Intramolecular Donor-Acceptor Regioregular Poly(hexylphenanthrenyl-imidazole thiophene) Exhibits Enhanced Hole Mobility for Heterojunction Solar Cell Applications

Intramolecular Donor–Acceptor Regioregular

Poly(hexylphenanthrenyl-imidazole thiophene) Exhibits

Enhanced Hole Mobility for Heterojunction Solar Cell

Applications

By

Yao-Te Chang, So-Lin Hsu, Ming-Hsin Su, and Kung-Hwa Wei*

Conjugated polymers possessing extended arrays of delocalized p

electrons are being investigated intensively for their potential use

in organic optoelectronic devices, with some studies focused on

solar-cell devices incorporating bulk heterojunctions using

conjugated polymers.

_{Polythiophene derivatives are at present}

among the most promising materials for solar-cell applications

because of their high light absorption and electronic conductivity.

For example, polymer solar cells containing blends of

poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C

-butyric acid methyl

ester (PCBM) have recently reached power conversion

efficien-cies of
4–5% under standard solar conditions (AM 1.5G,

100 mW cm

_{, 25 8C).}

_{If the power-conversion efficiency of}

these devices is to be improved further, the light absorption of the

active polymer must be improved, because P3HT absorbed only

20% of sunlight, that is, the band-gap of P3HT must be reduced

to meet the maximum photon flux of sunlight. Research into

conjugated polymers containing electron donor–acceptor (D–A)

pairs in the polymeric main chain has recently become quite

active

because such materials exhibit narrow band-gaps.

Alternatively, the introduction of an electron-acceptor unit—

usually a conjugated species that can absorb a different

wavelength of sunlight—onto the side chain of a conjugated

polymer can increase the breadth of wavelengths of light

absorbed, and can also lower the band-gap to some extent.

Additionally, the generated excitons can be readily dissociated

into electrons and holes in this type of conjugated polymer,

because of the internal field produced by the dipole moment built

on its D–A molecular structure and subsequent charge transfer to

nearby n-type nanoparticles (for example, PCBM). Therefore,

conjugated polymers that contain side-chain-tethered conjugated

acceptor moieties not only absorb light more effectively (multiple

absorption) but also exhibit enhanced charge-transfer ability—

two desirable properties for photovoltaics applications.

In a

heterojunction polymer solar cell, however, the photocurrent

depends not only on the rate of photogeneration of free electrons

and holes but also on the transport properties of the electrons and

holes in the acceptor and donor, respectively. In fact, the overall

performance of bulk-heterojunction solar cells is directly limited

by the ambipolar carrier transport.

In the P3HT/PCBM

system, the slower rate of hole transport governs the

recombina-tion process;

_{increasing the carrier mobilities results in both}

increased extraction of the charge carriers and increased

bimolecular recombination.

_{Therefore, the incorporation of}

electron-withdrawing moieties as side chains that are conjugated

with the polymeric main chains should also alleviate the

recombination problem, because such a molecular architecture

has the advantage in a heterojunction device of allowing charge

separation through sequential transfer of electrons from the main

chains to the side chains and then to PCBM. Hence, in this

present study, we synthesized a new kind of intramolecular D–A

thiophene-type

homopolymer

presenting

phenanthrenyl-imidazole moieties. Scheme 1 displays our synthetic approach

toward the planar phenanthrenyl-imidazole moiety-tethered

thiophene monomer and its polymerization. We expected that

the presence of the hexylphenanthrenyl-imidazole moieties

conjugated to the thiophene units would reduce the band-gap

of the polythiophene, and that the hexyl substituents would

improve the solubility of the polymer. The extent of reduction of

the band-gap of our synthesized polymer would, however, depend

on the effective conjugation length of the system, which is

sometimes reduced by steric hindrance.

