Synthesis of diketopyrrolopyrrole containing copolymers: A study of their optical and photovoltaic properties

Article abstract of DOI:10.1021/jp909183x

The diketopyrrolopyrrole-based copolymers PDPP-BBT and TDPP-BBT were synthesized and used as a donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed absorption in the range 500-600 nm with a maximum peak around 563 nm, while TDPP-BBT showed broadband absorption in the range 620 - 800 nm with a peak around 656 nm. The power conversion efficiencies (PCE) of the polymer solar cells based on these copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were 0.68% (as cast PDPP-BBT:PCBM), 1.51% (annealed PDPP-BBT:PCBM), 1.57% (as cast TDPPBBT:PCBM), and 2.78% (annealed TDPP-BBT:PCBM), under illumination of AM 1.5 (100 mW/cm²). The higher PCE for TDPP-BBT-based polymer solar cells has been attributed to the low band gap of this copolymer as compared to PDPP-BBT, which increases the numbers of photogenerated excitons and corresponding photocurrent of the device. These results indicate that PDPP-BBT and TDPP-BBT act as excellent electron donors for bulk heterojunction devices. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp909183x

J. Phys. Chem. B 2010, 114, 3095–3103
3095
Synthesis of Diketopyrrolopyrrole Containing Copolymers: A Study of Their Optical and
Photovoltaic Properties
Catherine Kanimozhi,† P. Balraju,‡ G. D. Sharma,*,‡,§ and Satish Patil*,†
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, Molecular
Electronic and Optoelectronic DeVice Laboratory, Physics Department, JNV UniVersity, Jodhpur 342005, India,
and Jaipur Engineering College, Kukas, Jaipur, India
ReceiVed: September 24, 2009; ReVised Manuscript ReceiVed: January 26, 2010
The diketopyrrolopyrrole-based copolymers PDPP-BBT and TDPP-BBT were synthesized and used as a
donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed
absorption in the range 500-600 nm with a maximum peak around 563 nm, while TDPP-BBT showed
broadband absorption in the range 620 - 800 nm with a peak around 656 nm. The power conversion efficiencies
(PCE) of the polymer solar cells based on these copolymers and [6,6]-phenyl C61 butyric acid methyl ester
(PCBM) were 0.68% (as cast PDPP-BBT:PCBM), 1.51% (annealed PDPP-BBT:PCBM), 1.57% (as cast TDPP-
BBT:PCBM), and 2.78% (annealed TDPP-BBT:PCBM), under illumination of AM 1.5 (100 mW/cm2). The
higher PCE for TDPP-BBT-based polymer solar cells has been attributed to the low band gap of this copolymer
as compared to PDPP-BBT, which increases the numbers of photogenerated excitons and corresponding
photocurrent of the device. These results indicate that PDPP-BBT and TDPP-BBT act as excellent electron
donors for bulk heterojunction devices.
Introduction
and synthesize new materials with maximum overlap of
absorption spectrum with the solar emission spectrum. It is often
found that the synthesis of low band gap polymer is not only
the solution to address this problem but the position of highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) limits the open-circuit voltage (Voc)
of the photovoltaic cell.19,20 These two properties of organic
materials can be controlled by introducing alternative electron-
rich and electron-deficient units in the polymer backbone. By
using a similar concept, various groups have synthesized low
band gap diketopyrrolopyrrole (DPP)-based copolymers.22-26
Many donor-acceptor (D-A) type copolymers have been used
in polymer solar cells to achieve power conversion efficiencies
above 5% with extensive device engineering efforts.27,28 Re-
cently, Yang and co-workers reported the family of alternating
copolymers based on the DPP as a central core unit with an
overall power conversion efficiency of 4.45%.29 Barbec et
al. reported30 a cyclopentabithiophene- and benzothiadiazole
(donor-acceptor)-based low band gap polymer which shows a
power conversion efficiency of ∼3.3% when blended with
PC70BM with Jsc ) 10-11 mA cm-2 and EQE of 25%. These
reports suggest that the structural manipulation is indeed
necessary to optimize the performance of organic solar cells.
