www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

A Hyperbranched Conjugated Polymer as the Cathode Interlayer for High-Performance Polymer Solar Cells

Menglan Lv , Shusheng Li , Jacek J. Jasieniak , Jianhui Hou , Jin Zhu ,* Zhan’ao Tan ,* Scott E. Watkins , Yongfang Li ,* and Xiwen Chen*

N

Organic photovoltaics (OPVs) have drawn great attention owing to their key advantages of low-cost, high throughput fabrica-tion, light weight physical characteristics, and the lucrative pos-sibility of integration directly into fl exible devices. [ 1 ] The power conversion effi ciency (PCE) of OPVs has rapidly increased in recent years and already reached effi ciencies of up to 9.2% with good ambient stability for single cell devices. [ 2 ] This has been based on high effi cient donor and acceptor blend with controlled morphology and improved device structures. [ 2 ] One key advance for achieving such effi ciency values was the use of an alcohol soluble conjugated polymer cathode interlayer. This kind of conjugated polymer cathode interlayer was fi rst used as an electron injection layer in polymer light-emitting diodes [ 3 ] and then applied to organic fi eld effect transistors [ 4 ] and recently in OPVs. [ 5–7 ] In addition, other organic electron collection materials have also been reported, for example, polyfl uorenes grafted with K + intercalated crown ethers (PFCn6:K + ), [ 8 ] fullerene derivatives, [ 9 ] and insulating polymers with amine group such as polyethylenimine (PEIE). [ 7d ] , [ 10 ] Polar solvent treatment can also improve the performance of the organic electronic devices. [ 11 ] It is believed that the molecular and interfacial dipoles or n-doped conducting nature are the

© 2013 WILEY-VCH Verlag Gm

M. L. Lv, Prof. J. Zhu Chengdu Institute of Organic Chemistry Chinese Academy of Sciences Chengdu , 610041 , China E-mail: jinzhu@cioc.ac.cn M. L. Lv, Dr. J. J. Jasieniak, Dr. S. E. Watkins, Dr. X. W. Chen CSIRO Materials Science and Engineering Clayton , VIC 3168 , AustraliaE-mail: xwchen5702@hotmail.com S. S. Li, Prof. Z. A. TanState Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources The New and Renewable Energy of Beijing Key Laboratory North China Electric Power University Beijing , 102206 , ChinaE-mail: tanzhanao@ncepu.edu.cn S. S. Li, Prof. J. H. Hou, Prof. Y. F. LiBeijing National Laboratory for Molecular Sciences Institute of ChemistryChinese Academy of Sciences Beijing , 100190 , ChinaE-mail: liyf@iccas.ac.cn M. L. LvUniversity of Chinese Academy of Sciences Beijing , 100049 , China .

DOI: 10.1002/adma.201302726

Adv. Mater. 2013, 25, 6889–6894

main reasons for the working mechanisms. However, a deeper understanding of how the interface modifi cation works is still needed. Previously, we reported three-dimensional alcohol-soluble conjugated polymers with pendant ammonium salts or neutral amines as cathode interlayers and showed that they work similarly or better than the linear analogues for OPVs. [ 12 ] However, they have not been applied to high performance OPVs, nor is it clear how they function.

Due to the easy synthetic accessibility -typical of one–pot syntheses, hyperbranched polymers have been extensively studied. [ 13 ] Hyperbranched polymers are highly branched mac-romolecules with three-dimensional dendritic architecture. They are imperfect architectural relatives of dendrimers but they are very similar to dendrimers for their properties such as excellent solubility, prominent processability, highly branched molecular structure and controllable densities of reactive or non-reactive end-groups.

In this communication, we synthesize a novel hyperbranched alcohol soluble polymer HBPFN ( Figure 1 ) by a one-pot “A2+B3” Suzuki polymerization and use it as a cathode interlayer in OPVs with the active blend layer of a high effi ciency donor, poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-thylhexanoyl)-thieno[3,4-b]thio-phen-4,6-diyl]} (PBDTTT-C-T) [ 14 ] (Figure 1 ), and [6,6-phenyl C 71 -butyric acid methyl ester (PC 71 BM)] acceptor. We demonstrate that incorporating the HBPFN cathode interlayer between the active layer and Al electrode causes a dramatic enhancement in V oc , J sc and FF values, resulting in an overall PCE enhancement from 4.8% to 7.55% (a 56% relative enhancement). The PCE of 7.55% for the OPV with HBPFN interlayer is ∼ 10% higher than that of the device with a typical linear interlayer PFN (6.88%). The PCE can be further enhanced to 7.70% by methanol treatment before incorporation of the HBPFN interlayer. The improved hole mobility, reduced dark current, good wetability of the interlayer on the active blend layer and the likely forma-tion of an interfacial dipole with the cathode contribute to the enhanced performance.

