Ellipsometric Spectroelectrochemistry: An In Situ Insight in the Doping

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Ellipsometric Spectroelectrochemistry: An In Situ Insight in the Doping of Conjugated Polymers Christoph Cobet, Kerstin T Oppelt, Kurt Hingerl, Helmut Neugebauer, Günther Knör, Niyazi Serdar Sariciftci, and Jacek Gasiorowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08602 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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The Journal of Physical Chemistry

Ellipsometric Spectroelectrochemistry: An In Situ Insight in the Doping of Conjugated Polymers

Christoph Cobet,

∗,†,‡

Günther Knör,

†Linz

¶

Kerstin Oppelt,

∗,¶

‡

Kurt Hingerl,

§

Niyazi Serdar Sariciftci,

§

Helmut Neugebauer,

and Jacek Gasiorowski

∗,‡,§

School of Education, Johannes Kepler Universität, Altenbergerstr 69, A-4040, Linz, Austria

‡Center

for Surface- and Nanoanalytics, Johannes Kepler Universität, Altenbergerstr 69, A-4040, Linz, Austria

¶Institute

of Inorganic Chemistry, Johannes Kepler Universität, Altenbergerstr 69, A-4040, Linz, Austria

§Linz

Institute for Organic Solar Cells, Johannes Kepler Universität, Altenbergerstr 69, A-4040, Linz, Austria

E-mail: [email protected]; [email protected]; [email protected]

1

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this work we introduce in situ UV-Vis-NIR transmission spectroscopic ellipsometry as an alternative spectroelectrochemical method to examine the π -electron orbital structure as well as intra- and inter-chain order in regioregular poly(3-hexylthiophene) (rr-P3HT) during electrochemical doping. With the suggested technique we measure the dielectric tensor of the P3HT lm through a typical electrochemical cell consisting of a glass cuvette. The method provides a stable controlled electrochemical environment with full control on the applied potential during the ellipsometric measurement. P3HT, which is a model semi-transparent conducting polymer, undergoes a sequence of oxidation steps with increasing electrochemical potential. In the presented work the doping is monitored up to a level where nally almost every second thiophene ring is positively charged. The reference free determination of the real and imaginary part of the dielectric tensor components allows a quantitative analysis of resonance energies as well as the orientation depended oscillator strength of inter-chain exciton and polaron resonances. This parameters are directly linked with (i) the intra-chain structure, (ii) the degree of electron localization (iii) the π -π stacking order, (iv) the average polymer orientation, and (v) the doping level. Additionally we could observe aggregation depending doping eciency. The lm optical properties appear fully reversible upon the cyclic electrochemical doping which proofs the stability of inter- as well as intra-molecular structure while going through very high doping levels.

1

Introduction

A lot of research eort has been addressed to the optimization of organic semiconductors concerning their physical and chemical properties as well as to the development of processing techniques.

13

Modications in the physical properties are not only achieved by adjusting

the chemical structure, but also oxidation or reduction processes are commonly used in order to achieve a so called doping of the polymers. Primarily, the doping induces charges which could yield in an increasing conductivity alongside with modication of the inner polymer

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structure and the electronic conguration (i.e. optical properties of the material). There are many ways to dope organic materials, but in general they can be divided into four groups:

3

1. Chemical oxidation (reduction) for p- (n-) doping

2. Electrochemical oxidation (reduction) for p- (n-) doping

3. Field induced doping in organic eld eect transistors (OFETs)

4. Photo-induced doping by photo-excited electron transfer from a donor onto an acceptor

From these four doping methods, electrochemical oxidation and/or reduction is of special interest for us since it provides an easy control of the charge density and thus the doping level.

This exibility to modify a variety of physical parameters can be used to obtain a

better understanding of the doping process in general. For these reasons a lot of attention is drawn to the combination of electrochemistry with dierent analytical techniques. This includes the development of dierent of the organic material properties.

in situ techniques aiming for a full characterization

Raman Spectroscopy,

Fourier Transform Infrared Spectroscopy (FTIR),

5

in situ UV-Vis-Spectroscopy, 4

Electron spin resonance (ESR),

6

Nuclear

Magnetic Resonance (NMR) or Impedance Spectroscopy (EIS) combined with cyclic voltammetry and/or potentiostatic measurements, allow studies of optical, molecular and electronic changes during doping processes in organic semiconductors. On the other hand, for optoelectronic applications of conjugated polymers, detailed knowledge of the optical properties in the spectral range of electronic excitations is of special importance.

Usually they are determined using transmission, reection UV-Vis, and/or

photoluminescence spectroscopies. Although these methods are simple and widely used for materials analysis, they are not able to determine the lm dielectric properties quantitatively correctly and without approximations and references. Thus, the optical properties in the visible spectral range are usually used more like as a nger print and for the discussion of relative changes. A detailed investigation of the physical nature of specic optical transition is dicult.

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Page 4 of 36

In this manner, a quantitatively correct determination of the complex dielectric function (or equivalently of the complex refractive index) and it's variations upon doping of the material, provides a more precise insight in the physical processes. Particularly desirable is, nevertheless, the accurate separation of the imaginary part of the dielectric function, since peaks in the line shape and their amplitude represent optical absorption resonances, dipole allowed transitions from initial to excited states.

79

i.e.

Their strength is determined by

the combined density of the electronic states and the transition probability

i.e. the dipole

matrix element. From fundamental aspects, it is important to know how these properties are connected to

e.g. the extension of

π -conjugation

as well as the rigidness and regularity

of the polymer backbone which are highly susceptible to changes in the side-groups. These connections become even more important while studding the doping induced increase of the electrical (DC) conductivity as the changes by several orders of magnitude depend also on conformational modications.

1012

The most precise way to determine the dielectric function of an organic semiconductor in the absolute scale is spectroscopic ellipsometry (SE). Commonly this optical method is implemented as a reection technique where the change in the light polarization upon the reection on interfaces is determined independently from the total intensity.

Up to date,

spectroscopic ellipsometry is established as one of the standard techniques for characterization of inorganic semiconductors.

