cimps05

8/7/2019 cimps05

http://slidepdf.com/reader/full/cimps05 1/14

Intensity Modulated Photo- and

Impedance Spectroscopy at

Polythiophenes on TiO2 and Pt in the

Reduced State

W. Plieth1 , C.A. Schiller 2* , U. Rammelt 1 , N. Hebestreit 1

1 Dresden University of Technology, Institute of Physical Chemistry and

Electrochemistry,

Mommsenstr. 13, D-010602 Dresden, Germany

2 ZAHNER-Messtechnik, Thüringer Str. 12, D-96317 Kronach, Germany

[email protected]

8/7/2019 cimps05


Plieth, Schiller, Rammelt, Hebestreit: Intensity Modulated Photo- and Impedance Spectroscopy on Polythiophenes ...

Outline Aims of this work:

Understanding organic polymer semiconductors with

the help of their photo-electrochemical properties.

Refinement of the intensity modulated photo-

spectroscopy IMPS: controlled intensity modulated

photo-spectroscopy ÃCIMPSµ.

Modeling experimental IMPS results by merging theresults of dynamic photo-potential, photo-current and

EIS measurements.

8/7/2019 cimps05



Overview of the Systems Under Test

and the Experimental Techniques Used

Systems

Methods

Ti-

TiO2-

PTP

Pt-

PTP

Ti-

TiO2

Photo-

Diode-

Network

CIMPS

Potential

CIMPS

Current

Photo-Pulse-

response

EIS

8/7/2019 cimps05



Preparation of Electrodes

TiO2

Electrolysis of 0.5 M K 2SO4 / 10 V, ca. 10 min.

(thickness: 25 nm)

PTP / PtElectropolymerisation of 0.1 M 2,2µ bithiophene / TBAPF6

(µCV-scanµ : 0 - 1.3 V vs SCE, scanrate = 20mV/s, 15 min.

thickness 200 nm)

PTP / TiO2- Pretreatment of TiO2-electrode with adhesion promotor TUTS

(11-(3-thienyl)undecyltrichlorsilane), ca. 30 min.- Oxidation of Thiophene with FeCl3 in CHCl3, ca. 90 min.,

(thickness: 50-100 nm)

8/7/2019 cimps05



Comparison Between EIS and IMPS

Principle of EIS:Modulation of the current I or the

voltage U at an electrochemical cell C.

The quotient of the dynamic

magnitudes leads to the Impedance Z*

(or Admittance Y*=1/Z*).

Principle of IMPS:Modulation of the light, illuminating an

electrochemical cell C. The quotient of one

electric magnitude (current or voltage) with

the dynamic light intensity P* leads to the

IMPS spectrum H*.

EIS IMPS

Z* H*

A m2

W

WV m

2H = 1UP

H = 1IP

C CU U

IIP

H ( ) =UP

U*

H ( ) =IP P*

I*[

[ P*

,

,I* = I0.e ( j[t+J )

U* = U0.e j[t1)

2)

*

I*

U

Z*

([)=

8/7/2019 cimps05



IMPS vs. Controlled Intensity

Modulated Photo Spectroscopy

Usual IMPS set-up:The function generator output of an FRA

supplies an LED light source with modulated

current. Mostly the cell current is correlated

with the LED current to calculate the photo-

electrochemical transfer function H*.

Principle of CIMPS:The LED light source is supplied by a

power-OPA, regulating the LED current in a

way, that the measured intensity tracks the

set value at the cellµs site. The cell voltage or

current is correlated with the measured

intensity to calculate H*.

IMPS

I

U U

I

CIMPS

H*C C

H*

LED UVBL or GRL

FRA

IM6-FRA

EPOTPD

PSA

PG

Ref

FRA: Frequency Response Analyzer

PG/IM6-FRA: Electrochemical Workstation IM6 *)

EPOT: Slave-Potentiostat / Power OPA *)

PD/PSA: UV-VIS Photodetector / Sense Amplifier *)

BUVZ/GRL: UV-VIS Multi-LED-Emitter *)

*) : Content of the ZAHNER CIMPS Equipment

8/7/2019 cimps05



Ti-PTP: CIMPS Current at 535nm.

Dynamic photo-current measurements at the binary system Ti-TiO2-PTP

under different cathodic polarization, starting at the OCP vs. Ag-AgCl.

Intensity: 10W/m2 at 535nm.