In search of such novel materials, we have synthesized two
copolymers with diketopyrrolopyrrole units by the palladium-
catalyzed Stille coupling reaction. Conceptually, the comono-
mers linked to the DPP unit were chosen on the basis of their
high p-type mobility as reported in the literature.31a The
photophysical properties of these copolymers in solution as well
as in solid state have been investigated in detail. The DPP
derivatives are known to exhibit distinct electronic properties
in the solid state. The electronic properties of both of the
polymers in the solid state have been discussed.
Solar cells based on the conjugated polymers have the virtue
of being lightweight and flexible.1,2 Hence, efforts were made
in the recent past to enhance the efficiency of organic solar
cells.3,4 An additional attractive feature of organic solar cells is
the compatibility with printing techniques which can be
exploited for roll-to-roll processes.5,6 The concept of polymer-
based bulk heterojunction (BHJ) improved the performance of
organic solar cells.7,8 In a typical BHJ device, the blend of
conjugated polymers as a donor and phenyl-C61-butyric acid
methyl ester (PCBM) as an acceptor have been used as an active
layer.9,10 So far, the blend of two materials (P3HT:PCBM) with
different electron affinities has provided a power conversion
efficiency up to 4-5% under 1.5 a.m. (100 mW/cm2)
illumination.11-13 The efficiency of organic solar cells is limited
due to several factors such as mismatch between the solar
spectrum and absorption spectrum of the active layer, the low
mobilities of charge carriers in organic materials, and the
morphology of a thin film in the case of the P3HT:PCBM
blend.14,15 However, in the past few years, a large number of
research groups have brought insights into the blend (P3HT:
PCBM) morphology by altering the annealing temperature and
the organic solvent and by adding a small amount of additives
in the blend.4,16-18,21 However, the relatively large band gap
(∼2.0 eV) of P3HT limits the absorption of near-infrared light
and thus lowers the light harvesting and efficiency of devices.
One of the fundamental features which limits the efficiency
in organic solar cells is the band gap of conjugated polymers.
The control over the band gap is necessary while designing the
new materials. This band gap engineering allows one to design
* To whom correspondence should be addressed. Phone: +91-80-
22932651. Fax: +91-80-23601310. E-mail: satish@sscu.iisc.ernet.in (S.P.);
sharmagd_in@yahoo.com (G.D.S.).
Furthermore, the copolymers were incorporated as an electron
donor in bulk heterojunction solar cells with PC60BM as an
electron acceptor. In the present contribution, we describe the
† Indian Institute of Science.
‡ JNV University.
§ Jaipur Engineering College.
10.1021/jp909183x  2010 American Chemical Society
Published on Web 02/16/2010

3096 J. Phys. Chem. B, Vol. 114, No. 9, 2010
Kanimozhi et al.
synthesis and photovoltaic properties of DPP-based copolymers
with different donor segments. The structure-property correla-
tion and its performance in the bulk heterojunction organic solar
cells were also evaluated.
(ppm): 7.46 (d, 2H), 7.33 (d, 2H), 4.26 (t, 4H), 1.87 (m, 4H),
1.561 (m, 4H), 1.37 (m, 8H), 0.91(m, 6H). IR (KBr) 2934.41(m),
1018.11(s).
2,6-Bis(tributyltin)-4,8-dihexylloxybenzo[1,2-b;3,4-
b]dithiophene (BBT) (6).31b A 0.50 g (1.28 mmol) portion of
compound 5 was taken in dry THF. A 0.34 g (3.33 mmol) (1.6
M) portion of n-BuLi was slowly added to this mixture at 0 °C
and stirred at room temperature for 1 h. After 1 h, 1.25 g (3.85
mmol) of tertiarybutyltinchloride was added at 0 °C and allowed
to stir at room temperature overnight. Then, the reaction mixture
was quenched with H2O and extracted with DCM. The DCM
layer was dried over anhydrous Na2SO4. The organic layer was
concentrated and used for the next step without purification.
Experimental Section
Materials. 1-Bromo hexane, 4-bromo benzonitrile, thiophene
3-carboxylicacid, n-butyllithium (1.6 M solution in hexane),
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), and tet-
rabutylammonium hexafluorophosphate ((TBA)PF6) were ob-
tained from Aldrich and used without further purification.
