Article pubs.acs.org/JPCC
Visible-Light-Induced Oxidation of Poly(3-hexylthiophene-2,5-diyl) Thin Films on ZnO Surfaces under Humid Conditions: Study of Light Wavelength Dependence Dae Han Kim,† Hyun Ook Seo,*,†,‡ Sang Wook Han,† Eun Ji Park,† Myung-Geun Jeong,† Kwang-Dae Kim,† Gerd Gantefoer,‡ and Young Dok Kim*,† †
Department of Chemistry, Sungkyunkwan University, 440-746 Suwon, Republic of Korea Department of Physics, University of Konstanz, D-78457 Konstanz, Germany
‡
S Supporting Information *
ABSTRACT: The oxidation behaviors of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films (∼60 nm thickness)/ZnO irradiated with three different wavelengths of visible light (blue, green, and red) in a humid atmosphere were studied using X-ray photoelectron spectroscopy (XPS) and UV−vis spectroscopy. The formation of sulfoxide states upon visiblelight irradiation was observed in the S 2p core-level XPS spectra, and more importantly, this oxidation became pronounced with decreasing wavelength of incident light. Photo-oxidation of P3HT films also resulted in a reduction in optical absorption. In contrast to the XPS results, changes in the UV−vis absorption spectrum were rather insensitive to the wavelength of incident visible light. The wavelength dependency of the photo-oxidation of P3HT films seen in the XPS spectra is attributed to the more pronounced photoinduced oxidation of locally disordered thiophene rings on the surfaces of P3HT films under irradiation with shorter-wavelength visible light. The population of local-disordered sites that increases the optical transition gap compared to that of the well-ordered bulk P3HT film decreases from the top surface to the interior of P3HT films due to stronger interchain interactions in the interior portion of the films. Therefore, changes in the optical absorbance seen in the UV−vis absorption spectra of the entire P3HT film upon photoinduced oxidation are less sensitive to the wavelength of incident light in the visible regime.
1. INTRODUCTION Semiconductive organic polymers have been drawing attention over the last 2 decades due to their importance in a variety of application fields, such as organic solar cells and organic lightemitting diodes.1−5 Semiconductive organic polymers have been widely applied to flexible electronic devices.6−8 Among various semiconductive organic polymers, frameworks based on polythiophene derivatives have been widely studied.7−9 Organic electronic devices initially show high performance; however, the performance of these devices rapidly decreases with time, particularly when the device is exposed to light.10−12 For example, organic solar cells consisting of a bulk heterojunction of a semiconductive organic polymer such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and an electron acceptor can show high power conversion efficiency; however, over time, a drastic decrease in the photovoltaic performance is often observed.12−15 Understanding the origin of the degradation of organic devices is essential for finding ways to increase the stability of organic electronic devices. Most semiconductive organic polymers have neighboring inorganic buffer layers, such as ZnO, TiO2, and WO3, which act as either electron- or hole-selective layers.16−18 The photo© XXXX American Chemical Society
stability of semiconductive organic polymers on these inorganic layers has been widely studied. In many studies, ultraviolet light was suggested to be responsible for the photodegradation of organic polymers. Either the organic polymers are directly photodegraded by UV light or UV light absorbed by the adjacent inorganic layer produces electron−hole pairs, which can yield highly potent oxidizing agents, such as hydroxyl radicals, that can oxidize organic polymers.12,19,20 Even though less attention has been paid to the photoinduced oxidation of organic polymers by visible light, some recent studies have shown that visible light absorbed by the P3HT layer can yield electron−hole pairs, which can initiate photoinduced reactions with oxygen and water in the atmosphere.21,22 Such reactions can produce oxidizing agents, such as superoxide and hydroxyl radical, which can oxidize the P3HT layer and thus reduce optical activity and electrical conductivity. Since this photoinduced oxidation is dependent on the lifetime of the electron−hole pairs created in the organic Received: April 6, 2016 Revised: August 22, 2016
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DOI: 10.1021/acs.jpcc.6b03530 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
ZnO samples prepared via the process described above were identical, indicating the high reproducibility of our sample preparation methods (Supporting Information, Figure S1). The surface morphology of bare and P3HT-covered ZnO ripple structures was studied using atomic force microscopy (AFM, XE-NSOM, Park Systems) and scanning electron miocrosocpy (SEM, JSM-7100F, JEOL). 2.3. Reaction Chamber. P3HT oxidation experiments were carried out in a high-vacuum chamber (base pressure ∼3.0 × 10−8 Torr) denoted as a reaction chamber in Figure 1. The
