Electrooptical Spectroscopy of Uniaxially Aligned Polythiophene Films

Apr 29, 2013 - Chem. Mater. , 2013, 25 (10), pp 2075–2082. DOI: 10.1021/ .... A. Pucci , J. Zaumseil. Journal of Materials Chemistry C 2017 5 (25), ...
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Electrooptical Spectroscopy of Uniaxially Aligned Polythiophene Films in Field-Effect Transistors Mi Jung Lee,†,§ Zhuoying Chen,†,∥ Riccardo di Pietro,†,⊥ Martin Heeney,‡ and Henning Sirringhaus*,† †

Cavendish Laboratory, Physics Department, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, U.K. Department of Chemistry, Imperial College of London, London, SW7 2AZ, United Kingdom

‡

S Supporting Information *

ABSTRACT: Charge carriers induced in field-effect transistors based on a uniaxially aligned polythiophene polymer, poly(2,5-bis(3alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), are investigated by electrooptical charge modulation spectroscopy (CMS). We used a zone-casting deposition method for uniaxial alignment of the nanoribbon crystalline domains of the polymer and examined the optical anisotropy of neutral and charge induced absorptions in FETs. We find that the charge-induced optical absorptions of the polaronic carriers at the interface exhibit a similar degree of uniaxial anisotropy as the absorption spectrum of the neutral polymer bulk suggesting similar degree of polymer alignment at the interface compared with the bulk. We observe subtle differences in the spectral shape of the polaron absorption polarized along and perpendicular to the polymer chain direction. We also detect an additional charge-induced absorption peak appearing at high charge carrier concentrations, which is similar to the charge-induced absorption that is typical for chemically doped films. These observations provide important insight into the interplay between polaron transport and polymer microstructure. KEYWORDS: charge induced absorption, aligning polymer, organic transistors, polarized spectroscopy

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INTRODUCTION The correlation between microstructure of organic semiconducting layers and electrical characteristics of field effect transistors (FETs) has been extensively studied to understand charge transport in organic semiconducting materials, and several models were suggested to account for it.1,2 Recently, as organic materials with high mobilities competitive with existing inorganic materials in terms of industrial applications are emerging,3−6 investigation of the intrinsic properties and behaviors of organic materials becomes more crucial in order to make further progress in organic electronics. In particular, the microstructure of crystalline polymer films is an important parameter because the way polymer chains pack and orient directly affect their charge transport properties. Although extensive studies have helped to achieve a deeper understanding of the structure−property relationships in conjugated polymers,1,2,7−10 many experimental characterization techniques have inherent limitations, when a molecular-scale understanding is aimed for. First, charge carriers in FETs are located at the interface between the gate dielectric and semiconducting layer. It is generally questionable to extract information about interfacial phenomena from techniques that are only bulk sensitive since the physical properties at the interface can be different from those in bulk. Moreover, the charge carrier density in polymer transistors is very low compared to that of neutral species, which makes charge carriers difficult to observe. © 2013 American Chemical Society

Optical spectroscopy has been extensively used to study the structures and properties of materials.10−12 Optical absorption spectroscopy of the charge carriers induced by the gate field into the FET channel can provide microscopic, spectroscopic information about the nature of charge transport in organic transistors.1,13 A modulation technique, the so-called charge modulation spectroscopy (CMS), is necessary to provide the necessary sensitivity to detect the charge carriers in the channel induced by an applied gate field.14−16 Here, we present an electrooptic study by CMS of charge carriers in uniaxially aligned poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (PBTTT) FETs. PBTTT is one of the most highly ordered and anisotropic semicrystalline conjugated polymers. We investigate the so-called nanoribbon phase comprising extended crystalline, ribbon-shaped domains in which the polymer chains are fully chain-extended and are laterally stacked along the π−π* direction. Previously, we showed that high field effect mobilities of this nanoribbon phase PBTTT can be achieved and a pronounced anisotropy of field-effect mobility in the direction parallel and perpendicular to polymer backbones can be detected.17 Optical anisotropy studies can provide more detailed information on charge generation and transport in PBTTT which can help discover Received: January 22, 2013 Revised: April 4, 2013 Published: April 29, 2013 2075

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further connections between the material properties and device performance.18 In particular, we can deduce information about the chain conformation in which the charge carriers that carry the current in an FET are formed, which is a key issue toward a proper understanding of the operation principle of semiconducting conjugated polymers FETs.

