A universal method to produce low-work function electrodes for organic electronics.
ABSTRACT Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low-work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.
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ABSTRACT: The work function (WF) of ZnO is modified by two types of dipole-bearing phenylphosphonate layers, yielding a maximum WF span of 1.2 eV. H3CO-phenyl phosphonate, with a positive dipole (positive pole pointing outwards from the surface), lowers the WF by ∼350 meV. NC-phenyl phosphonate, with a negative dipole, increases the WF by ∼750 meV. The WF shift is found to be independent of the type of ZnO surface. XPS data show strong molecular dipoles between the phenyl and the functionalizing (CN and OMe) tail groups, while an opposite dipole evolves in each molecular layer between the surface and the phenyl rings. The molecular modification is found to be invariant to supra-bandgap illumination, which indicates that the substrate's space charge-induced built-in potential is unlikely to be the reason for the WF difference. ZnO, grown by several different methods, with different degrees of crystalline perfection and various morphologies and crystallite dimensions, could all be modified to the same extent. Furthermore, a mixture of opposite dipoles allows gradual and continuous tuning of the WF, varying linearly with the partial concentration of the CN-terminated phosphonate in the solution. Exposure to the phosphonic acids during the molecular layer deposition process erodes a few atomic layers of the ZnO. The general validity of the treatment and the fine-tuning of the WF of treated interfaces are of interest for solar cells and LED applications.Physical Chemistry Chemical Physics 03/2014; · 3.83 Impact Factor
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ABSTRACT: Inorganic/organic hybrid radial heterojunction solar cells that combine vertically-aligned n-type silicon nanowires (SiNWs) with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) have great potential for replacing commercial Si solar cells. The chief advantage of such solar cells is that they exhibit higher absorbance for a given thickness than commercial Si solar cells, due to incident light-trapping within the NW arrays, thus enabling lower-cost solar cell production. We report herein on the effects of NW length, annealing and surface electrode on the device performance of SiNW/PEDOT:PSS hybrid radial heterojunction solar cells. The power conversion efficiency (PCE) of the obtained SiNW/PEDOT:PSS hybrid solar cells can be optimized by tuning the thickness of the surface electrode, and the etching conditions during NW formation and post-annealing. The PCE of 9.3% is obtained by forming efficient transport pathways for photogenerated charge carriers to electrodes. Our approach is a significant contribution to design of high-performance and low-cost inorganic/organic hybrid heterojunction solar cells.Nanoscale 05/2014; · 6.73 Impact Factor
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ABSTRACT: Inverted device architectures for OLEDs require suitable interfaces or buffer layers to enhance electron injection from high-work function transparent electrodes. We report on a solution-processable combination of ZnO and PEI that facilitates electron injection and enables efficient and air-stable inverted devices. Replacing the metal anode by highly conductive polymers enables transparent OLEDs.Advanced Materials 03/2014; · 14.83 Impact Factor
References and Notes
1. M. Butters et al., Chem. Rev. 106, 3002 (2006).
2. R. A. Sheldon, E. The, Green Chem. 9, 1273 (2007).
3. C. A. Busacca, D. R. Fandrick, J. J. Song, C. H. Senanayake,
Adv. Synth. Catal. 353, 1825 (2011).
4. J. Magano, J. R. Dunetz, Chem. Rev. 111, 2177 (2011).
5. C. D. Frohning, C. W. Kohlpaintner, in Applied Homogeneous
Catalysis with Organometallic Compounds, B. Cornils,
W. A. Herrmann, Eds. (Wiley-VCH, Weinheim, Germany
1996), pp. 29–104.
6. P. W. N. M. van Leeuwen, in Homogeneous Catalysis:
Understanding the Art (Kluwer, Dordrecht, Netherlands,
2004), pp. 139–174.
7. J. F. Bower, M. J. Krische, Top. Organomet. Chem. 34,
9. I. S. Kim, S. B. Han, M. J. Krische, J. Am. Chem. Soc. 131,
10. X. Gao, I. A. Townsend, M. J. Krische, J. Org. Chem. 76,
11. X. Gao, H. Han, M. J. Krische, J. Am. Chem. Soc. 133,
12. J. R. Zbieg, T. Fukuzumi, M. J. Krische, Adv. Synth. Catal.
352, 2416 (2010).
13. F. Shibahara, J. F. Bower, M. J. Krische, J. Am. Chem. Soc.
130, 6338 (2008).
14. J. R. Zbieg, J. Moran, M. J. Krische, J. Am. Chem. Soc.
133, 10582 (2011).
15. H. Alper, N. Hamel, J. Am. Chem. Soc. 112, 2803 (1990).
16. V. Komanduri, M. J. Krische, J. Am. Chem. Soc. 128,
17. G. L. Hamilton, E. J. Kang, M. Mba, F. D. Toste, Science
317, 496 (2007).
18. M. Rueping, A. P. Antonchick, C. Brinkmann, Angew.
Chem. Int. Ed. 46, 6903 (2007).
19. S. Mukherjee, B. List, J. Am. Chem. Soc. 129, 11336
20. S. Liao, B. List, Angew. Chem. Int. Ed. 49, 628 (2010).
21. G. Jiang, B. List, Chem. Commun. (Camb.) 47, 10022
22. B. Xu, S.-F. Zhu, X.-L. Xie, J.-J. Shen, Q.-L. Zhou,
Angew. Chem. Int. Ed. 50, 11483 (2011).