The chemical and device structures of the studied inter-layers and OPVs, respectively, are presented in Figure 1 . The synthetic procedure for HBPFN is shown in Scheme S1 in the supporting information (SI) and the detailed synthesis processes are described in SI as well. It was dissolved in methanol in the presence of small amount of acetic acid, but was insoluble in other common organic solvents such as chloroform, THF and toluene. The 1 H NMR shows chemical shifts at 7.64–7.78 ppm from protons on the fl uorene units, 7.20–7.63 ppm from the triphenylamine unit, 2.85 ppm from the methylene adjacent to the nitrogen atom, 2.55–2.66 ppm from the protons on the sp 3

bH & Co. KGaA, Weinheim 6889wileyonlinelibrary.com

689

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Figure 1. Molecule structures of the compounds studied in this work and schematic of the conventional organic solar cells.

S

S

S

S

O

S

S

R

R

n

R=2-Ethylhexyl

NN

**

n

C8H17C8H17

PBDTTT-C-TPFN

N

N

PC71BM

HBPFN

O

O

N

NN

N

N

NN N

N N

N

N

NN

N

N

N

N

N

N

N

N

N N

carbon connected to the fl uorene unit and 1.08–1.26 ppm from protons at the middle methylene and methyl groups. The ratio of integration is 3:4:2:2:8, which matches with the hyperbranched structure. The molecular weight of HBPFN was not available due to the solubility issue. However, we designed and synthe-sized a model hyperbranched polymer HBPFO (Scheme S2), in which the side amino- groups on HBPFN are replaced with octyl side chain. GPC analysis showed its weight-average molecular weight ( M w ) and polydispersity are 21,900 and 1.44, respectively. Their bandgaps and energy levels including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) were estimated and are listed in the SI. As a comparison, two linear polymers PFN and polydioctylfl uorene (PFO) were also studied. The small variations of the HOMO and LUMO levels between the amino-containing materials and the model polymers indicate that the side chain amino- groups do not change the electronic structure signifi cantly.

The hyperbranched polymer HBPFN and the linear polymer PFN were applied as cathode interfacial layers in OPVs. The optimized concentrations for spin casting HBPFN and PFN cathode interlayer solutions were found to be at 0.6 mg/mL and 0.4 mg/mL respectively, and the optimized device perfor-mance is shown along with other control devices for the cur-rent density-voltage ( J – V ) characteristics of the devices under illumination of AM 1.5G, 1000 W/m 2 ( Figure 2 a) and at dark (Figure 2 b), and for the spectral response profi les (Figure 2 c). The device structure was ITO substrate/PEDOT: PSS (30 nm)/PBDTTT-C-T: PC 71 BM (100 nm)/with or without interlayer (5 nm)/Al (100 nm). The extracted device performance metrics, including series resistance ( R s ) and shunt resistance ( R sh ), are summarized in Table 1 . From this table we can see that the PCE increases in the sequence according to the interface modifi ca-tions: methanol < PFN < HBPFN < methanol/HBPFN. We note

0 wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Wei

that all the interfacial treatments improved the effi ciency compared to the control device without interface modifi cation; and a com-bination of methanol treatment followed by HBPFN interlayer gave the highest PCE. All device parameters are improved signifi cantly from the control device to the best one: PCE from 4.83% to 7.70%, V oc from 0.69 to 0.78 V, J sc from 14.32 mA/ cm 2 to 17.15 mA/cm 2 , and FF from 48.8% to 57.5%. In addition, the series resistance is reduced from 19.86 Ω cm 2 to 1.84 Ω cm 2 and the shunt resistance is enhanced from 2.86 k Ω cm 2 to 10.25 k Ω cm 2 . The PCE of the device with Ca/Al cathode was 7.30% with V oc of 0.76 V, J sc of 15.96 mA/cm 2 and FF of 60.1%. [ 9c ] Collectively, these results show that the performance of OPVs with a methanol / HBPFN interlayer treatment is comparable to that with amine or ammonium iodide-modifi ed fullerene cathode interlayers and the traditional Ca/Al cathode. [ 9c ]