Also it proves its applicability for the determination of

the optical properties of organic molecules (small molecules) and polymers. An

ex situ SE

measurement of a polymer that has been chemically doped by iodine vapor was also recently reported by our group.

13

However,

in situ spectroelectrochemical application of ellipsome-

try was still missing. In latter measurements we gain a precise control of the doping in an extended range through the electrical potential. For SE in reection geometry a specically designed electrochemical cell and respective sample size is required.

To avoid limitations

related to such a purpose-made cell design, we introduce here a transmission conguration which permits ellipsometric spectroelectrochemistry through a standard glass cuvette

4


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in case of transparent substrate materials. Furthermore, this setup allows a comparison to the well-known intensity transmission measurements. In this work we use this method to investigate regioregular poly(3-hexylthiophene) (rrP3HT), which is a model donor material in optoelectronics. Regardless of the huge amount of characterization work done on this prototype semiconducting organic polymer, the fundamental understanding of the electronic structure and charge transport with increasing electrochemical doping level remains fragmentary.

From

e.g. cyclic voltammetry experi-

ments it is known that P3HT undergoes at least three distinct oxidation steps with raising anodic potentials.

14

Each of them is increasing the conductivity of the lm signicantly.

In

situ transmission ellipsometry is used here for a quantitative analysis of variations in the electronic structure as a function of the applied potential and transferred charges.

2

Experimental Details

2.1 Electrochemical Preparation For the experiment, we used regioregular poly(3-hexylthiophene) (rr-P3HT) (98%, Rieke Metals). In P3HT the thiophene rings have a hexyl side chain to increase solubility. The polymer was dissolved in chlorobenzene (99+%, Acros Organics) (10 mg/ml) and spin-casted on precleaned glass covered with conducting, transparent indium-tin oxide (ITO) (15

Ω/,

Kintec

Co.). The glass/ITO substrate was cleaned afore using sequential washing in the ultrasonic bath in acetone/isopropanol and deionized water. For the electrochemical measurements, an electrolyte solution of Bu4 NPF6 (≥99%, Fluka Analytical) in anhydrous acetonitrile (99.8%, Aldrich), was prepared (0.1 mol/l). In the setup the conducting ITO-coated glass covered with the thin layer of P3HT was used as working electrode (WE). A Pt foil was applied as the counter electrode (CE) and an Ag/AgCl wire is used as a quasi-reference electrode (QRE). The QRE reference was prepared using the recipe presented elsewhere.

5


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2.2 Spectroscopic Ellipsometry in Transmission Conguration For the

in situ ellipsometric measurements we use a commercial M2000 variable angle of

incidence spectroscopic ellipsometer (VASE) from the J.A.Woollam Co., Inc.. Two spectrograph's as well as a combined tungsten-halogen and deuterium light source allow with this instrument in principle a parallel measurement at 750 increments wavelengths in a range from 190 to 1675 nm which corresponds to a photon energy range from 6.5 to 0.74 eV. With spectroscopic ellipsometry we primarily determine the change of the light polarization upon reection and/or transmission through a sample. This change is recorded by means of the so called ellipsometric angles

Ψ

and

∆.

These two angle parameters account for a polarization

rotation and a phase shift between polarization components parallel and perpendicular to the plane of incidence.

In order to determine

Ψ

and

∆

most precisely, a xed polarizer,

rotating compensator, sample, and xed analyzer (PCRSA) conguration is used. In such setup we can determine

∆

◦ ◦ 16 in its full range from -90 to +90 with a uniform sensitivity.

Commonly, ellipsometric measurements are performed in reection geometry at angles of incidence around the Brewster angle. In this work we use a straight transmission conguration. As depicted in the graphical abstract and gure 1, the light beam is transmitted straight through the sample and all optical components. The sample surface is tilted by 45

◦

with respect to the incident light beam. The principle axes of polarization are referring to the plane of transmission which extents between the sample surface normal and the light beam. Such a conguration allows an

in situ optical characterization in a standard cuvette

(electrochemical cell; Fig. 1a). Light reection and polarization eects at the outer cuvette surfaces are suppressed by two 45

◦

refractive index matching liquid.

In this arrangement eective incident angle

glass prisms which are attached to the cuvette with a

φ(ω)

at the

◦ polymer surface is about 49 (Fig. 1b). The dielectric properties of P3HT are calculated applying an optical layer model. The therein required dielectric properties of the glass cuvette, the electrolyte, the glass substrate, and the ITO lm were determined stepwise in standard reection ellipsometric measurements

6


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(a)

WE: P3HT/ITO/glass CE: Pt QRE: Ag/AgCl

light in

(b)

light out

56 nm 100 nm

> 1 mm

1 mm

> 1 mm

sample Figure 1: (a) Image of the glass cuvette with connectors for the quasi reference electrode (QRE), the counter electrode (CE) and the ITO/glass working electrode (WE) with the P3HT lm. (b) Schematic diagram of the light propagation through the glass cuvette including the P3HT lm on the ITO/glass substrate.

prior to the experiment. The thickness of the P3HT (56 nm) and ITO (100 nm) layer has been independently measured with both ellipsometry and a stylus DEKTAK prolometer (Bruker Corp.). The thicknesses calculated from ellipsometric measurements and those determined with the prolometer coincided within a few percent deviation.

2.3 Optical Layer Model For the extraction of the dielectric properties of the P3HT lm from

Ψ

and

∆

we used an

optical layer model based on the classic electrodynamic theory of light reection/transmission at interfaces and light propagation in media. Six interfaces in the electrochemical cell are contributing to the measured change of the polarization: (i) the glass cuvette/electrolyte, (ii)

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Page 8 of 36

the electrolyte/ P3HT lm, (iii) the P3HT lm/ITO lm, (iv) the ITO lm/glass substrate, (v) the glass substrate/electrolyte, and (vi) the electrolyte/glass cuvette interface. Figure 1b shows a schematic diagram of the thereby dened light propagation in the cuvette. For the correct physical description, the coherence length (temporal coherence) of the light source is of a major importance. In our particular case the light sources/spectrographs have a spectral resolution of

∆λ=

of about 30-100

2 to 4 nm which results in a wavelength

µm (λ2 /2π∆λ).

dependent coherence length

Most of the above dened layers like the glass substrate

are much thicker than the coherence length. are smaller.

