Probing Device Degradation and Electric Fields in Polymeric Field

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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Probing Device Degradation and Electric Fields in Polymeric FieldEffect Transistors by SFG Vibrational Spectroscopy Silvia G. Motti,† Lilian S. Cardoso, Douglas J. C. Gomes, Roberto M. Faria, and Paulo B. Miranda* Sao Carlos Physics Institute, University of Sao Paulo, CP 369, Sao Carlos-SP 13560-970, Brazil S Supporting Information *

ABSTRACT: There is great interest on the study of the semiconductor/dielectric interface of organic field-effect transistors (OFETs), where a conducting channel is formed. Here, we use the interface selectivity, chemical sensitivity, and field-induced enhancement of sum-frequency generation (SFG) vibrational spectroscopy to probe interfacial molecular ordering and degradation processes in poly-3-hexylthiophene (P3HT) OFETs and also the electric field within their dielectric layer (poly(methyl methacrylate), PMMA). P3HT active layers fabricated by the Langmuir−Schaefer method are more orientationally ordered than spin-coated films. Upon electrical degradation of the device in ambient conditions, no noticeable changes were detected in the SFG spectra of the semiconductor/dielectric interface because the sensitivity of our experiment was not enough to detect degraded polymer chains due to loss of SFG electronic resonance enhancement. Perhaps for the same reason, we were also not able to detect any significant changes in the SFG spectra of the P3HT/dielectric interface upon charge accumulation induced by the gate bias. However, we found that upon polarizing the device, PMMA vibrational bands appeared due to field-induced reorientation of its polar groups. Therefore, SFG spectroscopy can be used to probe the electric field within the organic dielectric, including its sign, bringing the possibility of a complete device characterization by nonlinear spectroscopy/microscopy, mapping out the electric field both within the semiconductor and dielectric layers of the OFETs.

1. INTRODUCTION Organic field-effect transistors (OFETs) are considered the key component in modern organic-based microelectronics, and they have been successfully used in many applications, such as radio-frequency identification tags, drivers for flat panel displays, and sensor devices.1−3 OFETs offer an interesting balance between low manufacturing cost and good electrical performance, complemented by their flexibility, transparency, and chemical tunability. Particularly, the solution processability of the organic materials is the main advantage of these devices over the conventional inorganic technology because it enables the use of several deposition techniques, such as printing and/ or coating methods. OFET characteristics have been improving remarkably in the last decade, and recently, OFETs based on small-molecule semiconductors and polymer semiconductors have shown field-effect mobilities comparable to those of transistors based on amorphous silicon, ranging from 1 to 10 cm2/(V s).4−7 In spite of the huge progress in the OFET performance during the last years, the operation mechanism of these devices is still not completely clear. A deep understanding of the device physics is critical for further improvements in OFET performance and lifetime. The basic working principle of OFETs relies on the charge carrier accumulation in the semiconductor/ dielectric layer by a voltage applied in the gate electrode. These accumulated charge carriers are then driven along the interface © XXXX American Chemical Society

by a voltage applied between the source and drain electrodes. Most theoretical models proposed in the literature for organic transistors8−10 are based on the gradual channel approximation developed for conventional (inorganic) field-effect transistors,11 with the OFET operation usually occurring in the accumulation regime. However, comparisons of theoretical models for OFETs to experimental data are scarce in the literature and may be done in terms of IxV measurements,10 potential,12 or electric-field13 distribution along the channel. Therefore, mapping the potential, electric field, or charge density along the channel of operating organic transistors is very important for validating the theoretical models and understanding the physical mechanisms involved both in charge injection by electrodes and their transport along the channel. For that purpose, scanning Kelvin probe microscopy has been successfully applied for mapping the potential distribution in the OFETs channel.14 The charge density distribution along the channel has been mapped by scanning microwave conductivity measurements,15 charge modulation optical16 and infrared (IR) microscopy,17,18 and photoluminescence modulation microscopy.19 Direct electric-field measurements along the OFET channel may be performed with second-order Received: February 20, 2018 Revised: April 3, 2018

A

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the silicon wafers were oxygen plasma-treated at a power of 40 W for 10 min. Poly(3-hexylthiophene) (P3HT, Mn = 15−45 kDa and 95% regioregular) was purchased from Sigma-Aldrich. The semiconductor layer was deposited either by spin-coating (8 mg/mL in tetralin, 3-step spinning at 800 rpm for 60 s; 2000 rpm for 150 s; and 7000 rpm for 20 s, resulting in an ∼40 nm thick film, followed by 15 min of annealing at 120 °C) or by the Langmuir−Schaefer (LS) technique (spreading 0.1 mg/mL solution in chloroform) in a KSV 5000 trough in a class 10 000 clean room at a constant temperature of 22 °C and constant surface pressure of 30 mN/m. After the deposition of P3HT (one spin-coated film, or six LS layers), gold source and drain electrodes (thickness 50 nm, channel length L = 100 μm, channel width W = 2 mm, W/L = 20) were deposited by thermal evaporation (without annealing of the P3HT film). We have also fabricated OFETs with two stacked dielectric layers, using both poly(methyl methacrylate) (PMMA) and the SiO2 dielectric layer. For these devices, the experimental procedure was kept as described above, adding the PMMA deposition step before the semiconductor layer deposition. The PMMA (120 kDa) was purchased from Sigma-Aldrich and diluted in methyl−ethyl-ketone (30 mg/mL). The PMMA solution was deposited by spin-coating at 800 rpm for 60 s. After the deposition, the PMMA film was annealed at 100 °C for 10 min. The complete fabrication was performed in a glovebox with N2 atmosphere, after which the sample was sealed inside the glovebox in a custom chamber with a CaF2 window for the SFG measurements, which were performed under vacuum. The electrical measurements of the OFETs were carried out using a Keithley 4200 semiconductor characterization analyzer. For the photodegradation experiment, the electrical measurements were performed immediately after 5 h of air and light exposure (from a white-light light-emitting diode, intensity at the sample ∼5 mW/cm2) under constant bias stress (Vd = −12 V and Vg = −12 V). 2.2. Sum-Frequency Generation (SFG) Spectroscopy. SFG is a second-order nonlinear technique that provides the vibrational spectra of surfaces and interfaces because the SFG signal is produced only by dipolar molecules in a noncentrosymmetric arrangement.28,29 The SFG theory has been well discussed elsewhere,29,39 and here we will describe only the important points for our data analysis. In SFG spectroscopy, tunable infrared (IR) and visible beams are spatially and temporally overlapped in a sample to generate a signal at the sum of their frequencies. The intensity of the SFG signal is proportional to the visible and infrared intensities and to the square modulus of the effective second-order susceptibility of the interface (χ(2) eff ), which is composed by the (2) nonresonant background (χ(2) ) NR and the resonant term (χR ). The latter can be expressed in terms of the summation over all g vibrational resonances of the molecules at the interface