To understand the effect of the interfacial treatment from a topographic perspective, atomic force microscopy (AFM) was used (see Figure S5 and table S4 in the SI). The original PBDTTT-C-T: PC 71 BM fi lm exhibits

the surface root mean square roughness (RMS) of 2.08 nm. In the presence of HBPFN interlayer the RMS is very similar (RMS = 3.49 nm) both with and without methanol treatment. The RMS is slightly bigger than the pristine fi lm and also larger than that with methanol treatment and PFN interlayer, which are 2.38 and 3.04 nm, respectively.

Beside the morphological variation, we also investigated the wetting properties of the fi lms. Contact angle measurements were performed on different surfaces with drops of water, methanol, and interlayer solutions. The images of the drops and the contact angles are shown in Figure S6 and Table S5 in the SI. The contact angles of water on ITO/PEDOT: PSS/PBDTTT-C-T: PC 71 BM/Interlayer fi lms show that the active layer surface becomes less hydrophobic with the interfacial treatments than the pristine one. The water contact angle for the pristine fi lm is 100 o , and it was reduced to 95° after meth-anol treatment, 70° and 80° with PFN and HBPFN respec-tively. To see the wetting properties of the interlayers methanol solutions on the active layer, we measured the contact angles of the interlayer solutions on the active layer surface. HBPFN solution gave a contact angle of 10.4°, a little higher than PFN (8.7°) and methanol itself (8.2°). To simulate the condition for the best device, we also spin cast methanol on the active layer fi rst and then dropped the interlayer solutions on the meth-anol-modifi ed surface. The HBPFN solution gave lower contact angle of 7 o while PFN still remained at 8°. The better wetting of the HBPFN on the active layer after methanol treatment may be partially responsible for the improvement in the device per-formance. In general, the small contact angles of the interlayer methanol solutions on the active layer indicate the good wetting properties and the advantage of the dimethylamino-modifi ed interlayers for use in conventional solar cells.

nheim Adv. Mater. 2013, 25, 6889–6894

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 2. Device performance for various interfacial treatments as indi-cated: (a) The current density-voltage (J–V) characteristics under 1,000 W/m 2 AM 1.5G illumination and (b) under dark; (c) External quantum effi ciency (EQE) spectra.

-0.3 0.0 0.3 0.6 0.9

-20

-15

-10

-5

0

5

a

Cu

rren

t D

ensi

ty /

mA

cm-2

Voltage / V

MeOH/HBPFN/Al HBPFN/Al PFN/Al MeOH/Al Al

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.51E-4

1E-3

0.01

0.1

1

10

100

1000

MeOH/HBPFN/Al HBPFN/Al PFN/AlMeOH/Al

Al

b

Cu

rren

t d

ensi

ty /

mA

cm-2

Voltage / V

300 400 500 600 700 8000

10

20

30

40

50

60

70

c

EQ

E /

%

Wavelength / nm

MeOH/HBPFN/Al HBPFN/Al PFN/Al MeOH/Al Al

Table 1. Device performance under 1,000 W/m 2 AM 1.5G illumination with various interfacial treatments.

Interlayer V oc [V]

J sc [mA/cm 2 ]

FF [%]

PCE [%]

Average PCE [10 devices]

R s a) [ Ω cm 2 ]

R sh b) [k Ω cm 2 ]

None 0.69 14.32 48.8 4.83 4.70 19.86 2.86

MeOH 0.76 16.03 50.6 6.17 6.02 5.61 3.33

PFN 0.77 16.76 53.3 6.88 6.78 5.44 5.33

HBPFN 0.78 18.01 53.8 7.55 7.50 2.01 7.15

MeOH/ HBPFN 0.78 17.15 57.8 7.70 7.67 1.84 10.25

a) Series resistance R s and b) shunt resistance R sh were measured from slopes under dark currents at 1.5 V and at the voltage where the current is minimum, respectively.