N-Bromosuccinimide (NBS) was crystallized prior to use. THF,
CHCl3, and NMP have been dried by Na/benzophenone, P2O5,
and calcium hydride, respectively.
General Procedure for the Synthesis of Polymers PDPP-
BBT and TDPP-BBT. Equal amounts of both of the monomers
were taken in dry toluene and DMF (4:1). This mixture was
purged with argon for 15 min. It was evacuated and flushed
with argon three times. A catalytic amount of Pd(PPh3)4 was
added under an argon atmosphere and refluxed for 16 h. After
polymerization, solvent was removed under reduced pressure
and the solid was dissolved in CHCl3 and precipitated in MeOH.
This precipitate was washed several times with MeOH, and the
crude solid was washed on a Soxhlet apparatus with MeOH,
hexane, and acetone to remove low molecular weight impurities.
The solid was dried under a vacuum for 2 h. PDPP-BBT was
obtained as a dark brown solid, and TDPP-BBT was obtained
as a dark blue solid.
1
Instrumentation. H NMR and 13C NMR were recorded
using a Bruker 400 MHz instrument. Chemical shifts were
recorded with respect to TMS and given in parts per million.
Molecular weights were measured by gel permeation chroma-
tography in THF. Thermal gravimetric analysis (TGA) was
conducted on a Mettler Toledo TGA/SDTA 851 instrument.
Absorption spectra of monomers and polymers were recorded
by using a Perkin-Elmer UV-vis spectrometer for both solid
and solution. Solution spectra’s were recorded in chloroform.
Electrochemical properties of both of the polymers were
examined by using cyclic voltammetry (CH instrument).
Polymers were coated on a platinum disk and immersed in 0.1
M Bu4NPF6 acetonitrile solution. CVs were recorded using a
platinum disk as a working electrode and Ag/Ag+ as the
Bulk Heterojunction Photovoltaic Device Fabrication and
Characterization. Indium tin oxide coated glass substrates were
cleaned in acetone and in an ultrasonic bath. The blends of
copolymer PDPP-BBT or TDPP-BBT and PCBM in 1:1 ratio
in chloroform solution were coated over ITO glass substrates
by the spin coating method under a nitrogen atmosphere in a
glovebox. The top aluminum electrode was evaporated by
thermal evaporation under a vacuum of 10-6 Torr. The blended
active layers were thermally annealed at 100 °C for 1 min before
the deposition of the top Al electrode. The thickness of the active
layers and effective area of the devices were about 80 nm and
0.25 cm2, respectively. The J-V characteristics of the devices
in the dark and under illumination were measured by a
semiconductor parameter analyzer (Keithley 4200-SCS). A
xenon light source (Oriel, USA) was used to give a simulated
irradiance of 100 mW/cm2 (equivalent to AM1.5 irradiation) at
the surface of the device. The photoaction spectrum of the
devices was measured using a monochromator (Spex 500 M,
USA), and the resulting photocurrent was measured with a
Keithley electrometer (model 6514), which is interfaced to the
computer by LABVIEW software.
reference electrode at a scan rate of 20 mV s-1
.
Synthesis of Monomers. Thiophene-3-carbonyl Chloride,
N,N-diethylthiophene-3-carboxamide, and 4,8-dihydrobenzo[1,2-
b:4,5-b′]dithiophen-4,8-dione (4) have been synthesized by a
previously reported procedure.31b 3,6-Dithiophen-2-yl-2,5-di-
hydro-pyrrolo[3,4-c]pyrrole-1,4-dione, 1,4-diketo-3,6-bis(4-bro-
mophenyl)pyrrolo[3,4-c]pyrrole,1,4-diketo-2,5dihexyl-3,6-bis(4-
bromophenyl)pyrrolo[3,4-c]pyrrole25 (1), and 2,5-di-n-hexyl-3,6-
dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione32 (2)weresynthesized
by a previously reported procedure.