nonlinear optical techniques, such as second-harmonic generation (SHG)19 and sum-frequency generation (SFG) spectroscopy.20 For example, electric-field-induced SHG microscopy has been used to map the in-plane electric field (within the semiconductor) along the transistor channel13,21 and its transient dynamics upon device turn-on.22 SFG spectroscopy has also been successfully used to probe charge accumulation at the dielectric/organic semiconductor interface for OFETs based on pentacene,23 P3HT,24 phenyl-thiophene derivatives,25,26 and PTAA,27 because charge accumulation in the semiconductor leads to an enhanced nonresonant background in the SFG spectrum23−26 or to new vibrational features due to polaronic transitions.27 These nonlinear optical methods have the advantages of being noninvasive, sensitive to molecular orientation, and interface-specific, discriminating only the interfacial molecules and suppressing bulk contribution, at least in the absence of strong electric fields.28,29 In particular, SFG spectroscopy yields the vibrational spectrum of molecules in buried interfaces, in contrast to conventional vibrational spectroscopic techniques, and has been used by many research groups to study the interfacial arrangement of organic semiconductors.30−33 In this work, we aim at using SFG spectroscopy on operating P3HT-based OFETs not only to probe electric fields within the transistors but also for investigating chemical degradation processes at the semiconductor/dielectric interface induced by device operation (bias stress). Previous studies by Fouriertransform infrared spectroscopy in P3HT devices did show clear vibrational signatures of carriers accumulated at the semiconductor/dielectric interface34 or photodoped in presence of oxygen.35 Raman spectroscopy has also been used to probe interfacial charge transfer from P3HT to transition-metal oxides.36 These studies were possible most probably due to the high cross section for polaron IR absorption or Raman scattering.37,38 The poor discrimination of interfacial contributions against the bulk in such linear optical techniques would not allow enough sensitivity to probe chemical changes at the organic semiconductor/dielectric interface that are not associated with charged conjugated molecules (polarons), such as loss of conjugation by chemical oxidation. The much enhanced surface sensitivity of SFG spectroscopy could in principle allow us to detect such interfacial chemical changes. However, we will show that the obtained SFG data demonstrated a low sensitivity of our SFG experiment to detect conjugated polymer degradation. Nevertheless, our results have clearly shown that SFG spectroscopy can be used to probe the out-of-plane electric field within the dielectric layer of OFETs, generated by charge accumulation at the semiconductor/dielectric interface. Therefore, SFG microscopy could be used for mapping charge accumulation along the transistor channel, yielding not only its magnitude but also the sign of the accumulated charge.

(2) χeff

2. MATERIALS AND METHODS 2.1. Device Preparation and Characterization. Bottomgate top-contact OFETs were fabricated on heavily doped ntype silicon substrates covered by a 300 nm thermal oxide (SiO2) dielectric layer. The silicon wafers were cleaned using a Nochromix solution in sulfuric acid (Godax Laboratories, Inc.) and then washed in deionized water. The oxide on the back side of the wafer was removed in buffered oxide etch solution (HF/NH4F, 1:6 v/v), and a thin layer of aluminum was thermally evaporated to ensure good electrical contact. Finally,

2

=

(2) χNR

+

χR(2)

2

=

(2) χNR

+

∑ g

Ag

2

ωg − ωIR − i Γg (1)

where Ag, ωg, and Γg are the amplitude, frequency, and line width of the g-th vibrational mode. When the frequency of the IR beam (ωIR) coincides with the frequency of the vibrational mode (ωg), the magnitude of the χ(2) eff is maximized and an SFG intensity enhancement is observed. The molecular vibrational spectrum of the molecules at the interface is collected by monitoring the SFG signal as a function of the IR frequency. B

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Additionally, SFG spectra with different polarization combinations of the incident and output beams, such as SSP (Spolarized output, S-polarized visible input, and P-polarized IR input), PPP, and SPS, can be collected and used to determine molecular orientation at the interface.39 If a strong direct current (DC) electric field E0 is present in the bulk of an isotropic material, it breaks the inversion symmetry and leads to a significant increase of the SFG signal produced by the sample. This enhancement can be due to a field-induced orientation of the material that leads to a nonvanishing χ(2)(E0), which is usually proportional to E0 if this field is low enough, and/or by an interaction of E0 with the optical input fields via the third-order nonlinear susceptibility of the bulk material, χ(3). These two contributions can be lumped together as an effective third-order contribution or as an effective second-order susceptibility that is proportional to the DC field ISFG ∝ χeff ⃡ (2) (E0): E ⃗(ωvis)E ⃗(ωIR )

2

= χ ⃡ (2) (E0): E ⃗(ωvis)E ⃗(ωIR ) + χ ⃡ (3) : E ⃗(ωvis)E ⃗(ωIR )E0⃗ = χeff ⃡ (3) : E ⃗(ωvis)E ⃗(ωIR )E0⃗

Figure 1. SFG spectra with PPP polarization obtained for a P3HT film deposited by spin-coating and by Langmuir−Schaefer (six layers) onto SiO2/Si substrates. The inset shows the chemical structure of P3HT.

average of 100 laser pulses for each data point. All SFG spectra were normalized by a nonresonant spectrum from a ZnS thin film to compensate for the spectral dependence of beam overlap and signal detection efficiency. Finally, the SFG spectra were fitted using eq 1.