We polymerized the

2-(2,5-dibromothiophen-3-yl)-6,9-dihexyl-1-(4-hexylphenyl)-1H-phenanthro-[9,10-d]-imidazole

(HPIT)

monomer

using

a

Grignard metathesis approach. The presence of the bulky

hexylphenanthrenyl-imidazole unit appended to the thiophene

monomer led to very high selectivity during the Grignard

reaction, resulting in highly regioregular

poly(hexylphenanthre-nyl-imidazole thiophene) (PHPIT). The number molecular

weight (M

) of PHPIT was 15.3 kg mol

, and the polydispersity

index (PDI) was 1.35, indicating that PHPIT possessed
20

repeating units. The 5% thermal degradation temperature of this

polymer was 355 8C.

Figure 1 displays the UV–vis spectra of PHPIT/PCBM (1:1, w/w)

and the P3HT/PCBM (1:1, w/w) solid film obtained after

annealing at 120 8C for 30 min, as well as cyclic voltammogram

(CV) band-gap data for PHPIT, PEDOT, PCBM, Al, and ITO. The

peak at 305 nm was caused by the presence of conjugated

phenanthrenyl-imidazole moieties that were not fully coplanar

with the polythiophene chain due to steric hindrance. The

maximum absorption (l

) at
12 nm for the P3HT/PCBM thin

film resulted from p–p* transitions. The annealed PHPIT/PCBM

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[*] Prof. K. H. Wei, Y. T. Chang, S. L. Hsu, M. H. Su Department of Materials Science and Engineering National Chiao Tung University

1001 Ta Hsueh Road

Hsinchu, 30049 Taiwan (R.O.C.) E-mail: khwei@mail.nctu.edu.tw

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film exhibits a red-shift p–p* transition peak at 552 nm and two

additional absorption peaks at 596 and 641 nm, indicating that a

phase-separated structure developed after annealing at 120 8C.

The area under the spectrum in the visible absorption range

(400–750 nm) for PHPIT/PCBM after annealing at 120 8C for

30 min was 11% higher than that of the annealed P3HT/PCBM.

The optical band-gap of PHPIT was
1.85 eV, which is close to the

cyclic voltammogram (CV) band-gap (1.80 eV), with the highest

occupied molecular orbital (HOMO) at
4.70 eV and the lowest

unoccupied molecular orbital (LUMO) at
2.90 eV. We detected

no photoluminescence from the PHPIT film, suggesting that

charge transfer from the photoexcited polythiophene backbone to

the electron-withdrawing phenanthrenyl-imidazole side chains

was sufficiently rapid to compete with radiative recombination of

the excitons.

Figure 2 displays the photocurrents of diodes with the

structure ITO/PEDOT:PSS/polymer: PCBM (1:1, w/w)/Ca/Al

that were illuminated at 100 mW cm

under AM 1.5G and their

dark currents. Table 1 lists the short-circuit current densities (J

),

open-circuit voltages, and power conversion efficiencies of these

heterojunction polymer solar cells. The value of J

for the device

incorporating

the

PHPIT/PCBM

blend

improved

to

11.3 mA cm

from 8.3 mA cm

after the annealing time at

120 8C was increased from 20 to 30 min, probably because of

improved ordering of the blend structure. However, the value of

J

of the device decreased to 7.8 mA cm

when the blend

underwent thermal treatment at 120 8C for 45 min, probably

because of decomposition of the polymer structure. Figure S2

(Supporting Information (SI)) presents the device characteristics

of the blends that we subjected to annealing at temperatures of

130 and 150 8C. Among all of the systems we studied, the PHPIT/

PCBM blend thermally treated at 120 8C for 30 min exhibited the

highest power-conversion efficiency. From atomic force

micro-scopy images (Fig. S3, SI), we found that the root-mean-square

roughness of the PHPIT/PCBM film (2.27 nm) annealed at

120 8C for 30 min was larger than those (1.94 and 1.76 nm) of the

films annealed at 120 8C for 20 and 45 min. Hence, we suspect

that the rough surface effectively reduced the charge-transport

distance while providing a nanoscale texture that further

enhanced internal light absorption.

_The

power-conversion efficiency of the device incorporating PHPIT/PCBM

Scheme 1. Synthesis of the monomer and polymer; NBS: N-bromosuccinimide; THF: tetrahydrofuran; dppp: 1,3-bis(diphenylphosphino) propane.

