Photocurrent spectroscopy of conductive polymers

ELSEVIER

Synthetic

Photocurrent

Department

Metals

69 (1995) 3.53-354

Spectroscopy

of Conductive

Jukka Lukkari and Jouko Kankare of Chemistry, University of Turku, FIN-20500

Polymers

Turku, Finland

Abstract Photocurrent spectroscopy allows to obtain spectra of ultrathin conductive polymer deposits on electrode surfaces and to study the first steps of anodic electropolymerisation. With poly(3-methylthiophene), electropolymerisation takes place by deposition of solution formed oligomers but the results for poly(N-methylpyrrole) show gradual increase in conjugation length. The preparation

of thin conductive

carried out using anodic easy control of electropolymerisation,

polymer

electropolymerisation,

the the

reiative initiation

films is usually which provides

film process

thickness. determines

transmittance, and transformed proportional to quantum efficiency. No absolute quantum efficiencies were determined.

In the

quality and properties of the resulting thin film to a large extent. However, the mechanisms of film initiation and growth have received

little attention.

either the deposition

Two mechanisms of solution

formed

have been postulated, oligomers

or gradual

growth of surface-bound chains.[l] The former model is closely related to the classical concept of nucleation. The lack of data on the early stages of film formation partly stems from the experimental difficulties involved when studying ultrathin polymer deposits. The film initiation have been studied using classical

electrochemical

techniques

[2], scanning

probe

techniques [3], and ellipsometry[4]. Ellipsometry has been the only technique providing information about the chemical nature of the deposits but it requires expensive instrumentation complex model fitting. We have found that measuring photocurrent

0 PC

Figure I : The experimental photocurrent

set-up

for the measurement

of

spectra. The light source is a 450 W Xe lamp.

and the

of the deposits as a function of the excitation energy

is an excellent in situ technique for studying the early stages of the electropolymerisation reaction.[ l] The photocurrent spectrum (in quantum efficiency) is identical to the absorbance spectrum of the deposits but the sensitivity of the method enables us to record spectra of deposits monolayer.

corresponding

to less than a hypothetical

Experimental The polymerisation was carried out from acetonitrile or aqueous solutions containing the monomer (3-methylthiophene, thiophene, pyrrole, or N-methylpyrrole, all 0.1 M) and the electrolyte (usually tetrabutylammonium hexafluorophosphate TBAF, 0.2 M). The working electrode was either indium-tin oxide (ITO) or polished polycrystalline platinum disc. The details of the polymerisation procedure and the electrochemical cells have been described elsewhere.[l] Photocurrent spectra of neutral polymer deposits were recorded using the apparatus shown in Fig. I. The light chopping reference frequency was either 2.5 Hz or 0.5 Hz. The measured photocurrents were corrected for the spectrum and monochromator and cell IamP intensity

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Figure 2 : Cyclic voltammogram (100 mV/s) and photocurrent of poly(N-methylpyrrole) on IT0 (2.0 mC/cm* using 1.0 mA/cm2 in acetonitrile/TBAF). Results and Discussion The excitation of neutral polythiophene, PolY(3methylthiophene) or poly(N-methylpyrrole) deposits on the electrode surface produces a reductive photocurrent. The photocurrents are so low that residual oxygen or water can guarantee an electron acceptor concentration high enough even in

354

dry

J. Lukkari, J. Kankare I Synthetic Metals 69 (1995) 353-354

acetonitrile

nondegenerate

solutions.[l] ground

In

state,

conductive

photoinduced

polymers

from the formation of polarons and bipolarons excitation.[5] At least with thiophene-type formation

of these charged

chain distortions

with result

ca. 3.6 eV to cu. 2.75 eV when the polymerisation charge increases from 0.025 mC/cm2 to 1 mC/cm*. The spectra display,

after the initial polymers, the

however, a sharp rise below 350 nm with an absorption edge of 3.6 eV also at higher polymerisation charges. The same kind of

phenomena

takes place within

spectral

features

are present

in the UV-vis

spectra

of thick

the time scale of energy transfer, which enables the observation of photocurrent in thin polymer deposits on metal electrodes also.