λ

Only the P3HT and the ITO lm thickness

Thus only in these two layers interference eects of forward and backwards

traveling waves have to be taken into account. Otherwise the waves superimpose each other incoherently. The amplitude and phase of the transmitted waves are calculated by means of the so called Berreman transfer matrix formalism.

With this approach it is possible

to calculate the wave propagation in arbitrary anisotropic media.

1618

Double refraction

images from the two light beams with dierent Poynting vectors are negligible because the anisotropic polymer layers are thin. Interferences between layers thicker than the coherence length of the light are suppressed by averaging over a number of arbitrarily chosen layer thicknesses. Note, the results obtained in this way are independent from the chosen values. For the calculation we used the CompleteEASE software distributed by the J.A.Woollam Co., Inc.. The ellipsometric angles

Ψ

and

∆

for light, which is transmitted through the entire cell,

can be expressed in terms of two overall complex transmission coecients for parallel tp and perpendicular ts polarized light according to:

tp = tan Ψei∆ . ts For isotropic lms, the optical model for

Ψ

and

(1)

∆

nally contains only two unknown

parameters which are the real and imaginary part of the dielectric function or in an equivalent

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formulation the real and imaginary part of the complex refractive index (n

p

=n ˜ (ω) + iκ(ω) =

ε1 (ω) + iε2 (ω)) of the P3HT. These two parameters are determined in comparison with the

experimental results by means of a Levenberg-Marquardt t. In case of uniaxial anisotropic lms, we have to deal with 2 independent dielectric tensor components and thus 4 unknown parameters which requires additional assumptions as explained in the following.

3

Results and Discussion

3.1 Electrochemical Characterization E vs. Ag/AgCl (V) -0.2

I (A cm-2)

0.0

(a)

40 20

0.2 2

nd

0.4

0.6

0.8

1.0

1.2

1.4

3nd oxidation oxidation

2.4 mC cm-2 ~ 2.1*1021cm-3

1st oxidation

0 -20 -40

Q (mC cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5

0.5

2.0

1.2 mC cm-2 ~ 1.1*1021cm-3

0.6

(b)

0.4 mC cm-2 15 -2 ~ 3.4*10 2.4*1020 cm-3

1.5 1.0 0.5 0.0 0

20

40

60

80

100

120

140

160

t (s) Figure 2: (a) Cyclic voltammogram of poly(3-hexylthiophene) (P3HT) recorded with a scan rate of 10 mV/s and (b) the accumulating charge density in the P3HT lm during the anodic sweep from -0.2 V to 1.1 V.

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Page 10 of 36

A detailed electrochemical characterization of the P3HT lms with cyclic voltammetry (CV) as a function of the scan speed and vertex potentials, as well as electrochemical impedance spectroscopy (EIS) were already published and discussed from the electrochemical point of view in detail elsewhere.

14,19

Therefore we restrict the discussion to the most relevant

electrochemical results. A representative CV recorded with 10 mV/s is presented in gure 2. It shows a number of fully reversible redox peaks. We found that three of them contribute to the discussed optical results, although most previous publications from our group just discussed two as the major oxidation steps. 0.35 V

19

Below the redox peaks

i.e. at potentials below

vs. Ag/AgCl, the lm conductivity is very low and P3HT retains a semiconducting

behavior. The EIS measurements and a respective Mott-Schottky analysis (not shown here) revealed a donor concentration in the order of 1×10 to air oxygen induced doping.

18

cm

−3

in this range which is attributed

20,21

The time integral of the anodic current measured in CV disclose that the donor concentration increase by 3.4×10

20

cm

−3

upon the rst oxidation step. However, at this point

the conductivity increase, as determined by impedance spectroscopy, is almost negligible. A closer inspection of the anodic current around the rst oxidation (inset, Fig. 2) shows that this oxidation splits into two contributions which were also not yet discussed in the literature. The major second oxidation step in the CV has a very broad current maximum around 0.7 V

vs. Ag/AgCl while the third oxidation step again yields a more prominent

peak at 1.0 V

vs. Ag/AgCl. The second and third oxidation results in an overall anodic

charge transfer of 1.2 and 2.4 mC cm in the lm of 1.1×10

21

−2

. This corresponds to a total donor concentration

21 −3 and 2.1×10 cm , respectively. The real values are probably slig-

htly lower because of the close vicinity to the border of the electrochemical window. Thus decomposition of the electrolyte or other irreversible degradation processes in the electrochemical cell may start to superimpose the measured redox currents. However, a comparison of the measured donor concentrations with an approximated P3HT-monomer density of 4.5×10

21

cm

−3

shows that the second oxidation step already yields a positive charging of

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about 20% of the thiophene monomers.

After the third oxidation this value increases to

almost 50%. The P3HT-monomer density is approximated based on the lattice parameters which have been measured on P3HT crystals by X-ray.

22

In reference 19 we could further

show that the conductivity considerably increases only with the herein denominated second and third oxidation. All together the conductivity rises by almost three orders of

−1 magnitude up to 8 S cm .

3.2 Potentiodynamic Optical Response (b)

(a)

49 48

2.04 eV

Ψ (°)

(c) 47

1.40 eV 46 45

∆ (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.20 eV

*

max. polaron absorption at 1.4eV

44 9 8 * * 7 rd after 3 6 oxidation step 5 4 3 pristine 2 P3HT 1 0 -1 -2 -200 0 200 400 600 800 1000 1200 1200 1600 800 600 400 200

0

-200

E vs. Ag/AgCl (mV)

Figure 3: (a,b) Dynamic angles

Ψ

and

∆

in situ ellipsometric spectra recorded in terms of the ellipsometric

during oxidation of P3HT. Panel (c) shows potential transients at three

selected photon energies (2.04 eV - A1 inter-chain exciton resonance, 1.40 eV - plasmon resonance, and 1.20 eV).