23. S. E. Denmark, J. Fu, Chem. Rev. 103, 2763 (2003).
24. M. Yus, J. C. González-Gómez, F. Foubelo, Chem. Rev.
111, 7774 (2011).
25. R. W. Hoffmann, W. Ladner, Tetrahedron Lett. 20, 4653
26. H. C. Brown, K. S. Bhat, J. Am. Chem. Soc. 108, 293
27. H. C. Brown, K. S. Bhat, J. Am. Chem. Soc. 108, 5919
28. H. Kim, S. Ho, J. L. Leighton, J. Am. Chem. Soc. 133,
29. A. Dobson, S. R. Robinson, M. F. Uttley, J. Chem. Soc.,
Dalton Trans. 1975, 370 (1975).
30. J. R. Zbieg, E. L. McInturff, J. C. Leung, M. J. Krische,
J. Am. Chem. Soc. 133, 1141 (2011).
31. H. C. Maytum, B. Tavassoli, J. M. J. Williams, Org. Lett. 9,
32. K. Hiraki, N. Ochi, Y. Sasada, H. Hayashida, Y. Fuchita,
33. P. Xue et al., Organometallics 23, 4735 (2004).
Acknowledgments: The Robert A. Welch Foundation
(grant F-0038) and the NIH’s National Institute of General
Medical Sciences (grant RO1-GM069445) are acknowledged
for financial support.
Materials and Methods
Tables S1 to S4
17 January 2012; accepted 8 March 2012
A Universal Method to Produce
Low–Work Function Electrodes
for Organic Electronics
Yinhua Zhou,1Canek Fuentes-Hernandez,1Jaewon Shim,1Jens Meyer,2Anthony J. Giordano,3
Hong Li,3Paul Winget,3Theodoros Papadopoulos,3Hyeunseok Cheun,1Jungbae Kim,1
Mathieu Fenoll,1,4Amir Dindar,1Wojciech Haske,1Ehsan Najafabadi,1Talha M. Khan,1
Hossein Sojoudi,5Stephen Barlow,3Samuel Graham,5Jean-Luc Brédas,3Seth R. Marder,3
Antoine Kahn,2Bernard Kippelen1*
Organic and printed electronics technologies require conductors with a work function that is
sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices.
We show that surface modifiers based on polymers containing simple aliphatic amine groups
substantially reduce the work function of conductors including metals, transparent conductive metal
oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral
polymer, which turns the modified conductors into efficient electron-selective electrodes in organic
optoelectronic devices. These polymer surface modifiers are processed in air from solution,
providing an appealing alternative to chemically reactive low–work function metals. Their use can
pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.
and organic thin-film transistors (TFTs), hold great
economic potential; they may lead to a new gen-
eration of consumer electronic devices that could
be printed or processed at low cost on large areas,
have very low weight, and conform to free-form
and flexible substrates (1–4). However, most
printed optoelectronic devices require at least one
electrode with a work function (WF) that is suf-
ficiently low to either inject electrons into or collect
electrons from the lowest unoccupied molecular
orbital (LUMO) of a given organic semiconductor.
Low-WF metals, such as alkaline-earth metals (Ca,
Mg) or metals co-deposited or coated with alkali
rganic-based thin-film optoelectronic de-
vices, such as organic solar cells (OSCs),
organic light-emitting diodes (OLEDs),
elements (Li, Cs), meet this requirement; how-
ever, they are chemically very reactive and easily
oxidize in the presence of ambient oxygen and
Fig. 3. Proposed mechanism for
alkylation of butadiene, illustrating
1Center for Organic Photonics and Electronics (COPE), School
of Electrical and Computer Engineering, Georgia Institute of
Technology, Atlanta, GA 30332, USA.2Department of Elec-
trical Engineering, Princeton University, Princeton, NJ 08544,
USA.3Center for Organic Photonics and Electronics (COPE),
School of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, GA 30332, USA.4Solvay SA, rue de
Ransbeek 310, 1120 Brussels, Belgium.5Center for Organic
Photonics and Electronics (COPE), George W. Woodruff School
of Mechanical Engineering, Georgia Institute of Technology,
Atlanta, GA 30332, USA.
*To whom correspondence should be addressed. E-mail:
VOL 33620 APRIL 2012
water. Thus, their use in printed electronics presents
limitations that can only be overcome by the fab-
rication of devices in an inert atmosphere and
their subsequent encapsulation with barrier-coating
technologies, which increases both the cost and
complexity of the device architectures.
