponding D/A blend ratio dependency of the photovoltaic
Figure 2. (a) J−V curves of 1:1 PBDT-TFQ:PC BM solar cells with
71
high-molecular-weight (HM ) and moderate-molecular-weight
w
parameters, the V , J , FF, and PCE data are shown in Figure
oc sc
(
MM ) PBDT-TFQ under AM 1.5G solar illumination, and (b)
w
3. The superior performance of the high-molecular-weight
corresponding IPCE spectra of optimized 1:1 PBDT-TFQ:PC BM
7
1
PBDT-TFQ-based solar cells is associated with the high hole
mobility and better nanoscale morphology of the inter-
penetrating network (vide infra), allowing efficient exciton
dissociation and enhancing charge carrier transportation. The
above encapsulated devices were then sent to Center for
Measurement Standards located at Industrial Technology
Research Institute (ITRI) of Taiwan for further certification.
The corresponding PSCs certifications are shown in Figures
S15 and S16 in the Supporting Information). Certified PCEs of
devices made from HMw and MMw of PBDT-TFQ illuminated by
monochromatic light. Legend: light blue circles ( ) represent the
●
−
2
HM device (V = 0.76 V, Jsc = −18.2 mA cm , FF = 58.1%, and
w
oc
PCE = 8.0%) and olive green squares (
■
) represent the MM device
Voc = 0.78 V, J = −16.1 mA cm , FF = 57.5%, and PCE = 7.2%).
w
−2
(
sc
fabrication configuration without additional modified layers,
which is advisable for cost-effective PSCs.
The incident photon-to-current efficiency (IPCE) of the
solar cells was measured under the illumination of mono-
chromatic light. For the PSC devices fabricated from the 1:1
PBDT-TFQ:PC BM blend with high-molecular-weight and
7.3% and 6.3% for the devices with high-molecular-weight and
moderate-molecular-weight PBDT-TFQ, respectively, are
observed under 1 sun, AM 1.5G illumination. Comparing
these values with those measured in our laboratory, we found
an ∼9% drop in both certified PCEs, which mainly resulted
71
moderate-molecular-weight PBDT-TFQ, very broad panchro-
matic spectra over the entire excitation spectral range were
obtained from IPCE measurements, as illustrated in Figure 2b.
The optimum device very efficiently harvests solar light in the
maximum IPCE of 85.5% at 425 nm, and exhibits a broad
photoresponse of over 75% in the 407−640-nm range. The
calculated J values of 1:1 PBDT-TFQ:PC BM devices with
from the reduction in J and FF. The degradation of certified
sc
PSCs can be attributed to the nonoptimized encapsulation
process, together with the significant time delay between
sending the devices to ITRI and with the queuing time of the
ITRI measurement. Nevertheless, note that PBDT-TFQ-based
high-performance solar cells were accomplished via a simple
sc
71
high-molecular-weight and moderate-molecular-weight PBDT-
D
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Chemistry of Materials
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Figure 3. Photovoltaic parameters of the corresponding PSCs under AM 1.5G solar illumination: (a) open-circuit voltage, (b) short-circuit current
density, (c) fill factor, and (d) power conversion efficiency as a function of D/A blend ratios.
−
2
TFQ from IPCE curves are 16.3 and 14.6 mA cm ,
respectively, which confirm the high J values of the devices.
sc
The J mismatch could be ascribed to the following reasons:
sc
(
1) As measured by the IPCE instrument, <5%−10% error in
1a
spectral mismatch would take place.
(
2) Our IPCE instrument resolution below 400 nm is less
accurate. Regarding this compound, the spectral response
below 400 nm still has absorption and, thus, may be
reflected in part to the slight loss in IPCE spectra.
It is also clear that PBDT-TFQ devices exhibited distinguished
improvement across the entire photoresponse region with
increased PBDT-TFQ molecular weight, indicating that the
enhancement in Jsc resulted from the more-efficient exciton
dissociation and the robust charge collection from both the
Figure 4. Determination of the hole mobility from the dark current
densities for PBDT-TFQ:PC BM (1:1) diodes with high-molecular-
weight PBDT-TFQ (denoted by orange circles, ) and moderate-
molecular-weight PBDT-TFQ (denoted by purple squares, ■).