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EXPERIMENTS

Poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) (Mn = 25.6 KDa, Mw = 46.8 KDa, PDI = 1.83) is dissolved in anhydrous 1,2-dichlorobenzene (95%, ROMIL) and annealed at 110 °C for 30 min to make a 2 mg/mL solution in a nitrogen atmosphere. A PTFE filter of 0.1 μm pore size is used to filter the solution for zone casting on gold patterned glass substrates. Glass substrates are cleaned by ultrasonication in D.I. water, acetone, and IPA for 10 min each followed by a nitrogen blow and oxygen plasma treatment for 10 min. Photolithography is used to fabricate a gold interdigitated electrode pattern on glass substrate, and gold was deposited by thermal evaporation. Detail of the zone casting process of PBTTT films are described elsewhere.17 Films are annealed at 275 °C under nitrogen atmosphere to induce ribbon formation. The FET devices for the CMS measurements are fabricated in a bottom contact, top gate configuration with polymer dielectric layers of either polymethymethalacrylate (PMMA), polystyrene (PS), or polyvinylphenol (PVP). For the CMS measurements interdigitated source and drain electrodes with a 40 μm channel length and 26 mm channel width are defined. An aluminum gate electrode, which is only 7 nm thick, is used so that it is semitransparent; i.e., light can penetrate through it and reach the channel interface where the charge carriers are accumulated. CMS in the UV−visible spectral range is performed in either vacuum or an inert gas atmosphere for low temperature measurements. Light is generated by a 12 V, 100 W tungsten−halogen lamp and focused by a concave mirror onto the entrance slit of a doublegrating monochromator, which can produce monochromatic light in a spectral range of 1.0−2.8 eV. The transmitted light is collected by a Si photodiode detector. The signal is passed onto a Stanford SR530 lockin amplifier. A modulating voltage (VAC) is generated by a Tektronix frequency generator and is added to the DC gate voltage. By varying the DC voltage, it is possible to measure the transmission in different regimes: accumulation, depletion, and full depletion. Variable temperature optical absorption spectra of the semiconducting films have been obtained using a Varian Cary 6000i spectrophotometer, fitted with an Oxford Instruments CF-V continuous helium flow cryostat. Temperature is controlled using an Oxford Instruments ITC-503 temperature controller.

Figure 1. (a) Transfer curves of a PBTTT/PMMA transistor, PBTTT molecular structure (inset, upper right), and transistor structure (inset, lower left), (b) CMS spectra for accumulation region (−20 V) and depletion region (0 V) for isotropic nanoribbon PBTTT/PMMA transistors deposited by spin-coating and schematic diagram of charge transitions in delocalized states (inset), and (c) CMS of a nanoribbon PBTTT film in accumulation (−30 V, black empty square) and full depletion (+10 V, blue filled square) regimes when the sample was placed perpendicular to the incoming light beam. Spectra in accumulation (−30 V, red empty triangle) and full depletion (+10 V, pink filled triangle) regimes after the sample was tilted by 20° with respect to the original position. (d) UV−visible absorption (red line) and first derivative of the absorption (black line) at 300 K.

absorption spectra of nanoribbon reported here and terraced PBTTT FETs reported in Zhao et al.’s study suggests that the local chain conformation and intermolecular stacking in terraced and nanoribbon PBTTT films is similar despite significant difference in surface and grain morphology. This is consistent with the results from electrical characterization presented in our previous work.17 CMS spectra of aligned nanoribbon PBTTT films prepared by zone-casting are measured in accumulation regime at −30 V and full depletion regime at +10 V with a modulating voltage of ±1 V (Figure 1c). A pronounced shoulder is observed on the lower energy side of the bleaching peak of the spectrum in the depletion regime which is absent in the accumulation regime. This shoulder is still present when the sample is tilted by 20° away from normal incidence. Tilting the sample alters the path length which the light needs to travel through the various layers of the device. This is a simple test for interference artifacts in these CMS spectra. If, like in this case, the obtained spectrum is invariant under sample tilting, we can exclude the effect of interference artifacts induced by the dielectric layer.20 Neutral normal absorption spectra provide basic information about the bulk material properties, such as position and width of the absorption peak, which can help in the interpretation of the CMS spectra. Neutral UV−visible absorption spectroscopy is carried out on PBTTT at room temperature, 300 K (Figure 1d). We note that these spectra show more pronounced vibronic structure than the ones presented in the CMS bleaching as they are measured with a different apparatus with high spectral resolution which is capable of measurement at low temperature.