3,6-Bis-(5-bromo-thiophen-2-yl)-2,5-di-n-hexyl-pyrrolo[3,4-c]-
pyrrole-1,4-dione (3).32 A 0.50 g (1.07 mmol) portion of TDPP
was taken in dry chloroform. Under an argon atmosphere, 0.418
g (2.35 mmol) of N-bromosuccinimide was added portionwise
at 0 °C. It was stirred at room temperature for 48 h. This mixture
was poured into an excess of MeOH and stirred for 2 h. Solid
was vacuum filtered and washed with MeOH and H2O to remove
excess N-bromo succinimide and polar impurities. The crude
solid was purified through column chromatography using DCM
as a solvent. Dibrominated DPP (BrDPP) was obtained as a
dark purple flaky solid (0.430 g). Yield: 65%. 1H NMR (CDCl3,
400 MHz), δ (ppm): 8.66 (d, 2H), 7.24 (d, 2H), 3.98 (t, 4H),
1.71 (m, 4H), 1.31-1.43 (m, 12H), 0.89 (t, 6H). IR (KBr)
2951.56 (m), 2924.71 (m), 1655.27 (s).
Results and Discussion
Synthesis of Monomers. The synthetic approach to monomers
(TDPP and PDPP) is outlined in Scheme 1. The diketopyrrolo-
prrrole moiety was prepared by a pseudo stobbe condensation
between isopropyl succinicester and p-bromobenzonitrile (or)
thiophene carbonitrile as reported in the literature.25
4,8-Dihexyl Benzo[1,2-b;3,4-b]dithiophene (5).31b A 0.50 g
(2.27 mmol) portion of compound 4 and 0.44 g of Zn (6.81
mmol) were taken in 50 mL of water. NaOH (0.44 g × 4) was
added and refluxed until it was dissolved. After 2 h, 1.12 g
(6.82 mmol) of hexyl bromide and a catalytic amount of
tetrabutylammoniumbromide were added to this mixture and
refluxed for 12 h. This mixture was poured into cold water and
extracted with DCM. The DCM layer was dried over anhydrous
Na2SO4. The organic layer was concentrated and purified by
column chromatography using ethyl acetate:hexane (1:99) as
the eluent. Compound 5 was obtained as a white crystalline
This DPP core consists of two fused lactam rings. Inter- and
intramolecular H-bonding due to the lactam hydrogen atoms is
responsible for insolubility of DPP in common organic solvents
such as chloroform, dichloromethane, and tetrahydrofuran. The
solubility of this product was enhanced by alkylation on lactam
nitrogen atoms of DPP which was subsequently converted into
a dibromo derivative by using N-bromosuccinimide in dry
1
solid (1.33 g). Yield: 75%. H NMR (CDCl3, 400 MHz), δ

Synthesis of DPP Containing Copolymers
J. Phys. Chem. B, Vol. 114, No. 9, 2010 3097
SCHEME 1: (a) KBuOt, Dry DMF, 60°C, Hexyl Bromide, 10 h; (b) N-Bromo Succinimide, Chloroform, 2 Days; (c)
Zn/NaOH, H2O, Hexyl Bromide, Reflux, 12 h; (d) Butyl Lithium, 0 °C, 1 h, Tributyl Tin Chloride, Overnight
TABLE 1: Molecular Weight and Thermal Properties of the
Polymers
SCHEME 2: Synthesis of DPP Copolymers by Stille
Coupling by Using Pd (PPh3)4, Toluene:DMF (4:1) at
110°C for 24 h
a
b
polymer
Mw
Mn
PDIMw/Mn
Td (°C)
PDPP-BBT
TDPP-BBT
9700
19000
4700
11560
2.06
1.64
355
318
a Mn, Mw, and PDI of the polymers were determined by gel
permeation chromatography using polystyrene standards in tetra-
hydrofuran. b The 5% weight-loss temperatures in N2 atm. The heating
rate is 10 °C/min.
chloroform, dichloromethane, and tetrahydrofuran. The chemical
structural characterization of PDPP-BBT and TDPP-BBT was
carried out by 1H NMR, as shown in Figure 1. The copolymers
of DPP show broad peaks due to their coil-like nature unlike
their monomers, whose signals are sharp. Aromatic signals
appear between 7.27 and 9.02 ppm. In the polymer PDPP-BBT,
the aromatic signals get split compared to its monomer which
shows a singlet for all eight aromatic protons (not shown).