Figure 1. a) UV–vis spectra of P3HT/PCBM as cast and PHPIT/PCBM annealed at 120 8C in the solid state, and the solar spectrum. b) CV band-gap data for PHPIT, PEDOT, PCBM, Al, and ITO.

Figure 2. Current–voltage characteristics of illuminated (AM 1.5G, 100 mW cm2) polymer/PCBM (1:1, w/w) solar cells.

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increased dramatically to 4.1% from 3.1% when the annealed

time at 120 8C was increased from 30 to 20 min, but it decreased

to 2.7% when annealed for 45 min, presumably because of

decomposition of the polymer.

We performed a control

experiment in which we subjected commercially available

high-molecular-weight P3HT (M

¼
33 000, about 200

repeat-ing units) to the same annealrepeat-ing conditions as those experienced

by PHPIT. The power-conversion efficiency of the device

incorporating commercially available P3HT and PCBM was

2.9% (Fig. 2). Thus, although thermal treatment at 120 8C for

30 min is optimal for PHPIT/PCBM, that is not necessarily the

case for commercial P3HT/PCBM.

We investigated the photophysics of the devices incorporating

the synthesized copolymers by determining their external

quantum efficiencies (EQEs). Figure 3 displays the EQEs of

the PHPIT/PCBM devices in which the blends were annealed at

120 8C for various annealing times. At wavelengths from 400 to

650 nm, the absolute EQEs of the device prepared from PHPIT/

PCBM annealed at 120 8C for 30 min were
20% higher than

those of the corresponding blends annealed for 20 and 45 min.

For example, the EQE at an incident wavelength of 400 nm for the

device incorporating PHPIT/PCBM annealed at 120 8C for 30 min

improved from 53% to 79% for the corresponding device

annealed for 20 min—an increase of 50%. The maximum EQEs

at 460 nm for the devices containing PHPIT annealed at 120 8C

for 30 and 20 min device were 80 and 52%, respectively—a 53%

increase for the former over the latter; at a much longer

wavelength of 620 nm, the corresponding values were 48 and

30%, respectively—almost a 60% increase.

Figure 4 displays the dark J–V curves for electron- and

hole-dominated carrier devices. The electron and hole mobilities

were determined by fitting the dark J–V curves into the

space-charge-limited current (SCLC) model for electron- and

hole-dominated carrier devices based on the equation

J ¼

9"o"r

m

V

8L

(1)

where e

is the permittivity of free space, e

is the dielectric

constant of the polymer, m

is the hole (electron) mobility, V is

the voltage drop across the device, and L is the polymer

Table 1. Photovoltaic properties of polymer solar cells annealed at 120 8C for various lengths of time and of P3HT/PCBM annealed at 120 8C for 30 min.

Blend annealing at 120 8C Voc[V] Jsc[mA cm2] Fill Factor [%] PCE [%]

PHPIT/PCBM (1:1, w/w) for 20 min 0.6 8.3 62 3.1

PHPIT/PCBM (1:1, w/w) for 30 min 0.61 11.3 60 4.1

PHPIT/PCBM (1:1, w/w) for 45 min 0.61 7.8 58 2.7

P3HT/PCBM (1:1, w/w) for 30 min 0.58 7.6 66 2.9

Figure 3. EQEs of devices containing polythiophene side-chain-tethered hexylphenanthrenyl-imidazole/PCBM blends (1:1, w/w) annealed at 120 8C for various times.

Figure 4. DarkJ–V curves for a) electron- and b) hole-dominated carrier devices incorporating PHPIT/PCBM (1:1, w/w) annealed at 120 8C for various times.

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thickness.

_{Table 2 lists the hole mobilities, electron mobilities,}

and the ratio of hole and electron mobilities that are

determined from Figure 4 and Equation (1). We obtained hole

mobilities for the PHPIT/PCBM system (from 6.5  10

_to

1.9  10

cm

V

s

) that were
three to ten times greater

than that of the P3HT/PCBM system (1.8  10

cm

V

s

)

when both blends experienced the same thermal treatment. The

device containing the PHPIT/PCBM blend annealed at 120 8C for

30 min exhibited the highest mobility, indicating that

mor-e-ordered PHPIT/PCBM films facilitate hole transport. Thus, the

lowest electron-to-hole mobility ratio for the PHPIT/PCBM blend

results in the highest photocurrent.