Employing the transmission spectroscopic ellipsometry setup it was possible to monitor the UV-Vis-NIR lm optical response

in situ simultaneously to the CV measurement. The

spectral and potential evolution of the as determined ellipsometric angles already provide

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Page 12 of 36

several pieces of information about the P3HT lm properties (Fig. 3). The experiment is optimized to obtain an accessible spectral window between

~ω =1.5 and 3 eV (827-413 nm).

At

the smallest photon energies the ellipsometric response is superimposed by a strong spectral feature related to a so called Berreman mode in the thin ITO lm at the electronic plasmon frequency of ITO.

23,24

At higher energies the spectral range is limited by total light reection

at the glass-electrolyte interfaces. If necessary this border can be shifted more to the UV by optimizing the angle of incidence. However, all signicant

π -electronic

are found below 3 eV and thus an extension was not necessary.

excitations of P3HT

The observed excitation

features are entirely related to transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the P3HT polymer. All other materials in the light path through the electrochemical cell have no optical resonances in the spectral range from 1.5 to 3 eV. They contribute to the ellipsometric angles only with a monotonous constant background and all spectral features in the ellipsometric angles are thus P3HT lm properties. The analysis of the potential dependency of

Ψ and ∆ reveals, rst of all, that the recorded

spectra appear fully reversible over the potential cycles. This is a strong proof of the stability of the P3HT lm upon the cyclic electrochemical oxidation and reduction.

Any chemical

or conformational modication would also change the HOMO-LUMO optical response. As it will be discussed in more detail, even a change in the intra-molecular order and the

π -π

stacking of the polymer strands would result in a signicant modication of the measured optical properties.

25,26

The stability of P3HT is in particular remarkable by considering

the very high doping levels after the second and third oxidation step. The electrochemical oxidation, on the other hand, mainly results in a changing population of

π -orbitals while the

chemical bonds, that hold the polymer strands together, are essentially

σ -type.

The

Ψ

and

∆

transients furthermore disclose that the P3HT lm (optical) properties

do not change at all upon potential variations below 0.35 V

vs. Ag/AgCl. Above 0.35 V

vs. Ag/AgCl, however, the optical response changes signicantly.

12


We can identify three

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characteristic optoelectronic congurations of the P3HT in the potential range between 0.2 and 1.3 V

vs. Ag/AgCl. Starting from the pristine P3HT, the spectra change almost

monotonously throughout the rst and second oxidation step where a new spectral feature appears between 1.3 and 1.5 eV. With the third major oxidation step,

i.e. above 0.8 V vs.

Ag/AgCl, this new spectral feature almost disappears again. The three characteristic

Ψ

and

∆

spectra and the respective electrochemical potentials

are emphasized in gure 3a and 3b in green, blue, and red. In gure 3c we additionally show transients at three distinctive photon energies in order to illustrate the dynamic evolution with the electrochemical potential. The

Ψ

and

∆

transients at 2.04 and 1.40 eV concisely

resemble the transformation between the three characteristic optical congurations.

But

the transients recorded at 1.2 eV disclose a peculiarity in the optical response (asterisk, Fig. 3c) in clear coincidence with one of the smaller oxidation steps at 0.5 V

vs. Ag/AgCl

(inset, Fig. 2). A detailed analysis in this photon energy range is unfortunately not possible due to the overlapping ITO lm plasmon singularity in our samples. However, the

∆

Ψ

and

transients demonstrate that even the smaller oxidation steps induce specic change in

the optoelectronic properties of the P3HT lm. Worth mentioning in this connection is the abundance of smaller oxidation steps and their eect on the lm optical properties. Both are not explainable by an oxidation of isolated monomers or polymers. The electrochemical as well as the optical properties are dened by intra- and intermolecular electronic interactions to a major extent. Finally it is worth mentioning, that the ellipsometric angle potentials above 1.1 V

∆

is almost constant at

vs. Ag/AgCl although the current measured in CV is continuously

increasing. The anodic current as well as changes in the lm impedance at such potentials were already attributed to irreversible degradation processes in the electrochemical cell while the measured ellipsometric angle

∆

primarily depends on the absorption coecient of the

P3HT or ITO lm. The ellipsometric response is thus a specic indicator for modications in the lm only.

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Page 14 of 36

3.3 Potentiostatic Ellipsometry Based on the information obtained from cyclic voltammetry and ellipsometric transient measurements we have identied three characteristic potentials at -0.2, 0.8, and 1.3 V

vs.

Ag/AgCl which are studied in more detail. For these potentials and a number of potentials in-between, potentiostatic measurements were performed and the current behavior over time (60 s) was measured. After 40 s of applying the respective potential, when electrochemical equilibrium was obtained, an ellipsometric spectrum was recorded. The thereby determined potentiostatic

Ψ and ∆ spectra are in close agreement with the spectra recorded in transient

measurements (green, blue, and red line, Fig. 3a, 3b) and are used in order to determine the potential dependent dielectric properties of the P3HT lm. Therefore we use the optical layer model which has been described in the experimental details. For optically isotropic media the real and imaginary part of the dielectric function (or equivalently

n ˜

and

κ)

can be unambiguously calculated from the two ellipsometric angles.

A thereby determined dielectric function of our P3HT lm, however, turned out not to be Kramers-Kronig consistent.

The KK-inconsistency is assigned to an out-of-plane optical

anisotropy. Such an assumption is in agreement with previously published tric measurements on spin-coated or drop-cast P3HT lms on glass or Si.

ex situ ellipsome-

27

An out-of-plane

anisotropy is obtained if the P3HT polymer strands in the lm are predominantly aligned parallel to the substrate, since the near band gap optical transitions in each P3HT polymer can only be excited by the electric eld component along the conjugated chain.