Protons of the hexyl group appeared between 0.71 and 1.97
ppm, the methylene group attached to nitrogen appears between
3.72 and 4.16 ppm, and the methylene group attached to oxygen
appears between 4.32 and 4.48 ppm.
CHCl3. The other comonomer 6 was prepared by stannylation
of compound 5 and used for polymerization without further
purification.
Synthesis of Polymers. The synthetic protocol for the
polymerization of DPP copolymers from the relevant monomers
(1 or 3 + 6) is depicted in Scheme 2. Polymerization via Stille
coupling was carried out between dibrominated DPP monomers
(1 or 3) and compound 6 in the presence of tetrakis (triph-
enylphosphine) palladium (0). Stille coupling gave PDPP-BBT
as a dark brown solid with a molecular weight of 9700 g/mol
and a polydispersity (PDI) of 2.06. A similar experimental
protocol has been used to obtain TDPP-BBT as a dark blue
solid with a molecular weight of 19 000 g/mol and a PDI of
1.64.
The molecular weights of these two copolymers were
measured by gel permeation chromatography (GPC) using
tetrahydrofuran as an eluent. The average values and polydis-
persity index (PDI) of PDPP-BBT and TDPP-BBT are given
in Table 1. These two polymers show very good solubility in
Thermal Properties. The thermal stability of organic material
is a very important parameter in organic solar cells. High thermal
stability is required to prevent the degradation of polymers in
photovoltaic devices upon annealing. We have investigated the
thermal properties of these two polymers by thermo gravimetric
analysis. These two polymers show a very good thermal
stability. PDPP-BBT is stable up to 355 °C and TDPP-BBT is
stable up to 318 °C, as shown in Figure 2.
Optical Properties of Copolymers. Figure 3 shows the
UV-visible spectra of PDPP-BBT and TDPP-BBT in chlo-
roform solution and in the thin film. Both of the polymers show
well-defined vibronic features in solution at room temperature
with λmax at 539 nm for PDPP-BBT and λmax at 638 nm for
TDPP-BBT. The peak between 280 and 400 nm corresponds
to π-π* transition and between 470 and 660 nm corresponds
to the intramolecular charge transfer (ICT)33,34 from the donor

3098 J. Phys. Chem. B, Vol. 114, No. 9, 2010
Kanimozhi et al.
1
Figure 1. 400 MHz H NMR spectra of the two polymers (left) PDPP-BBT and (right) TDPP-BBT.
Figure 2. TGA of the PDPP-BBT and TDP-BBT.
(thiophene or phenyl) to the central core acceptor unit.35,36 We
also found that the absorption peaks of TDPP-BBT are
bathochromically shifted. This can be attributed to enhanced
intramolecular charge transfer due to incorporation of the
thiophene unit as a strong donor. Both of the copolymers show
unusual features in the absorption spectrum upon thin film
formation. The absorption peaks correspond to π-π* transition
and ICT significantly red-shifted and show a broad extended
absorption edge toward the near-infrared region. This indicates
that these materials have strong electronic coupling and π-π
interactions in the solid state.37,38 The absorption spectrum of
both of the polymers covers the maximum photon flux in the
solar spectrum. PDPP-BBT and TDPP-BBT also exhibit an
intramolecular charge transfer (ICT) band at 539 nm (solution),
563 nm (film), 638 nm (solution), and 656 nm (film), respec-
tively. Both of the polymers show a broad absorption spectra
with an extended absorption edge in the solid state. The detailed
optical data of PDPP-BBT and TDPP-BBT are summarized in
Table 2. The optical band gap of these polymers has been
calculated from the absorption edge of the film using Egopt
)
1240/λonset, with λonset being the onset wavelength of the
materials.
The absorption band maximum of TDPP-BBT is more red-
shifted as compared to PDPP-BBT, which indicates that there
is an excited state delocalization in TDPP-BBT polymer relative
to PDPP-BBT and more exciton interaction between adjacent
π-conjugated chromophores in TDPP-BBT. The presence of a
strong TDPP donor lowers the band gap, resulting a substantial
red shift in the absorption maximum band.

Synthesis of DPP Containing Copolymers
J. Phys. Chem. B, Vol. 114, No. 9, 2010 3099
Figure 3. UV-vis absorption spectra of polymers (PDPP-BBT and TDPP-BBT) in chloroform and thin film on a quartz plate.