2

2

(2)

In any case, the effective SFG field amplitude would be proportional to E0. Therefore, the SFG intensity can also be used to investigate the distribution of electric fields in organic devices. For example, Miyamae et al.40 have probed charge accumulation at interfaces within a multilayer device such as an organic light-emitting diode because it changes the field distribution within the layers and therefore the region with the strongest field dominates the SFG spectrum. Our SFG experiments were performed using a commercial SFG spectrometer (EKSPLA, Lithuania) based on a picosecond high-energy Nd3+:YAG laser (λ = 1064 nm, 30 mJ/pulse, 20 Hz repetition rate). An optical parametric amplifier coupled to a difference-frequency generation unit was employed to generate a tunable mid-IR beam (spectral resolution of 3 cm−1). The second harmonic of the Nd3+:YAG laser output (532 nm) was used as a visible input beam. The IR and visible beams were focused and overlapped at the sample, and the outgoing SFG beam was spatially filtered and spectrally selected by a monochromator and then detected by a gated photomultiplier. The incident angles of the IR and visible beams were 55 and 65°, respectively. For non-FET samples (spectra shown in Figures 1 and 7), to avoid photodamage, the pulse energies were set to 150 μJ for IR (spot size of ∼0.5 mm diameter) and 8 μJ for the visible beam (spot size of ∼1.0 mm diameter). For measurements in the OFETs, we tried to minimize the strong nonresonant SFG contribution from the gold electrodes by using two f = 150 mm cylindrical lenses to further focus the visible beam into the transistor channel and recollimate the emitted SFG beam. This measurement configuration is illustrated in Figure S1 of the Supporting Information and corresponds to the channel length L perpendicular to the incidence plane. We have not controlled the relative orientation of the LS film compression on the trough with respect to the channel length. However, because we used the PPP polarization combination in this study, which probes mainly χ(2) zzz due to IR transition dipoles and Raman polarizabilities perpendicular to the film, any small in-plane anisotropy of the P3HT film due to monolayer compression should have a minor effect on the SFG spectra. The SFG spectra were collected at 3 cm−1 steps with an

3. RESULTS AND DISCUSSION The Langmuir−Schaefer technique is characterized by a horizontal lifting method, which allows the transfer of a polymer monolayer at the air/water interface (Langmuir film) to a solid substrate. We have chosen this technique because it yields highly ordered thin polymer films,41 and hence higher SFG intensity can be observed from these P3HT films with respect to films deposited by spin-coating, as shown in Figure 1. The PPP-polarized SFG spectra for P3HT has vibrational modes at ∼1440 and 1380 cm−1 assigned to CC symmetric stretch and C−C (ring), respectively,42,43 although the CC band may actually have several contributions.30 The PPP polarization was chosen in this study because it provides higher SFG signal intensity (see Figure S2 of the Supporting Information for spectra with different polarization combinations) and it probes mainly χ(2) zzz due to IR transition dipoles and Raman polarizabilities perpendicular to the film. Hereafter, all data shown for P3HT devices are fabricated by the Langmuir− Schaefer technique, with six-layer deposition. The output curves (Id vs Vd) for the Si/SiO2/P3HT and Si/ SiO2/PMMA/P3HT OFETs used in the SFG measurements are shown in Figure 2. Both OFETs show a typical p-type transistor behavior with linear and saturation regimes. However, the maximum drain current for the OFET based only on SiO2 as a gate dielectric was about 200 times lower than that for the device with SiO2/PMMA dielectric. This is mostly related to the polythiophene ring orientation at the P3HT/dielectric interface. A quantitative analysis of molecular ordering of polythiophene rings onto SiO2 substrates modified by self-assembled monolayers with different surface energies has been performed by Anglin et al.42 Using the polarization dependence of the SFG spectrum of the CC stretches (at ∼1440 cm−1), these authors demonstrated that the thiophene rings are tilted away from the surface (almost edge-on configuration) on the substrates with lower surface energy, whereas they lie down closer to the surface for high-surfaceenergy substrates, such as the bare SiO2. Because the edge-on ring alignment is favorable for electrical transport along the C

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. SFG spectra of (a) Si/SiO2/P3HT and (b) Si/SiO2/ PMMA/P3HT OFETs under different bias voltages (Vd and Vg). (c) Cartoon of the expected charge density and field distribution along the transistor channel with only gate bias or both gate and drain bias. Figure 2. Output curves of (a) Si/SiO2/P3HT and (b) Si/SiO2/ PMMA/P3HT OFETs.

for the Si/SiO2/PMMA/P3HT transistor (Figure 3b). Such difference in the bias response of the SFG spectra for the two types of devices could be related to their electrical performance because the Si/SiO2/P3HT transistor had indeed much poorer electrical characteristics. Upon applying only the gate voltage (Vg), there is a slight enhancement of the P3HT bands due to channel formation in the transistors (Figure 3b, blue squares), which decrease almost to the original levels when the drain voltage (Vd in the saturation regime) is simultaneously applied. This reduction is associated with the charge redistribution along the channel upon applying a drain voltage, leading to a smaller fraction of the channel length accumulating a significant charge density (see Figure 3c). Similar behavior was found by Manaka et al.47 in their study of pentacene OFETs using second-harmonic generation (SHG). In addition to the enhancement of the peaks at 1380, 1440, and 1510 cm−1, there are changes in the line shape of the SFG spectra for the Si/SiO2/PMMA/P3HT device with bias, such as a marked increase of the high-frequency shoulder of the 1440 cm−1 band, thus appearing to blue-shif t for the polarized device. Although the interpretation of this change is not straightforward, it could be attributed to conjugation changes in the polymer chain due to the accumulation of holes (polaron formation). Since the selection rule for SFG spectroscopy is that a mode must be both Raman and IR active to appear in the SFG spectrum,28,29 we first note that no changes in the IR spectrum in this spectral range are observed upon doping.34 In the Raman spectra with near-IR excitation,36,48 doping leads to a new red-shif ted band at ∼1420 cm−1 and an increase of the 1380 cm−1 peak, together with a broadening in this spectral region. However, Louarn et al.49 have investigated the Raman