Several strategies for replacing low-WF metals
have been explored. In one approach, a film of a
conducting or semiconducting material, typical-
ly thicker than 10 nm and displaying a low WF, is
coated on a high-WF electrode. This film, an elec-
tron transport material, mediates charge injection
and transport between the higher-WF conductive
electrode onto which it is coated and a semicon-
ducting layer in the device. Common examples
of this approach have included coating indium
tin oxide (ITO) with thin metal-oxide films—
such as ZnO (5), In2O3(6), Al-doped ZnO (7),
or In-doped ZnO (8)—that present a lower WF
(around 4.3 eV) than ITO. In another approach,
the surface of the conductive electrode is coated
with an ultrathin layer (≤10 nm) of a material that
is chemically or physically adsorbed onto the con-
ductor surface; the surface modifier is chosen in
such a way as to create strong interface and/or
molecular dipoles that induce a vacuum-level shift
and modify the WF of the conductor. In this con-
text, the chemisorption of small molecules onto the
surface of conductors has been the most common
route. For example, the WF of ITO could be de-
creased from 4.4 to 3.9 eVwhentreatedwith abasic
solution of N(C4H9)4OH (9). The chemisorption
of amine-containing conjugated small molecules
such as tetrakis(dimethylamino)ethylene (TDAE)
led to even greater reductions (up to 0.9 eV) of
the WF of ITO (10), Au (11, 12), and poly(3,4-
(PEDOT:PSS) (13, 14). However, molecules such
as TDAE are highly unstable in air in their neu-
tral state and undergo spontaneous oxidation,
which limits their practical use. Chemisorbed
self-assembled monolayers (SAMs) of dipolar
molecules can also substantially modify the WF
of metals and metal oxides (15–17), but specific
surface chemistry is required to ensure chemisorp-
tion; a similar problem is faced by other inorganic-
based modifiers such as Cs2CO3when processed
from solution (18). Polyethylene oxide and con-
jugated polymers have also been used but yield
reductions in WF that are generally smaller (0.3
to 0.5 eV) (19–21).
ture of PEIE and PEI.
(B) Photoemission cut-
off obtained via UPS for
PEDOT:PSS PH1000, ITO,
and Au samples, with
and without PEIE. (C) WF
change, relative to bare
ITO, of ITO/PEIE after dif-
ferent washing conditions.
(D)WF change, relativeto
bare ITO, upon modifica-
tionfrom PEIE water solu-
tion, PEIE with HPF6water
solution, and PEIE with
NaOH water solution be-
fore (solid squares) and
after (open circles) wa-
ter washing. (E) N1s core
level recorded via XPS on
the samples in (D) be-
fore and after washing. (F)
Proposed model of molec-
ular dipole–induced and
surface dipole–induced WF
reduction on ZnO surface.
-18 -17 -16
E w.r.t. Fermi level (eV)
Au + PEIE
ITO + PEIE
PH1000 + PEIE
PEIE + HPF6
PEIE + NaOH
0 10 20 30 40 50
∆ WF (eV)
405 400 395
Binding energy (eV)
4 6 8 10 12 14
pH value of water solution
Table 1. Work function of conducting materials with and without polymer modifiers, as independently mea-
sured by Kelvin probe in air and by UPS. Empty cells indicate no measurement for the corresponding sample.
Work function (eV)
Kelvin probe in air UPS
PristineWith PEIE With PEI Pristine With PEIEWith PEI
ITO 4.62 T 0.06
5.16 T 0.06*
4.26 T 0.06
4.68 T 0.06
3.60 T 0.06
3.60 T 0.06*
3.28 T 0.06
3.80 T 0.06
3.50 T 0.06
3.10 T 0.06
3.60 T 0.06
5.10 T 0.10
4.60 T 0.06
3.40 T 0.06
4.90 T 0.06
4.60 T 0.06
3.90 T 0.06
3.70 T 0.06
2.75 T 0.06
3.58 T 0.06
3.80 T 0.10
3.94 T 0.06
3.60 T 0.06
3.88 T 0.06
*Substrate was treated with an O2plasma for 2 min prior to measurements or polymer modifier deposition.
20 APRIL 2012 VOL 336
Here, we report on what appears to be a “uni-
versal” approach to reducing the WF of a con-
ductor, in which an ultrathin layer (1 to 10 nm)
of a polymer containing simple aliphatic amine
groups is physisorbed onto the conductor surface.
In contrast to the p-conjugated amine-containing
small molecules and polymers considered earlier,
the polymers exploited in this work are large
band-gap insulators and should not be regarded
as charge-injection layers but rather as surface
modifiers. The intrinsic molecular dipole mo-
ments associated with the neutral amine groups
contained in such an insulating polymer layer,
and the charge-transfer character of their inter-
action with the conductor surface, together re-
duce the WF of a wide range of conductors. The
commercially available polymer modifiers can
be easily processed in air, from dilute solutions in
environmentally friendly solvents such as water
or methoxyethanol. Their low cost and ease of
large-area mass production techniques and suited
for organic or printed electronic devices. To illus-
ing the demonstration of all-polymeric OSCs),
organic and metal-oxide TFTs, and OLEDs.
Figure 1A shows the chemical structure of
polyethylenimine ethoxylated (PEIE) and branched
polyethylenimine (PEI). The high content of amine
groups (primary, secondary, and tertiary) in the
polymer structures yields high pH values in water
and methoxyethanol solutions; the pH values were
measured to be 10.3 for PEIE and 10.5 for PEI
in water, and 10.1 for PEIE and 10.3 for PEI in
methoxyethanol solutions with a polymer con-
centration of 0.4 weight percent. Figure 1B dis-
plays the results of ultraviolet photoemission
spectroscopy (UPS) measurements on a series of
conductors before and after deposition of an ul-
trathin layer of PEIE; the spectra revealed WF
reductions from 4.95 to 3.32 eV for PEDOT:PSS
(high-conductivity grade PH1000), from 4.40 to
3.30 eV for ITO, and from 4.70 to 3.40 eV for Au.