26,28,29

Azi-

muthal scans reveal, on the other hand, an isotropic in-plane dielectric function (DF) and thus in average no further in-plane directional arrangement of the P3HT. With the postulation of out-of-plane anisotropy, the lm dielectric function is replaced by a tensor, which contains four independent parameters,

i.e. the real and imaginary part

of the in- and out-of-plane dielectric functions. A rigorous point by point t of the

∆

Ψ

and

spectra in a respective layer calculation, is therefore not possible anymore. But without

reducing the generality for the investigated system, we can further approximate that the two

14


-200mV 2

800mV

1300mV

A2

2

ε2

A1

Polaron

A1 A2

1

1

0

0

ε|| (in plane)

4

ε⊥ (out of plane)

4

ε1

ε1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ε2

Page 15 of 36

3

3

2

2 1.5

2.0

2.5

3.0

photon energy (eV)

1.5

2.0

2.5

3.0

photon energy (eV)

Figure 4: Real (ε1 ) and imaginary (ε2 ) part of the dielectric function for in-plane and outof-plane electric elds. The three spectra were recorded at -0.2 V (olive), 0.8 V (blue), and 1.3 V (red)

vs. Ag/AgCl and represent the undoped, polaronic, maximal doped lm optical

properties.

dielectric functions consist of a number of equal Gauÿ-Lorentzian oscillators which dier only in the oscillator strength. In this way we ensure, furthermore, that the in-plane and outof-plane dielectric function exhibit a Kramers-Kronig consistent spectral line shape. Under these conditions, it is nally possible to obtain a lm dielectric function for the in-plane and out-of-plane light polarizations as presented in gure 4. The error of the in-plane DF is approximated with max. 10% while we have to consider an error of about 50% for the out-of-plane DF. For all three applied potentials we determined a very strong out-of-plane anisotropy. In the undoped state, the imaginary part of the in-plane DF is in average 10 times larger than in the out-of-plane component. Based on the measured anisotropy we can estimate the volume faction of the P3HT polymer chains aligned perpendicular to the surface. For this approximation it is important that the transition dipole moments are zero for light polarizations perpendicular to the polymer strands.

26

We further assume that the lm consist of polymer

bunches which dier only concerning the P3HT strand orientation parallel or perpendicular

15



Page 16 of 36

to the surface while the dielectric properties of each polymer bunch are the same. In such a framework, the in-plane and out-of-plane lm DF can be obtained in terms of a Bruggeman eective medium.

16,30

In a self-consistent approach (where we use a depolarization factor of

0.33) we got an average volume fraction of the polymer strands with an orientation perpendicular to the surface of approximately 8%. The observed strong orientational anisotropy is remarkable if we consider that the planar orientation is initially induced only by the P3HTITO interface. After the rst mono-layers the planar orientation has to be mediated by the

π -π

stacking of the polymer strands over the entire 56 nm lm.

27,31,32

Worth mentioning is the fact that absorption peaks in the imaginary part of the eective out-of-plane DF (dashed lines, Fig. 4) appear blue shift in comparison to the much stronger in-plane DF. This shift is mainly a consequence of the minimum in the real part of the DF (lower left panal in Fig. 4) and the respective minimum in the screening properties. HamidiSakr

et al. observed a similar blue shifted in absorption experiments at P3HT lms with an

in-plane alignment of the polymer chains.

33

They explain the blue shift in the smaller tensor

component with a spectral similarity to the absorption of coiled P3HT in solution. In our case, conformational dierences are also conceivable. Polymer chains with a perpendicular orientation to the surface may also coil up and nally both eects may play a role. The DF of the undoped P3HT lm was measured at -0.2 V

vs. Ag/AgCl (left spectra,

Fig. 4) contains a series of electronic excitations and compares well with spectra measured

ex situ. The choice of the potential was driven by the fact that this potential is closest to 0 V

vs. a standard hydrogen electrode. The resonances energies (wavelength) of the contri-

buting excitations can be identied by the respective peak position in part of the DF (Fig. 5a).

ε2

i.e. the imaginary

For the following discussion it is appropriate to distinguish ex-

citations related to isolated P3HT molecules from those which emerge in bulk P3HT lms. Therefore we compare our ellipsometric results again with the absorption spectrum of the solvated P3HT. In gure 5b the absorption spectrum measured through P3HT diluted in chlorobenzene (0.1 mg/ml) is compared with a lm absorption spectrum calculated from our

16


Page 17 of 36

ε2 (in plane)

3.5

A2 (A1+1LO)

@ -200mV A1

3.0

(a)

A3 (A1+2LO) "A4" (A1+3LO)

2.5

HOMO ↔ LUMO

2.0 1.5 1.0 0.5

Polaron

0.0

Absorbance α (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

solution film

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

photon energy (eV) Figure 5: Optical transitions in pristine P3HT: a comparison of the imaginary part of the lm dielectric function (a) with lm absorbance calculated from the dielectric function (b black line) and the absorbance measured for P3HT diluted in chlorobenzene (b - violet line). The blue, orange, and gray lines in the upper penal (a) represent the Gaussian oscillators of our parametric t.

ellipsometric results by

α(ω) = κ(ω) where

c

2ω , c

(2)

is the light velocity.

The absorption spectrum of P3HT in solution possesses just one broad absorption peak around 2.7 eV, which is attributed to the intra-chain HOMO-LUMO transition (conjugated

π -π ∗

transition) of the isolated P3HT polymer chain.

11

This intra-chain HOMO-LUMO

band-to-band transition is found also in the P3HT lm. The onset of the P3HT lm absorption, however, is at much lower energies. The rst distinct resonance in been assigned to a delocalized inter-chain singlet exciton (A1 ).

3437

ε2

at 2.04 eV has

The following two peaks

at A2 and A3 are vibronic side-bands. The inter-chain exciton with vibronic side-bands are mentioned in literature also as aggregate absorption/emission bands .

38

Spano

et al. have

theoretically discussed the relative intensity of the vibronic replica based on the FranckâCondon principle and how it depends on the microscopic order in the polymer (P3HT)

17



lms.

26,39

Page 18 of 36

Attention should be paid to the dierence between the here discussed inter-chain

excitons and the before mentioned photo-induced doping obtained by an optical transition between dierent donor and acceptor polymers/molecules with a charge separation and much longer live times. The inter-chain character of the A1 -A3 resonances and it's dependency on polymer strand conformation, was already studied on pristine P3HT lms.