interface, low-surface-energy dielectrics usually yield higher mobility OFETs.44 They also present less charge traps than higher energy surfaces, such as SiO2/Si.45 Moreover, a considerable drain current is observed in both output curves when the transistor is off (Vd = 0). This behavior is related to the leakage current through the dielectric layers. The OFET parameters, such as field-effect mobility (μ), threshold voltage (VT), Ion/Ioff, and subthreshold swing (S), were calculated in the saturation regime from the Id vs Vg curves (Figure S3) using the gradual channel approximation model,46 and they are listed in Table S1. The lower slope factor S value and the higher μ and Ion/Ioff values observed for the transistor with the SiO2/PMMA dielectric indicate that the electrical performance of the OFETs can be improved by employing an additional dielectric layer of PMMA. A drawback of such double-layer dielectrics is its increased thickness, leading to higher operating voltages. The SFG spectra obtained from the channel of the Si/SiO2/ P3HT and Si/SiO2/PMMA/P3HT OFETs under different applied voltages (Vd and Vg) are displayed in Figure 3. In all SFG spectra hereafter, points represent the data, whereas the solid lines are curve-fittings using eq 1. At Vg = 0 V and Vd = 0 V, the SFG spectra for both devices show two broad vibrational resonances near 1440 and 1380 cm−1, which are assigned to the CC symmetric stretch and C−C stretch (ring), as discussed previously (Figure 1). Upon applying drain and/or gate voltages, no appreciable change in the SFG spectrum was observed for the Si/SiO2/P3HT OFET (Figure 3a) and the slight intensity changes are within the experimental uncertainty. However, more pronounced SFG spectral changes were seen D

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. SFG spectra in the region of (a, b) C−O and (c, d) CO stretching modes for the Si/SiO2/PMMA/P3HT OFET under different gate voltages (Vg). The insets in (a) and (c) represent the peak amplitude (A/Γ) dependence with the gate voltage.

Nevertheless, the most important feature of the SFG spectra for the Si/SiO2/PMMA/P3HT device is a new peak at ∼1240 cm−1, which appears only with bias and is absent in the Si/ SiO2/P3HT device. This suggests that this new prominent peak is associated to the PMMA dielectric layer. To investigate in more detail the origin of this peak, SFG spectra were acquired for Si/SiO2/PMMA/P3HT OFET at different gate voltages in the region of 1170−1320 and 1660−1780 cm−1, as shown in Figure 4. For these SFG measurements, we kept Vd = 0 while positive and negative gate voltages were applied to the device. According to the literature, the resonant peaks at 1240 and 1720 cm−1 are assigned to the C−O and CO stretches of the ester and carbonyl groups of the PMMA, respectively.54,55 The SFG spectra show significant changes in the line shape due to interference between the nonresonant background and peak amplitudes, which vary with the gate voltage. It is important to emphasize that PMMA is a transparent, nonconjugated polymer so that resonant enhancement of the SFG signal on visible and SFG wavelengths is not present. Therefore, a measurable SFG signal with such low pump beam intensities used in these experiments must be due to a field-induced contribution of the bulk PMMA film, as described in Section 2.2 (eq 2). A linear dependence of the SFG peak amplitude as a function of the gate voltage (maybe offset by some threshold voltage) is expected on the basis of eq 2 for the PMMA dielectric layer. Indeed, our amplitudes extracted from the spectral fittings of both C−O and CO stretches for the Si/SiO2/PMMA/P3HT device are consistent with this linear dependence, as shown in the insets of Figure 4a,c. A similar (quadratic) dependence of the SFG intensity on the gate voltage was reported in the SFG study of pentacene/PVP/SiO2 OFETs.23 No appreciable change was observed in the nonresonant background of the spectra in Figure 4 as a function of the gate voltage, in contrast

spectra for neutral and doped poly(3-decylthiophene) with several excitation wavelengths, showing that with green excitation (514 nm), the observed CC stretch is not sensitive to doping because only the neutral chains are being probed due to resonant enhancement, which does not occur for the doped ones. Conversely, for doped chains, the red (676 nm) or nearIR (1064 nm) excitations are resonant with their electronic transitions, so the CC stretches are quite sensitive to doping: a red shif t and broadening is observed, similar to the other reports mentioned above.36,48 They further concluded that doping leads to a slight quinoid character for the bond alternation of the doped polymer, similar to other works on thiophene oligomers.50 Therefore, because our visible beam is at 532 nm and the SFG output beam is at ∼495 nm, we expect double-resonance enhancement in the visible region (plus the usual IR resonance) in the SFG spectrum for neutral chains,51 whereas for doped chains, this enhancement should be lost. Indeed, the visible beam energy used in our experiment (8 μJ) was much lower than what we normally use for nonconjugated organic materials (∼900 μJ)52 but still yielded a strong SFG signal. This implies that with our experimental configuration, most likely we are not sensitive to polaron vibrations at the P3HT/dielectric interface but we are probing the vibrations of the remaining neutral chains at the interface. In such a case, the blue shift of the 1440 cm−1 band could be due to a lower average conjugation length of these remaining chains, as it is well known these the CC stretches blue-shift for shorter oligomers.53 Indeed, the Raman spectra with 1064 nm excitation for dimethylsexithiophene show higher-frequency shoulders on the main CC stretch band for neutral (1452 and 1477 cm−1) and doped (1438, 1466, and 1480 cm−1) molecules,50 which could be an indication of a finite conjugation length. E