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RESULTS AND DISCUSSION General Observation of CMS in Nanoribbon PBTTT. Figure 1a shows the I−V characteristics of a PBTTT/PMMA transistor which has a linear mobility of 0.10 cm2 V−1 s−1 and a saturated mobility of 0.32 cm2 V−1 s−1. CMS spectra of the isotropic nanoribbon PBTTT transistors, shown in Figure 1b, are congruent with those of terraced PBTTT transistors shown in previous work by Zhao et al.18 The neutral absorption is bleached around 2.2 eV, showing up as a positive peak in the ΔT/T plot, and a charge induced absorption with two pronounced negative peaks around 1.2−1.3 eV and 1.8−1.9 eV is observed. As shown in a previous study18 these two peaks are considered to originate from the same charge species because the shape of the spectra was independent of the modulation frequency or magnitude of the gate voltage. The smooth bleaching peak, which does not exhibit a vibronic structure like the one seen in P3HT, is ascribed to the large interchain interaction in PBTTT which broadens the exciton bandwidth.19 The similarity between the charge induced 2076

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Electroabsorption in CMS of Nanoribbon PBTTT. When the absorption spectrum is superposed onto a CMS spectrum with the maximum of the neutral absorption normalized to the maximum of the bleaching peak (Figure 2a), the two plots should approximately match since the change

spectral peaks due to the induced charges may only be shifted by a small amount with respect to the neutral absorption and may overlap at least partly with the bleaching signal. Next, we would like to comment on the origin of the shoulder in the CMS curve in depletion around 2.0 eV, which does not match well with the 0−0 vibronic peak in the thin-film absorption spectrum which appears at slightly higher energy. When analyzing the origin or shape of peaks in CMS spectra it is important to consider potential electroabsorption signals, particularly in spectra acquired under depletion conditions.20,23 We could observe a peak around 2.0 eV in the first derivative of the neutral absorption in Figure 1d which is close to the shoulder in the CMS spectrum in depletion. To analyze further the shoulder peak in the CMS spectrum in full depletion, we carry out CMS at low temperatures of 150 K (Figure 3a). At low temperatures more complete depletion

Figure 2. Superposed normalized CMS spectra, with a bleaching peak in accumulation (red) and depletion (blue) regimes, and normal absorption spectrum (black) where (a) the zero levels in both cases are matched and (b) the zero level of the normal absorption spectrum is shifted down.

of neutral absorption due to the modulating gate field at the channel interface in CMS is due to the same phenomena which determine the neutral absorption in the bulk of the film.14 In this case, as seen in Figure 2a, the overlap is not good in accumulation, since the bleaching peak of the CMS spectrum is much narrower than the neutral absorption one. This broader neutral absorption is not caused by different resolution of apparatus used in each spectra. The broad shape of neutral absorption compared with CMS bleaching peak was consistently observed in measurement using machine with low spectral resolution as well. (Supporting Information Figure 1). In depletion, the overlap appears to be better; however, the two curves match only on the low energy side of the peak (around 2.0 eV). However, the energies at which the shoulders in the neutral absorption and CMS spectra in depletion occur do not seem to be identical, as the shoulder in the CMS spectrum is slightly red-shifted. In addition, in both accumulation and depletion regimes, the value of the normalized CMS spectra at high energies (above 2.3 eV) is much smaller than that of the normal absorption. The UV−visible neutral absorption curve overlaps better with the bleaching peak of the CMS spectrum when the two are normalized in such a way that not only the maximum values of both curves are made to coincide but also the normal absorption curve is rescaled so that the onset of normal absorption matches the charge induced absorption peak around 1.85 eV (Figure 2b). In this case the two curves are plotted on different y-axes, which have different zero levels. This better match can be interpreted as an indication of the presence of a broad and more or less flat charge-induced background absorption in the energy range between 2.0−2.7 eV which pulls down the ΔT/T signal in the bleaching region toward more negative values. When additional charges are induced in conjugated polymer films the energy states of the molecules are altered from their neutral levels.21,22 These new levels can be probed by spectroscopic techniques and appear as peaks close to the neutral absorption peaks. In materials with a high degree of order, such as PBTTT, the charge carriers tend to be more delocalized, which implies that the amount by which the molecular energy levels in such films would change due to the induced charges is reduced. Hence in such materials, the

