Unveiling the Complexity of the Degradation Mechanism of

Article pubs.acs.org/JPCC

Unveiling the Complexity of the Degradation Mechanism of Semiconducting Organic Polymers: Visible-Light-Induced Oxidation of P3HT Films on ZnO/ITO under Atmospheric Conditions Tae Gyun Woo,† Hyun Ook Seo,‡ Il Hee Kim,† Sang Wook Han,† Byeong Jun Cha,† and Young Dok Kim*,† †

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea Department of Chemistry and Energy Engineering, Sangmyung University, Seoul 03016, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The oxidation of poly(3-hexylthiophene) (P3HT) films deposited on ZnO/indium tin oxide (ITO) under blue light irradiation in either dry or humid atmospheres was studied using X-ray photoelectron spectroscopy in combination with UV−vis absorption spectroscopy. From results up to 12 h of reaction, ring-opening was hardly found, and it is suggested that the water molecules chemisorbed competitively against O2 (i.e., the major oxidizing agent), thereby decreasing the oxidation of P3HT. Beyond 12 h, thiophene ring-opening took place at the topmost surface layer of P3HT, and the humidity facilitated the ring-opening of P3HT. Regarding the oxidation of the entire P3HT thin film, the humidity did not have a large influence on the oxidation behavior of P3HT. Here, the degree of oxidation of P3HT abruptly increased when the reaction time exceeded 12 h. This suggests that the rate of oxidation of the entire P3HT film is determined by the slow diffusion of the activated oxygen species into the deeper layers of the P3HT films. We also demonstrate that the photoinduced degradation of P3HT can be retarded by turning off light between irradiation, which may be due to the reversible desorption of activated oxygen species under dark conditions.

1. INTRODUCTION Since the development of conducting or semiconducting organic polymers, there has been tremendous interest in the fundamental physicochemical properties of semiconducting polymers as well as their applications in electronic devices.1−10 Semiconducting organic polymers are compatible with solution-based processes; therefore, they have the potential to be fabricated in a relatively inexpensive manner. Additionally, they can easily be applied to flexible electronic devices. Organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs) have drawn much interest in terms of fundamental science explorations and industrial applications over the past several decades.1−10 Among the various semiconducting organic polymers, polymers consisting of thiophene derivatives have been studied extensively due to their wide applicability, which is based on their excellent electrical properties and relatively high chemical stability.11−15 There have been concerns related to the fact that devices consisting of organic polymers can easily undergo degradation, and this point has been considered as a hurdle preventing the broader application of organic devices. Recently, extensive studies have been devoted to unveil the mechanisms of © XXXX American Chemical Society

degradation in organic electronic devices that occur over time. For example, degradation of OSCs has been the subject of many previous studies, and organic polymers in OSCs can undergo structural changes after reacting with the atmosphere (e.g., oxygen and water vapor), especially under light illumination.16−21 It has been argued that UV light is mostly responsible for the photoinduced degradation of semiconducting organic polymers;22,23 however, it was suggested recently that visible-light-induced oxidation of poly(3-hexylthiophene) (P3HT) cannot be ruled out.24−27 P3HT absorbs visible light to create excitons, which can interact with O2 and H2O to form O2− and OH radicals (i.e., strong oxidizing agents). These active species can attack organic polymers, leading to their degradation.24−27 The stability of semiconducting organic polymers has been shown to depend on the polymer structure.17,21,28 In addition to the photoinduced degradation of a single polymer species, structural changes at the donor− acceptor interfaces in OSCs have also been addressed.19,20,29−31 Received: June 15, 2017 Revised: July 29, 2017 Published: August 9, 2017 A