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C to the work of Ye et al.,26 who reported an electric-field enhancement of the SFG background signal as result of accumulated charge carriers at the semiconductor/dielectric interface. Furthermore, another interesting aspect of these SFG spectra is the modification of the line shape for positive and negative gate bias (Figure 4b,d) due to a change of sign for the mode amplitudes (fitting parameters available in the Supporting Information). As shown by eq 2, there are two possible interpretations for the change of sign of the SFG amplitude with the sign of the gate voltage: (i) an up/down flip of the orientation of C−O and CO groups within the whole PMMA layer under the action of the electric field in the dielectric, yielding an χ(2) that flips sign with the applied field; (ii) the SFG signal comes from a χ(3)E0 contribution, in which case the sign of E0 determines the sign of the SFG amplitude and no molecular reorientation needs to be invoked. The exact origin of such field-induced SFG signal will be the subject of a forthcoming publication. In either case, these results demonstrate that SFG spectroscopy is a powerful tool to probe the electric field (including its sign) not only in the semiconductor but also in the dielectric layer of OFETs. The chemical degradation of P3HT during device operation is also an important aspect to be investigated using SFG spectroscopy. It is well known that organic devices are susceptible to degradation of electrical properties when operating under ambient conditions.56 The output curves of the pristine Si/SiO2/P3HT device, after air and light exposure under operation and recovery in vacuum for 30 h, are displayed in Figure 5. For the degradation experiment, the output measurement (Figure 5b) was carried out in the device immediately after 5 h of air and light exposure. As can be seen in Figure 5, the typical output characteristics of the P3HT-based transistor is lost after being exposed to ambient conditions. The output curves show only an almost straight-line current−voltage characteristic with higher current values, which can be attributed to an increase of the charge carrier density in the P3HT layer due to an oxygen doping process.57 Reversible and irreversible degradation processes of P3HT by oxygen are proposed in the literature.58,59 The irreversible doping process involves a direct chemical reaction of oxygen with P3HT, resulting in the formation of carbonyl and carboxyl groups.59 In contrast, the reversible process involves the formation of weak P3HT−O2 complex, which yields a distinct absorption band in the visible region.57,58 In this reversible doping process, the electron transfer from the P3HT to the oxygen molecule results in an immobilized electron on the oxygen molecule and a mobile hole on the P3HT chain, increasing thus the charge carrier density and electrical current in P3HT-based devices. For probing irreversible chemical degradation at the P3HT/ dielectric interface, SFG spectroscopy was also performed on the Si/SiO2/P3HT transistor whose electrical measurements are reported in Figure 5, before and after exposure to air and light. These results are shown in Figure 6. The SFG spectra show a slight decrease in the peak intensities of the CC and C−C stretches after the device was exposed to ambient conditions, with a reduction of the high-frequency side of the CC band at ∼1440 cm−1. Nevertheless, because of the relatively low signal-to-noise ratio of the SFG spectra, we are not able to analyze in detail such changes. However, no new peaks were observed in the region of carbonyl stretches (data not shown). This could be either due to the absence of irreversible degradation or to a low sensitivity of our

Figure 5. Output curves of the Si/SiO2/P3HT OFET: (a) pristine device; (b) after air and light exposure under bias for 5 h; (c) recovered from (b) after 30 h in vacuum (10−6 mbar).

Figure 6. SFG spectra for the Si/SiO2/P3HT OFET (Vd = Vg = 0) before and after exposure to air and light (same conditions as Figure 5).

experiment to detect such carbonyl groups, which break the chain conjugation and suppress the double-electronic resonance enhancement for our SFG spectra, as discussed above. To investigate the sensitivity of our setup to chemical degradation, SFG spectroscopy measurements were carried out for a P3HT film on a glass substrate before and after being exposed to oxygen plasma (Plasma Etch PM-313, with 20% of the nominal power of 100 W) for different periods (15−90 s). Brief oxygen plasma exposure should induce severe oxidation F

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C effects from the topmost surface chains.26 Because the P3HT sample became nearly transparent for relatively short exposure times such as 45 s, we can conclude that full sample degradation and etching occurred at long times (∼90 s). The SFG spectra of the P3HT film for different exposure times to oxygen plasma are shown in Figure 7.

have shown that SFG spectroscopy can be used to probe the electric field within the organic dielectric layer (PMMA), including its sign, in contrast to previous works by SHG and SFG spectroscopy that always probed the in-plane electric field within the organic semiconductor layer of OFETs.19,23 This result brings the possibility of a complete device characterization by SFG and SHG spectroscopy/microscopy to map out the electric field both within the semiconductor and dielectric layers of the OFETs, which will be the focus of our future study.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01760. Sketch of the beam configuration for SFG spectra acquisition in OFETs; SFG spectra for a P3HT film on SiO2/Si substrates with SPS, SSP, and PPP polarization combinations; transfer curves and electrical parameters for P3HT OFETs and complete fitting parameters to the SFG spectra as a function of the gate voltage in Figure 4b,d (PDF)

Figure 7. SFG spectra collected from a spin-coated P3HT film on glass before and after oxygen plasma treatment for the times indicated in the graph.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +55 16 33739825.

The SFG spectra show a reduction in the peak intensities for the CC and C−C stretches for exposure times to oxygen plasma beyond 15 s. A small change in line shape of the spectrum was observed only at long exposure times (t = 60 s), right before the SFG spectrum completely vanished. No new peak around 1720 cm−1 due to CO groups was observed in the spectra even for low exposure times, when oxidation of the topmost surface should yield a surface with strongly oxidized chains. Therefore, these results demonstrate the low sensitivity of our SFG measurements to evaluate the degradation process in P3HT chains due to the loss of electronic resonance upon chain degradation. We suggest that such an investigation should be performed with near-IR excitation wavelengths and work in this direction is ongoing in our laboratory.