Separate Kelvin probe measurements in air fur-
ther indicate that PEIE reduced the WF of metal
oxides (ITO, FTO, ZnO), metals (Au, Ag, Al),
and PEDOT:PSS PH1000 as well as graphene.
Table 1 summarizes the values of the WF obtained
by Kelvin probe and UPS experiments for sev-
eral conductors modified with 10-nm-thick PEIE
or PEI layers. Discrepancies between the two tech-
niques are caused by the different atmospheres
and the different experimental conditions under
which the Kelvin probe and UPS experiments
were conducted (22).
The thermal stability of the WF of PEIE- and
PEI-coated ITO substrates was studied by Kelvin
probe measurements in air. The WF of PEIE-
coated ITO substrates did not suffer any change
until a temperature of 190°C, making them com-
patible with the processing of printed electronic
devices on plastic substrates (typically at temper-
atures below 200°C) (fig. S1). For PEI-coated ITO
electrodes, the WF was unaffected until 150°C
(fig. S1). PEIE-coated ITO electrodes also remained
fairly stable under normal ambient conditions
for more than 4 weeks; during that period, the WF
varied by less than 0.2 eV (fig. S1). Furthermore,
aqueous solutions of PEIE or PEI are stable in
air (i.e., remain effective agents for reducing the
WF) for more than 1 year.
Inverse photoemission spectroscopy (IPES)
and UPS measurements on PEIE-coated Au in-
dicate electron affinity and ionization potential
energy values of 0.3 eV and 6.5 eV, respectively
(fig. S2). These results imply that, in contrast
to water/alcohol-soluble polymers based on a
p-conjugated main chain that can display good
electron-transporting properties (23–26), PEIE is
likely to function as an insulator with a gap of
6.2 eV, and to exhibit large barriers for both hole
and electron injection. We investigated the PEIE
thickness dependence (from 2 to 22 nm) of the
WF modification (DWF) of ITO and found a
variation of <10% (fig. S3). However, even if
the thickness of the PEIE layer did not influ-
ence DWF, its insulating nature would cause
thicker polymer layers to adversely affect device
Atomic force microscopy (AFM) measure-
ments on PEIE-coated, non–plasma-treated ITO
show that a 10-nm-thick PEIE layer did not uni-
formly cover the ITO surface, but formed islands
separated by areas with a much thinner PEIE
coating (fig. S4, C to F). These PEIE islands
could be easily washed away by subjecting the
PEIE-coated ITO substrate to a mild flow of run-
ning distilled water for 1 min (fig. S4, G and H).
After washing, |DWF| decreased by less than
0.1 eV. This observation indicates that only an
ultrathin layer of PEIE is needed to produce a
large DWF and that the processes leading to such
modifications are truly confined to the surface
of the conductor.
To explore the strength of the binding be-
tween PEIE and substrate, we monitored the WF
of a PEIE-coated ITO substrate over time as a
function of controlled washing cycles with water.
Figure 1C shows the evolution of DWF when the
substrates were subjected to a total of 50 min of
mild washing conditions (22). After such a period
of time, |DWF| became less than 0.34 eV. The
apparent resilience of the PEIE layer on the sur-
face of ITO might at first glance point to a strong
binding interaction between the polymer and the
electrode; however, the WF reduction entirely
disappeared after the PEIE-coated ITO substrate
was subjected to a 50-min water wash in an ul-
trasonic bath (Fig. 1C). This result suggests that
the PEIE layer is physisorbed on the conduc-
tor’s surface, which would be consistent with the
seemingly universal ability of PEIE to substan-
tially reduce the WF of such a diverse group of
conducting materials (Table 1).
To further explore the nature of the in-
teraction between PEIE and the conductor sur-
face, we modified the pH values of the original
PEIE/water solution by adding either hexafluoro-
phosphoric acid (HPF6) or NaOH. PEIE was
deposited on ITO from solutions with pH values
of 4.5, 7.1, 9.2, 10.3, and 13. Figure 1D displays
the DWF values induced before and after wash-
ing for 1 min with running water. Differences in
the solution pH values were expected to primar-
ily affect the degree of protonation of the amine
groups in PEIE. The degree of PEIE protonation
was followed with x-ray photoelectron spectros-
copy (XPS) by tracking the position and shape
of the N1s peak and the appearance of the F1s
peak for PEIE solutions with pH values below
CuPc (100 nm)
Mg (30 nm)/Ag
PEIE (12 nm)
CuPc (100 nm)
Mg (30 nm)/Ag
Current Density (mA/cm
at V >0
WF = 3.3 eV
WF = 3.46 eV
EA = 3.1 eV
IE = 4.9 eV
WF = 4.4 eV
WF = 4.4 eV
EA = 3.1 eV
IE = 4.9 eV
Fig. 2. (A) Structure of devices using ITO with and without PEIE as the bottom electrode. (B) J-V characteristics of these devices (without correction for
built-in potential); inset shows the J-V characteristics on a semilogarithmic scale at low voltage. (C and D) Energy level alignment of CuPc (10 nm) on top
of (C) ITO and (D) ITO/PEIE.