TABLE 2: Optical Properties of the Polymers
Band gaps estimated from cyclic voltammetry data are com-
λmax.abs (nm)
solution film
λonset (nm)
parable with the optical band gaps estimated from the optical
absorption data. Similar kinds of CV plots were reported in the
literature for DPP copolymers.24,29 TDPP-BBT exhibits a smaller
band gap (1.46 eV) compared to PDPP-BBT (1.73 eV). This
difference reflects the strong donor property of TDPP moieties,
which increases the HOMO level more than the LUMO level,
leading to a decrease in the band gap.
polymer
film
Eogpt a (eV)
PDPP-BBT
TDPP-BBT
539
638
563
656
728
866
1.70
1.43
a Band gap (Eogpt) measured at the onset of electronic absorption
of the polymer film (Eopt ) 1240/λonset eV).
Electrical and Photovoltaic Properties. The focus of the
paper is on the performance of the copolymers in BHJ solar
cells; therefore, we have measured the current-voltage (J-V)
characteristics of the devices having configuration ITO/PDPP-
BBT or TDPP-BBT/Au to estimate the hole mobility. The J-V
data were analyzed using nonlinear square fitting to the modified
Mott Gurney equation, described as39
Electrochemical Properties. Cyclic voltammetry is em-
ployed to calculate the oxidation and reduction potential of
polymers and the HOMO-LUMO energy levels of the poly-
mers. The cyclic voltammograms of TDPP-BBT and PDPP-
BBT are shown in Figure 4. Both of the polymers show
reversible oxidation (p-doping /rereduction) and irreversible
reduction (n-doping/reoxidation) waves. All of these electro-
chemical values are given in Table 3. Potentials have been
measured with respect to the Ag/Ag+ electrode. HOMO and
LUMO levels were calculated from the following equation
9
8
V2
J ) εε0µ exp
0.89ꢀ
√
V
(1)
L3
√
L
(
)
HOMO ) -e(Eox + 4.7)LUMO ) -e(Ered + 4.7). Electro-
ox
chemical band gaps have been calculated from EEglc ) e(E
where J is the current density, V is the applied voltage, L is the
thickness of the active layer, µ is the zero field mobility, ε is
the dielectric constant, ε0 is the permittivity of free space (8.85
× 10-12 F/M), and ꢀ is the field activation factor. From the
analysis of the J-V current in the dark and using eq 1, the zero
field hole mobilities of PDPP-BBT and TDPP-BBT were
determined to be 3.2 ((0.4) × 10-5 and 6.3 ((0.5) × 10-5
cm2/(V s), respectively. The hole mobilities in both of the
onset
red
- E ). PDPP-BBT has an onset reduction potential and
onset
oxidation potential at -0.96 and 0.77 V vs Ag/Ag+, respectively.
The calculated values are HOMO ) 5.47 eV and LUMO )
3.74 eV. TDPP-BBT has reduction and oxidation potentials at
-1.01 and 0.45 V, respectively. It has a HOMO level at 5.15
eV and a LUMO level at 3.69 eV. The electrochemical band
gap is 1.73 eV for PDPP-BBT and 1.46 eV for TDPP-BBT.
Figure 4. Cyclic voltammograms of the polymers PDPP-BBT and TDPP-BBT. The scan rate is 20 mV s-1. T ) 30 °C.

3100 J. Phys. Chem. B, Vol. 114, No. 9, 2010
Kanimozhi et al.
TABLE 3: Electrochemical Properties of the Polymersa
ox
red
polymer
(V)
E
E
HOMO LUMO
Eeglc
(eV) (eV)
Eogpt
onset
onset
(V)
(eV)
(eV)
(eV)
PDPP-BBT
TDPP-BBT 0.45
0.77
-0.96
-1.01
-5.47
-5.15
-3.74
-3.69
1.73 1.70
1.46 1.43
a The HOMO-LUMO gap was calculated according to the
equations -ELUMO ) Eonset(red) + 4.7 eV and -EHOMO ) Eonset(ox)
+
4.7 eV, where Eonset(ox) and Eonset(red) are the onset potentials for the
oxidation and reduction processes of polymer thin films vs the Ag/
Ag+ electrode.