DOI: 10.1021/acs.jpcc.7b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

electron microscopy (SEM, JSM-7100F, JEOL) and highresolution atomic force microscopy (HR-AFM, SPA-300HV, SII). The P3HT film thickness was determined by UV−vis absorbance measurements and AFM measurements.26 2.3. Experimental Setup Used To Study the Oxidation Behavior of P3HT Thin Films. The experimental setup used for the P3HT oxidation experiment is described in greater detail with schematic descriptions elsewhere.27 P3HT oxidation experiments were conducted using a high-vacuum system (base pressure ∼1.0 × 10−5 Torr) equipped with gauges for gas pressure measurements (ionic gauge and Pirani gauge) and two gas lines for injecting H2O vapor and dry air, respectively, into the reactor chamber. A quadrupole-mass spectrometer (QMS) was installed in the reactor chamber to determine the purity of each gas that was added into the reactor chamber. A P3HT/ ZnO/ITO glass sample (lateral size: 6 × 6 mm2) was placed in the reactor, and the sample surface was oriented toward a quartz window, which allows transmission of the light (blue light-emitting diode [LED] with λcenter at 455 nm). The distance between the sample and blue LED was ∼10 m, and the light intensity on the sample was estimated as ∼1129 W/cm2 based on the relationship between light intensity and sample− LED distance which was experimentally determined in our previous work.26 We used a blue LED as a light source for photoinduced oxidation of P3HT, since we are interested in unveiling the mechanism of photoinduced oxidation of P3HT initiated by the photon-absorption of P3HT, not by the ZnO substrate. Instead of a white light source, we used a blue LED, since this study can be a cornerstone for further comparative studies of photoinduced oxidation of P3HT using various wavelengths in the visible-light range.26 Photodegradation experiments of the P3HT layers were carried out under either dry or humid atmospheric conditions. Before the gas injection process, the reactor was pumped down to its base pressure. For the experiments under dry conditions, only dry air was injected into the reactor; this was done until the pressure inside the reactor chamber reached 760 Torr. Alternatively, the humid atmospheric conditions were achieved by first filling the evacuated reactor chamber with water vapor until the pressure reached 8 Torr. Then, dry air was added to keep the total pressure inside the reactor at 760 Torr (i.e., the same as what was used for the dry conditions). The humidity of our humid conditions corresponds to 34% relative humidity (RH) at room temperature. In order to accurately control the partial pressure of gas, leak valves were used during the injection of each gas. The distance between the sample and the light source was about 10 cm. Light was irradiated onto the samples for 3, 12, and 18 h, cumulatively, under dry and humid conditions. The samples exposed to blue LED irradiation for various times under dry or humid conditions were analyzed by XPS (S 2p, C 1s, and O 1s). The samples were transferred from the reactor chamber into the XPS analysis system without exposing the samples to air outside of the chamber; the reactor, load-lock (base pressure: 5 × 10−9 Torr), and XPS analysis (base pressure: 1 × 10−10 Torr) chambers were sequentially connected with gate valves, and a magnetic transfer system allowed for back-and-forth transfer of the samples between the reactor and XPS analysis chambers. We used two different sample holders, allowing for the detection of photoelectrons with emission angles of either 0° or 60° from the axis normal to the sample surface. It is worth noting that a higher detection angle of emitted photoelectrons with respect to the axis normal to the surface allows for a more surface-sensitive measure-

Most of the studies devoted to unveiling the degradation mechanism of organic electronic devices such as OSCs have used devices with complex structures; therefore, unveiling the degradation mechanism of organic devices on a physical− chemical basis has been limited.30,32−37 We adopted a simple model system consisting of P3HT films deposited on ZnO/ indium tin oxide (ITO). Previously, we had studied the oxidation behaviors of P3HT films supported by wet-chemically prepared ZnO ripple structures using X-ray photoelectron spectroscopy (XPS).26,27 In contrast to our previous work, this time, a relatively flat ZnO layer was deposited on ITO glass using atomic layer deposition (ALD) and used as a substrate for P3HT films. Most importantly, we performed systematic studies on how oxidation behaviors of P3HT differ from the top surface to the deeper layers of P3HT films by using angledependent X-ray photoelectron spectroscopy (XPS) and UV− vis absorption spectroscopy. Angle-resolved XPS can be used to compare oxidation behavior of P3HT of surface topmost layers and deeper layers with a slab thickness of 5−10 nm, whereas UV−vis absorption spectroscopy can monitor the changes caused in the entire P3HT film with a thickness of ∼50 nm. It is worth mentioning that use of angle-dependent XPS is meaningful, only when the surface structure is flat; i.e., rippled surfaces cannot be properly studied using angle-resolved XPS. Identification of the major oxidizing agents, as well as the oxidation mechanism during the visible-light-induced oxidation process of the P3HT film, was achieved under dry and humid conditions. We also provide a starting point for developing strategies to retard photoinduced degradation of semiconducting organic polymers.