VOL 33620 APRIL 2012
10.3 (fig. S5). When PEIE was processed from
the most basic solution, DWF was –0.92 eVand
the presence of neutral amine groups was indi-
cated by the N1s peak at 399.8 eV (Fig. 1E). The
same N1s peak appeared in the PEIE layers
processed from a pH = 10.3 solution that yielded
DWF = –0.97 eV. These results indicate that neu-
tral amine groups are primarily involved in the
interactions leading to the formation of the in-
terface dipoles and the substantial changes in
WF. In contrast, when the PEIE layers were pro-
cessed from the more acidic solutions with a
higher proportion of protonated amines, smaller
WF reductions were observed. In films processed
from solutions with pH values of 7.1 and 4.1,
the appearance of a shoulder in the N1s peak
at 402.5 eV (Fig. 1E) is indicative of proton-
ated amines; at the same time, an F1s peak
appeared, which is consistent with the inclusion
of PF6–counterions accompanying the proton-
ated amines (fig. S5). After a 1-min water wash,
with the exception of the PEIE layer coated
from the most basic solution (which was com-
pletely removed from the ITO surface), all lay-
ers yielded similar DWF values around –0.86 eV
(Fig. 1D). The XPS spectra of these layers show
N1s peaks with a similar shape and a small
shoulder at 402.5 eV, as well as the disappear-
ance of the F1s peak (fig. S5). Similar trends were
found when the polymer layers were deposited
on FTO (fig. S6A) or Au (fig. S6B), or with the
use of HCl instead of HPF6(fig. S6C). Inter-
estingly, these results are different from those
in previous reports where poly(amidoamine)
dendrimers reduced the WF of ITO (27, 28). In
those reports, it was speculated that protonated
amine groups were primarily responsible for DWF
of ITO. Our results show that in the case of PEIE,
neutral amine groups are primarily responsible
for the largest DWF observed.
To understand the possible mechanism lead-
ing to the WF decrease, we carried out density
functional theory (DFT) calculations for the Au
(111) surface and the polar (0002) and nonpolar
(10-10) ZnO surfaces (22). A SAM of ethylamine
(C2H5NH2) molecules was used to model the
amine-containing thin molecular layer adsorbed
on the electrode surface (fig. S7). The adsorp-
tion energy per amine group is on the order of
–0.47 eV for Au (111), –0.88 eV for ZnO (0002),
and –0.78 eV for ZnO (10-10). In all cases, physi-
sorption of the ethylamine groups is calculated
to be energetically favored over dissociative chemi-
sorptions (for example, by 0.7 eV on the ZnO
(0002) surface), which is consistent with the ex-
perimental observations. The DWF values of the
three model surfaces induced by a SAM of eth-
ylamine molecules are calculated to be –1.3 eV
for Au (111), –1.7 eV for ZnO (0002), and –1.7 eV
for ZnO (10-10); these results are fully in line
with the UPS and Kelvin probe data.
The mechanism leading to DWF has been
analyzed by decomposing it into contributions
from (i) the ethylamine molecular dipole (mMD)
within the SAM along the direction perpendic-
ular to the surface (leading to an electrostatic
potential energy change denoted as ∆VMD), and
(ii) the dipole (mID) formed at the interface be-
tween the molecular SAM and the electrode
surface (∆VID) (29–31) (Fig. 1F and table S1). In
all instances, the contributions to DWF from the
molecular dipole and the interface dipole were
of the same order of magnitude—for instance,
–0.5 eV and –0.8 eV, respectively, on Au (111).
The contribution to DWF from the interface dipole
is attributed to a slight electron transfer [0.16 e
for ethylamine on Au (111), 0.07 e on ZnO (0002),
and 0.06 e on ZnO (10-10)] from the amine-
containing molecules to the electrode surface.
We tested PEIE-coated conductors as elec-
trodes in various device geometries. First, we
investigated the ability of PEIE-coated ITO sub-
strates to inject electrons into an organic semi-
conductor. The devices comprised a 100-nm-thick
layer of copper(II) phthalocyanine (CuPc, with
an electron affinity of 3.1 eV) deposited on top
of a glass/ITO/PEIE substrate with a top Mg/Ag
electrode (WF = 3.6 eV), as shown in Fig. 2A.
A comparison of the current density–voltage (J-V)
characteristics of such devices to identical de-
vices without the PEIE layer is shown in Fig.
2B. The voltage was applied to the top Mg/Ag
contact. Devices without the PEIE layer present
J-V characteristics resembling those of a diode
because of the large energy barrier for electron
injection from ITO to CuPc. Figure 2C illustrates
that the energy level alignment of a CuPc (10 nm)
layer deposited on top of ITO, as determined by
UPS and IPES measurements, yielded a value of
Fig. 3. (A) Device structure of inverted solar cells. (B to D) J-V characteristics
of devices with PEIE-modified ITO (B), Ag (C), and PH1000 (D) electrodes
under air mass (AM) 1.5 illumination (100 mW/cm2). (E) Device structure and
two photographs of all-polymeric solar cells. (F) J-V characteristics of all-
PCE of all-polymeric solar cells after continuous bending tests with different
dark and under illumination on a semilogarithmic scale.
20 APRIL 2012VOL 336
1.3 eV for the height of this barrier. In contrast, as
shown in Fig. 2D, in a device with an ITO/PEIE
electrode, this barrier was greatly reduced to a
value of 0.36 eV. Thus, the J-V characteristics be-
came nearly symmetric, with the current injected
from the ITO/PEIE electrode slightly greater than
that injected from Mg/Ag (Fig. 2B). This result
confirms that, despite the insulating nature of
PEIE, electrons can be injected efficiently into the
semiconductor through the PEIE layer by pro-
cesses such as tunneling or thermionic injection.
