Journal of the American Chemical Society
ARTICLE
ferrocenium redox couple (Fc/Fcþ). The electrochemical onsets were
determined at the position where the current starts to differ from the
baseline. The HOMO in electron volts was calculated from the onset of
the oxidation potential (Eox) according to the following equation:
XRD curves, SEM images with EDS analysis. This material is
’ AUTHOR INFORMATION
þ
HOMO ¼ - ½4:8eV þ eðEox - E
Þꢂ
Corresponding Author
Fc=Fc
Microwave reactions were performed using a CEM Discover Bench-
mate microwave reactor.
Polymer Solar Cell Fabrication and Testing. Glass substrates
coated with patterned tin-doped indium oxide (ITO) were purchased
from Thin Film Devices, Inc. Prior to use, the substrates were subjected
to cleaning with ultrasonication in acetone, deionized water, and
2-propanol successively for 20 min each. The substrates were dried
under a stream of nitrogen and subjected to the treatment of UV-Ozone
for 15 min. A 0.45 μm filtered dispersion of PEDOT:PSS in water
(Baytron PH500) was then spun cast onto clean ITO substrates at
4000 rpm for 60 s and then baked at 140 °C for 10 min to give a thin film
with a thickness of 40 nm. A 1:2 w/w blend of polymer:PCBM at a
12 mg/mL concentration of polymer was dissolved in trichlorobenzene
with heating at 140 °C overnight, filtered through a 1 μm poly-
(tetrafluoroethylene) (PTFE) filter, and spun cast between 400 and
1200 rpm for 60 s onto the PEDOT:PSS layer. The substrates were then
dried at room temperature under nitrogen for 12 h. The devices were
finished for measurement after thermal deposition of a 30 nm film of
calcium and then a 100 nm aluminum film as the cathode at a pressure of
∼1 ꢀ 10-6 mbar. There are eight devices per substrate, with an active
area of 12 mm2 per device. The thicknesses of films were recorded by a
profilometer (Alpha-Step 200, Tencor Instruments), and AFM Images
were taken using an Asylum Research MFP3D atomic force microscope.
Device characterization was carried out under AM 1.5G irradiation with
the intensity of 100 mW/cm2 (Oriel 91160, 300 W) calibrated by a
NREL certified standard silicon cell. Current density vs potential (J-V)
curves were recorded with a Keithley 2400 digital source meter. External
quantum efficiencies (EQE) were detected under monochromatic
illumination (Oriel Cornerstone 260 1/4 m monochromator equipped
with Oriel 70613NS QTH lamp), and the calibration of the incident
light was performed with a monocrystalline silicon diode. All fabrication
steps after adding the PEDOT:PSS layer onto ITO substrate and
characterizations were performed in a glovebox under nitrogen atmo-
sphere. For mobility measurements,28 the hole-only devices in a con-
figuration of ITO/PEDOT:PSS (45 nm)/polymer-PCBM/Pd
(40 nm) were fabricated. The experimental dark current densities J of
polymer-PCBM blends were measured when applied with voltage from
0 to 6 V. The applied voltage V was corrected from the built-in voltage
Vbi, which was taken as a compensation voltage Vbi = Voc þ 0.05 V, and
the voltage drop Vrs across the ITO/PEDOT:PSS series resistance and
contact resistance, which is found to be around 35 Ω from a reference
device without the polymer layer. From the plots of J0.5 vs V, hole
mobilities of copolymers can be deduced from the equation:
’ ACKNOWLEDGMENT
This work was supported by a NSF CAREER award (DMR-
0954280), ONR (grant no. N000140911016), and a DuPont
Young Professor Award. S.C.P. gratefully acknowledges a Car-
olina Energy Fellowship. A.C.S. was supported as part of the
UNC EFRC: Solar Fuels and Next Generation Photovoltaics, an
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
under award number DE-SC0001011. We thank Dr. Shubin Liu
of Research Computing Center at UNC Chapel Hill for DFT
calculations. We also want to thank Prof. Richard Jordan and Mr.
Zhongliang Shen of the University of Chicago for GPC
characterization.
’ REFERENCES
(1) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu,
L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.
(2) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.;
Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297.
(3) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.;
Beaujuge, P. M.; Frꢀechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 7595.
(4) Zhao, G.; He, Y.; Li, Y. Adv. Mater. 2010, 22, 4355.
(5) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk,
M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc.
2009, 131, 16616.
(6) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009,
1, 657.
(7) Zhang, F.; Jespersen, K. G.; Bj€orstr€om, C.; Svensson, M.;
Andersson, M. R.; Sundstr€om, V.; Magnusson, K.; Moons, E.; Yartsev,
A.; Ingan€as, O. Adv. Funct. Mater. 2006, 16, 667.
(8) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk,
P. Appl. Phys. Lett. 2002, 80, 1288.
(9) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.;
Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841.
(10) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res.
2010, 43, 1396.
(11) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.;
Dante, M.; Heeger, A. J. Science 2007, 317, 222.
(12) Dennler, G.; Scharber, M. C.; Ameri, T.; Denk, P.; Forberich,
K.; Waldauf, C.; Brabec, C. J. Adv. Mater. 2008, 20, 579.
(13) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. Adv. Funct.
Mater. 2005, 15, 1617.
(14) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat. Mater. 2005, 4, 864.
(15) Price, S. C.; Stuart, A. C.; You, W. Macromolecules 2010,
43, 4609.
(16) Zhou, H.; Yang, L.; Price, S. C.; Knight, K. J.; You, W. Angew.
Chem., Int. Ed. 2010, 49, 7992.
9
V2
L3
J ¼ εrε0μh
8
where ε0 is the permittivity of free space, εr is the dielectric constant of
the polymer which is assumed to be around 3 for the conjugated
polymers, μh is the hole mobility, V is the voltage drop across the
device, and L is the film thickness of active layer.
(17) Zhou, H.; Yang, L.; Liu, S.; You, W. Macromolecules 2010,
43, 10390.
’ ASSOCIATED CONTENT
(18) Zhou, H.; Yang, L.; Xiao, S.; Liu, S.; You, W. Macromolecules
2010, 43, 811.
(19) Shockley, W.; Queisser, H. J. J. Appl. Phys. 1961, 32, 510.
(20) Balan, A.; Gunbas, G.; Durmus, A.; Toppare, L. Chem. Mater.
2008, 20, 7510.
S
Supporting Information. Synthesis of monomers and
b
polymers with NMR spectra, images of PBnDT-HTAZ,
PBnDT-FTAZ, P3HT, and PBnDT-DTPyT in solution,
0.5
J
vs V plots of mobility measurement of polymers and
polymer/PCBM blends, AFM images of thin films of blends,
(21) Tanimoto, A.; Yamamoto, T. Macromolecules 2006, 39, 3546.
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dx.doi.org/10.1021/ja1112595 |J. Am. Chem. Soc. 2011, 133, 4625–4631