ESR03: Vapor Phase Infiltration (VPI) and Doping of Conducting Polymers / Thesis

CIC NanoGUNE, Spain  

Weike Wang
PhD Thesis

Abstract / Thesis download here

Over the past years, conductive polymers have been extensively investigated due to
their tremendous importance as integral part of a wide range of electrical devices. For
instance, studies have reported on enhanced mobilites of charge carriers in field-effect
transistors (FETs) achieved by trap filling, improved charge injection in light-emitting diodes
(LEDs) and, more recently, superior power conversion efficiencies in organic-photovoltaics.
During the fabrication of the electrical devices, for improving their efficiency and lifetime, a
crucial step is the realization of stable and controllably doped transparent highly conducting
polymer thin films. Nowadays, the typical doping strategies for conducting polymers usually
rely on charge-transfer redox chemisty (chemical or electrochemical n-/p-doping processes),
and non-redox acid-base chemistry (protonation of polyaniline), which result in permanent
electrical conductivity. However, these doping processes are usually performed in the liquid
state (solvents or solutions), which can introduce various chemical species, such as solvents
or byproducts of chemical reactions, into the conducting polymer matrix, and in this way
bring about negative consequences for the conductivity and stability of the polymer. In
addition, doping processes in liquid state can also influence the morphology, structure and
purity of the conducting polymer, which are very important for various applications. In this
thesis, an alternative approach is introduced, which can avoid such negative influences and
provide better control of the doping process. The ¨ Vapor Phase infiltration ¨ (VPI), a vacuum
based process derived from the atomic layer deposition (ALD), is used for the top-down
infiltration and doping of conducting polymers.
In the first part of this thesis, a single precursor vapor phase infiltration (VPI) process
to dope polyaniline (PANI) is presented. As dopants, the vaporized Lewis acidic precursors
SnCl4 and MoCl5 were used at a process temperature of 150 °C. The conductivities are
extracted from four-point current-voltage measurements. From the room-temperature I-V
plots, it was found that the conductivities of the infiltrated PANI are a function of the
infiltration cycle number. After 100 cycles, the MoCl5-infiltrated PANI showed the highest
conductivity, 2.93 × 10-4 S/cm, which is a significant enhancement of up to 6 orders of
magnitude in comparison to undoped PANI. SnCl4-infiltrated PANI showed highest
conductivity after 60 cycles with a value of 1.03 × 10-5 S/cm. The enhancement amounted to
5 orders of magnitude. After heating the infiltrated samples at 150 °C in a vacuum
environment for 100 min, it was found that the conductivity of traditionally doped PANI (1M
HCl) decreased by nearly 7 orders of magnitude, which is likely to be due to the
deprotonation of the doped PANI and evaporation of HCl. The conductivities of the VPI
doped PANI decreased to a much lower extent, which indicates that the infiltrated metal
chlorides were trapped inside the polymer matrix, resulting in a higher stability of the doped
polymer even in a harsh environment. SEM images showed that the VPI process did not alter
the PANI morphology, thus the process can indeed be used for top-down doping after a
desired morphology of PANI has been obtained. According to the FTIR, Raman and UV-vis
spectra, the doping process with MoCl5 and SnCl4 results in an oxidation of PANI and
presumably in complexation of the metal chlorides with the nitrogen atoms of PANI. As a
result, the electron mobility along the polymer chains is significantly enhanced and the
structure is stabilized even at an elevated temperature in a vacuum environment.
The second part of this thesis describes the single precursor VPI and doping of poly(3-
hexyl)thiophene (P3HT). For the infiltration processes, the Lewis acid MoCl5 was used as
precursor at a process temperature of 70 °C. The room-temperature I-V plots show a
dependency on the VPI cycles numbers. The highest values show an increase of 5 orders of
magnitude for the conductivity, namely from 1.44 × 10-5 S/cm in the as-prepared P3HT to
3.01 S/cm after 100 infiltration cycles. SEM images show a homogeneous film of P3HT in
the pristine state, while the MoCl5-infiltrated P3HT thin film became decorated with
nanoscale islands. A possible reason for this change is the intercalation of MoCl5 into the
polymer martrix, resulting in a swelling of the polymer. Elemental mapping of Mo showed a
considerable amount of the metal inside the bulk of the infiltrated polymer. It was further
found that the MoCl5-infiltrated P3HT is temporally very stable. The absorption spectra, the
perceived color, and the measured conductivities only marginally changed upon exposure of
the samples to air in ambient conditions for 30 days. The FTIR and XPS spectra show that the
doping with MoCl5 from the vapor phase results in a local oxidation of P3HT, owing to the
Lewis basic character that P3HT shows in conjuction with MoCl5. The lone electron pairs at
the sulfur atoms of the thiophene rings in P3HT can donate negative charge to the MoCl5 with
themselves becoming rather positively charged.
In the last part of this thesis, the multiple pulsed vapor phase infiltration (MPI)
process was applied to dope polyaniline (PANI). For the process, the two typical ALD
precursors diethylzinc (DEZ) and deionized water (H2O) were used at a process temperature
of 155 °C. The room-temperature I-V polts showed the conductivity of Zn-infiltrated PANI
increasing to 18.42 S/cm, up to three orders of magnitude higher than obtained upon
conventional doping with 1 M HCl in wet-chemical ways (8.23 × 10-2 S/cm). SEM images did
not show any obvious change of the polymer after infiltration, except a slight variation in the
PANI fiber diameters, which is due to the unavoidable surface-deposited ZnO. The TEM and
EDS scans of cross-sectioned regions of individual Zn-infiltrated PANI fibers show a
significant presence of Zn in the bulk of the polymer. From FTIR spectra, it was found that a
thermally actived chemical reaction between the polymer and precursor takes place, which at
temperatures equal or above 155 °C becomes pronounced. Raman spectra additionally
showed an evolution of bands, which are associated with the C-N vibrational A1g mode,
indicating the formation of a bond between Zn and N. Infiltration of PANI with Zn from the
vapor phase results in a hybrid or composite material consisting of ZnO and PANI in the
subsurface area of the polymer. Being chemically bound to each other, the inorganic and
organic components mutually dope each other for the benefit of the resulting conductivity.