Full Length ArticleLaser nanostructuring of thin films of PEDOT:PSS on ITO: Morphology, molecular structure and electrical properties
Graphical abstract
Introduction
Conducting and semiconducting polymers are being investigated for uses in applications which were classically restricted to silicon-based devices. In particular, polymers like poly(3-hexylthiophene) (P3HT) or its blends with [6], [6]-phenyl C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) have been widely studied for applications in the photoactive layer in organic photovoltaics (OPV) [1], [2], [3]. Among the conducting polymers, poly(3,4-ethylenedioxythiophene) complexed with poly(styrenesulfonate) (PEDOT:PSS) can be highlighted: a water-soluble material which presents good processability, high electrical conductivity, optimal optical transparency in the visible region when processed as thin films, high thermal stability and good mechanical flexibility [4], [5], [6], [7], [8]. Thus, PEDOT:PSS can form conducting and transparent thin films by solution processing techniques as spin-coating [9], solution cast [10], inkjet printing [11] or electro spinning [12] among others.
Due to its electrical conductivity, thin films of PEDOT:PSS spin-coated over Indium Tin Oxide (ITO) have been extensively used in the field of organic electronics, for instance as a hole extracting layer in organic solar cells [13], in organic light emitting diodes [14] and organic field-effect transistors [15]. Also, different oxidation levels lead to changes in the visible absorption spectra of PEDOT:PSS, making it suitable for electrochromic windows [16]. Controlled stimuli of PEDOT:PSS films to water ambient opens the possibility of fabricating humidity sensors [17]. Furthermore, the water-component offers potential applications in biology and medicine [18], [19]. In fact, PEDOT‐based conducting polymers have found applications for electrically controlled protein conformation toward the control of the cellular functions of electroactive cells [20], for cell-based assays to monitor barrier tissue properties [21], for cell response to specific biomolecules [22] and for electroactive cell recording [23].
For most of the previous applications, it is beneficial to nanostructure the polymer surface. Several methods to nanostructure PEDOT:PSS have already been reported such as the use of ultrathin porous alumina membranes to produce nanopores and nanopillars [24], nanoimprint lithography (NIL) [25], [26] and electro-spinning [12]. Nowadays, there is an increasing interest in alternative ways of producing nanostructures more economic, quicker and reproducible, avoiding the necessity of stringent environmental conditions like those implied in using clean rooms, high vacuum or complex mask fabrication. Laser based techniques are attractive alternatives, and in particular for nanostructuring of PEDOT:PSS laser methods have been reported including Direct Laser Interference Patterning (DLIP) [27], selective ablation patterning [28], [29] or the combination of optical near field-enhancement through self-assembled silica nanospheres and laser interference lithography [30].
The mentioned laser-based techniques typically involve ablation of the material. However, the formation of laser induced periodic surface structures (LIPSS) is an alternative non-ablative technique for polymer nanostructuring. LIPSS originate from the interference of the incident and reflected/refracted laser light with the scattered light near the interface. The interference between the different waves leads to an inhomogeneous energy input which, together with positive feedback mechanisms, can cause surface instabilities. LIPSS have been reported on the surface of different polymers by irradiation with a polarized laser within a narrow fluence range, well below the ablation threshold [31]. In fact, nanostructured polymers using LIPSS have been proposed for different applications [31], [32], [33], [34], [35], [36], [37], [38].
In this work the objective is the formation of LIPSS on thin films of PEDOT:PSS spin-coated on ITO substrates and we report on the characterization of the nanostructured material by atomic force microscopy (AFM), synchrotron grazing incidence X-ray scattering and Raman spectroscopy. Finally, the electrical properties at the nanoscale have been analyzed at the nanoscale by conductive AFM.
Section snippets
Materials and film preparation
A PEDOT:PSS aqueous dispersion (Heraeus Clevios™ AI 4083 from Ossila, UK, PEDOT:PSS ratio 1:6) was used to prepare thin films on ITO covered glass squares (S111 from Ossila Ltd., UK). The dispersion as-received was filtered with a syringe through a 0.2 µm cellulose filter and then sonicated for 5 min. The substrates were sonicated in acetone for 10 min and then in isopropyl alcohol for other 10 min before using them, and subsequently, they were rinsed in deionized water. Finally, the substrates
Results and discussion
A detailed study on the formation of LIPSS on PEDOT:PSS deposited on ITO has been performed to determine the optimal processing parameters leading to optimal nanostructures formation. Fig. 1 shows AFM images of PEDOT:PSS irradiated with a fluence of 12.4 mJ/cm2 as a function of the number of pulses.
The AFM image of the sample irradiated with 4800 pulses already shows linearly oriented structures parallel to the polarization of the laser beam and the corresponding Fast Fourier Transformation
Conclusions
Laser induced periodic surface structures were formed on thin films of PEDOT:PSS deposited on ITO covered glass substrates, and the structures obtained had periods close to the irradiation wavelength of the laser used (266 nm) and were aligned parallel to the laser polarization. Raman spectroscopy results indicated that polymer was structured without chemical damage during the process, suggesting that the heat treatment caused by laser irradiation induced the molecular transition from benzoid
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been supported by Ministry of Economy and Competitiveness (MINECO, Spain), Spanish State Research Agency (AEI) and European Regional Development (FEDER) under the projects MAT2014-59187-R, MAT2015-66443-C02-1-R-MINECO/FEDER, UE and CTQ2016-75880-P-AEI/FEDER, UE. E.R. also thanks for the tenure of a Ramón y Cajal contract (No. RYC-2011-08069). GISAXS and GIWAXS experiments were performed at NCD-SWEET beamline at ALBA Synchrotron with the collaboration of ALBA staff. We thank J.V.
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