2. EXPERIMENTAL METHODS 2.1. Preparation of ZnO Thin Films Using Atomic Layer Deposition (ALD). ZnO thin films were deposited on ITO-coated glass by an ALD method. ITO-coated glass with a lateral size of 25 × 25 mm2 was placed in an ALD reactor that was evacuated with a rotary pump. The substrate was then exposed to diethyl zinc (DEZ, precursor of Zn) and water vapor (oxidizing agent) in an alternating manner. Prior to the ALD process, the ITO glass was sonicated in acetone for 15 min and then dipped into an isopropyl alcohol (IPA) solution at 150 °C for 15 min. During the ALD process, the ITO-coated glass was exposed to DEZ at 1.6 Torr for 15 s. This was followed by a purging process for 3 min. After the purging process, the sample was exposed to H2O vapor at 2.8 Torr for 15 s and again purged for 3 min. This constitutes one ALD cycle, which results in the deposition of ∼1.6 Å of ZnO per cycle.38,39 Here, we used 200 ALD cycles for the ZnO film deposition. The reactor temperature was kept at 150 °C during the ALD process. 2.2. Preparation of P3HT Films on ZnO/ITO. P3HT (regioregularity ∼94%, 4002-EE, Rieko Metals, Inc.) was used as-purchased without further purification. To spin-cast P3HT on the ZnO/ITO glass substrate, a P3HT solution was prepared by the following process. A 20 mg portion of regioregular P3HT was dissolved in 1 mL of 1,2-dichlorobenzene (Sigma-Aldrich) at 60 °C, and the resulting solution was stirred overnight using a magnetic stirrer. A 20 μL portion of the solution was dropped onto a ZnO/ITO glass substrate with a lateral size of 6 × 6 mm2 and spin-cast at 3500 rpm for 40 s. The sample was then dried at room temperature for 20 min under dark conditions. The surface morphologies of bare and P3HT-deposited ZnO/ITO were analyzed by scanning B


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The Journal of Physical Chemistry C ment.40 We estimate that the 60° measurements monitor changes in the surface of the structure (1−2 nm from the topmost surface layer), whereas the 0° measurements measure significantly deeper layers (a monitoring depth of ∼5−10 nm). XPS data were obtained at room temperature using a Mg Kα line (1250.0 eV) as an X-ray source and a concentric hemispherical analyzer (CHA, PHOIBOS-Has 2500, SPECS). The pass energy of 10 eV was used to obtain XPS spectra shown in this work, and the analyzer is specified to deliver the resolution of 0.8 eV for Mg Kα excitation. The spot size of the X-ray beam is specified as 1−2 cm2 depending on the sample− anode distance. X-ray power of 300 W of Mg Kα was used, and the estimated photon density on the sample was 7.5 × 1013 s−1 cm−2. The analyzer was calibrated with Ag 3d core-level spectra (3d5/2 368.3 eV and 3d3/2 374.3 eV), and the XPS spectra shown in this work were shifted with reference to C 1s (C 1s of sp2 carbon at 285.5 eV). The Gaussian−Lorentzian product function was used for the XPS spectra fitting process. The Gaussian and Lorentzian functions were multiplied with the ratio of 7:3. 2.4. UV−Vis Absorption Spectroscopy. Ex situ UV−vis absorption spectra of P3HT/ZnO/ITO were obtained using a UV−vis absorption spectrophotometer (OPTIZEN 3220UV, MECASYS Co., Ltd.) before and after irradiating the samples with light from a blue LED under dry or humid conditions with various light exposure durations. After each P3HT degradation experiment, the sample was taken out of the reaction chamber and exposed to ambient air before measuring the UV−vis absorption spectrum. Each spectrum was collected in the wavelength range 250−700 nm. ZnO/ITO without P3HT was used as a reference cell, which allowed us to remove the UV− vis absorbance signal of ZnO; therefore, it was easier to detect changes in the structure of P3HT that resulted from photoinduced degradation.