Table 2. Hole and electron mobilities of the devices with various inter-facial treatment.

Interlayer Electron-only devices a) Hole-only devices b)

Electron mobility [cm 2 V −1 s −1 ] Hole mobility [cm 2 V −1 s −1 ]

None 7.2 × 10 −4 8.3 × 10 −5

MeOH 8.9 × 10 −4 4.6 × 10 −4

PFN 9.2 × 10 −4 5.2 × 10 −4

HBPFN 8.9 × 10 −4 7.2 × 10 −4

a) Structure of electron-only device: ITO/TIPD (12 nm)/PBDTTT-C-T: PC 71 BM (100 nm)/with or without interlayer (5 nm)/Al (100 nm); b) Structure of hole-only device: ITO/PEDOT:PSS (38 nm)/PBDTTT-C-T: PC71BM (100 nm)/with or without interlayer (5 nm)/MoO x (3 nm)/Au (40 nm).

With the interface modifi cations, the J sc and external quantum effi ciency (EQE) of the devices were changed (Figure 2 c). To explore their origins, the charge transport

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, 25, 6889–6894

mobilities were approximated using space charge limited cur-rent measurements of electron-only and hole-only devices. Their device structures were: ITO substrate/TIPD (12 nm)/PBDTTT-C-T:PC 71 BM (100 nm)/ with or without interlayer (5 nm) /Al (100 nm) and ITO substrate/PEDOT:PSS (38 nm)/PBDTTT-C-T:PC 71 BM (100 nm)/with or without interlayer (5 nm)/MoO x (3 nm)/Au (40 nm), respectively. Here TIPD represents a titanium chelate which was used for the electron collection layer. [ 15 ] J-V characteristics of single-carrier devices were measured (curves in fi gures S3 and S4 in the SI), and the mobilities were calculated by fi tting to the Mott-Gurney law, [ 16 ] and are listed in Table 2 . From this table, it is clear that the electron mobility remains similar (from 7.2 × 10 −4 to 9.2 × 10 −4 cm 2 /Vs) after interfacial modifi cation, but the hole mobility increases by almost one order of magnitude from 8.3 × 10 −5 cm 2 /Vs for pristine fi lm to 7.2 × 10 −4 cm 2 /Vs for the fi lm with HBPFN, the highest among the various surface treatments. The more balanced charge transport in the device is believed to contribute to the enhanced J sc and FF observed in the solar cells by reducing the build-up of space charges and charge recombination. Reduced surface traps, optical spacer effect, and increased built-in potential might also be the possible reasons.

Finally, scanning Kelvin probe microscopy (SKPM) was used to explore if the interfacial dipole exists with various interlayer treatments. SKPM provides the contact poten-tial difference (CPD) between the probe tip and the surface, which for a conductive fi lm is related to a relative difference of the work functions. This technique has also been applied

6891wileyonlinelibrary.commbH & Co. KGaA, Weinheim

6892

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON Table 3. Work functions of polymers on various substrates measured by

Scanning Kelvin Probe Microscopy in air. The work function of ITO was set as 4.70 eV, and errors of measured work function are ±0.025 eV.

Top layer Substrate none [eV]

MeOH [eV]

PFN [eV]

HBPFN [eV]

PStN [eV]

ITO/PEDOT: PSS/Active

layer

4.50 4.41 4.19 4.45 4.18

ITO 4.70 4.70 4.22 4.47 4.16

Evaporated Au a) 4.60 — 4.16 4.07 3.91

a) air exposed.

for the study of conjugated polymer fi lms and photovoltaic blend surfaces, although the interpretation is at times not as straightforward. [ 6a ] , [ 10a ] , [ 11b,c ] Our SKPM results are summarized in Table 3 . Each value was calculated relative to ITO, which was set at a nominal value of 4.70 eV. [ 17 ] At this reference value, evaporated gold and PEDOT-PSS fi lms possessed work func-tions of 4.60 ± 0.025 eV [ 18 ] and 5.25 ± 0.025 eV respectively. The consistency of these values with previous literature encouraged us to study the active layer surfaces treated by methanol or inter-layers PFN and HBPFN. Methanol treatment reduced the work function by 0.09 eV, which is similar to literature reports and implies modifi cation of the interface composition and/or defect structure; [ 11 ] PFN reduced the work function by 0.31 eV, also in good agreement with literature report; [ 6a ] however, HBPFN reduced the work function only by 0.05 eV. Considering that the work function shift of PFN has been described to arise from a dipole, [ 6a ] it seems surprising that the HBPFN exhibits such a small work function reduction despite also exhibiting a high V oc within solar cells.

