Paper
Journal of Materials Chemistry A
30 min. PEDOT:PSS (Clevios P VP Al 4083) was drop-ltered
from a 0.45 mm PVDF lter atop the cleaned ITO-coated glass
and spun at 2500 rpm to produce a 50 nm thick lm, which was
then annealed on a hotplate set at 120 ꢀC for 10 min, transferred
into a glovebox, and annealed for another 3 min at 180 ꢀC.
Unless otherwise noted, a 20 mg mLÀ1 solution of CP (the
precursor to BP) or a 30 mg mLÀ1 solution of CuCP (the
precursor to CuBP) in 1 : 2 chloroform : chlorobenzene was
batch-ltered using a 0.22 mm PTFE lter, spun at 1500 rpm,
and subsequently annealed on a hotplate set to 180 ꢀC for BP or
210 ꢀC for CuBP for 20 min. For donor thickness dependent
bilayer OPV devices, precursor solution concentrations ranged
from 2 to 30 mg mLÀ1, otherwise using the same processing
conditions. Films were allowed to cool before casting PCBM.
PCBM lms were cast from 0.22 mm PTFE batch-ltered solu-
tions of 12 mg mLÀ1 PCBM in chlorobenzene at 1500 rpm.
Where noted, a 3 nm thick layer of bathocuproine (BCP) was
deposited in an Angstrom Engineering thermal evaporator at a
Acknowledgements
We thank Alexander Sharenko, Dr Martijn Kuik, and Dr Loren
Kaake for helpful discussions. Funding of this work was
provided by Mitsubishi Chemical Center for Advanced Mate-
rials (MC-CAM). Portions of this research were conducted at the
Stanford Synchrotron Radiation Lightsource user facility,
operated by Stanford University on behalf of the U.S. Depart-
ment of Energy, Office of Basic Energy Sciences under Contract
No. DE-AC02-76SF00515. We thank the NSF-DMR SOLAR
(1035480) for the support of the modeling of exciton diffusion.
MG thanks the ConvEne IGERT program at UCSB (NSF-DGE
0801627). CMP thanks the National Science Foundation Grad-
uate Research Fellowship Program. TQN thanks the Camille
Dreyfus Teacher Scholar Award.
References
pressure below 1 Â 10À6 Torr at a rate of 0.2 A sÀ1 using an open
1 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and
E. D. Dunlop, Prog. Photovoltaics, 2014, 22, 1–9.
2 J. You, C.-C. Chen, Z. Hong, K. Yoshimura, K. Ohya, R. Xu,
S. Ye, J. Gao, G. Li and Y. Yang, Adv. Mater., 2013, 25,
3973–3978.
3 B. Azzopardi, C. J. M. Emmott, A. Urbina, F. C. Krebs,
J. Mutale and J. Nelson, Energy Environ. Sci., 2011, 4, 3741–
3753.
4 W. R. Mateker, J. D. Douglas, C. Cabanetos, I. T. Sachs-
Quintana, J. A. Bartelt, E. T. Hoke, A. E. Labban,
P. Beaujuge, J. M. J. Frechet and M. D. McGehee, Energy
Environ. Sci., 2013, 6, 2529–2537.
5 A. Anctil, C. Babbitt, B. Landi and R. P. Raffaelle, in
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE,
2010, pp. 000742–000747.
6 J. E. Coughlin, Z. B. Henson, G. C. Welch and G. C. Bazan,
Acc. Chem. Res., 2014, 47, 257–270.
7 Z. B. Henson, G. C. Welch, T. van der Poll and G. C. Bazan,
J. Am. Chem. Soc., 2012, 134, 3766–3779.
8 Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu,
J. Am. Chem. Soc., 2009, 131, 56–57.
9 A. B. Tamayo, B. Walker and T.-Q. Nguyen, J. Phys. Chem. C,
2008, 112, 11545–11551.
˚
mask that exposed most of the lm surface. All bilayer OPVs
were completed by evaporating a patterned top electrode of
100 nm Al evaporated at a pressure below 1 Â 10À6 Torr at a rate
of 0.3 A sÀ1 for the rst 10 nm and then gradually increasing to
˚
À1
˚
2.3 A s
.
Conducting atomic force microscopy (c-AFM)
Topographic and current c-AFM images were collected on an
Asylum MFP-3D AFM under a nitrogen atmosphere in contact
mode using a conductive, Au-coated silicon probe with a reso-
nant frequency of ꢁ13 kHz and a force constant of ꢁ0.2 N mÀ1
(Budget Sensors).
Scan sizes of 20 Â 20 mm collected at a rate of ꢁ2 lines
per second were used for this study. The device structure of
ITO/PEDOT:PSS/donor/Au tip was used to selectively probe
hole current through the donor lm. Aer compensating
for parasitic voltages and subtracting the baseline current,
a bias of +0.01 V was applied. This corresponds to hole
injection into the donor lm from the ITO/PEDOT:PSS
substrate.
Samples for c-AFM were prepared by following the procedure
described above for bilayer OPVs with BP or CuBP, but omitting 10 Y. Matsuo, Y. Sato, T. Niinomi, I. Soga, H. Tanaka and
the procedure aer the donor layer deposition and annealing
(PCBM and top electrodes were not added).
E. Nakamura, J. Am. Chem. Soc., 2009, 131, 16048–16050.
11 S. Ito, T. Murashima, N. Ono and H. Uno, Chem. Commun.,
1998, 1661–1662.
12 S. M. Borisov, G. Nuss and I. Klimant, Anal. Chem., 2008, 80,
9435–9442.
Exciton diffusion length modelling
The external quantum efficiency was calculated by modeling the 13 M. H. Hoang, Y. Kim, S.-J. Kim, D. H. Choi and S. J. Lee,
optical eld in the bilayer OPVs using the indices of refraction Chem. - Eur. J., 2011, 17, 7772–7776.
of the materials from ellipsometry measurements. The exciton 14 I. Kim, H. M. Haverinen, Z. Wang, S. Madakuni, Y. Kim,
generation rate in each layer of the device was factored into
J. Li and G. E. Jabbour, Chem. Mater., 2009, 21, 4256–
tting the computed EQE to the experimental EQE, using the
4260.
effective exciton diffusion length of the donor material and the 15 A. S. Dhoot, S. Aramaki, D. Moses and A. J. Heeger, Adv.
effective exciton diffusion length of the acceptor material as the Mater., 2007, 19, 2914–2917.
two variables in the tting process. The reported values are the 16 S. Aramaki, Y. Sakai, H. Yanagisawa and J. Mizuguchi, Acta
average LD and standard deviation across several samples of
varying donor lm thickness.
Crystallogr., Sect. E: Struct. Rep. Online, 2006, 62, m2616–
m2617.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 7890–7896 | 7895