ORCID

Paulo B. Miranda: 0000-0002-2890-0268 Present Address †

Center for Nano Cience and Technology@Polimi, Istituto Italiano di Tecnologia, via Pascoli 70/3, Milano 20133, Italy (S.G.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Brazilian agencies FAPESP (Process Numbers 2014/01595-0, 2014/50869-6, and 2015/24908-7) and CNPq (Grant number 465572) for financial support. L.S.C. was supported by a CAPES postdoctoral fellowship, and D.J.C.G. by a Ph.D. fellowship from FAPESP (2013/07328-1) and S.G.M. acknowledges CAPES for an M.Sc. scholarship.

4. CONCLUSIONS The surface/interface selectivity, chemical sensitivity, and fieldinduced enhancement make SFG spectroscopy a powerful method to probe interfacial molecular ordering, degradation processes, and electric field distribution in OFETs. In this work, we have shown with SFG spectroscopy that P3HT active layers fabricated by the Langmuir−Schaefer method tend to be more orientationally ordered than spin-coated films. We also attempted to use SFG spectroscopy to investigate molecular changes in P3HT-based OFETs upon electrical degradation of the device in ambient conditions. The obvious degradation of the electrical performance of the OFET was not accompanied by noticeable changes in the SFG spectra of the semiconductor/dielectric interface. We have demonstrated that the sensitivity of our SFG measurements with visible wavelengths is not enough to detect degraded polymer chains, probably due to loss of electronic resonance enhancement of the SFG signal as a result of breaking the conjugation of the P3HT backbone. Perhaps because of the same reason, we were also not able to detect any significant changes in the SFG spectra of the P3HT/dielectric interface upon charge accumulation induced by the gate bias. However, our results

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REFERENCES

(1) Battiato, P.; Abdinia, S.; Jacobs, S.; Chartier, I.; Coppard, R.; Klink, G.; Cantatore, E.; Ragonese, E.; Palmisano, G.; Fiore, V. An Integrated 13.56-MHz RFID Tag in a Printed Organic Complementary TFT Technology on Flexible Substrate. IEEE Trans. Circuits Syst. I 2015, 62, 1668−1677. (2) Yagi, I.; Hirai, N.; Miyamoto, Y.; Noda, M.; Imaoka, A.; Yoneya, N.; Nomoto, K.; Kasahara, J.; Yumoto, A.; Urabe, T. A Flexible FullColor AMOLED Display Driven by OTFTs. J. Soc. Inf. Display 2008, 16, 15−20. (3) Wu, X.; Ma, Y.; Zhang, G.; Chu, Y.; Du, J.; Zhang, Y.; Li, Z.; Duan, Y.; Fan, Z.; Huang, J. Thermally Stable, Biocompatible, and Flexible Organic Field Effect Transistors and Their Application in Temperature Sensing Arrays for Artificial Skin. Adv. Funct. Mater. 2015, 25, 2138−2146. (4) Kimura, Y.; Nagase, T.; Kobayashi, T.; Hamaguchi, A.; Ikeda, Y.; Shiro, T.; Takimiya, K.; Naito, H. Soluble Organic Semiconductor Precursor with Specific Phase Separation for High-Performance Printed Organic Transistors. Adv. Mater. 2015, 27, 727−732.

G

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (5) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C. A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A Stable SolutionProcessed Polymer Semiconductor with Record High-Mobility for Printed Transistors. Sci. Rep. 2012, 2, No. 754. (6) Umeda, T.; Kumaki, D.; Tokito, S. Surface-Energy-Dependent Field-Effect Mobilities up to 1 cm2/V s for Polymer Thin-Film Transistor. J. Appl. Phys. 2009, 105, No. 024516. (7) Tseng, H.-R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J. High-Mobility Field-Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers. Adv. Mater. 2014, 26, 2993−2998. (8) Braga, D.; Horowitz, G. High-Performance Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 1473−1486. (9) Kumar, B.; Kaushik, B. K.; Negi, Y. S. Organic Thin Film Transistors: Structures, Models, Materials, Fabrication, and Applications: A Review. Polym. Rev. 2014, 54, 33−111. (10) Schmechel, R.; Ahles, M.; Von Seggern, H. A Pentacene Ambipolar Transistor: Experiment and Theory. J. Appl. Phys. 2005, 98, No. 084511. (11) Sze, S. M. Physics of Semiconductor Devices; Wiley & Sons: New York, 1981. (12) Weis, M. Gradual Channel Approximation Models for Organic Field-Effect Transistors: The Space-Charge Field Effect. J. Appl. Phys. 2012, 111, No. 054506. (13) Manaka, T.; Nakao, M.; Yamada, D.; Lim, E.; Iwamoto, M. Optical Second Harmonic Generation Imaging for Visualizing in-Plane Electric Field Distribution. Opt. Express 2007, 15, 15964−15971. (14) Hu, Y.; Pecunia, V.; Jiang, L.; Di, C.-A.; Gao, X.; Sirringhaus, H. Scanning Kelvin Probe Microscopy Investigation of the Role of Minority Carriers on the Switching Characteristics of Organic FieldEffect Transistors. Adv. Mater. 2016, 28, 4713−4719. (15) Babajanyan, A.; Melikyan, H.; Kim, J.; Lee, K.; Iwamoto, M.; Friedman, B. Direct Imaging of Conductivity in Pentacene Field-Effect Transistors by a Near-Field Scanning Microwave Microprobe. Org. Electron. 2011, 12, 263−268. (16) Sciascia, C.; Martino, N.; Schuettfort, T.; Watts, B.; Grancini, G.; Antognazza, M. R.; Zavelani-Rossi, M.; McNeill, C. R.; Caironi, M. Sub-Micrometer Charge Modulation Microscopy of a High Mobility Polymeric N-Channel Field-Effect Transistor. Adv. Mater. 2011, 23, 5086−5090. (17) Chin, X. Y.; Yin, J.; Wang, Z.; Caironi, M.; Soci, C. Mapping Polarons in Polymer FETs by Charge Modulation Microscopy in the Mid-Infrared. Sci. Rep. 2014, 4, No. 3626. (18) Khatib, O.; Mueller, A. S.; Stinson, H. T.; Yuen, J. D.; Heeger, A. J.; Basov, D. N. Electron and Hole Polaron Accumulation in LowBandgap Ambipolar Donor-Acceptor Polymer Transistors Imaged by Infrared Microscopy. Phys. Rev. B 2014, 90, 1−8. (19) Koopman, W. W. A.; Toffanin, S.; Natali, M.; Troisi, S.; Capelli, R.; Biondo, V.; Stefani, A.; Muccini, M. Mapping of Charge Distribution in Organic Field-Effect Transistors by Confocal Photoluminescence Electromodulation Microscopy. Nano Lett. 2014, 14, 1695−1700. (20) Motti, S. G.; Maia, F. B.; Miranda, P. B. In Ultrafast Dynamics in Molecules, Nanostructures and Interfaces; Gurzadyan, G. G., Lanzani, G., Soci, C., Sum, T. C.; Eds.; World Scientific Publishing: Singapore, 2014; Chapter 11, pp 183−218. (21) Manaka, T.; Lim, E.; Tamura, R.; Yamada, D.; Iwamoto, M. Probing of the Electric Field Distribution in Organic Field Effect Transistor Channel by Microscopic Second-Harmonic Generation. Appl. Phys. Lett. 2006, 89, No. 072113. (22) Manaka, T.; Lim, E.; Tamura, R.; Iwamoto, M. Direct Imaging of Carrier Motion in Organic Transistors by Optical Second-Harmonic Generation. Nat. Photonics 2007, 1, 581−584. (23) Nakai, I. F.; Tachioka, M.; Ugawa, A.; Ueda, T.; Watanabe, K.; Matsumoto, Y. Molecular Structure and Carrier Distributions at Semiconductor/Dielectric Interfaces in Organic Field-Effect Transistors Studied with Sum Frequency Generation Microscopy. Appl. Phys. Lett. 2009, 95, No. 243304.