To further investigate this apparent discrepancy, we meas-ured the work functions of the interlayers on ITO and on evap-

Figure 3. Schematic energy levels of the devices with various interfacial treatments: (a) none (with surface traps), (b) methanol treatment (with surface traps removed), (c) PFN, (d) HBPFN.

orated gold. The work functions of PFN on both ITO and evaporated Au were reduced to ∼ 4.2 eV, indicating interfacial dipole forma-tion. Similarly, the work functions of HBPFN on ITO and on evaporated Au were reduced to 4.47 eV and 4.07 eV, respectively. This indi-cates that interfacial dipole formed at these “metallic” interfaces do indeed occur.

It was reported that insulating amine-polymers can reduce the work functions of metal oxides and metals by the molec-ular dipole and interfacial dipole. [ 10 ] Those amine-containing polymers consist of pri-mary, secondary, tertiary amines or hydroxy groups. [ 10 ] To confi rm if tertiary amine only can work similarly, we made an insulating amorphous polymer, PStN, which has only dimethylamino- moiety on the side chain of polystyrene (Scheme S3). Indeed it reduced the work functions of ITO, Au and the active layer signifi cantly, comparable to the PFN with an ordered structure. [ 2 ]

Based on the SKMP results we summa-rize the energy level landscape of the various devices studied in Figure 3 . We propose the

wileyonlinelibrary.com © 2013 WILEY-VCH Verlag Gm

importance of two interfaces: one is between the interlayer and the active blend layer and the other between the interlayer and the cathode. While in reality both will be important, the interfacial modifi cation provided by PFN has clear evidence of the former. In contrast, this contribution to the operation of HBPFN layers is negligible. Since both amorphous PStN and ordered PFN form dipoles with the active layer, that no dipole formation is found on hyperbranched polymer HBPFN with the active layer could not be ascribed to its amorphous structure of the HBPFN. For the interlayer-cathode interface, we argue that the known complexation of Lewis bases to metal surface justi-fi es the formation of the interfacial dipole between the inter-layer and Al cathode. Notably, it was reported that molecular dipole and interfacial dipole contributed to the work function reduction of electrodes in the same order of magnitude with insulating amine-containing polymer surface modifi ers. [10a] It would be interesting to know how these two kinds of dipoles contribute to the interfaces at active layer/interlayer and interlayer/cathodes.

For completeness, one of the factors not yet considered is the interaction of the evaporated aluminium cathode with the interlayer. Evaporated aluminium on conjugated polymer sur-faces has been shown to modify the extent of conjugation at the surface of conjugated polymer fi lms, [ 19 ] which may infl uence the interfacial electronic structure and the resulting recombina-tion dynamics. Understanding such effects is outside the scope of the current work; however, based on the above fi ndings we believe that they are likely to explain the observed electrical dif-ferences between devices which utilize PFN and HBPFN, and those that only use methanol treatment.

In conclusion, an alcohol soluble hyperbranched polymer HBPFN has been synthesized and used as a cathode interlayer for high effi ciency OPVs. A PCE of 7.7% was obtained, which was an improvement compared to previously utilized, linearly

bH & Co. KGaA, Weinheim Adv. Mater. 2013, 25, 6889–6894

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

conjugated, polymeric interlayer PFN and comparable to tradi-tional devices fabricated with a highly reactive Ca/Al cathode. No interfacial dipole was found between the active layer and the interlayer HBPFN but interfacial dipole formation between the interlayer and the cathode could be one of the reasons for the enhanced performance.