Figure 3. (a) CMS spectra of isotropic nanoribbon PBTTT at 150 K, (b) absorption spectra at 100 K (two different scales) overlaid with the CMS spectrum of nanoribbon PBTTT in accumulation at 150 K, and (c) absorption spectrum and first derivative of absorption overlaid on the CMS spectrum in full depletion regime.

conditions are easier to achieve because the turn-on voltage is shifted to further negative value and as a result the chargeinduced absorption signal around 1.3 −1.7 eV is much lower than at room temperature. Helium gas is used to cool a cryostat containing the sample. Some features of these spectra, such as the ratio between the two peaks of the charge induced absorption in accumulation regime, are a bit different from the ones shown in Figures 1 and 2. However, a CMS spectrum of this sample in the accumulation regime at room temperature is consistent with that at 150 K, and the difference seen here is only attributed to batch to batch variations. The low temperature CMS data can be directly compared to the low temperature neutral absorption. In Figure 3b, the absorption spectrum at 100 K is superimposed on the normalized CMS spectrum at 150 K in the accumulation regime. A difference of two low temperature measurements was caused by a restriction of apparatus. At 150 K, the shape of the rescaled neutral absorption spectrum (purple line) and the 2077

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CMS spectrum in accumulation become very similar. Although not very well-defined, the CMS bleaching peak even includes the characteristic shoulder at 2.05 eV. This shoulder does not appear at room temperature which might be attributed to thermal broadening and lower resolution of the CMS measurement setup than the one for absorption spectra measurements. In contrast, it becomes even more apparent at low temperature that the peaks in the bleaching region of the CMS spectrum in depletion do not match the profile of the neutral absorption spectrum (Figure 3c). Hence, these peaks must have a different origin. Electroabsorption in conjugated polymer films is the change in absorption attributed to the change of electronic structure due to an applied electric field.24 It is mainly due to the Stark effect, which leads to a spectral shape that can be described as the linear sum of the absorption spectrum, its first derivative (quadratic Stark effect) and its second derivative (linear Stark effect), which depend on the induced dipole moments and permanent dipole moments, respectively. Normally in organic materials quadratic Stark effect is dominant, and the linear Stark effect can be added in disordered films.25 However, the only way to determine the contribution of each stark effect for a particular material is to collect empirical data, as in most cases it is impossible to make a theoretical prediction.26 The easiest way to find out whether the Stark effect is present in CMS data is to compare CMS curves to the first and second derivatives of the neutral absorption curve of the same material. The derivatives required for the quantitative analysis are obtained by smoothing the absorption spectrum by a thirdorder polynomial using a Savizky-Golay filter method (Figure 3c) which is known to produce well-defined curves and calculating the derivative of the smoothed spectrum.27 In the depletion regime and at low temperatures, several of the peaks in the CMS spectrum match very well with some of the peaks of the first derivative of the neutral absorption. In particular, the shoulder on the lower energy side of the bleaching peak in the CMS spectrum in depletion coincides closely with the lowest energy peak of the first derivative of the neutral absorption. On this basis we assign this shoulder to the effect of electroabsorption due to the quadratic Stark shift. Polarized Absorption in Aligned Nanoribbon. For the CMS measurement of uniaxially aligned PBTTT, the semiconductor material is zone casted onto glass substrates prepatterned with source and drain electrodes. In this case the metal electrode thickness is reduced to 10 nm (normally is it 20−30 nm) in order to minimize the perturbation of the film alignment during the casting process. In addition the width of the electrodes is also reduced to 5 μm to maximize the channel area in the spectroscopy window. The AFM images in Figure 4 prove that this modified electrode structure is effective in achieving highly aligned nanoribbons without much disruption over the electrodes. In particular, the phase image in Figure 4 (right) shows that the ribbons are aligned uniformly despite the morphological variations of substrates (gold and glass). It should be noted, however, that for all CMS devices measured in this section, unlike the device shown in Figure 4, the zone-casting direction is parallel to the channel length (i.e., the polymer backbone lies along the source-drain field) since this is the direction of higher mobility. Neutral absorption spectra of PBTTT films in the UV− visible range taken with a film polarizer show that uniaxial alignment of the nanoribbon phase is induced in zone-cast films

