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H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
yield 40 mg/mL solutions that were stirred in a glove box
under a nitrogen atmosphere overnight. The blended solu-
tions were spin-coated at 1000 rpm for 60 s for a thickness
of 100 nm on top of the PEDOT:PSS layer after filtration
electrochemical analysis showed that the highest occupied
molecular orbital (HOMO) energy level of PONTBT is
ꢀ5.34 eV, the lowest unoccupied molecular orbital (LUMO)
energy level of PONTBT is ꢀ3.60 eV, and its band gap is
1.74 eV. Table S1 provides a summary of the electrochem-
ical and electrical properties of PONTBT. The spectrum of
the PONTBT film contains a high energy absorption maxi-
mum at 595 nm, which indicates that PONTBT is a low
band gap polymer. Structural analysis with atomic force
microscopy (AFM), transmission electron microscopy
(TEM), and grazing incidence wide angle X-ray scattering
(GIWAXS) was performed to determine the molecular
ordering and the film morphologies of PONTBT and
PONTBT:PC71BM. The films were prepared by dissolving
PONTBT and PONTBT:PC71BM in DCB, followed by spin-
coating at 1000 rpm onto ITO glass coated with PEDOT:PSS.
An AFM image of a PONTBT film is shown in Fig. S3(a). The
PONTBT film has an r.m.s. roughness of 0.459 nm. The bulk
morphology of the PONTBT:PC71BM film was analyzed
with TEM. Fig. S3(b) shows a TEM image of a homogeneous
PONTBT:PC71BM domain. The dark areas are attributed to
PCBM-rich regions because their electron scattering den-
sity is higher than that of the polymer. Fig. S3(c) shows
GIWAXS patterns of PONTBT and PONTBT:PC71BM films.
Neither GIWAXS pattern contains any Bragg refraction
peak, which confirms that both films are amorphous.
The measured current–voltage (J–V) curves in Fig. S4
shows that the device performance is strongly dependent
on the ratio of PONTBT and PC71BM. The photovoltaic
results for all of the investigated blends under illumination
with an intensity of 100 mW cmꢀ2 are summarized in
Table S2. The device with a 1:3 weight ratio of conjugated
polymer to PC71BM was found to exhibit the best PCE.
with a 0.2 lm PTFE filter for all solvents, then annealed
at various temperatures for 20 min on a hot plate in the
glove box. In all the devices, the PONTBT:PC71BM layers
had similar thicknesses, on the order of 100 nm, and the
P3HT:PC61BM layers had similar thicknesses of 120 nm,
as measured with an alpha-stepper. LiF and Al cathodes
were thermally deposited to thicknesses of 1 and 100 nm
respectively onto the surface of each active layer.
The J–V characteristics were measured by using a Keith-
ley 4200 source measurement unit in the dark and under
AM 1.5G solar illumination (Oriel 1 kW solar simulator)
with respect to a reference cell PVM 132 calibrated at the
National Renewable Energy Laboratory at an intensity of
100 mW/cm2.
UV–visible (Cary, Varian Co.) and PL (FR 650, JASCO Co.)
measurements were used to analyze the optical properties
of the conjugated polymer/fullerene derivative blends.
AFM (Multimode IIIa, Digital Instruments) was performed
in tapping mode to acquire images of the surfaces of the
conjugated polymer/fullerene derivative blend films. In
the transmission electron microscopy (TEM) measure-
ments, the conjugated polymer/fullerene derivative layers
on a water-soluble PEDOT:PSS substrate were floated on
the surface of deionized water and picked up using
200-mesh copper TEM grids. TEM images were obtained
by using a HITACHI-7600 operated at 100 kV. GIWAXS data
were obtained at the 5A beamline (E = 11.57 keV) of the
Pohang Accelerator Laboratory (PAL) [46]. Total external
reflection angle of GIWAXS measurements is 0.14° to unify
the penetration depth of thin films. 6 Circle diffractometer
and scintillation detector are used to control external
reflection angle with no calibration standard. The exposure
time was 1 s at each measured angles.
3.2. Optical properties of the conjugated polymer/fullerene
derivative blends with various processing additives
Solar cell performance can be optimized by the addition
of processing additives to the conjugated polymer/fuller-
ene derivative blend layers. The conjugated polymer/
fullerene derivative ratios were optimized at 1:3 (wt:wt)
for PONTBT:PC71BM in DCB and 1:1 (wt:wt) for P3HT:
PC61BM in CB. In this study, processing additives with var-
ious alkyl chain lengths and end-group electronegativities
were incorporated into these optimized blends.
As shown in Fig. 2, the absorption maximum of the
PONTBT:PC71BM blends is at 550 nm. When 2 vol% of the
additives are added to the host solvent, DCB, the absorp-
tion intensity in the region 600–700 nm increases, which
is indicative of the close packing of PONTBT and a red-shift
3. Results and discussion
3.1. Properties of PONTBT and the optimization of the
PONTBT:PC71BM Solar Cells
The amorphous polymer, PONTBT, was synthesized
with the procedures shown in Scheme S1 and described
in the supporting information. The number average molec-
ular weight (Mn) of PONTBT is 16.2 kDa. The resulting
copolymer is soluble in toluene, chloroform, CB, and DCB.
The thermal properties of PONTBT were investigated with
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC), as shown in Fig. S1. This molecule
exhibits excellent thermal stability: the decomposition
temperature for 5% weight loss is approx. 344 °C. The
DSC traces for this compound contain a peak indicative of
phase transitions at 222 °C.
The electrochemical and optical properties of PONTBT
were determined in order to optimize the PONTBT:PC71BM
blend solar cells. Fig. S2 shows the UV–visible absorption
spectra of PONTBT in chloroform solution and in the film
state and the cyclic voltammogram for PONTBT. Our
in the p–p absorption band. The intensities of the intermo-
lecular interactions of PONTBT are increased by the addi-
tion of the processing additives. Moreover, the intensities
of the shoulders at 650 nm in the spectra of the PON-
TBT:PC71BM blends increase with increases in the alkyl
chain length (Fig. 2(a)) and in the electronegativity
(Cl > Br > I) of the end group (Fig. 2(b)) of the processing
additives. The incorporation of the processing additives
into the PONTBT:PC71BM (1:3, w/w) blend results in the
close packing of PONTBT and better phase separation of
PONTBT and the fullerene derivative.