In a second step, the low-WF electrodes were
as the bottom electrode to demonstrate their elec-
tron selectivity (Fig. 3A). The top Ag layer was
150 nm for the devices with glass/ITO/PEIE and
polyethersulfone (PES)/PH1000/PEIE, and it
was 20 nm for the devices with glass/Ag/PEIE.
A mixture of poly(3-hexylthiophene) (P3HT) and
was used as the photo-active layer (32). Figure 3,
B to D, displays the J-V characteristics of these
solar cells in the dark and under illumination. In
all cases, the J-V characteristics in the dark show
a large rectification and small reverse saturation
currents; this result demonstrates the excellent
Solar cells with PEIE-coated ITO electrode (Fig.
of 5.9% [open-circuit voltage (VOC) = 0.81 V,
short-circuit current density (JSC) = 11.0 mA/cm2,
fill factor (FF) = 0.66], averaged over 10 devices.
Such a large FF value also provides indirect evi-
coated ITO electrode (33, 34). Note that the PCE
previously reported in other inverted solar cells
that use the same active layer (35).
We studied the shelf air stability of cells with
an ITO/PEIE electrode and found that the PCE
remained nearly constant after 30 days in air and
was still about 70% of its initial value after 102
days (fig. S8). PEIE-coated ITO electrodes were
also tested in devices with an active layer com-
prising a blend of poly[(4,8-bis-(2-ethylhexyloxy)-
(PBDTTT-C) and 3′-phenyl-3′H-cycloprop-
[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl
ester (PC60BM). Devices with an inverted archi-
tecture consisting of glass/ITO/PEIE/PBDTTT-
C:PC60BM/MoO3/Ag yielded an average PCE of
PSS(4083)/PBDTTT-C:PC60BM/Ca/Al yielded a
PCE of 6% (fig. S9 and table S2).
In the case of the solar cells with a PEIE-coated
Ag bottom electrode (Fig. 3C), light enters the
device through the MoO3(15 nm)/Ag (20 nm)
top electrode. In these devices, a relatively high
FF of 0.60 was also measured but JSCwas lower
(5.5 mA/cm2) because of the low transmittance
of the top electrode (fig. S10); as a result, the
PCE was only 2.6%. Devices with a PEIE-coated
PH1000 bottom electrode yielded a PCE of 3.5%
(averaged over five devices), with VOC= 0.79 V,
FF = 0.57, and JSC= 7.6 mA/cm2. The slightly
smaller FF value is attributed to the increased
series resistance introduced by the relatively low
conductivity of PH1000 (600 S/cm) relative to
ITO or Ag, whereas the relatively small JSCis
mainly caused by the lower transmittance of
PH1000 (36–38) and differences in the thick-
ness of the active layer (fig. S10).
Having transformed PEDOT:PSS into an effi-
substrates [Fig. 3E and fig. S11). As shown in
Fig. 3F, OSCs with PEDOT:PSS/PEI electrodes
show excellent rectification in the dark in con-
trast to samples without PEI (fig. S12). Under il-
lumination, semitransparent all-polymeric OSCs
showed a PCE of 3.0% averaged over seven de-
vices, with VOC= 0.80 V, FF = 0.52, and JSC=
7.1 mA/cm2. When a thick Ag layer was placed
behind the semitransparent OSC to reflect some
of the light back into the active layer, JSCin-
creased to 8.2 mA/cm2, yielding a PCE of 3.4%
(table S3 and fig. S13), comparable to that of a
device that uses a MoO3/Ag hole-collecting elec-
trode (Fig. 3D). Because the all-polymeric OSCs
only present polymer-polymer interfaces, they
demonstrate excellent mechanical properties un-
der multiple bending conditions (Fig. 3G). They
provide a proof of principle that OSCs can be
fabricated using in-line, all-additive solution-based
coating and/or printing of polymers. However,
the design of large-area cells and modules could
require the integration of some highly conductive
electrodes to compensate for the lower sheet re-
sistance of conducting polymers (38).
Air-stable, low-WF electrodes are also needed
for organic semiconductor–based or metal oxide–
based n-channel TFTs. In a TFT, the existence
of large energetic barriers for electron injection
at the source electrode and collection at the drain
electrode can lead to a large threshold voltage
(VTH); it can also potentially result in low field-
effect mobility (m) when the contact resistance ex-
ceeds the channel resistance (39), a particularly
critical issue for TFTs with short channel lengths.