In summary, we have synthesized PHPIT, a new kind of

intramolecular D–A side-chain-tethered

hexylphenanthreny-l-imidazole polythiophene. The visible-light absorption of the

PHPIT/PCBM blend is enhanced by the presence of the

electron-withdrawing hexylphenanthrenyl-imidazole. The EQE

of the device was maximized when the PHPIT/PCBM blend

experienced annealing at 120 8C for 30 min. The more-balanced

electron and hole mobilities and the enhanced visible- and

internal-light absorptions in the devices consisting of annealed

PHPIT/PCBM blends both contributed to a much higher

short-circuit current density, which in turn led to a

power-conversion efficiency as high as 4.1%, despite the fact that PHPIT

is only comprised of
20 repeating units.

Experimental

Materials: Chemicals were purchased from Aldrich, TCI, or Lancaster. PCBM was purchased from Nano-C.

Preparation of Monomers: Scheme 1 illustrates the synthetic route followed for the preparation of the monomer HPIT. 6,9-Dihexyl-1-(4-hexylphenyl)-3a,11b-dihydro-2-(thiophen-3-yl)-1H-phenanthro[9,10-d] imidazole (2) was isolated in 80% yield from the reaction of compound (1) with hexylmagnesium bromide and Ni(dppp)Cl2under reflux. HPIT was

isolated in 93% yield from the reaction between 2 and NBS [15]. Detailed synthetic procedures and characterization data are provided in the Supporting Information.

Preparation of Polythiophene Derivatives [26]: The Grignard metathesis polymerization of 2-(2,5-dibromothiophen-3-yl)-1-phenyl-1H-phenanthro [9,10-d]-imidazole is illustrated in Scheme 1. Detailed synthetic procedures and characterization data are provided in the Supporting Information.

Characterization: 1_{H and} 13_{C NMR spectra were recorded using a}

Varian Unity-300 NMR spectrometer. Infrared spectra were recorded from KBr disks using a Nicolet Prote´ge´-460 FTIR spectrophotometer. Elemental analyses (EA) of the polymers were performed using a Heraeus CHN-OS Rapid instrument. Thermogravimetric analyses of the polythiophene derivatives were performed using a DuPont TGA 2950 instrument operated at a heating rate of 10 8C min1 under a nitrogen purge. Differential scanning calorimetry (DSC) was performed using a DuPont DSC 2010

instrument operated at a heating rate of 10 8C min1 _{under a nitrogen}

purge. Samples were heated from 30 to 200 8C, cooled to 20 8C, and then heated again from 30 to 200 8C; the glass-transition temperatures (Tg) were

determined from the second heating scans. The redox behavior of each polymer was investigated through cyclic voltammetry using a BAS 100 electrochemical analyzer operated at a potential scan rate of 40 mV s1_and

an electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) in acetonitrile. In each case, a glassy disk carbon electrode

coated with a thin layer of the polymer was used as the working electrode, a platinum wire was used as the counter electrode, and a silver wire was used as the quasi-reference electrode. All potentials quoted herein are referenced to the Ag wire as the quasi-reference electrode; the electrochemical potential of Ag is 0.02 V versus SCE. The HOMO and LUMO energy levels were determined using the equations EHOMO¼ Eox 4.4 eV and ELUMO¼ Ered 4.4 eV, where Eoxand Ered

are the onset potentials of the oxidation and reduction peaks (vs. saturated calomel electrode (SCE)), respectively, and the value of 4.4 eV relates the SCE reference to a vacuum[15a,19a]. UV–vis spectra were measured using an HP 8453 diode array spectrophotometer. The molecular weights of the polythiophene derivatives were measured through gel-permeation chro-matography (GPC) using a Waters chrochro-matography unit interfaced to a Waters 2414 differential refractometer. Three 5 mm Waters styragel columns were connected in series in decreasing order of pore size (104, 103_{, and 10}2_{A˚); tetrahydrofuran (THF) was the eluent, and standard}

polystyrene samples were used for calibration. AFM samples were prepared by spin-coating solutions of polymer/PCBM blends in dichloro-benzene onto ITO glass substrates, followed by annealing in an oven at 120 8C for 20, 30, or 45 min.

