Photon Factory Activity Report 2016 #34 (2017)

BL-11D /2015G109

Local Electronic Structure of EG treated PEDOT：PSS Thin Films
Koji K. Okudaira1,*, Yu Urabe1, and Masato Tanaka1
1Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan
1 Introduction
Poly (3,4ethylenedioxythiophene) : poly (4styrenesulfonate) (PEDOT: PSS) has attracted considerable
attention due to its superior electrochemical stability andtransparency, which can be used as a possible alternativeto ITO(Indium Tin Oxide). [1] As-prepared ﬁlm fromthe PEDOT:PSS dispersion typically has a conductivityof ∼ 0.1 − 1 S/cm, which is three or four orders ofmagnitude lower than that of ITO Recently It wasreported that the conductivity of the PEDOT:PSS thinfilm could be increased by 2 to 3 orders of magnitude byadding certain additives into the dispersion prior to ﬁlm
deposition or by treating the ﬁlms with certain solvents oracids post deposition such as ethylene glycol (EG),diethylene. [2]were sonicated in acetone 10 min. The deposited
PEDOT:PSS films were immersed in EG solution atvarious concentration (1wt%, 3wt%, and 10wt%).
The sample was irradiated with p-polarized SR with anincidence angle of 84°from the surface normal at thebeamline 11C of the SR facility in KEK [Photon Factory(PF)]. A coincidence analyzer that can be used for Auger,PES, and APECS. Auger, PES, and APECS wereobserved at hv = 203eV. The energy resolution (E/E) of
both the ASMA and the DP-CMA is ∼ 36 in the electron
kinetic energy range of 20–100 eV [3]3 Results and DiscussionFigure 2 shows sulfur (S) (2p) PE spectra ofPEDOT:PSS films dipped EG solution. The spectra
display two contributions. The S(2p) signal for PSS
appears at higher binding energy (BE) of 168.5eV due tothe three electronegative oxygen atoms withdrawing theelectron density of the S atom. The S(2p) doublet at 164.2
eV comes from PEDOT. The probing depth of thespectroscopy is defined by the inelastic mean free path ofthe S(2p) photoelectrons, is several Å for a kinetic energyof about 30eV. Upon increasing DG concentration theamount of PEDOT at the film surface increases.
Furthermore, it is found that the difference of peak
position of S(2p) of PSS at a BE of 168eV and that of
PEDOT at about 164eV becomes smaller upon increasing
EG concentration due to the promotion of charge transferfrom PEDOT to PSS. (doping)Fig.1 Molecular structure of PEDOT and PSS

2 Experiment
The PEDOT:PSS films were spin cast from the
PEDOT:PSS aqueous dispersion (Clevios P AI4083,
weight ratio of PEDOT to PSS = 1:6) onto ITO, which

EG10wt%EG3wt%
EG1wt%
Pristine

Normalized intensity (arb.u)

The electrical conductivity strongly depends on theelectronic structure of the valence region. It is important
to clarify the electronic structure of valence of PEDOT,
which is responsible for conductivity of PEDOT:PSSsystem
In this paper, we report on the surface-site-selective
sulfur (S) L VV Auger electron spectroscopy, S 2p
photoelectronspectroscopy(PES)
and
Auger
photoelectron coincidence spectroscopy (APECS) ofPEDOT:PSS system. The S L VV–S 2p APECS indicatedthat the electronic structure of valence band in the vicinityof the S atom not only in PEDOT but also PSS.

170

168

166

164

Binding Energy(eV)Fig.2 Sulfur (2p) PE at hv = 203.3 eV of PEDOT:PSS
pristine and after treated (dipping EG solution withvarious concentration ) films

Normalized APECS intensity(arb.unit)

Photon Factory Activity Report 2016 #34 (2017)

EG1wt% Auger electron
2000
PSS S2p APECSPEDOT S2p APECS-4Auger electron intensity (cps)1.5x101.0x10-4

1500

5.0x10-51000
0.0125

130

135

140

145

150

155

Auger electron kinetic energy (eV)160

Fig.3Sulfur(S) L VV–S 2p APECS of PEDOT:PSS
film (dipping to EG solution with concentration of 1wt%)
measured in coincidence with S 2p photoelectrons at a BEof 168.5eV (PSS) (■) and of 164.2 eV (PEDOT) (▲)

Normalized APECS intensity(arb.u)

Figures 3 shows S L VV–S 2p APECS of PEDOT:PSS
film (dipping to EG solution with concentration of 1wt%)
measured in coincidence with S 2p photoelectrons at a BEof 168.5eV (PSS) and of 164.2 eV (PEDOT). The
spectral feature of APECS in coincidence with S2p
photoelectrons from PSS is largely different from thatfrom PEDOT. The strong peak of APECS in coincidencewith S 2p photoelectrons from PEDOT appear at about
kinetic energy (Ek) of 143eV, which is larger than that ofAPECS in coincidence with S 2p photoelectrons fromPSS. It reflects the difference between the electronicstructure at the vicinity of S atoms in PEDOT, where theS atoms consist of  conjugated system, and that of S
atoms in PSS, where the S atoms are part of sulfo group.
It shows large density of states distributed in the vicinityof S atoms in PEDOT at relatively lower binding energy
region.
PEDOT(EG1wt%) S2p APECSPEDOT(EG10wt%) S2p APECS

135140

145

150

155

Auger electron kinetic energy (eV)Fig. 4 Sulfur (S) L VV–S 2p APECS of PEDOT:PSS
film (dipping to EG solution with concentration of
1wt% (upper panel) and 10wt% (lower panel))measured in coincidence with S 2p photoelectrons at a
BE of 164.2 eV (PEDOT).
Figures 4 shows S L VV–S 2p APECS of PEDOT:PSS
film (dipping to EG solution with concentration of 1wt%and 10wt%) measured in coincidence with S 2pphotoelectrons at a BE of 164.2 eV (PEDOT). TheAPECS of PEDOT:PSS film by dipping to 10wt% EG
solution shows a stronger peak at Ek of 154 eV for thecomparison with that of PEDOT:PSS film by dipping
1wt% EG solution. It indicates that the new states
distributed in the vicinity of S atom of PEDOT bydipping to high concentration EG solution appears near
the top of valance region. The conductivity of PEDT:PSS
by dipping to 10wt% EG solution becomes higher by 2-3orders magnitude than that by dipping to 1wt% EG
solution. The states distributed in the vicinity of S atom
in PEDOT at near valence band top can be considered tobe related to the high conductivity of treated PEDOT:PSS.
4 Conclusion
The local electronic structure of valence region in thevicinity of S atom of PEDOT:PSS film surface can beobtained by APECS. It is found that the electronicstructure in the vicinity of S atoms of PEDOT is different
from that of PSS、The states in the vicinity of S atoms in
PEDOT for high concentration EG treated PEDOT PSS
film can be detected at most valence top region and are
related to the origin of high conductivity of PEDOT :PSSfilm.
Acknowledgements
The authors are thankful to Prof. K. Mase about IMSS(Institute of Materials Structure Science) for his help withhis homemade coincidence analyzer.
References[1]T. Takano et al Macromolecules, 45, 3859-3865(2012)[2]X. Crispin et al Chem. Mater. 18, 4354-4360(2006)[3]S. Arae et al J. Vac. Soc. Jpn, 56, 507-510(2013)* okudaira@faculty.chiba-u.jp