Experimental Section Device fabrication : The device structure was ITO substrate/PEDOT:

PSS/PBDTTT-C-T:PC 71 BM/with or without interlayer/Al. ITO substrates were cleaned sequentially with detergent aqueous solution, deionized water, acetone and isopropyl alcohol. The cleaned ITO substrates were then treated with UV/ozone at 30 °C for 10 min. PEDOT: PSS thin layer (about 30 nm) was spin-coated on the treated ITO substrate and annealed at 150 °C for 10 min in air. After that, the PBDTTT-C-T: PC 71 BM (the ratio is 1:1.5 by weight) blend fi lm was spin-coated from its 1,2-dichlobenzene/1,8-diiodoctane (97:3 vol%) mixed solution on top of the PEDOT:PSS thin layer at 900 rpm for 90 s. For the case of methanol solvent treatment, methanol was spin-coated on top of the PBDTTT-C-T: PC 71 BM active layer at 5000 rpm for 30 s. In the case of PFN and HBPFN interlayers, their solutions with various concentrations in the mixed solvent of methanol/acetic acid (99:1 vol%)) were spin-coated on top of the active layer. Finally, the devices were completed after deposition of 100 nm Al as the electrode in a vaccum thermal evaporator at a pressure of less than 2 × 10 −6 Pa. The active area of the OPV device is 4 mm 2 . For the interlayer thickness, assuming the layer of HBPFN (on glass/PEDOT:PSS/active layer) to be continuous and having a uniform thickness we can estimate a nominal thickness value of 4 – 5 nm using a simple overlayer calculation. The calculation is based on the exponential attenuation of the N 1s photoelectron signal and a value of 3.25 nm for the effective attenuation length of N 1s photoelectrons in organic materials.

The measurements of J–V characteristics and EQE : Current density-Voltage (J–V) characteristics of the devices were measured with a computer-controlled Keithley 236 Source Measure Unit. A Xenon lamp coupled with AM 1.5G solar spectrum fi lters was used as the light source. The illumination intensity of 1000 W/m 2 irradiation was calibrated using a standard monocrystal Si reference cell to make sure the strict light intensity. The external quantum effi ciency (EQE) spectra were measured by Stanford Research Systems model SR830 DSP lock-in amplifi er coupled with WDG3 monochromator and 500W Xenon lamp.

Other Characterization methods : 1 H and 13 C NMR spectra were recorded on a Bruker AV-400X spectrometer operating at 400 and 100 MHz, in deuterated chloroform or deuterated methanol respectively. Molecular weights of the polymers were measured by Gel Permeation Chromatography (GPC) using a Waters 2695 Separations Module, with a Waters 2414 Refractive Index Detector and a Waters 2996 Photodiode Array Detector, a series of four Polymer Laboratories PLGel columns (3 × 5 μ m Mixed-C and 1 × 3 μ m Mixed-E), and Empower Pro Software. The molecular weights were calibrated with narrow polydispersity polystyrene standards (Polymer Laboratories EasiCal, MW from 264 to 256000), and molecular weights are reported as polystyrene equivalents based on the refractive index detector using THF as the eluent at a rate of 1.0 mL/min at 30 °C. Recycling preparative GPC was performed in chloroform (3.5 mL/min) at room temperature using a JAI LC-9201 separation module, with a RI-50s refractive index detector and a UV-3740 single wavelength detector, and a series of 2 JAI preparative column: JAIGEL-2H (20 × 600 mm) and a JAIGEL-2.5H (20 × 600 mm). The surface potentials and the work functions of the active layer surfaces and other polymer fi lms were measured on Scanning Kelvin Probe Microscopy (Kelvin Probe 5050 Technology) in air. The work functions were achieved from the average value of 200 points for each sample. X-ray photoelectron spectroscopy (XPS) characterizations were performed in an AXIS Ultra-DLD spectrometer (Kratos Analytical Ltd. Manchester) equipped with a monochromated

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, 25, 6889–6894

Al K α X-ray source at a power of 144 W (12 mA, 12 kV). Ionization potentials of the fi lms were measured by Photo Electron Spectroscopy in Air (PESA) on a Riken Keiki AC2 spectrometer with a light power of 5 nW and a power number of 0.5. UV-Vis spectra were measured with Cary 5E UV-Vis-NIR spectrophotometer. The roughness analysis and atomic force microscopy (AFM) images were carried out on a MFD-3D AFM instrument in AC mode with a NSC15/AIBS Si cantilever (resonant frequency around 325 kHz from μ -masch). The contact angles were measured with a CAM 200 (KSV Instrument LID.) and the photos were taken with a BASLER A602f-2 camera.