poly(N-methylpyrrole) films.[7] This shows that the chemical nature of the polymer films is not uniform, and already very small

However, contrary to thiophenes,

poly(N-methylpyrrole)

no photocurrents

were observed

with poly(N-methylpyrrole) on platinum electrodes. A good signal was obtained with the N-methyl derivative on IT0 but we were

unable

to detect

photocurrent

with

polypyrrole

conjugation

deposits

contain

species

with different

lengths.

itself,

contrary to some previous reports.[6] In general, the absolute quantum efficiencies are rather low, and electrochemically induced charges can quench photocurrents already at low doping levels (Fig. 2). In the case of 3-methylthiophene spectra

and thiophene,

photocurrent

(Fig. 3) show that on native IT0 the absorption

edge

(band gap edge) and h,, attain the values characteristic of highly conjugated films (cu. 1.95 eV and 550 nm, respectively) already in the first observed deposits (polymerisation charge cu. 0.1 mC/cmz). This indicates that the electropolymerisation takes place by deposition of long oligomers formed in the solution. The determination of the actual conjugation length of the deposits is rendered difficult by the lack of suitable reference spectra. Solidstate effects induce a bathochromic shift in the spectra of solid films, as compared to the solution spectra of corresponding oligomers. complicates

On the other hand, splitting of energy levels the use of highly ordered evaporated films of known

oligomers.

Figure 4 : Normalised photocurrent spectra of poly(Nmethylpyrrole) deposits on IT0 (prepared using 1 mA/cm*). The behaviour thiophene-type interactions

of N-metbylpyrrole

polymers. between

species have an electropolymerisation.[l]

contrasts

We have previously

the electrode

surface

with that of shown

that the

and the deposited

important role in the Strong interactions

course favour

of the

deposition of short oligomers and even with thiophenes the initial step might be a reaction with the species adsorbed on the surface. On the other hand, weak interactions

lead to the deposition

of

long oligomers. Therefore, the differences can be explained by the higher affinity of pyrrole-type oligomers toward the hydrophilic IT0 surface. References [l] J.Lukkari,

photocurrent spectra of poly(3Figure 3 : Normalised methylthiophene) deposits on IT0 (prepared using 1 n&cm*). With poly(N-methylpyrrole) on native ITO, the results imply a somewhat different mechanism (Fig. 4). Photocurrent spectra could be obtained at much lower deposition charge densities than with thiophene-type polymers. The absorption edge of these initial deposits is co. 3.6 eV, indicating a very short conjugation length. At higher charge densities a shoulder starts to form at cu. 375 nm. The absorption edge experiences a gradual shift from

M.Alanko,

V.Pitk%nen,

K.Kleemola,

J.Kankare,

J.Phys.Chem., in press; J.Kankare, V.Vuorinen, M.Alanko, J.Lukkari, J. ChemSoc., Chem. Commun. (1993) 241. [2] AR Hillman, EF Mallen, J. Electround. Chem. 220 (1987) 35 1 [3] J.Lukkari, M.Alanko, L.HeikkilZI, R.Laiho, J.Kankare, Chem. Muter. 5 (1993) 289; F.Chao, MCosta, CTian, SynthMet. 53 (1993) 127. [4] A.Hamnett, AR Hillman, J.Electrochem.Soc. 135 (1988) 25 17 [5] G. Yu, SD Phillips, H.Tomozawa, AJ Heeger, PhyxRev. B42 (1990) 3004. [6] Y.Yang, ZG Lin, SynthMet. 64 (1994) 43. [7] DL Feldheim, M. Krejcik, SM Hendrickson, CM Elliot, J.Phys.Chem.

98 (1994) 5714.