33,40,41

Accordingly, the A1 -A3

resonances are absent for totally disordered lms or P3HT-blend lms where the single P3HT polymer strands have no direct contact.

The amplitude ratio A1 /A2 is shown to

increase if the level of intra-chain order (the eective conjugation length) increase while it decrease, on the other hand, if the inter-chain exciton coupling decrease.

et al. achieved a certain degree of crystallization due to the strands after annealing of their lms.

41

π -π

26,33,39

Karagiannidis

stacking of the polymer

The thereby obtained relative A1 oscillator strength

is comparable to our spin-casted lm which is measured in electrolyte at potentials below 0.35 V

vs. Ag/AgCl. Hamidi-Sakr et al. achieved even higher ordering with an in-plane

alignment of the P3HT strands with a high-temperature rubbing method yielding in an further increased relative A1 amplitude.

It is worth noting that the oscillator strength of

A1 do not change after our electrochemical redox cycles although we attained very high doping levels in-between. We conclude that likewise also the degree of crystallization and the arrangement of the polymer chains in the lm does not change. The reversibility of the A1 and A2 amplitude as well as the strong optical anisotropy demonstrate the stability of the

π -π

stacking.

For the interpretation of the presented results it is crucial that the inter-chain exciton resonances including the vibronic side-bands (A1 -A3 ) as well as all absorption features at even lower photon energies are solely P3HT lm excitation. P3HT fractions which are possibly diluted in the electrochemical cycle, do thus not compromise the ellipsometric results below 2.4 eV (above

≈540

nm). It is further worth mentioning that peaks in an absorption

spectrum do not necessarily coincide with the resonance energy of the underlying electronic

18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or vibrionic excitation. Absorption spectra, which are recorded in transmission experiments, are convoluted furthermore with the intensity losses due to reections

e.g. on the lm boun-

daries. With the presented ellipsometric measurement we determine the absolute values of the material dielectric function which can easily be translated in all other equivalent representations,

i.e. in the complex (i) polarizability, (ii) susceptibility, (iii) optical conductivity,

(iv) electron loss function, and (v) in the complex refractive index. Furthermore, it is possible to calculate the lm thickness and additionally the color impression for certain viewing angles under consideration of the substrate and surrounding material properties. The insets in gure 4 depict the result of such a color calculation for white light transmitted through the P3HT lm deposited on ITO/glass and measured at dierent potentials during electrochemical process. The result excellently matches with the color changes observed by eye during the electrochemical experiment (a short video is provided as supplementary material). The formation of polarons in conjugated polymers is also known to be connected with the formation of a characteristic absorption band in the IR spectral range.

13

The IR opti-

cal properties are typically investigated in attenuated total reection (ATR) experiments. Corresponding spectra for our P3HT lms with iodine as well as electrochemical doping are published in reference 42 and 14. The IR polaron absorption band can be understood in terms of a LO-phonon excitation plus the excitation of the self-trapped electron from its ground state in an excited or continuum state regarding its own trapping potential. onset of the IR polaron absorption band is thus expected at photon energies of where

ωLO

43,44

The

~ω > ~ωLO ,

is the long-wavelength LO-phonon frequency. From our ATR experiments (not

shown here) we deduce an

ωLO

of 0.17±0.01 eV. The excitation of the polaron in the IR

and the excitation of vibronic side-bands of the inter-chain exciton resonances are both mediated by Froehlich interaction

i.e. by the electron-LO-phonon coupling. The vibronic

side-bands are thus determined by the same long-wavelength LO-phonon frequency which is introduced in 26 as a symmetric stretching and ring-breathing mode with (ω0

=0.173

ω0 =1400

−1 cm

meV). The correlation of the spacing of the vibronic side-bands with the IR

19



Page 20 of 36

polaron absorption is used in our parametric t of the P3HT DF with harmonic oscillator. The A1 to A4 absorption resonances are tted with an common photon energy distance

ωLO

which turned out to be 0.17±0.02 eV. The relative big error accounts for variations due to a undetermined choice in the type of resonance. Because of the conjugation and so call critical point line shapes like in anorganic semiconductors are suggested.

π -stacking

32,45

We use,

however, harmonic oscillators with a symmetric Gaussian broadening (Fig. 5). The Gaussian broadening considers disorder in the P3HT lm while the more commonly used Lorentzian life time broadening seems to be relatively small in our lms.

Figure 6: Evolution of the imaginary part of the dielectric function (in-plane component) in dependence of the applied electrochemical potential.

While electrochemical doping, the oscillator strength of the inter-chain exciton resonances (A1 -A3 ) and the HOMO-LUMO band-to-band transition continuously decrease by a factor of 2 between 0.35 and 0.8 V

vs. Ag/AgCl. At the same time an additional transition re-

sonance appears at 1.4 eV (Fig. 4 and Fig. 6). Both changes in the optical response are a result of the formation of self-localized, positively charged, spin 1/2 polarons.

46,47

The local

relaxation at the polaron sites yields, accordingly, in a new HOMO-LUMO band pair, which is energetically located in the gap of the undisturbed polymer. The transition between these

∗ 7,8,42 polaron p(g)-p(u) states is observed at 1.4 eV as an additional absorption. In order to

20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distinguish the polaronic form the pristine electronic states, the polaron orbitals are denoted

∗ here with p and p , respectively, although the

π

character is preserved. The change in the

symmetry of involved electronic states is important in this context. The

π (u)-π (u)∗

tion in pristine P3HT is a dipole transition from odd to even conjugated

π

polaronic excitation is a transition from even to odd localized orbitals. transitions from the half occupied polaron p(g) states to

π (u)∗

48

excita-

states while the

Electronic dipole

are thus symmetry forbid-

den. The unpaired electron spin in the polaronic state was verify for our lms by electron paramagnetic resonance (EPR) measurements. At electrochemical potentials around 0.8 V

14

vs. Ag/AgCl (the potential with the highest

amplitude of the polaron band-to-band excitation) in maximum every 4th P3HT-monomer is positively charged. absorption at 1.4 eV.