Figure 1. SEM images of the ZnO/ITO (a) before and (b) after P3HT spin-casting. Height plots of the ZnO/ITO (c) before and (d) after P3HT spin-casting.

photodegradation processing times up to 12 h, under either dry or humid conditions, are shown in Figure 2. After the samples were exposed to light under dry or humid conditions for 3 h, XPS spectra were collected. The samples were then treated under the same respective conditions for another 9 h, reaching a cumulative reaction time of 12 h. All of the XPS core-level spectra shown here were measured with a detection angle that was 60° from the axis normal to the surface; this allows for highly surface-sensitive measurements with a detection depth of ∼1−2 nm from the topmost surface layer.40 Figure 2a,b corresponds to the S 2p and C 1s spectra collected before and after photodegradation experiments with various oxidation times under dry conditions, whereas Figure 2c,d shows the same type of data obtained under humid conditions. With increasing oxidation time under blue LED irradiation in either dry or humid conditions, both the S 2p and C 1s spectra became broader, and additional states appeared at higher binding energies; this indicates the oxidation of both S and C in P3HT. It is noteworthy that samples exposed to atmospheric conditions without light and samples exposed to light under vacuum conditions did not show any changes in their core-level spectra, indicating that both visible light and air are needed to oxidize P3HT;27 the optically excited states of P3HT may interact with O2 and H2O vapor, yielding strongly oxidizing agents such as O2− and OH radicals, which can attack and degrade the P3HT.24−27 For deeper analysis, each spectrum in Figure 2a−d was deconvoluted using various components, and each component consisted of a linear combination of Gaussian and Lorentzian functions with a ratio of 7:3. The fitting was conducted after Shirley background subtraction using the CASA-XPS program. To fit each S 2p core-level spectrum, four different chemical species were considered: the nonoxidized S state, sulfoxide (SO with S bound in the ring structure), sulfone (OSO with S bound to the ring structure), and the totally oxidized state of S resulting from the ring-opening.18,24−27 It should be noted that each S 2p state consists of a doublet structure of S 2p1/2 and S 2p3/2; this was caused by the spin−orbit splitting of the final state effects. Therefore, two components with a fixed

3. RESULTS AND DISCUSSION 3.1. Surface Structure Analyses of ZnO/ITO and P3HT/ ZnO/ITO before Photodegradation. Figure 1 displays SEM images of bare ZnO/ITO glass and P3HT/ZnO/ITO samples before the photodegradation experiment. The surface of the ZnO deposited on the ITO was not perfectly flat, but instead consisted of mosaic structures with islands of different heights (Figure 1a). The mean thickness of ZnO, as measured by ellipsometry, was 35 nm. This is roughly in line with the expected thickness value based on the number of ALD cycles used for ZnO deposition and the well-known deposition rate of our ZnO deposition process (200 ALD cycles, ∼1.6 Å/cycle).38,39 After spin-coating the P3HT onto the ZnO/ITO glass, the P3HT polymer appeared to coat the surface evenly (Figure 1b). In our previous study, we determined that our spin-coating process results in the formation of a P3HT layer with a mean film thickness of ∼50 nm.26 In order to further investigate the surface structures of bare and P3HT-deposited ZnO/ITO, AFM measurements were also performed (Figure 1c,d). The root-mean-square (RMS) roughness values of the ZnO/ITO and P3HT/ZnO/ ITO samples were 2.8 and 2.4 nm, respectively, exhibiting the relatively flat surface structures of both samples. 3.2. XPS Analyses of the Photodegradation of P3HT/ ZnO/ITO under Dry and Humid Conditions Using XPS with a Monitoring Depth of 1−2 nm from the Surface. The XPS core-level spectra of P3HT surfaces taken after C


