Plasma induced degradation and surface electronic structure modification of Poly(3-hexylthiophene) films

https://doi.org/10.1016/j.polymdegradstab.2017.12.010Get rights and content

Highlights

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    Exposure of P3HT films to oxygen plasma causes extended oxidation of surface and deep surface region.

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    Oxygen plasma causes oxidation of sulfur atom of the thiophene ring.

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    Exposure to argon and hydrogen plasma leads to the formation of oxidized species located exclusively at the polymer surface.

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    Upon surface oxidation Fermi level is shifted closer to the HOMO energy indicating p-type doping.

Abstract

Plasma treatment is an environmentally friendly solution for modifying or nanostructuring the surface of several materials including photoactive polymers. The detailed characterization of the effect of plasma treatment on chemical and optoelectronic properties of photoactive polymers is, therefore, of specific interest. Herein, the effect of the exposure of poly(3-hexylthiophene) (P3HT) thin films to plasma created in three different gases (oxygen, argon and hydrogen) was studied. A range of spectroscopic techniques, such as x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy in conjunction with UV–vis absorption, Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopies, are employed to quantify the extent of chemical modification occurring in each particular case. It is shown that oxygen plasma treatment leads to the disruption of the π-conjugation via the direct oxidation of the sulfur atom of the thiophene ring while the aliphatic side chain remains nearly unaffected. An oxidation mechanism is proposed according to which the sulfur atom of the thiophene ring is oxidized into sulfoxides and sulfones, which subsequently degraded into sulfonates or sulfonic acids in a relatively small degree. For argon and hydrogen plasma treatments some oxidation products are detected only at the polymer surface. In all cases the polymer surface Fermi level is shifted closer to the highest occupied molecular orbital (HOMO) energy after plasma treatment indicating p-type doping arising from surface oxidation.

Introduction

Poly(3-hexylthiophene) (P3HT) has been intensely investigated due to its attractive electrical and electronic properties for application in organic optoelectronic devices, such as photovoltaics, photodetectors and field-effect transistors [1,2]. Moreover, it is among the semiconducting polymers with the largest database on degradation and thus serves as a model system for degradation studies. In the field of organic photovoltaics (OPVs), the P3HT-donor/methanofullerene (PCBM)-acceptor couple has been the selection material for the photoactive bulk heterojunction (BHJ) layer for over a decade [[3], [4], [5], [6]]. However, to increase the device efficiency a surface modification strategy can be applied. Oxygen (O2) plasma treatment has been occasionally used to tune the surface properties of the P3HT:PCBM layer. For instance, Wang et al. used O2 plasma to etch away the PCBM remnants from the free surface of a P3HT:PCBM blend layer before depositing the Ag anode in inverted solar cells [7]. In this way they achieved better interfacial contact and higher electrode selectivity leading to increased power conversion efficiency (PCE). Similarly, other groups have reported that exposure to O2 plasma of the same BHJ layer in organic photodetectors and organic solar cells with an inverted structure turned the photoactive surface more hydrophilic, thus facilitating deposition of PEDOT:PSS hole extracting layer on top of it without substantially affecting the overall electrical characteristics of the optoelectronic device [8,9].

On the other hand, it is highly critical to tailor P3HT/PCBM heterojunction interface in bilayer devices, which generally show lower performance than BHJ ones, in order to achieve better compatibility and larger interfacial contact between organic layers and increase the device efficiency [10]. To this end, one approach is the plasma induced nanoscale roughening (nanotexturing) of the interface in order to overcome the problem of limited contact area of donor and acceptor [11]. At low bias powers (low ion energy) plasma can direct the assembly of organized structures on the polymer surface without damaging the electronic properties of the material [12]. However, despite the obvious potential benefits arising from the direct plasma treatment of P3HT or its blends with fullerenes, there is a lack of understanding on the details of the plasma-induced chemical changes and possible electronic structure modification of P3HT, which are relevant to its application in organic electronic devices.