49

Such a high polaron density explains the strength of the polaron It gains the same amplitude as the remaining oscillator strength of

the inter-chain and intra-chain HOMO-LUMO transitions above 2.0 eV (Fig. 6). The strength of the transitions above 2.0 eV decrease by approximately one half between 0.35 and 0.8 V

vs. Ag/AgCl. The well distinguishable vibronic side-bands of the inter-chain exciton and the band-to-band transitions appear now broadened. by Hamidi-Sakr

A similar broadening was observed

et al. 29 for chemically doped P3HT. A possible reason for this broadening

is the coexistence of electronic states of the remaining pristine P3HT which overlap with renormalized

π

states at the polaron sites. A splitting of the polaronic resonance in an intra-

and an inter-chain excitonic contribution is not observed. Such a splitting of the polaronic transition was measured for photo-induced doping of P3HT lms.

50

Concerning peak width

and position, the electrochemical induced polaronic spectrum is actually very similar to those obtained for iodine p-type doped P3HT lms. anions,

i.e. PF6

in electrochemistry and I3

42,50

In both cases the incorporation of counter

in the case of chemical doping in iodine vapor

respectively, may prevent the formation of inter-chain resonances at the polaron sides. Remarkable is the fact that the polaron transition appears only in the in-plane dielectric tensor components (left panal Fig. 4) and thus in polymer strands laying parallel to the

21



surface. It seems that either the PF6

Page 22 of 36

doping itself or the formation of polarons depend on

the polymer orientation. In literature it is shown that the electron transfer from P3HT to acceptor molecules and/or the induced conductivity depend on the intra-chain order which is in aggregates enhanced and diers between regioregular and regiorandom P3HT. The chemically doped P3HT lms studied by Hamidi-Sakr

11,12,51

et al. contained lamellars where

the P3HT strands are in-plane and parallel to each other orientated.

29

In absorption mea-

surements they showed that the polaron resonance appears only for light which is polarized along the ordered strands in the lamellars. The absorption is absent for the perpendicular light polarization which is sensitive to coiled chain segments in the amorphous inter-lamellar zones.

The fact that the polaron transition appears in our spin coated lms only in the

in-plane dielectric tensor component could be explained accordingly. P3HT strands which are in parts or entirety perpendicular orientated to the surface may are also stronger warped and less ordered. Surprisingly, the polaron transition at 1.4 eV diminish again above 0.8 V

vs. Ag/AgCl in

connection with the third oxidation peak although the p-type doping is further increasing in the lm (Fig. 6). The red spectrum in gure 4 and 6 was recorded at a potential of 1.3 V

vs.

Ag/AgCl. It is representative for the whole electrochemical potential range above 1.0 V

vs.

Ag/AgCl. Considering these second change in the dielectric properties it is apparent that the electronic conguration in the polymer strands change again upon the third major oxidation. A closer inspection of the potential evolution of DF (Fig. 6) reveals that the discussed broadening of the inter-chain exciton structures has two maxima at potentials where the polaron resonance change - around 0.5 V and around 0.9 V

vs. Ag/AgCl where the polaron absorption increase

vs. Ag/AgCl where it decrease again. It is thus apparent the morphological

and/or electronic structure of P3HT changes two times upon doping. In a widely excepted model, the self-localization of charges in P3HT extends in average over three monomers which determine the polarons as new quasi particles.

The bond conguration converts in

this part of the P3HT strands from an aromatic to a more quinoid like structure.

22


46,52

On

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the other hand, we discussed already that the doping level is increasing at potentials above 1.0 V

vs. Ag/AgCl to a level, where up to 50% of the thiophene units in the lm are

positively charged.

Thus the polarons should strongly overlap after the third oxidation.

Whether this overlapping state is a bipolaron in terms of an energy minimization, which exceeds the Coulomb repulsion between two positively charged polarons, is questionable. The total Gibbs free energy minimization includes, in case of the electrochemical doping, also the electrical potential which could compensate the Coulomb repulsion. However, the transformation in a new electron conguration with a more quinoid like structure, where the polaronic states overlap, is unambiguous.

Both scenarios, the formation of bi-polarons as

well as the formation of unoccupied polaronic bands, are consistent with the disappearing EPR signal.

14

We would explain the doping induces changes of the polaron absorption feature above 0.8 V

vs. Ag/AgCl as following. Due to the overlap of the polarons the respective electronic

states form bands which would explain also the small redshift and broadening of the polaron absorption peak between 0.6 V and 0.9 V

vs. Ag/AgCl (g. 6). The resonance disappears,

if the lower polaron band rearrange almost entirely above the Fermi level.

47

The diminish

broadening at the high doping concentration structures is consistent with the assumption that the P3HT chains transform in a homogeneous conguration. This transformation expresses itself also in the photon energy position of the A1 resonance, as it is determined by our t with Gaussian oscillators. Below the rst oxidation, it is located at 2.040±0.001 eV. After the third oxidation

i.e. above 0.8 V vs. Ag/AgCl it is 2.014±0.002 eV. The small red shift

could be an indication for an extended intra-chain

π -conjugation.