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Figure 2. S 2p and C 1s core-level XPS spectra of P3HT layers under (a, b) dry and (c, d) humid conditions. (e) The area ratio of S 2p XPS peaks.

obtained under humid conditions for reaction times up to 12 h are qualitatively analogous to those obtained under dry conditions; with increasing photodegradation time, the relative areas of the nonoxidized states of S and C decreased, whereas those of sulfoxide and the oxidized C species increased in relative area. Figure 2e compares the results of the dry and humid conditions more quantitatively; i.e., the relative area of the oxidized S 2p states (sulfoxide, sulfone, and totally oxidized species) with respect to the nonoxidized states is displayed as a function of the reaction time. One can see that the oxidation of S to sulfoxide was more pronounced under dry conditions than under humid ones. Similar behaviors were also observed in C 1s spectra; the oxidation of carbon atoms was facilitated under dry conditions compared to humid conditions up to a reaction time of 12 h (Figure S1). However, the difference in the oxidationtime-dependent results under dry and humid conditions up to reaction time of 12 h is less pronounced in C 1s than S 2p spectra. This might be due to the fact that the C 1s spectrum consists of a large signal from the P3HT framework; therefore, it is difficult to see the subtle changes that result from the partial oxidation of C of P3HT. Our observation that dry conditions are more efficient than humid ones for the oxidation of S of P3HT (Figure 2) can be rationalized as follows. O2 is the major oxidizing agent, and the adsorption of water and O2 molecules onto the surface of P3HT is competitive, rather than independent or cooperative; thus, the humid conditions reduce

area ratio and binding energy separation should be taken into account to represent each S component.40 Therefore, eight different functions should be used to fit each S 2p spectrum. The binding energies of S 2p3/2 of the four aforementioned S species were centered at 164.7, 165.2, 166.7, and 169.4 eV, respectively, on the basis of previous results.18,24−27 Regarding the C 1s core-level spectra, six components were used to fit each spectrum: CC, CCC, CCS, COH, CO, and COOH at binding energies of 284.9, 285.5, 285.7, 286.5, 287.9, and 289.4 eV, respectively.18,24−27 Detailed results of the quantitative analyses based on these fitting processes can be found in the Supporting Information (Tables S1 and S2). The trace amount of sulfone in the S 2p spectrum, which was present even before the photodegradation experiment, was most likely caused by a reaction between the P3HT, solvent, and atmosphere; the relative area ratio of sulfone with respect to the entire S 2p peak area was barely changed during the photodegradation experiment. With increasing photodegradation time (up to 12 h under dry conditions), the relative area of the nonoxidized S state decreased, whereas that of sulfoxide increased. The totally oxidized species of S, formed by thiophene ring-opening, were negligible for reaction times up to 12 h under dry conditions. Regarding the C 1s spectra, the COH, CO, and COOH species became more abundant as the reaction time increased from 0 to 12 h. This can be mostly attributed to the oxidation of C atoms in the alkyl side chains of P3HT.15,17 The results D


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Figure 3. O 1s core-level XPS spectra of P3HT layers under (a) dry and (b) humid conditions. O 1s core-level XPS spectra were fitted considering chemical species of CO, SO, CO, and OH group of either water or the graphite−OH complex. The peak area of OH groups is filled with gray-dashed pattern in parts a and b. (c) The area ratio of the molecular oxygen-related component O 1s peak.