(24) Anglin, T. C.; O’Brien, D. B.; Massari, A. M. Monitoring the Charge Accumulation Process in Polymeric Field-Effect Transistors via in Situ Sum Frequency Generation. J. Phys. Chem. C 2010, 114, 17629−17637. (25) Ye, H.; Abu-Akeel, A.; Huang, J.; Katz, H. E.; Gracias, D. H. Probing Organic Field Effect Transistors in Situ during Operation Using SFG. J. Am. Chem. Soc. 2006, 128, 6528−6529. (26) Ye, H.; Huang, J.; Park, J.-R.; Katz, H. E.; Gracias, D. H. Correlations Between SFG Spectra and Electrical Properties of Organic Field Effect Transistors. J. Phys. Chem. C 2007, 111, 13250−13255. (27) Anglin, T. C.; Sohrabpour, Z.; Massari, A. M. Nonlinear Spectroscopic Markers of Structural Change during Charge Accumulation in Organic Field-Effect Transistors. J. Phys. Chem. C 2011, 115, 20258−20266. (28) Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40, 103−145. (29) Shen, Y. R. Fundamentals of Sum-Frequency Spectroscopy, Cambridge University Press: Cambridge, U.K., 2016. (30) Maia, F. C. B.; Miranda, P. B. Molecular Ordering of Conjugated Polymers at Metallic Interfaces Probed by SFG Vibrational Spectroscopy. J. Phys. Chem. C 2015, 119, 7386−7399. (31) Dhar, P.; Khlyabich, P. P.; Burkhart, B.; Roberts, S. T.; Malyk, S.; Thompson, B. C.; Benderskii, A. V. Annealing-Induced Changes in the Molecular Orientation of Poly-3-Hexylthiophene at Buried Interfaces. J. Phys. Chem. C 2013, 117, 15213−15220. (32) Oh-E, M.; Ogata, H.; Fujita, Y.; Koden, M. Anisotropy in Amorphous Films of Cross-Shaped Molecules with an Accompanying Effect on Carrier Mobility: Ellipsometric and Sum-Frequency Vibrational Spectroscopic Studies. Appl. Phys. Lett. 2013, 102, No. 101905. (33) O’Brien, D. B.; Anglin, T. C.; Massari, A. M. Surface Chemistry and Annealing-Driven Interfacial Changes in Organic Semiconducting Thin Films on Silica Surfaces. Langmuir 2011, 27, 13940−13949. (34) Furukawa, Y.; Takao, H.; Yamamoto, J.; Furukawa, S. Infrared Absorption Induced by Field-Effect Doping from Poly(3alkylthiophene)s. Synth. Met. 2003, 135−136, 341−342. (35) Ogawa, S.; Naijo, T.; Kimura, Y.; Ishii, H.; Niwano, M. Photoinduced Doping of Organic Field Effect Transistors Studied by Displacement Current Measurement and Infrared Absorption Spectroscopy in Multiple Internal Reflection Geometry. Jpn. J. Appl. Phys. 2006, 45, 530−533. (36) Yamamoto, J.; Furukawa, Y. Raman Study of the Interaction between Regioregular Poly(3-hexylthiophene) (P3HT) and Transition-Metal Oxides MoO3, V2O5, and WO3 in Polymer Solar Cells. Chem. Phys. Lett. 2016, 644, 267−270. (37) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P. Solitons in Conducting Polymers. Rev. Mod. Phys. 1988, 60, 781−850. (38) Fincher, C. R.; Ozaki, M.; Heeger, A. J.; MacDiarmid, A. G. Donor and Acceptor States in Lightly Doped Polyacetylene, (CH)x. Phys. Rev. B 1979, 19, 4140−4148. (39) Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H. Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Int. Rev. Phys. Chem. 2005, 24, 191−256. (40) Miyamae, T.; Takada, N.; Tsutsui, T. Probing Buried Organic Layers in Organic Light-Emitting Diodes under Operation by ElectricField-Induced Doubly Resonant Sum-Frequency Generation Spectroscopy. Appl. Phys. Lett. 2012, 101, No. 073304. (41) Pandey, R. K.; Singh, A. K.; Upadhyay, C.; Prakash, R. Molecular Self Ordering and Charge Transport in Layer by Layer Deposited Poly (3,3′-dialkylquarterthiophene) Films Formed by Langmuir-Schaefer Technique. J. Appl. Phys. 2014, 116, No. 094311. (42) Anglin, T. C.; Speros, J. C.; Massari, A. M. Interfacial Ring Orientation in Polythiophene Field-Effect Transistors on Functionalized Dielectrics. J. Phys. Chem. C 2011, 115, 16027−16036. (43) Yun, J.-J.; Peet, J.; Cho, N. S.; Bazan, G. C.; Lee, S. J.; Moskovits, M. Insight into the Raman Shifts and Optical Absorption H