Notable in this connection is the fact that the overall optical transition strength decays signicantly. As we are measuring the dielectric function, this observation can be quantied. Therefore we use the eective number of electrons

23

nef f

which is connected to the imaginary



part of the dielectric function

ε2 (ω)

through the sum rule relation:

nef f

where

Page 24 of 36

2ε0 me = N πe2

53

Z ωε2 (ω)dω,

(3)

ε0 is the free space permittivity, e is elementary charge, and me is the electron mass. N

is chosen to be the number of thiophene monomers per unit volume which was approximated already before. For the undoped P3HT lm we thus obtain an eective number of electrons

nef f =1.96

per monomer. There are two double bonds (4

π

electrons) in each thiophene ring

and each of the double bonds seems to contributing with one eective electron to the

π -π ∗

transitions in the investigated spectral range. According to the electrochemically determined charging of every second thiophene monomer the number of conjugated double bonds (π electrons) decrease by 1/8 after the third oxidation step. The eective number of electrons calculated from the measured dielectric function, on the other hand, decrease to

nef f =1.1

per monomer which is roughly a factor of 1/2 (4/8). In other words the optical oscillator strength decrease much more than expected by counting

π

electrons. The missing 3/8 of the

oscillator strength should show up at energies (wave lengths) outside the measured spectral range. Possible are contributions of the Drude response of free electrons as well as of the polaron excitations - both in the IR spectral range. Spano has shown already that the relative amplitude of A1 and the A2 vibronic side band depend on the exciton coupling between the polymer strands and thus on aggregation amount/strength in the P3HT lm. exciton bandwidth

W.

linear aggregates like the

26

As a bench mark parameter he is using the proportional

The latter is connected to the A1 and A2 oscillator strength for

π -π -stacked

P3HT through:

|hPA1 i|2 = |hPA2 i|2 where

|hPA1 i|2

and

Fermi Golden Rule,

|hPA2 i|2

26

1 − 0.24W/~ωLO 1 + 0.073W/~ωLO

2 ,

are the dipole transition matrix elements.

ε(ω) is proportional to |hP i|2 /ω 2 . 53 24

(4)

According to the

We calculate the exciton bandwidth


Page 25 of 36

W,

(~ω)2

therefore, with the A1 and A2 amplitudes divided by the respective

both obtained

from the Gaussian oscillator t.

-200

0

200

400

600

800

1000 1200

(a)

Amp/Ampmax

1.00

0.75

A1 A2

0.50

Polaron

0.25

80

(b)

(c)

8

10

4 1

2

100 0

600

1200

σ (S cm-1)

6

W (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

120 0.01

-200

0

200

400

600

800

1000 1200

E vs. Ag/AgCl (mV) Figure 7: (a) Evolution of the peak amplitudes of the A1 (orange squares), the A2 (magenta circles), and the polaron transition (olive triangles) depending on the applied electrochemical potential. Panel (b) depicts the evolution of the exciton bandwidth lm conductivity in logarithmic scale.

19

W

in comparison to the

The inset (c) shows the same conductivity data but

in a linear scale.

Figure. 7a shows a transient plot of the normalized A1 , A2 , and polaron amplitudes (orange squares and green circles) together with the exciton bandwidth Fig. 7b).

The rst oxidation step between

≈0.4

V and

almost synchronous decay of the A1 and A2 amplitude,

≈0.6 W

V

W

(magenta circles,

vs. Ag/AgCl yields in an

decrease moderately, and the

discussed polaron resonance shows up. During the second oxidation step between and

≈0.8

V

≈0.6

V

vs. Ag/AgCl on can observe an oscillator strength retransfer from A2 to A1 ,

25



which yields in a stronger decrease in much less than before. Above

W

Page 26 of 36

while the polaron amplitude further increase but

≈0.8 V vs. Ag/AgCl the relative A1

and A2 amplitude and the

exciton bandwidth remains almost constant while the polaron amplitude diminishes again. According to these results, the exciton coupling strength decrease upon the entire doping process but in particular in connection with the second oxidation. It was pointed out already that exciton coupling strength increase with increasing inter-chain order while it decrease with higher intra-chain order.

26

The decrease of the exciton bandwidth

W

matches very well with the logarithmic increase

of the conductivity of the lm. In order to demonstrate this coincidence we plot in gure 7b the exciton bandwidth together with the evolution of the lm conductivity (black triangles) which has been determined in previously published impedance measurements.

19

For the

comparison we have shifted the electrochemical potentials from reference 19 by -250 mV so that the respective CV results matches. The correlation of exciton bandwidth conductivity was already reported

W

and the

e.g. in 54. On the other hand it is known that the doping

induced transformation from an aromatic to quinoid like structure yields in a planarization of the monomer units in P3HT.

50,52

Two scenarios are conceivable.

the P3HT strands could (i) induce an enhances intra-chain disorder which enhances the

π -π

The planarization of

stacking and/or (ii) could decrease the

π -conjugation.

If we plot the conductivity change

on a linear scale (Fig. 7c), one can see that the conductivity primarily increase in relation to the second oxidation which coincides with the observed oscillator strength retransfer from A2 to A1 and thus the distinct decrease of

W.

The polaron resonance increase just slightly

in this potential range. We would explain all these by an increasing inter-chain order. An enhanced

π -π

stacking order would increase the

W

value which is not observed.

We have attribute the decrease of the polaron oscillator strength to a reduced number of polaron p-states above the Fermi level. The conductivity, however, increase further and the exciton bandwidth

W

remains almost constant. Additionally we would note again that

the polaron transition emerge mainly in connection with the rst oxidation step where we

26


Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observe only a relatively small conductivity increase. The formation of a polaronic transition in the near infrared spectral range is thus not an unambiguous indicator for a increasing conductivity while aspects correlating with the solid-state order play a much bigger role.

4

Summary

We introduce a transmission spectroscopic ellipsometry approach in order to study the electrochemical doping of semitransparent polymer thin lms in a standard cuvette electrochemical cell, in a stable environment and with the possibility to control the applied electrochemical potential. On P3HT lms, we could demonstrate the ability of the technique to determine the anisotropic UV-Vis-NIR lm dielectric function

in situ. Knowing the com-

plex dielectric function and the lm thickness, we can unequivocally predict

e.g. the results

of attenuated total reection (ATR), reection, or transmission/absorption measurements for comparison. The determination of individual resonance energies and amplitudes as well as the respective polarization dependent transition probabilities permits information about the chemical, electronic, and conformational properties. resonances are related to the conjugated

π

Thereby detected P3HT optical

electrons.

The suggested method is used in this work, to determine and discuss the P3HT dielectric response as a function of the applied electrochemical potential.

In anodic sweeps, P3HT

passes through three major oxidation steps which correspond to a positive charging of about

Ellipsometric Spectroelectrochemistry: An In Situ Insight in the Doping

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