Device Fabrication: The current density–voltage (J–V) characteristics of the polymers were measured using devices with a sandwich structure (ITO/ PEDOT:PSS/polymer:PCBM (1:1, w/w)/Ca/Al). The ITO-coated glass substrate was precleaned and treated with oxygen plasma prior to use. The polymer/PCBM layer was spin-coated at 700 rpm from a dichlor-obenzene solution (20 mg mL1_{). Dichlorobenzene was a better solvent for}

these polymers than were toluene, chloroform, and THF. The thickness of the polymer/PCBM layer was
100 nm. The active layers of our devices were thermally annealed at 120 8C for 30 min prior to electrode deposition. Using a base pressure below 1  106torr (1 torr ¼ 133.32 Pa), a layer of Ca (30 nm) was vacuum-deposited as the cathode and then a thick layer of Al (100 nm) was deposited as the protecting layer; the effective area of one cell was 0.04 cm2. Testing of the devices was performed under simulated AM 1.5G irradiation (100 mW cm2) using a xenon lamp-based Newport 66902 150W solar simulator. A xenon lamp equipped with an AM1.5 filter was used as the white-light source; the optical power at the sample was 100 mW cm2_{, detected using an OPHIR thermopile 71964. The J–V}

characteristics were measured using a Keithley 236 electrometer. The spectrum of the solar simulator had a mismatch of less than 25%; it was calibrated using a PV-measurement (PVM-154) mono-Si solar cell (NREL calibrated), and a Si photodiode (Hamamatsu S1133) was employed to check the uniformity of the exposed area. AM 1.5G (ASTM G173) [27] light intensity was calibrated through thermopile and PV-measurements. The mismatch factor (M) of 1.34 was obtained by taking the PVM-154 as the reference cell. The PVM-154 combined with a KG-5 filter (350–700 nm passed, Newport) was used to simulate a reference solar cell exhibiting spectral responsivity from 350 to 700 nm. Reported efficiencies are the averages obtained from four devices prepared on each substrate. The external quantum efficiency (EQE) was measured using a Keithley 236 Table 2. Hole mobilities, electron mobilities, and hole-to-electron-mobility ratios of P3HT/PCBM annealed at 120 8C for 30 min and PHPIT/PCBM annealed at 120 8C for various lengths of time.

Blend annealing at 120 8C Hole mobility [mh, cm2Vs1] Electron mobility [me, cm2Vs1] [me/mh]

P3HT/PCBM (1:1, w/w) for 30 min 1.8  0.1  106 _{1.8  0.1  10}5 ₁₀

PHPIT/PCBM (1:1, w/w) for 20 min 9.0  0.3  106 _{2.6  0.1  10}5 _2.9

PHPIT/PCBM (1:1, w/w) for 30 min 1.9  0.1  105 _{4.2  0.1  10}5 _2.2

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electrometer coupled with an Oriel Cornerstone 130 monochromator. The light intensity at each wavelength was calibrated using an OPHIR 71580 diode. Hole-only devices, used to investigate the hole transport in polymer/ PCBM, were fabricated following the same procedure presented above, except that the top electrode was replaced with gold (Au, 100 nm). Electron-only devices were fabricated by spin-coating the active layer on top of glass/Ag (100 nm) followed by evaporation of the Al (100 nm) top electrode. TheJ–V curve was measured using a Keithley 236 electrometer under inert condition.

Acknowledgements

We are grateful for the financial support provided by the National Science Council through Project NSC 96-2120-M-009-005. We thank H.-S. Wang for assisting in the synthesis of the polymers and C.-C. Chen for assisting in the study of devices of this paper. Supporting Information is available online from Wiley InterScience or from the author.

Received: August 15, 2008 Revised: January 16, 2009 Published online: March 2, 2009

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