3 were exposed to light under the respective atmospheric conditions for an additional 6 h and then analyzed using XPS with a photoemission detection angle of 60°. It is obvious from the bare spectra of Figure 4a that the peak at ∼171 eV, corresponding to the totally oxidized S with thiophene ringopenings, became pronounced for both spectra. This peak is more pronounced in the spectrum of the sample that was exposed to humid conditions. Quantitative analyses based on the deconvolution process shown in Figure 4b,c also agree with these observations. In summary, up to a reaction time of 12 h, almost no thiophene ring-opening was found, and S was transformed mostly into sulfoxide species. O2 (forming O2− upon charge transfer from the optically excited P3HT) rather than H2O is suggested to be the oxidizing agent, and H2O retarded the oxidation rate of P3HT due to the competitive adsorption of O2 and H2O on the surface of P3HT. When the reaction time exceeded 12 h, thiophene ring-opening became pronounced; in this case, water vapor is the active species for ring-opening and the further oxidation of S. Water vapor, or OH radicals, formed as a consequence of interaction between the hole of optically excited P3HT and water, is not active for oxidation of P3HT at the initial stage of P3HT oxidation. In this step, rather, superoxide seems to be a more important oxidizing agent, transforming S of P3HT into sulfoxide and sulfone. Once sulfone and sulfoxide are formed and interchain stacking is perturbed, then water molecules can play an active role for further oxidation of P3HT and opening of the thiophene ring.

the adsorption of O2 onto the surface of P3HT, thereby decreasing its oxidation rate. Figure 3 shows the O 1s spectra taken from the same samples as those detailed in Figure 2. Again, it is obvious that the increase in the O 1s peak intensity with time is more pronounced under dry conditions (Figure 3c). Chemical species of CO, SO, CO, and OH groups of either water or the graphite−OH complex were considered in the deconvolution process of each O 1s core-level spectrum. These species were centered at binding energies of 532.6, 531.4, 533.3, and 534.4 eV, respectively.24−27,41 It is worth mentioning that the peaks above 534 eV can be attributed to molecular water or the hydroxyl groups bound to graphitic carbon; however, these two different species cannot be clearly differentiated in the O 1s core-level spectra.41 Graphite−OH-like species can be formed, when active oxygen species attack carbon atoms in the thiophene ring forming an OH−C bond at the thiophene ring. It is notable that the hydroxyl-group-related peak at 534.4 eV becomes more pronounced under dry conditions than it does under humid conditions. Oxygen molecules adhered to the surface of the P3HT film likely underwent transformation into activated oxygen species such as O2− via charge transfer from the optically excited P3HT. These reacted with P3HT to form the graphite−OH species. Figure 4a shows the S 2p spectra of P3HT that underwent photodegradation for 18 h under dry or humid conditions. The samples that were photo-oxidized and analyzed in Figures 2 and E


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Figure 4. (a) S 2p core-level XPS spectra of P3HT layers under dry and humid conditions. The area ratio of (b) sulfoxide + sulfone and (c) totally oxidized state of S 2p.

Figure 5. S 2p and C 1s core-level XPS spectra of P3HT layers under (a, b) dry and (c, d) humid conditions.