polymers are comparatively higher than other copolymers
reported in the literature.40,41
We have investigated both copolymers PDPP-BBT and
TDPP-BBT for their utility as light absorbing and electron
donating materials with the PCBM as an electron acceptor for
BHJ photovoltaic devices. The J-V characteristics of the devices
in the dark and under AM 1.5 irradiation (100 mW/cm2) are
shown in Figure 6. The optimized photovoltaic parameters for
each device are compiled in Table 4. The open circuit voltage
of the photovoltaic device based on TDPP-BBT is slightly lower
than the device based on PDPP-BBT. This difference of Voc
could result from the upward shift of the HOMO energy level
of TDPP-BBT because the Voc is related to the energy difference
between the LUMO of the acceptor (PCBM) and the HOMO
of the donor (the conjugated polymer).41,42 The energy offset
between the HOMO of PDPP-BBT (-5.47 eV) or TDPP-BBT
(-5.15 eV) and the LUMO of PCBM (3.9 eV) is estimated to
be 1.57 and 1.25 eV for PDPP-BBT:PCBM and TDPP-BBT:
PCBM active layers, respectively, which are the theoretical limit
for Voc for devices using a blend layer. Similar results have been
reported for P3HT/fullerene solar cells.11,15,43 The experimentally
measured lower value of Voc as compared to the theoretical value
may be attributed to charge carrier losses at the electrodes,44
energy level alignment of the polymer at the polymer/electrode
interface,45 and lowered effective band gap of the blends due
to the formation of the charge transfer complexes.46 Since the
HOMO levels of both polymers are not aligned to the work
function of ITO, it causes a band bending at the interface
between ITO and polymer and results in a voltage loss that is
responsible for the lower value of Voc as expected theoretically.
The short circuit photocurrent and overall power conversion
Figure 6. (a) Current-voltage (J-V) charactersitics of the ITO/PDPP-
BBT:PCBM/Al device in the dark, under light. (b) Current-voltage
(J-V) characteristics of the ITO/TDPP-BBT:PCBM/Al device in the
dark, under light.
TABLE 4: Photovoltaic Parameters of the Devices Based on
Different Active Layers Sandwiched between ITO and Al
Electrodes
short circuit open circuit fill
power
current (Jsc) voltage (Voc) factor
conversion
active layer
(mA/cm2)
1.77
(V)
(FF) efficiency (η) (%)
PDPP-BBT:PCBM
(as cast)
PDPP-BBT:PCBM
(annealed)
TDPP-BBT:PCBM
(as cast)
TDPP-BBT:PCBM
(annealed)
0.84
0.46
0.52
0.53
0.56
0.68
1.51
1.57
2.78
3.73
3.8
0.78
0.78
0.74
6.72
efficiency for the device based on the TDPP-BBT:PCBM blend
is higher than that for PDPP-BBT:PCBM under similar condi-
tions. This can be attributed to the larger difference in the
LUMO of the donor and acceptor in TDPP-BBT:PCBM than
PDPP-BBT:PCBM, leading to more efficient photoinduced
charge transfer in the device based on the TDPP-BBT:PCBM
blend.
The devices were also tested for their incident photon to
current efficiency (IPCE) and shown in Figure 7. Both of the
Figure 5. Energy band diagram of ITO/copolymer: PCBM/Al
electrode.

Synthesis of DPP Containing Copolymers
J. Phys. Chem. B, Vol. 114, No. 9, 2010 3101
Figure 7. IPCE spectra of the ITO/PDPP-BBT:PCBM/Al and ITO/
TDPP-BBT:PCBM/Al devices using as cast and annealed films.
devices show the IPCE peak, which corresponds to the peak in
the absorption spectra of the polymers. The similarity of the
absorption spectrum with IPCE response demonstrates that the
excitons produced by the absorption in the polymers are
dissociated into free charge carriers at the interfaces between
PDPP-BBT or TDPP-BBT and PCBM in the active layer and
subsequently collected at the electrodes.