The hole and electron mobilities were calculated by fi tting to the Mott-Gurney law, which is expressed by J = 9 ε r ε 0 μ V 2 /8 L 3 , where ε r ε 0 is the dielectric permittivity of the active layer, L is the thickness of the active layer, and μ is the mobility. [ 16 ] Under the space charge limited current, we fi tted the experimental J–V data with the equation to get the slope of 2 in the logarithmical scales and the mobilities were obtained as fi eld independent values.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by CSIRO through CAS-CSIRO joint project and the Victorian Organic Solar Cell Consortium (Victorian Department of Primary Industries, Sustainable Energy Research and Development Grant and Victorian Department of Business and the Australian Solar Institute), and Chinese Academy of Sciences, CAS-CSIRO joint project (Project No. GJHZ1124), the Ministry of Science and Technology of China (No. 2010DFB63530). J. Jasieniak acknowledges fi nancial support through the Australian Research Council grant DP110105341. M. L. Lv acknowledges fi nancial support from Chinese Scholarship Council. M. L. Lv and S. S. Li contribute equally to this work.

Received: June 14, 2013 Revised: August 20, 2013

Published online: October 7, 2013

[1] R. Steim , F. R. Kogler , C. J. Brabec , J. Mater. Chem. 2010 , 20 , 2499 .

[2] Z. C. He , C. M. Zhong , S. J. Su , M. Xu , H. B. Wu , Y. Cao , Nat. Pho-tonics 2012 , 6 , 591 .

[3] a) F. Huang , H. B. Wu , D. L. Wang , W. Yang , Y. Cao , Chem. Mater. 2004 , 16 , 708 – 716 ; b) F. Huang , Y. H. Niu , Y. Zhang , J. W. Ka , M. S. Liu , A. K. Y. Jen , Adv. Mater. 2007 , 19 , 2010 ; c) F. Huang , H. Wu , Y. Cao , Chem. Soc Rev. 2010 , 39 , 2500 .

[4] a) J. H. Seo , A. Gutacker , B. Walker , S. Cho , A. Garcia , R. Q. Yang , T. Q. Nguyen , A. J. Heeger , G. C. Bazan , J. Am. Chem. Soc. 2009 , 131 , 18220 ; b) L. Lan , J. Peng , M. Sun , J. Zhou , J. Wang , Y. Cao , Org. Electron. 2009 , 10 , 346

[5] a) J. Luo , H. Wu , C. He , A. Li , W. Yang , Y. Cao , Appl. Phys. Lett. 2009 , 95 , 043301 ; b) C. He , C. Zhong , H. Wu , R. Yang , W. Yang , F. Huang , G. C. Bazan , Y. Cao , J. Mater. Chem. 2010 , 20 , 2617 .

[6] a) Z. C. He , C. M. Zhong , X. Huang , W. Y. Wong , H. B. Wu , L. W. Chen , S. J. Su , Y. Cao , Adv. Mater. 2011 , 23 , 4636 ; b) Z. He , C. Zhang , X. Xu , L. Zhang , L. Huang , J. W. Chen , H. B. Wu , Y. Cao , Adv. Mater. 2011 , 23 , 3086 ; c) L. Zhang , C. He , J. Chen , P. Yuan , L. Huang , C. Zhang , W. Cai , Z. Liu , Y. Cao , Macromolecules 2010 , 43 , 9771 .

[7] a) Z. Tang , L. M. Andersson , Z. George , K. Vandewal , K. Tvingstedt , P. Heriksson , R. Kroon , M. R. Andersson , O. Inganas , Adv. Mater.

6893wileyonlinelibrary.commbH & Co. KGaA, Weinheim

6894

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

2012 , 24 , 554 ; b) S. H. Oh , S. I. Na , J. Jo , B. Lim , D. Vak , D. Y. Kim ,

Adv. Funct. Mater. 2010 , 20 , 1977 ; c) C. H. Duan , K. Zhang , X. Guan , C. M. Zhong , H. M. Xie , F. Huang , J. W. Chen , J. B. Peng , Y. Cao , Chem. Sci. 2013 , 4 , 1298 ; d) K. Zhang , X. Guan , F. Huang , Y. Cao , Acta Chim. Sinica. 2012 , 70 , 2489 .