F


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The Journal of Physical Chemistry C In the previous study, a P3HT film deposited on a rippled ZnO surface only showed enhanced oxidation of P3HT by humidity at the very initial stage of visible-light-induced oxidation, which is not in line with the results of the present work.27 The surface of the flat and compact P3HT layer can be partially photooxidized by the molecular oxygen species during the initial stage of oxidation, and water vapor only plays an active role in P3HT oxidation once the surface of P3HT is oxidized to some extent and becomes defective. Alternatively, a P3HT film with a rough and open surface (such as P3HT layers on rippled ZnO with a ripple width and height of several tens of namometers) can be attacked by both oxygen and water, which are derived during the initial stage of photoinduced oxidation. 3.3. XPS and UV−Vis Absorption Spectroscopy Analyses of the Photodegradation of P3HT/ZnO/ITO under Dry and Humid Conditions: Oxidation of the Deeper Layers of P3HT. In order to investigate the photoinduced oxidation behavior of deeper P3HT layers (5− 10 nm from the topmost surface layer of P3HT), XPS analysis with a detection angle of 0° with respect to the axis normal to the surface was performed to detect photoelectrons. S 2p and C 1s core-level XPS spectra collected with visible-light irradiation times of 0, 3, 12, and 18 h are shown in Figure 5. Each spectrum was further analyzed with a fitting process, in the same manner that was used to obtain the data in Figure 2 (Tables S1, S2 and Figures S2, S3). Here, the difference in the oxidation behaviors of P3HT under dry and humid conditions is not as pronounced as was observed in Figures 3 and 4. We also studied the photoinduced oxidation behavior of P3HT thin films under dry and humid conditions using UV−vis absorption spectroscopy, which can be used to monitor the entire 50 nm thick P3HT film (Figure 6). The pristine P3HT film showed a broad absorbance spectrum in the range 400−700 nm with a maximum intensity at ∼520 nm and two shoulders at ∼550 and ∼610 nm. The single monomer of P3HT is responsible for the absorption peak at ∼520 nm, and the conjugation length within a strand and stacking of P3HT strands represent the absorbance peaks at ∼550 and ∼600 nm, respectively.18,27 Less significant oxidation of the P3HT film was observed after 3 and 12 h of visible-light irradiation; however, after 18 h of photo-oxidation, a drastic decrease in the absorbance over the entire wavelength range was observed. Also, there was almost no difference in the change of UV−vis absorption spectra with increasing reaction time under dry and humid conditions. This pronounced reduction of the UV−vis absorbance between 12 and 18 h can be attributed to the disturbed π−π interactions between neighboring P3HT strands, the shortened polymer conjugation length of the P3HT strands upon oxidation, and the oxidation of the P3HT monomer. It is notable that this abrupt change in the P3HT structure between 12 and 18 h of photodegradation was only observed in the UV−vis absorption spectra; no such changes were observed in the XPS spectra (Figures S2 and S3). The following scenario can be used to rationalize the aforementioned results. Up to 12 h of the photoinduced reaction, oxidation of the topmost surface layer of P3HT occurs very easily, whereas that of the deeper layers is not as significant. However, after a certain amount of oxidation takes place on the surface, which makes the surface of P3HT more defective after 12 h of photodegradation, oxygen molecules can more easily diffuse into the deeper layers of the P3HT film, thereby accelerating the oxidation of the entire film. Although

Figure 6. UV−vis absorption spectra of P3HT layer on the ZnO film obtained after 0, 3, 12, and 18 h of blue LED irradiations under (a) dry and (b) humid conditions.

oxidation of the topmost surface layer of the P3HT film is sensitive to humidity in the atmosphere, the oxidation behavior of the deeper layers, which were studied by our less surfacesensitive XPS studies (normal emission of photoelectrons) and UV−vis absorption spectroscopy, is independent of the humidity. Therefore, oxygen molecules can diffuse into the deeper layers once the surface of the P3HT film becomes sufficiently defective after partial oxidation; however, this diffusion of O2 (or O2−) into the deeper layers of P3HT is very slow. Therefore, this is the rate-determining step for the oxidation of the entire P3HT films. 3.4. Photodegradation of P3HT/ZnO/ITO under On− Off Light Irradiation Cycles. Figure 7a,b shows the S 2p spectra taken at a detection angle of 0° with respect to the axis normal to the surface (lower surface sensitivity) and the UV− vis absorption spectra, respectively, before and after exposing the samples to blue light under dry conditions with a total irradiation time of 18 h. In the first experiment, which used 18 h of light exposure, the sample underwent the following sequential procedure: 3 h of light exposure under atmospheric conditions, evacuation for XPS analyses under UHV conditions, 9 h of light exposure under atmosphere, XPS analyses, and another 6 h of light exposure. In the second experiment, denoted as 18 h (interval) in Figure 7, the sample experienced exposure to a dry atmosphere and repeated on−off light cycles; here, a cycle consisted of 3 h of light irradiation followed by 1 h of exposure to dark conditions. This cycle was repeated six times. Even G


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Figure 7. (a) S 2p core-level XPS spectra. (b) UV−vis absorption spectra taken before and after 18h light irradiation under humid atmospheric conditions. Two different methods were used for light illumination.

