Biennial Report 2016/2017

Reports
Low Temperature Photochemical Conversion of Metal-organic Precursor
Layers to Metal Oxide Thin Films
P.C. With, U. Helmstedt, S. Naumov, A. Sobottka, A. Prager, U. Decker, R. Heller, B. Abel, L. Prager
Introduction
The demand of large-area and flexible metal
oxide thin films - especially as composites with
thermally sensitive substrates, i.e. polymer films -
is steadily increasing. Such composite materials
offer multiple functions, necessary for
contemporary and novel applications in the field of
flexible electronics, which rapidly expands over
the last decades. This includes applications in
flexible electronic devices, such as a)
antireflective coatings to increase the efficiency of
solar cells, b) as gas barrier layers for protection
of flexible solar-modules, UV-protection and
protection of organic lighting-devices, c) or as part
of functional coatings for thermochromic window
films, sensors, self-cleaning surfaces, organic
light-emitting (OLED) and organic photovoltaic
(OPV) devices. Nevertheless, high costeffectiveness,
large-area applicability and lowtemperature
processing associated with the
desired final film properties are criteria not yet
fully met by existing technologies.
State of the art approaches to deposit inorganic
thin films include chemical or physical vapour
deposition methods (CVD, PVD), reactive
sputtering, atomic layer deposition (ALD), and solgel
techniques. But either high processing
temperatures >350 °C, e.g. in sol-gel methods
and atmospheric pressure CVD, or low
processing pressures, e.g. in most CVD, ALD and
PVD methods, are required. A photochemical
conversion approach represents an attractive
alternative especially for inorganic thin film
deposition on flexible and thermally sensitive
polymeric substrates, such like polyethylene
terephthalate (PET), polyethylene naphthalate
(PEN) and polycarbonate (PC) because a liquid or
solid precursor coating can be converted directly
on the substrate. However, there is less
knowledge of this process available and it needs
to be developed regarding accessible inorganic
materials and an energy efficient precursor
conversion to utilize it for roll-to-roll (R2R) coating
processes, e.g. for printed electronics.
Prager et al. [1,2] have shown that chemically
pure silicon(IV) oxide thin films can be obtained
from polysilazanes, e.g. perhydropolysilazane
(PHPS) as precursors, using different
commercially available vacuum ultraviolet (VUV)
and far ultraviolet (UVC) lamps (E Ph about 7.2, 6.7
and 5.6 eV, respectively). This photochemically
triggered process proceeds at temperatures
below 80 °C. For this approach, further studies
showed that roll-to-roll processes became
technically feasible due to the required low radiant
exposures of about H e = 0.5 J cm -2 . In this
manner, ~100 nm thick PHPS layers can be
converted to SiO x (x ≈ 1.8–1.95) with high
densities of 2.1 g cm -3 directly on thermally
sensitive polymer substrates (PET, PEN), to
produce transparent and flexible gas-barriers. [2]
Besides polysilazanes, further studies were done
using a polymeric metal organic aluminium
hexanoato complexes as precursor to obtain
aluminium (III) oxide photochemically. [3]
However, the necessary radiant exposure for
complete mineralization was two orders of
magnitude higher than for PHPS and reaches
values as high as H e = 36 J cm -2 , which is
unattractive for technical applications such as
R2R coating. Thus, a contemporary challenge is
the development of this technology with the aim to
minimize the needed radiant exposure for
decomposition and mineralization of appropriate
precursors, especially at low processing
temperatures.
Herein, it is shown that photochemical conversion
of a titanium metal-organic precursor can be
applied for deposition of pure TiO x semiconductor
thin films onto various substrates, including
thermally sensitive polyester films. [4] In addition
an insight into the conversion kinetics, formed
intermediates, and reaction products is given by a
combined experimental and theoretical approach.
Experiment
The experiments were conducted under inert
conditions inside of a glovebox, a 0.75 mol l -1
solution of Ti(OEt) 4 (95 wt.%) in n-dibutylether
(water content