It is well documented that P3HT undergoes severe degradation upon oxygen uptake [13,14]. In particular, the exposure of P3HT to light and oxygen (i.e. photooxidation) leads to the disruption of the π-conjugated system in both, solution and the solid state [[15], [16], [17], [18], [19], [20]]. The conclusion was that an adduct between oxygen and a thiophene ring of the π-conjugated chain is formed, generating carbonyl and sulfonic derivatives and breaking down the polythiophene backbone [[15], [16], [17]]. Literature reporting on the quantitative influence of illumination, temperature, and atmospheric conditions is available [21,22], because these are key factors in polymer aging. Besides these irreversible effects, it was also shown by several groups that the exposure of P3HT to oxidative conditions resulted in partially reversible p-type doping via a polymer-oxygen charge transfer complex [[23], [24], [25], [26]]. With respect to the plasma treatment, however, no data pertaining to the possible degradation reactions of P3HT or other conjugated polymers have been published.

In this work, motivated by the possibility to adopt a much simpler approach towards direct plasma-induced nanotexturing/nanoroughening of the polymer surface and/or towards controlled modification of the surface properties of organic semiconductors for possible application in organic electronics, we demonstrate the first in depth investigation of the degradation mechanisms and subsequent surface electronic structure modification of P3HT induced by plasma treatment. The dependence of the degradation reaction pathways occurring in P3HT films on the nature of plasma gas is thoroughly studied. In particular, we study the effects of different gases, namely oxygen (O2), argon (Ar) and hydrogen (H2), plasma treatment on P3HT thin films. The plasma treated films are characterized in terms of surface morphology, crystallinity and surface energy, which are directly relevant to their application in bilayer organic photovoltaics. The elucidation of plasma induced degradation mechanism is obtained through probing the optical properties (absorption, emission) of P3HT films prior and after plasma treatment, while XPS and FTIR spectroscopies enabled detailed information on chemical modification of the polymer film. It is found that oxygen plasma causes the direct oxidation of the thiophene unit of P3HT film through the formation of sulfoxides and sulfones; the latter subsequently degrades into sulfonates or sulfonic acids in a relatively small degree. For argon and hydrogen plasma treatments the oxidation reaction is limited at the polymer surface. However, UPS measurements revealed that in all cases changes in the surface electronic structure take place which are equivalent to irreversible surface p-type doping.

Section snippets

Film formation and plasma treatment

Poly(3-hexylthiophene-2,5-diyl) (P3HT, regioregular, provided by Rieke Metals) was dissolved in chloroform (CF) in a concentration of 20 mg/mL and stirred at 30 °C for 3 h. P3HT was spin-coated on either Si or quartz substrates at spin speed of 500 rpm for 40 s and then annealed in air on a hot plate at 130 °C for 15 min. Next, the films were plasma treated using different gases for 15 s. The plasma conditions were: gas flow 100 sccm for oxygen and hydrogen and 25 sccm for argon, source power

Effect of plasma treatment on surface morphology, crystallinity and surface energy of P3HT

Due to the relevance to organic electronic devices, we first investigated possible changes in the morphology and surface energy of as-deposited and O2, Ar or H2 plasma treated P3HT films. The surface topography of these films was probed with atomic force microscopy (AFM). Fig. 1a shows the as-deposited polymer surface, which is rather smooth and homogeneous with a very low root mean square (RMS) roughness of 1.5 nm. The topography is significantly altered upon exposing the film to O2 (Fig. 1b),

Conclusions

Using a combination of UV–vis, FTIR, XPS and UPS spectroscopies we studied the effect of plasma treatment of P3HT films and we confirmed that upon O2 plasma treatment direct oxidation of the sulfur atoms of thiophene rings takes place which results to a reduction of the π-conjugation. In addition, the exciton formation and charge generation efficiency are significantly suppressed in the O2 plasma treated sample while the exciton lifetime is reduced due to its increased nonradiative deactivation

Acknowledgements

This work was supported by the Research Excellence Project “Plasma directed assembly of nanostructures and applications - Plasma Nano Factory” which is implemented under the "ARISTEIA I″ Action of the "OPERATIONAL PROGRAMME EDUCATION AND LIFELONG LEARNING″ and is co-funded by the European Social Fund (ESF) and National Resources. Prof. M. Fakis would also like to acknowledge that part of this work also has been supported by Grant E.028 from the Research Committee of the University of Patras

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