oxidation proceeded for 18 h, ring-opening became significant, and the water vapor facilitated the ring-opening process. In contrast to the top surface layer of the P3HT film, oxidation of the deeper layers of the film was not sensitive to the humidity level of the atmosphere; this indicates that O2 rather than H2O is the major oxidizing agent of the P3HT film. XPS spectra taken via normal detection, which is sensitive to the region 5−10 nm from the topmost surface layer of P3HT, show continuous oxidation of S and C over time; however, the UV−vis absorption spectra show that the oxidation of the entire P3HT film, with a mean thickness of ∼50 nm, was abruptly increased between oxidation times of 12 and 18 h. We suggest that O2, which appears to be the major oxidizing agent, transforms into O2− upon charge transfer from the optically excited P3HT. Additionally, slow diffusion of O2− into the deeper layers of P3HT, followed by oxidation of P3HT, is the rate-determining step of oxidation for the entire P3HT film. The interaction between O2 and the surface of the P3HT film does not appear to be a rate-determining step for this process. We also examined the effect of continuous light exposure on the photoinduced oxidation of P3HT films. This was accomplished by exposing the samples to cycles that consisted of 3 h of light illumination and 1 h of exposure to dark conditions. This cyclic light exposure resulted in less oxidation of P3HT with respect to illumination time as compared to samples that were exposed to light for the same overall period of time without applying any intermittent dark conditions. We suggest that continuous illumination can cause a high concentration of O2− to be maintained in the P3HT film; O2− can slowly diffuse into the deeper layers of P3HT and induce oxidation of the entire film. Exposure to dark conditions between illumination conditions can result in oxidation of O2− to O2 and the subsequent desorption of O2 from the P3HT film, thereby retarding the oxidation rate of P3HT.

though the cumulative light illumination time was the same (18 h) for both experiments, the sample that underwent six on−off illumination cycles showed much less oxidation of P3HT, as determined by the S 2p and UV−vis absorption spectra. These results could be explained by the fact that the oxygen molecules incorporated into the P3HT film transform into O2− due to charge transfer from the optically excited P3HT, and this species may be responsible for the oxidation of the entire P3HT film. Diffusion of O2− into deeper layers of P3HT appears to be very slow, acting as the rate-determining step for the oxidation of the entire P3HT film. Under repeated on−off light illumination, the O2− species formed within the P3HT films under irradiation to visible light can be oxidized into O2 under dark conditions by giving excess charge back to the sample (most likely to ITO via tunneling through the ZnO layer), and the resulting O2 desorbs from the P3HT film. Continuous exposure of the P3HT film to light can maintain a sufficiently high concentration of O2− within the P3HT layer, which can slowly diffuse into the deeper layers of P3HT and oxidize the deeper material. Using a light on−off process can delay the oxidation rate of P3HT, which can be used to develop strategies to increase the lifetimes of organic electronic devices such as OSCs.

4. CONCLUSION The blue-light-induced oxidation of P3HT thin films with a thickness of 50 nm on ZnO/ITO was studied under dry and highly humid conditions using angle-resolved XPS with varying surface sensitivities and ex situ UV−vis absorption spectroscopy. We found that the oxidation behavior of the topmost surface layer (1−2 nm from the surface) of the P3HT film is complex; for photoinduced oxidation times of up to 12 h, humidity plays a negative role in the oxidation of P3HT, which is most likely due to the fact that O2 is the major oxidizing agent. During this period, H2O and O2 molecules competitively adsorb onto the P3HT surface, which results in reduced O2 adsorption on the P3HT surface at increased humidity. Up to 12 h, thiophene ring-opening was negligible, and the S of the thiophene ring and C (most likely that of the alky chains) transformed into sulfoxide and partially oxidized C (CO, COH, and COOH), respectively. When photoinduced

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05849. Figures and tables detailing area ratios and fitting results (PDF) H


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Young Dok Kim: 0000-0003-1138-5455 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT, and Future Planning (2015R1A2A2A01003866).

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