We fabricated examples of organic and metal-
oxide n-channel TFTs that use air-stable PEIE-
coated Au source and drain electrodes. In thefirst
example, top-gate bottom-contact organic-based
n-channel TFTs with a bilayer gate dielectric were
fabricated (40) (Fig. 4A). For comparison purposes,
two sets of TFTs were fabricated with Au source
and drain electrodes, either coated with a PEIE
layer or uncoated. In both instances, the organic
alt-5,5´-(2,2´-bithiophene) [P(NDI2OD-T2), also
called N2200] (4) was inkjet-printed on top of the
Gate contact (Ti/Au)
IGZO (30 nm)
PEIE (10 nm)
-10 0 10 20 30 40 50
10 nm PEIE
VDS = 50 V
W/L = 600 µm/200 µm
VTH= 1.5 V,
µ = 1.2 cm2/Vs
VTH= 38.7 V,
µ = 0.004 cm2/Vs
N2200 (100 nm)
-2 0 2 4 6 8 10
1.5 nm PEIE
W/L = 2550 µm/180 µm
VDS = 10 V
VTH= 0.4 V,
µ = 0.1 cm2/Vs
VTH= 4.5 V,
µ = 0.04 cm2/Vs
Fig. 4. (A) Device structure of N2200 OTFTs. (B)
Transfer characteristics of N2200 OTFTs with and
without PEIE. (C) Device structure of IGZO TFTs.
(D) Transfer characteristics of IGZO TFTs with and
without PEIE. CYTOP (CTL-809M) is a perfluorinated polymer purchased from Asahi Glass.
VOL 33620 APRIL 2012
source and drain electrodes. Figure 4B displays a
comparison of the transfer characteristics of both
TFTs. In the TFTs with PEIE-coated source and
drain electrodes, VTHdropped from 4.5 to 0.4 V,
the average m increased from 0.04 to 0.1 cm2
V–1s–1and the device yield improved from 60%
to 95%. We note that whereas m values obtained
after the PEIE modification are comparable to
those of similar TFTs previously reported, the
VTHin our devices is lower (4). In this example,
PEIE also coats the gate insulator inside the
channel. PEIE layers thicker than 1.5 nm led to
n-doping of the organic semiconductor channel.
Similar doping was also observed on bottom-gate
bottom-contact PC60BM TFTs that used PEIE-
coated Au electrodes with PEIE thicknesses greater
than 10 nm (fig. S14). This doping could assist
the injection/collection of carriers by producing
band-bending in the vicinity of the conductor sur-
face; this effect is also likely present in OSCs
containing fullerene-based acceptors (Fig. 3).
In a second example, bottom-gate top-contact
amorphous InGaZnO (IGZO) TFTs were fabri-
cated as shown in Fig. 4C. In contrast to the de-
vices described above, PEIE was first deposited
directly on top of the semiconductor (to prevent
any damage from the radio frequency–sputtering
deposition of IGZO) and the Au source and drain
electrodes were evaporated on top of the PEIE
layer. Figure 4D provides a comparison of the
transfer characteristics of IGZO TFTs with and
without PEIE. As in the n-channel organic-based
TFTs, the VTHof the IGZO TFTs dropped from
38.7 to 1.5 Vand m increased from 0.004 to 1.2
cm2V–1s–1in the devices with the PEIE-modified
Finally, we tested the use of PEIE in OLEDs
by replacing a LiF/Al cathode with PEIE/Al in
benchmark devices based on 4,4´-bis(carbazol-
9-yl)biphenyl (CBP) and an emitter of fac-tris(2-
phenylpyridinato-N,C2′) iridium [Ir(ppy)3], and
achieved an external quantum efficiency of 15%
(fig. S15). Although the performance of these
devices was not optimized, it illustrates the ap-
plicability of this method to OLED platforms.
Polymers containing simple aliphatic amine
groups such as PEIE and PEI appear to be “uni-
versal” surface modifiers that allow the fabrica-
tion, at very low cost and from environmentally
friendly solvents, of air-stable low-WF electrodes.
This approach should enable the mass produc-
tion of low-WF electrodes from processes that
are compatible with the large-area roll-to-roll
manufacturing techniques required for the com-
mercialization of low-cost organic and printed
electronic devices. The specific properties of the
polymers can be further optimized for other ap-
plications, and conceptually the approach could
be applied to the development of polymers for
References and Notes
1. H. Y. Chen et al., Nat. Photonics 3, 649 (2009).
2. R. H. Friend et al., Nature 397, 121 (1999).
3. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger,
Science 270, 1789 (1995).
4. H. Yan et al., Nature 457, 679 (2009).
5. H. Cheun et al., J. Phys. Chem. C 114, 20713 (2010).
6. L. Wang et al., Nat. Mater. 5, 893 (2006).
7. J. Meyer et al., Appl. Phys. Lett. 93, 073308 (2008).
8. H. Cheun et al., Opt. Express 18 (suppl. 4), A506 (2010).
9. F. Nüesch, L. J. Rothberg, E. W. Forsythe, Q. T. Le, Y. Gao,
Appl. Phys. Lett. 74, 880 (1999).
10. W. Osikowicz et al., Appl. Phys. Lett. 85, 1616 (2004).
11. L. Lindell et al., Appl. Phys. Lett. 92, 163302 (2008).
12. B. Bröker et al., Appl. Phys. Lett. 93, 243303 (2008).
13. L. Lindell et al., Chem. Mater. 18, 4246 (2006).
14. F. L. E. Jakobsson et al., Chem. Phys. Lett. 433, 110 (2006).
15. A. Sharma, P. J. Hotchkiss, S. R. Marder, B. Kippelen,
J. Appl. Phys. 105, 084507 (2009).
16. X. Bulliard et al., Adv. Funct. Mater. 20, 4381 (2010).
17. S.-N. Hsieh et al., Org. Electron. 10, 1626 (2009).
18. J. Huang, Z. Xu, Y. Yang, Adv. Funct. Mater. 17, 1966 (2007).
19. Y. Zhou et al., Sol. Energy Mater. Sol. Cells 93, 497 (2009).
20. G. Jo et al., Appl. Phys. Lett. 97, 213301 (2010).
21. S. I. Na et al., Appl. Phys. Lett. 97, 223305 (2010).
22. See supplementary materials on Science Online.
23. F. Huang, H. B. Wu, Y. Cao, Chem. Soc. Rev. 39, 2500 (2010).
24. F. Huang et al., Adv. Mater. 19, 2010 (2007).
25. S. H. Oh, D. Vak, S.-I. Na, T.-W. Lee, D.-Y. Kim, Adv. Mater.
20, 1624 (2008).