[8] S. H. Liao , Y. L. Li , T. H. Jen , Y. S. Cheng , S. A. Chen , J. Am. Chem. Soc. 2012 , 134 , 14271 .

[9] a) K. M. O’Malley , C. Z. Li , H. L. Yip , A. K. Y. Jen , Adv. Energy Mater. 2012 , 2 , 82 ; b) C. Z. Li , C. C. Chueh , H. L. Yip , F. Ding , X. Li , A. K. Y. Jen , Adv. Mater. 2013 , 25 , 2457 ; c) S. S. Li , M. Lei , M. L. Lv , S. E. Watkins , Z. A. Tan , J. Zhu , J. H. Hou , X. W. Chen , Y. F. Li , Adv. Energy Mater. DIO: 10.1002/aenm.201300425 .

[10] a) Y. Zhou , C. Fuentes-Hernandez , J. Shim , J. Meyer , A. J. Giordano , H. Li , P. Winget , T. Papadopoulos , H. Cheun , J. Kim , M. Fenoll , A. Dindar , W. Haske , E. Najafabadi , T. M. Khan , H. Sojoudi , S. Barlow , S. Graham , J. L. Bredas , S. R. Marder , A. Kahn , B. Kippelen , Science 2012 , 336 , 327 ; b) Y. H. Zhou , C. Fuentes-Hernandez , J. W. Shim , T. M. Khan , B. Kippelen , Energy Environ. Sci. 2012 , 5 , 9827 ; c) H. Kang , S. Hong , J. Lee , K. Lee , Adv. Mater. 2012 , 24 , 3005 .

[11] a) X. Liu , W. Wen , G. C. Bazan , Adv. Mater. 2012 , 24 , 4505 ; b) H. Zhou , Y. Zhang , J. Seifter , S. D. Collins , C. Luo , G. C. Bazan ,

wileyonlinelibrary.com © 2013 WILEY-VCH Verlag G

T. Q. Nguyen , A. J. Heeger , Adv. Mater. 2013 , 25 , 1646 ; c) Q. Wang , Y. Zhou , H. Zheng , J. Shi , C. Li , C. Q. Su , L. Wang , C. Luo , D. Hu , J. Pei , J. Wang , J. Peng , Y. Cao , Org. Electron. 2011 , 12 , 1858 .

[12] Y. Chen , Z. Jiang , M. Gao , S. E. Watkins , P. Lu , H. Wang , X. Chen , Appl. Phys. Lett. 2012 , 100 , 203304 .

[13] C. Gao , D. Yan , Prog. Polym. Sci. 2004 , 29 , 183 . [14] L. J. Huo , S. Q. Zhang , X. Guo , F. Xu , Y. F. Li , J. H. Hou , Angew.

Chem. Int. Ed. 2011 , 50 , 9697 [15] Z. A. Tan , W. Zhang , Z. Zhang , D. Qian , Y. Huang , J. H. Hou ,

Y. F. Li , Adv. Mater. 2012 , 24 , 1476 . [16] P. W. M. Blom , M. C. J. M. Vissenberg , Mater. Sci. Eng. 2000 , 27 ,

53 . [17] a) J. S. Kim , B. Lagel , E. Moons , N. Johansson , I. D. Baikie ,

W. R. Salaneck , R. H. Friend , F. Cacialli , Synth. Met. 2000 , 111 , 311 ; b) R. J. Davis , M. T. Lloyd , S. R. Ferreira , M. J. Bruzek , S. E. Watkins , L. Lindell , P. Sehati , M. Fahlman , J. E. Anthony , J. W. P. Hsu , J. Mater. Chem. 2011 , 21 , 1721 .

[18] S. Braun , W. R. Salaneck , M. Fahlman , Adv. Mater. 2009 , 21 , 1450 . [19] a) P. Dannetun , M. Boman , S. Stafstrom , W. R. Salaneck ,

R. Lazzaroni , C. Fredriksson , J. L. Bredas , R. Zamboni , C. Taliani , J. Chem. Phys. 1993 , 99 , 664 ; b) W. R. Salaneck , J. L. Bredas , Adv. Mater. 1996 , 8 , 48 .

mbH & Co. KGaA, Weinheim Adv. Mater. 2013, 25, 6889–6894