71
PBDT-TFQ and PC BM domains. Furthermore, the results
●
7
1
also demonstrate that the solar energy response for the current
generation extends to 750 nm, as expected from the absorption
spectrum of the active layer (see Figure S17 in the Supporting
Information).
carrier transport in the active layer, which is expected to
In consideration of charge-carrier mobilities in blend films,
hole-only devices in a configuration of ITO/PEDOT:PSS/
active layer/Pd were measured by using the space-charge-
contribute in part to the high J and FF values observed in
optimized BHJ devices (Figure 2a).
sc
The charge-carrier mobility and the nanoscale morphology of
the BHJ film are the two major factors that could be
significantly affected by the polymer molecular weight.
Tapping-mode atomic force microscopy (TM-AFM) studies
were conducted to investigate the film morphology of the
PBDT-TFQ:PC BM (1:1) active layers with and without DIO
18
limited current (SCLC) model at low voltage. The device
characteristics were extracted by modeling the dark current
under forward bias, described by the Mott−Gurney law. For
comparison, the hole mobility of 1:1 PBDT-TFQ:PC BM
7
1
−
4
2
−1 −1
−3
2
devices varied from 3.8 × 10 cm V
s
to 2.3 × 10 cm
71
−
1 −1
V
s
as the molecular weight of PBDT-TFQ increased from
additive after thermal annealing at 140 °C for 10 min. The 1:1
3
7 kg mol− to 71 kg mol shown in Figure 4. There was a
1
−1
blend film exhibits unevenly aggregated polymer and PC BM
7
1
difference of ∼6 times in mobility between the devices with
high-molecular-weight and moderate-molecular-weight PBDT-
TFQ. The higher hole mobility yielded a balanced charge
domains, forming large islands ∼100 nm in diameter and a
root-mean-square (rms) roughness of 3.10 nm (see Figure 5).
Consequently, this adverse morphology reduces the effective
E
dx.doi.org/10.1021/cm302861s | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
relatively high hole mobility together with the excellent
morphology mentioned above, results in a high J and FF.
sc
CONCLUSIONS
■
In summary, we have designed and synthesized a new medium-
band gap conjugated polymer of PBDT-TFQ, and we have
demonstrated excellent photon harvesting and efficient photo-
generation of charge-separated states when it is blended with
PC BM. The resulting copolymer revealed the desired deep
7
1
HOMO and LUMO energy levels, allowing high Voc and Jsc
values. Moreover, the high charge carrier mobility and better
continuous percolation pathways for charge transport lead to a
superior power conversion efficiency (PCE) of up to 8.0%,
which applies to a high-performance polymer solar cell (PSC).
The prospect of its future extension is promising; this includes
optimization of the molecular weight of PBDT-TFQ, rational
chemical modification, and engineering the interfacial layers
and device architectures in an effort to achieve record
performance.
Figure 5. Morphology characterization of (a−c) TM-AFM topography
and (d−f) phase images of PBDT-TFQ:PC BM blend films with
7
1
various compositions: (panels a and d) 1:0.5, (panels b and e) 1:1, and
(
panels c and f) 1:2. The imaging size is 2 μm × 2 μm for each panel.
D/A interface and thus limits the exciton dissociation
probability, resulting in exciton loss, geminate charge
7
e
ASSOCIATED CONTENT
Supporting Information
recombination, and poor charge mobility. In contrast,
addition of a small amount of additive to the blend film results
in smoother (rms roughness of 2.01 nm) surfaces without large
domains (very fine domains ∼10 nm in size), as shown in
Figures 6a and 6c. It is noteworthy that the blend film based on
■
*
S
Details for the synthesis and characterization of PBDT-TFQ
copolymer and experimental protocols for the devices are given
in the Supporting Information. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail addresses: austinchen@ntu.edu.tw (H.-C.C.), chop@
ntu.edu.tw (P.-T.C.).
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of this work by
the National Science Council and Ministry of Economic Affairs
of Taiwan. We also thank Dr. I-Wen P. Chen and Prof. Chun-
hsien Chen, National Taiwan University, for the TM-AFM
support.
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Figure 6. TM-AFM topography images (upper row) and phase images
■
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