Modification of Indium−Tin Oxide Electrodes with Thiophene Copolymer Thin Films:  Optimizing Electron Transfer to Solution Probe Molecules

Abstract

We describe the modification of indium-tin oxide (ITO) electrodes via the chemisorption and electropolymerization of 6-{2,3-dihydrothieno[3,4-b]-1.4-dioxyn-2-yl methoxy}hexanoic acid (EDOTCA) and the electrochemical co-polymerization of 3,4-ethylenedioxythiophene (EDOT) and EDOTCA to form ultrathin films that optimize electron-transfer rates to solution probe molecules. ITO electrodes were first activated using brief exposure to strong haloacids, to remove the top approximately 8 nm of the electrode surface, followed by immediate immersion into a 50:50 EDOT/EDOTCA co-monomer solution. Potential step electrodeposition for brief deposition times was used to grow copolymer films of thickness 10-100 nm. The composition of these copolymer films was characterized by solution depletion studies of the monomers and atomic force microscopy (AFM), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy (reflection-absorption infrared spectroscopy (RAIRS)) of the product films. The spectroscopic data suggest that the composition of the copolymer approaches 80% EDOTCA when electropolymerization occurs from concentrated (10 mM) solutions. AFM characterization shows that electrodeposited poly(EDOT)/poly(EDOTCA) (PEDOT/PEDOTCA) films are quite smooth, with texturing on the nanometer scale. RAIRS studies indicate that these films consist of a combination of EDOTCA units with noninteracting -COOH groups and adjacent hydrogen-bonded -COOH groups. The EDOTCA-containing polymer chains appear to grow as columnar clusters from specific regions, oriented nearly vertically to the substrate plane. As they grow, these columnar clusters overlap to form a nearly continuous redox active polymer film. ITO activation and formation of these copolymer films enhances the electroactive fraction of the electrode surface relative to a nonactivated, unmodified "blocked" ITO electrode. Outer-sphere solution redox probes (dimethylferrocene) give standard rate coefficients, kS > or = 0.4 cm.s-1, at 10 nm thick copolymer films of PEDOT/PEDOTCA, which is 3 orders of magnitude greater than that on the unmodified ITO surface and approaches the values for kS seen on clean gold surfaces.

Electrochemical Copolymerization and Spectroelectrochemical Characterization of 3,4-Ethylenedioxythiophene and 3,4-Ethylenedioxythiophene−Methanol Copolymers on Indium−Tin Oxide

“…These results become especially significant in those cases where electrode modification using copolymers of PEDOT and PEDTM, or related PEDOT polymers (e.g., those with carboxylic acid side chains) is attempted, to optimize electron-transfer processes at ITO and related electrodes. 34 …”

Work function control of hole-selective polymer/ITO anode contacts: an electrochemical doping study

“…Energy barriers at the interfaces between TCO substrates and organic layers have been controlled through the addition of both small molecules and polymers. [3][4][5][6][7][8][9][10] These barriers are often characterized by UVphotoemission spectroscopies (UPS) or Kelvin probe measurements of work function (F). 1,2,[11][12][13][14][15][16] In cases where polymers have been added as hole-selective interlayers to OPV and OLED contacts, some control over their effective work functions is possible through electrochemical doping; [17][18][19][20][21][22][23][24][25] however, the role played by substrate work function and surface chemistry in determining energy level alignment with other organic layers is still a subject of intense study.…”

Organic/Organic′ Heterojunctions: Organic Light Emitting Diodes and Organic Photovoltaic Devices

“…Optimized BHJ OPVs therefore use interlayers between the D/A thin film and the bottom and top contacts, respectively, to restore ''contact selectivity'' ( Figure 1B). [64,77,[166][167][168][169][170][171][172] Ideally such interlayers render the bottom contact ''electron-blocking'' and another interlayer renders the top contact ''holeblocking.'' [46,58,63,64,66,72,86,103,156,163,166,170,[173][174][175][176][177][178] The bottom contact interlayers are typically hole-transporting conducting polymers (e.g., poly(thiophene) dispersions like PEDOT:PSS), and top contact interlayers can be thin films of alkali metal halides (e.g., LiF), and/or large bandgap electron transporting materials [e.g., bathocuproine (BCP) or aluminum quinolate (Alq 3 )].…”