We have also investigated the effect of thermal annealing on
the photovoltaic response of the BHJ devices. The J-V
characteristics under illumination and IPCE spectra of the
devices are shown in Figures 6 and 7, respectively. The overall
PCE for the devices based on thermally annealed PDPP-BBT:
PCBM and TDPP-BBT:PCBM are 1.51 and 2.78%, respec-
tively. The increase in the IPCE and short circuit current in the
devices based on annealed blends was attributed to the increase
in the crystallinity of the blend and hole mobility upon thermal
annealing and discussed in the following part of the paper.
After the photoinduced electron transfer at the donor/acceptor
interfaces present in the bulk of the photoactive layer in BHJ
photovoltaic devices and subsequent dissociation, the electrons
are localized in the PCBM phase and holes remain in the
copolymer phase, i.e., PDPP-BBT or TDPP-BBT in the present
case. Subsequently, the free electrons and holes must be
transported via percolated PCBM and copolymer pathways
toward Al and ITO electrodes, respectively, to produce the
photocurrent. Therefore, the electron transport in PCBM and
hole transport in copolymer are crucial for understanding the
photogeneration process in the BHJ device. We have also
investigated the effect of thermal annealing on the performance
of the photovoltaic active layer in terms of charge carrier
mobility as reported earlier in the literature for P3HT:PCBM
blends.47 To measure the space charge limited current (SCLC)
of only one type of the charge carrier in the blend, the other
one must be suppressed by a large injection barrier, resulting
in an electron or hole only device. We have measured the dark
current density-voltage curves of TDPP-BBT:PCBM blends
for hole only and electron only devices for both devices using
annealed and as cast blend thin films. To fabricate the hole only
device, gold (Au) was evaporated as the top electrode having a
structure of ITO/TDPP-BBT:PCBM/Au. The work function of
ITO (-4.9 eV) is very close to the HOMO level of TDPP-
BBT, forming Ohmic contact for hole injection, whereas the
Au strongly suppress the electron injection into the LUMO of
PCBM owing to the large mismatch between its work function
Figure 8. (a) Current-voltage characteristics of devices based on
TDPP-BBT:PCBM for a hole only device using as cast and annealed
blends. (b) Current-voltage characteristics of devices based on TDPP-
BBT:PCBM for an electron only device using as cast and annealed
blends.
(-5.0 eV) and the LUMO of PCBM (-4.0 eV). A schematic
diagram of the hole only device is shown in the inset of Figure
8a. To suppress the hole injection into TDPP-BBT, the bottom
electrode must have a low work function. Therefore, we have
fabricated an electron only device by sandwiching the TDPP-
BBT:PCBM blend between two Al electrodes, as shown in the
inset of Figure 8b. Similar results have been observed for the
devices based on PDPP-BBT:PCBM blends.
The charge carrier mobilities of the TDPP-BBT:PCBM blend
films were evaluated by space charge-limited current (SCLC)
measurements using eq 1. Figure 8 shows the relationship
between dark current density (J) and voltage (V) in the hole
only devices of TDPP-BBT:PCBM blends. The solid lines in
the figure represent the fitting curves using this eq 1. The
dielectric constant was assumed to be 3 in the calculation.48
The zero field mobilities of the hole for PDPP:PCBM and
TDPP-BBT:PCBM are quite comparable with the value mea-
sured for pristine polymers. However, the values of hole
mobilities are about 2.8 × 10-4 and 5.2 × 10-4 cm2/(V s) for
annealed PDPP-BBT:PCBM and TDPP-BBT:PCBM blends,
respectively. The electron mobility for as cast and annealed
TDPP-BBT:PCBM are in the range 10-3 cm2/(V s) similar to
that reported for P3HT:PCBM blends.47
The increase in hole mobility upon thermal annealing leads to
better balanced charge transport and reduces the charge recombina-
tion during their transportation toward the electrodes. The increase
in the photovoltaic performance with the annealed blends is likely

3102 J. Phys. Chem. B, Vol. 114, No. 9, 2010
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formation of crystalline domains that is ideal for both charge carrier
separation and their collection at the electrodes. It has been shown
in polymer-based BHJ solar cells that the degree of phase separation
and surface roughness in blended films significantly affect the
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Supporting Information Available: H NMR of compounds
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