polymer layer, the degree of photoinduced oxidation is also dependent on the organic/inorganic interface structure; when electron−hole pairs in the organic polymer are easily quenched at this interface, initiation of the photoinduced reaction is less efficient. Previously, we studied the photoinduced oxidation behaviors of P3HT films on various underlying substrates [e.g., bare indium−tin oxide (ITO) glass, TiO2-covered ITO glass, flat ZnO, ZnO ripple, TiO2-covered ZnO ripple] under blue light exposure, and our experimental results revealed that the photoinduced oxidation behaviors of P3HT are strongly influenced by the interface structure. At the interface between P3HT and bare ITO, which is metallic, electron−hole pairs optically generated in P3HT layers can be efficiently recombined. The recombination of electron−hole pairs at the interface can be suppressed by the presence of additional thick TiO2 (thickness of ∼19 nm) layers between P3HT and ITO, resulting in a more pronounced photo-oxidation of P3HT films.23 The additional TiO2 films deposited between the P3HT and ZnO ripple structure showed a similar influence on the oxidation behaviors of P3HT layers under visible-light irradiation. TiO2 layers reduced the number of defects sites on the ZnO ripple surface where the recombination of electron−hole pairs takes place, and therefore, the P3HT films on TiO2/ZnO ripple substrate oxidized much more easily than on bare ZnO ripple surface.29 The previous study of the photo-induced-oxidation behaviors of P3HT layers on ZnO ripple structure under dry and humid air conditions showed the acceleration of P3HT oxidation in the presence of H2O vapor, indicating the significant role of H2O vapor in the photooxidation of P3HT layers.21 ZnO is one of the most widely used electron-collecting layers neighboring with semiconductive polymers in organic-based electronic devices to improve the electron-collecting efficiency. In the present work, we studied the oxidation behavior of P3HT layers (average thickness ∼40 nm) deposited on ZnO substrates, which would be relevant for many organic/inorganic electronic devices. The oxidation behavior of P3HT films was studied using photoelectron spectroscopy and UV−visible absorption spectroscopy. In particular, we were interested in determining whether photoinduced oxidation of the P3HT layer is dependent on the wavelength of visible light.
Figure 1. Experimental setup for the study of the oxidation behaviors of P3HT layers on ZnO surfaces under visible-light irradiation in an atmosphere of humid air is schematically described.
chamber was equipped with an ionic gauge, a quadrupole mass spectrometer (QMS, HAL 201, HIDEN), and three gas lines (H2O, O2, N2) such that the gas composition could be wellcontrolled by adjusting the partial pressure of each gas. The purity of each gas was monitored by QMS. The atmosphere of humid air (a total pressure of 760 Torr) was prepared by mixing N2 and O2 gas at a pressure ratio of 4:1 with H2O vapor, resulting in a relative humidity of 20% (20% RH) at room temperature. The roof of the reaction chamber was equipped with a quartz window port to allow light transmission from an external light source. The surfaces of the as-prepared P3HT/ZnO ripple samples were irradiated at three different wavelengths of light (λcenter: 455, 520, and 630 nm) using blue-, green-, and redlight-emitting diodes (LEDs) under an atmosphere of humid air at room temperature in the reaction chamber (Supporting Information, Figure S2). The LED lamps were supported by a spacer on the quartz window port such that the intensity of light from each LED illuminating the sample surface could be adjusted by controlling the distance between the lamp and the sample surface. The reaction chamber was also connected to an XPS analysis chamber (base pressure of ∼3.0 × 10−10 Torr) via a gate valve, allowing sample transfer between the two chambers without exposure of the sample to air. XPS spectra were obtained at room temperature using a Mg Kα (1253.6 eV) X-ray source and a concentric hemispherical analyzer (CHA, PHOIBOS-Has 2500, SPECS).
2. EXPERIMENTAL METHODS 2.1. ZnO Ripple Film Preparation. To fabricate the ZnO ripple film, first a 0.75 M ZnO sol−gel solution was prepared by dissolving zinc acetate [Zn(CH3COO)2·2H2O] in 2methoxyethanol solvent containing ethanolamine as a stabilizer. Next, 60 μL of the sol−gel solution was spin-coated onto an ITO-coated glass plate at 2000 rpm for 40 s. After spin-coating, the sample was heated to 350 °C at a constant heating rate of 22 °C/min in a furnace to form the ZnO ripple film.24 2.2. P3HT Film Preparation. P3HT (regioregularity ∼94%, 4002-EE) synthesized by Rieko Metal, Inc. (batch BS20-69) was used without further purification. First, 20 mg of the regioregular P3HT was dissolved in 1 mL of 1,2-dichlorobenzene (Sigma-Aldrich) at 60 °C, and the solution was stirred overnight. A P3HT layer was spin-coated onto the surface of the fabricated ZnO ripple film with 20 μL of the P3HT solution at 3500 rpm for 40 s, and then the P3HT film was dried at room temperature for 20 min under ambient conditions in the dark. Of note, X-ray photoelectron spectroscopy (XPS) core-level spectra (S 2p and C 1s) of three P3HT/ B
DOI: 10.1021/acs.jpcc.6b03530 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C 2.4. UV−Vis Absorption Spectroscopy. UV−vis absorption spectra of the P3HT/ZnO films were obtained using an UV−vis spectrophotometer (OPTIZEN 3220UV, MECASYS Co., Ltd.) before and after 18 h of light irradiation in humid air. Each spectrum was collected in the range of 380−800 nm with a resolution of 1 nm and a scan speed of 300 nm/min. The ZnO ripple structure on an ITO glass plate was used as a reference sample for UV−vis absorption spectra.
formed on the concave region of the ripple structure, whereas thinner films of P3HT (
