New application of spectroscopic imaging ellipsometry 

New application of spectroscopic imaging ellipsometry
in the field of organic photovoltaic (OPV) materials
B
 Introduction
 Sample Preparation
Since several years the development of organic semiconductor materials as active layer for organic solar
cells has become one of the main challenges in chemistry and applied physics. π-conjugated polymers are one
of most efficient exponent regarding optical and electronic properties for organic solar cells.
The πconjugated polymers stand out due to their structural diversity and optoelectronic properties. The modification of
the donor-acceptor-donor (DAD) architecture is a simple
and efficient way to construct a well-defined
π-conjugated system with low oxidation potential, broad
absorption spectrum and low band-gap. Controlling the
morphology, homogeneity and the optical properties of
the organic photovoltaic material is essential for optimizing the efficiency of organic solar cells when employing
π-conjugated polymers as photoactive layer.
 Monomers (DBQHT and PHEHT) were synthesized
Here we present the optical properties of two DAD type
π-conjugated
polymer
films
(poly-10,13-bis(4hexylthiophen-2-yl)dibenzo[a,c]phenazine (poly-PHEHT)
and
poly-5,8-bis(4-hexylthiophen-2-yl)-2-(2,3dihydrobenzo[b][1,4]dioxin-7-yl)quinoxaline
(polyDBQHT) Fig. 1) on indium tin oxide (ITO) doped glass
slide .
Fig. 1 Chemical formula of two DAD type π-conjugated polymers .
 Imaging ellipsometry
Fig. 2 Schematic setup of an imaging ellipsometer.
Ellipsometry is a sensitive, nondestructive optical method for determining the thickness and optical properties
of thin layers. It makes use of the fact that light changes
the state of polarization when it is reflected from a surface. Thereby the change in polarization is influenced by
the entire optical system of film (or a stack of films) and
substrate.
For nulling ellipsometry the ellipsometric parameters Δ
and Ψ are determined at the “nulling” point out of the
positions of the optical components (Fig. 2). The combination of a classical nulling ellipsometer with a microscope (objective and CCD-camera) leads to a powerful
visualization tool which is very sensitive to thin and/or
structured films.
via Stille coupling.
 Polymer layers were produced by electropolymeriza-
tion using ITO doped glass slides as working electrode.
 Spectroscopic Ellipsometry
All UV/VIS spectra of Delta and Psi were performed with
an nanofilm_ep4 with 10x objective, at an angle of incidence (AOI) of 50° and purposing a beam cutter to avoid
backside reflections. The mean value of the thickness
and the optical properties of selected regions of interests
were obtained by a fitting process using nanofilm_ep4
model software.
As reference a blank
ITO glass electrode
was measured under
the same conditions
(Fig. 3) to determine
the thickness and optical properties of the
ITO layer and the dispersion of the glass.
The dispersion of the
glass is represented
by a Cauchy function
3 Simulated and measured UV/VIS Delta Psi
with k = 0 as a trans- Fig.
spectra of the ITO (d = 140.7 nm) glass electrode.
parent material.
The optical properties of the ITO layer was parameterized by a superposition of one Tauc-Lorentz and one
Drude oscillators.
The π-conjugated polymers were measured
on special selected
Region
of
interest
(ROI). Figure 4 shows a
comparison
of
the
ellipsometric
high
contrast image (top)
(10x
objective,
AOI = 50°, λ = 570 nm,
nanofilm_ep4) of a thin
film of poly-PHEHT on
ITO/glass and a classical microscope image
(bottom) (including the
region
of
interest
(ROI)). UV/VIS spectra
of Δ and Ψ of both poly4 Ellipsometric high contrast image (top)
mers are shown in Fig. Fig.
(10x objective, AOI
= 50°, λ = 570 nm,
4
5. The optical proper- Accurion, EP -SE) of poly-PHEHT film
~ 40 nm) on
ITO/glass (ROI = green
ties (Fig. 6) of the ab- (d
square) and a classical microscope image
(bottom).
sorbing material is
parameterized by a superposition of two Lorentz and
one Drude oscillators.
The dispersion of the DAD type π-conjugated polymers
shows a local maximum in the absorption (π-π* transition), λmax (poly-PHEHT) = 560 nm and λmax (polyDBQHT) = 535 nm ).
The combination of high speed and high sensitivity
for similar samples is ideal for process control applications. Hence the setup was tested with Graphene-Monolayer on a Si-substrate. To be able to compare the measurements with a simulation, the spectra
were corrected by the source-intensity and the detectorsensitivity.
Fig. 5
Simulated and measured Δ and Ψ wavelength spectra (AOI= 50°) of polyPHEHT (left side, d = 43.4 nm) and poly-DBQHT (right side, d = 240.5 nm) from the nanofilm_ep4 model software.
Fig. 6
Simulation of the optical
properties of the π-conjugated polymers
(blue = poly DBQHT, red = poly-PHEHT)
consisting of two Lorentz and one Drude
oscillators (nanofilm_ep4 model software).
 Conclusion
 Imaging ellipsometry is a powerful tool for characteri
zation of thin organic absorbing materials (thickness,
optical properties) on transparent substrates, fast
observation of surface morphology and homogeneity
at microscopic scale.
 The π-π* absorption maximum in the modeled
dispersion of poly-PHEHT shows a 25 nm shift
compared to poly-DBQHT.
Acknowledgement:
Financial support of N-Bank (WA3-801226015) is kindly
acknowledged.
The samples were provided by Levent Toppare, Department of Polymer Science and Technology, Middle East
Technical University, 06531 Ankara, Turkey.
 Contact
C. Röling, P. H. Thiesen
[email protected]
Accurion GmbH
Stresemannstraße 30
D-37079 Göttingen, Germany