DOI: 10.1021/acs.jpcc.8b01760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Changes upon Annealing Polymer/Fullerene Solar Cells. Appl. Phys. Lett. 2008, 92, No. 251912. (44) Walter, S. R.; Youn, J.; Emery, J. D.; Kewalramani, S.; Hennek, J. W.; Bedzyk, M. J.; Facchetti, A.; Marks, T. J.; Geiger, F. M. In-Situ Probe of Gate Dielectric-Semiconductor Interfacial Order in Organic Transistors: Origin and Control of Large Performance Sensitivities. J. Am. Chem. Soc. 2012, 134, 11726−11733. (45) Benson, N.; Melzer, C.; Schmechel, R.; Seggern, H. Von. Electronic States at the Dielectric/Semiconductor Interface in Organic Field Effect Transistors. Phys. Status Solidi A 2008, 205, 475−487. (46) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. (47) Manaka, T.; Lim, E.; Tamura, R.; Iwamoto, M. Modulation in Optical Second Harmonic Generation Signal from Channel of Pentacene Field Effect Transistors during Device Operation. Appl. Phys. Lett. 2005, 87, No. 222107. (48) Baibarac, M.; Lapkowski, M.; Pron, A.; Lefrant, S.; Baltog, I. SERS Spectra of Poly(3-hexylthiophene) in Oxidized and Unoxidized States. J. Raman Spectrosc. 1998, 29, 825−832. (49) Louarn, G.; Trznadel, M.; Buisson, J. P.; Laska, J.; Pron, A.; Lapkowski, M.; Lefrant, S. Raman Spectroscopic Studies of Regioregular Poly(3-alkylthiophenes). J. Phys. Chem. 1996, 100, 12532−12539. (50) Casado, J.; Hernández, V.; Hotta, S.; López Navarrete, J. T. Vibrational Spectra of Charged Defects in a Series of α,α′-Dimethyl End-Capped Oligothiophenes Induced by Chemical Doping with Iodine. J. Chem. Phys. 1998, 109, 10419−10429. (51) Miyamae, T.; Ito, E.; Noguchi, Y.; Ishii, H. Characterization of the Interactions between Alq3 Thin Films and Al Probed by TwoColor Sum-Frequency Generation Spectroscopy. J. Phys. Chem. C 2011, 115, 9551−9560. (52) Silva, H. S.; Uehara, T. M.; Bergamaski, K.; Miranda, P. B. Molecular Ordering in Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy: The Effects of Drying Procedures. J. Nanosci. Nanotechnol. 2008, 8, 3399−3405. (53) Tsoi, W. C.; James, D. T.; Kim, J. S.; Nicholson, P. G.; Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J. S. The Nature of in-Plane Skeleton Raman Modes of P3HT and Their Correlation to the Degree of Molecular Order in P3HT:PCBM Blend Thin Films. J. Am. Chem. Soc. 2011, 133, 9834−9843. (54) Liu, Y.; Hu, W.; Lu, Z.; Li, C. M. Photografted Poly(methyl methacrylate)-Based High Performance Protein Microarray for Hepatitis B Virus Biomarker Detection in Human Serum. MedChemComm 2010, 1, 132−135. (55) Hossain, U. H.; Lima, V.; Baake, O.; Severin, D.; Bender, M.; Ensinger, W. On-Line and Post Irradiation Analysis of Swift Heavy Ion Induced Modification of PMMA (Polymethyl-Methacrylate). Nucl. Instrum. Methods Phys. Res., Sect. B 2014, 326, 135−139. (56) Seemann, A.; Sauermann, T.; Lungenschmied, C.; Armbruster, O.; Bauer, S.; Egelhaaf, H.-J.; Hauch, J. Reversible and Irreversible Degradation of Organic Solar Cell Performance by Oxygen. Sol. Energy 2011, 85, 1238−1249. (57) Hintz, H.; Peisert, H.; Egelhaaf, H. J.; Chasse, T. Reversible and Irreversible Light-Induced P-Doping of P3HT by Oxygen Studied by Photoelectron Spectroscopy (XPS/UPS). J. Phys. Chem. C 2011, 115, 13373−13376. (58) Abdou, M. S. A.; Orfino, F. P.; Son, Y.; Holdcroft, S. Interaction of Oxygen with Conjugated Polymers: Charge Transfer Complex Formation with Poly(3-alkylthiophenes). J. Am. Chem. Soc. 1997, 119, 4518−4524. (59) Manceau, M.; Rivaton, A.; Gardette, J. L.; Guillerez, S.; Lemaître, N. The Mechanism of Photo- and Thermooxidation of Poly(3-hexylthiophene) (P3HT) Reconsidered. Polym. Degrad. Stab. 2009, 94, 898−907.

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