Left: Modulus/phase diagram. Right: Complex plane diagram.

fr equency / z

30p

100p

300p

1n

3n

10 1 100 10Kr eal par t / n -1 2

-1 0 1 2 3 4 5 6 7

4K z

6 z

-4

-2

0

2

i a gi n ar y

p ar t / n

-1

2

535pcur+007535pcur -150535pcur -250535pcur -350

-100

-150

-50

100

150

0

50

535pcur+007535pcur -150535pcur -250535pcur -350

| p h o t o c u r r e n t | /

- 1

2

ph a s e

0

8/7/2019 cimps05


8/7/2019 cimps05



Ti-PTP: EIS Measurements.

EIS measurements of the binary system Ti-TiO2-PTPunder light of different wave lengths at the OCP (left)

and under green light with different cathodic polarization (right).

The intensity was 10W/m2 except in the case of the 429 nm

illumination (2.5 W/m2).

100

1K

10K

100K

1M

100m 10 1K 100K

370eisocpb

429eisocpa535eis-150

0

15

30

45

60

75

90

100m 10 1K 100K

535eis-150535eis-250535eis-350

frequency / Hz

| ph a s e | / 0

| i m p e d a n c e | / ;

8/7/2019 cimps05



Pt-PTP CIMPS Current

Dynamic photo-current measurements of the system Pt-PTP

under light of 535nm wavelength at different cathodic polarization.

Intensity: 10W/m2.

Left: Modulus/phase diagram. Right: Complex plane diagram.

f requency / Hz

m m

| p h o t o c u r r e n t | / n

W

-¨

m 2

K

2

2

28

2

-8

-

8

ph a s e

rea par t / n W - © m2

-2 - - -

-

m a gn ar y

p ar t /

nW -

m2

pcur+pcur+28

pcur-

8/7/2019 cimps05



Schematic Section, Band Structure, and

Equivalent Circuit of a Semiconductor Electrode

TiO2

EF

Electrolyte

W

DLSBTi

Rox

Cox

Csb

Rsb

Isb RctWct

Cdl

CB

VB

RelRmet

SB: Schottky barr ier junction

DL: Electrode double layer

CB: Conductance band

VB: Valence band

EF: Fer mi level in equilibr ium

Csb, Rsb, Isb: Impedance elements

and curr ent sour ce in the photo-

active junction ar ea

Cox, Rox: Impedance elements of

the semiconductor bulk

Cdl, Rct, Wct, Rel: Impedance elements of the electrode-

electrolyte system (Randles

cir cuit)

8/7/2019 cimps05



Ti-TiO2-CIMPS Current Measurements and Model Fit

Dynamic photo-current measurements of the system Ti-TiO2

under light of 370nm wavelength at different anodic polarization.

Intensity: 10W/m2. The measurement series was fitted with the model

displayed in the inlay of the complex plane diagram (right).

frequency / Hz

3

.5

5

7

10

1 10 100 1 3 10 30 100 4 6 8 10 1 14

370pcur+100

370pcur+300370pcur+400370pcur+500370pcur+600370pcur+700

370pcur+200

| p h o t o c u

r r e n t | / n A W -

1

2

0

-2

2

4

6

8

photocurrent ' / nAW -1 2

ph o t o c ur r en t ' '

/ nA W-1

2

0

15

30

45

60

75

90

| ph a s e | / 0

W

RctWct

C l

Rel

Cs

Rs

Is

Rox

Cox

Oxide Schottky-barr ier

Doublelayer

8/7/2019 cimps05



Pt-PTP-CIMPS Current Measurements and Model Fit

Dynamic photo-current measurements of the system Pt-PTP

under light of 535nm wavelength at different cathodic polarization.

Intensity: 10W/m2. The measurement series was fitted with the model

displayed in the inlay of the complex plane diagram (right).

f r equency /Hz

0

-20

20

40

80

100

200

150

1m 3m 10m 100m 1 3 10 30 100

ph a s e / o

| p h o t o c u

r r r e n t | / n A W - 1 m

2 535pcur+

535pcur+100535pcur-100

80 100 120 140 160 180 200 220

photocurrr ent '/n AW -1m2

100

50

0

ph o t o c ur r r en t ' ' / nA W -1 m2

Rct

Csb

RsbIsb

C l

Rel

Rpoly

Cpoly

Schottky-barr ier

Semicond.

bulkpolymer

Por ous

systempolymer-electr olyte

CadRad

8/7/2019 cimps05



Conclusions

Semiconductor-electrodes like conducting polymers can be modeled

with the same strategy as their two-pole impedance: for that,current sources, representing the photoactive process, have to be

introduced.

Photocurrent and photovoltage reflect different parts of the total

electrode network.

An equivalent circuit model, common to potentiostatic and

galvanostatic produced CIMPS and EIS spectra under identical

current load conditions promise to prove a proper modeling.

Measurements at the OCP under illumination as well as

measurements with resistance load complicate the analysis

Future aims: repeating the measurements under orthogonal

conditions.

cimps05

Documents