26. W. Ma et al., Adv. Mater. 17, 274 (2005).
27. R. Schlapak et al., Langmuir 23, 8916 (2007).
28. G. Latini, M. Wykes, R. Schlapak, S. Howorka, F. Cacialli,
Appl. Phys. Lett. 92, 013511 (2008).
29. G. Heimel, L. Romaner, J.-L. Brédas, E. Zojer, Phys. Rev. Lett.
96, 196806 (2006).
30. G. Heimel, L. Romaner, E. Zojer, J.-L. Brédas, Acc. Chem. Res.
41, 721 (2008).
31. H. Li, P. Paramonov, J.-L. Brédas, J. Mater. Chem. 20,
32. Y. He, H.-Y. Chen, J. Hou, Y. Li, J. Am. Chem. Soc. 132,
33. H. Schmidt et al., Appl. Phys. Lett. 96, 243305 (2010).
34. A. Wagenpfahl, D. Rauh, M. Binder, C. Deibel, V. Dyakonov,
Phys. Rev. B 82, 115306 (2010).
35. Y. J. Cheng, C.-H. Hsieh, Y. He, C.-S. Hsu, Y. Li, J. Am.
Chem. Soc. 132, 17381 (2010).
36. Y. H. Zhou et al., Appl. Phys. Lett. 97, 153304
37. O. Inganäs, Nat. Photonics 5, 201 (2011).
38. S. Choi, W. J. Potscavage Jr., B. Kippelen, Opt. Express 18
(suppl. 3), A458 (2010).
39. A. Rolland, J. Richard, J. P. Kleider, D. Mencaraglia,
Jpn. J. Appl. Phys. 35, 4257 (1996).
40. D. K. Hwang et al., Adv. Mater. 23, 1293 (2011).
Acknowledgments: Supported by the Center for Interface
Science: Solar Electric Materials, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under award
DE-SC0001084 (Y.Z., J.S., J.M., H.C., H.L., P.W., S.B., J.-L.B.,
S.R.M., and S.G.), the NSF Science and Technology Centers
program under agreement DMR-0120967 (C.F.-H., J.K.,
E.N., and A.D.), Office of Naval Research grant N00014-
04-1-0120) (T.M.K. and B.K.), NSF grants DMR-1005892 (A.K.)
and CMMI-0927736 (H.S.), the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under
award DE-FG02-05ER46165 (W.H. and T.P.), the Deutsche
Forschungsgemeinschaft postdoctoral fellowship program
(J.M.), and the National Defense Science and Engineering
Graduate Fellowship program and NSF graduate research
fellowship DGE-0644493 (A.J.G.).
Materials and Methods
Figs. S1 to S16
Tables S1 to S3
6 January 2012; accepted 29 February 2012
Dislocation Damping and Anisotropic
Seismic Wave Attenuation in
Earth's Upper Mantle
Robert J. M. Farla,1*† Ian Jackson,1John D. Fitz Gerald,1Ulrich H. Faul,2Mark E. Zimmerman3
Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with
passing seismic waves. Although these defects, known as dislocations, potentially contribute to the
attenuation of seismic waves in Earth’s upper mantle, evidence for dislocation damping from laboratory
studies has been circumstantial. We experimentally determined the shear modulus and associated
Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation
creep in either compression or torsion, the enhancement being greater for prior deformation in torsion.
These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks
where olivine is or was deformed at relatively high stress by dislocation creep in Earth’s upper mantle.
plates on Earth [e.g., (1)]. Laboratory experi-
rheologically weak sublithospheric man-
tle (the asthenosphere) is widely invoked
to explain the motion of the tectonic
ments underpin an emerging understanding of
the anomalous seismic properties of the astheno-
sphere (2, 3). In particular, the seismic anisotro-
py of this part of the upper mantle is attributed
to crystallographic preferred orientation in olivine-
rich rocks—testimony to their deformation by
dislocation creep (4–11). Several studies have
demonstrated that anelastic relaxation attributed
to grain-boundary sliding can affect the shear
modulus and attenuation of upper mantle rocks
(12–16), but evidence for strain-energy dissipa-
tion from mechanically forced vibrations of dis-
locations has been largely circumstantial until now.
1Research School of Earth Sciences, Australian National Uni-
versity, Canberra, Australian Capital Territory 0200, Australia.
2Department of Earth Sciences, Boston University, Boston, MA
002215, USA.3Department of Geology and Geophysics, Uni-
versity of Minnesota, Minneapolis, MN 55455, USA.
*To whom correspondence should be addressed. E-mail:
†Present address: Department of Geology and Geophysics,
Yale University, New Haven, CT 06520, USA.
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