Figure 4. AFM topology (left) and phase (right) image of PBTTT zone-cast on prepatterned thin electrode of 10 nm, 5 μm × 5 μm. The yellow arrow indicates the zone casting direction and the direction of alignment of the polymer backbones.

upon annealing. The transition dipole moment for the lowest π−π* optical absorption, which is aligned along the polymer backbone axis shows a dichroic ratio of around 1:10 in absorbance. This is similar to the value of dichroism in the optical absorption of rrP3HT aligned by rubbing, reported by Yang et al.28 For the uniaxially aligned devices, the CMS spectra measured without a polarizer are not different from the spectra of unaligned PBTTT films. However, the intensity of the spectra, ΔT/T, is increased remarkably when the incident light is polarized along the polymer backbone, while it is decreased when the light is polarized along the nanoribbon which is perpendicular to the polymer backbone axis (Figure 5a). Neutral absorption probes the bulk of the film, while CMS probes only the interfacial layer. Therefore comparing the anisotropy in the neutral absorption spectra to the anisotropy in the CMS spectra can reveal information about the film morphology and the degree of polymer alignment in the bulk and at the surface. In principle, the two anisotropies can be

Figure 5. (a) CMS spectra at −30 V of gate bias of uniaxially aligned nanoribbon PBTTT transistors without polarizer (black line), with polarizer parallel to polymer chain backbone direction (blue line), and with polarizer parallel to nanoribbon direction (red line), (b) spectra of (a) normalized to the peak of the charge-induced absorption peak at 1.8−1.9 eV, (c) polarized UV−visible absorption spectra of aligned film in each direction scaled in powers of 10 to be compared with the ratio of ΔT/T, and (d) normalized spectra of (c). 2078

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when the polarizer is aligned with the ribbon direction.19,22 Another reason for the more pronounced blue shift for polarization along the polymer chain alignment direction is that there are specific chain conformations incorporated into the crystalline domains or on the surface of the crystalline domains, in which the chains are still aligned, but they exhibit significant torsion/short conjugation length which results in higher exciton energy. In any case our measurements indicate that both in the bulk and at the interface the conformation of chains that are aligned perpendicular to the zone casting direction is significantly different from those that are aligned along the main alignment direction. Charge Induced Absorption at High Charge Carrier Concentration. The CMS measurements discussed above were all taken at relatively small DC gate bias. When applying larger gate bias of VG = −60 or −80 V additional spectral features were observed (Figure 6). As the gate bias voltage is

different for several reasons, for example, because the degree of uniaxial alignment of the polymer chains is different at the interface from that in the bulk. The absorption A is defined as A = −log(T) where T is transmission. Using this equation, the ratio of the polarized absorption can be expressed in terms of the transmission, A − A⊥ = −log(T /T⊥)

We measured the anisotropy in the UV−visible absorption spectra of aligned nanoribbon PBTTT which detects the bulk of the films in order to see if any difference in optical anisotropy exists between bulk properties and interface properties. Figure 5c shows the absorption spectra of the bulk aligned film to make direct comparison with the bleaching signal in CMS in ΔT/T. For UV−visible absorption measurements to be compared to CMS, PBTTT film was zone cast on top of glass substrates with gold electrode patterns for CMS device in order to make both films comparable in different measurements. The shape of spectra in absorption measurement, however, looks a bit different, which seems to be due to absorption of gold around 2.0−2.5 eV in normal absorption spectra. The dichroic ratio of neutral absorption in transformed scale in Figure 5c (I∥/I⊥) is 10.9. The dichroic ratio of the bleaching signals in the CMS spectra, (ΔT∥/T∥)/(ΔT⊥/T⊥) is also approximately 10.4. The dichroic ratio can be expressed in terms of the optical cross-sectional area σ through the formula ΔT /T = −C(σ /eA)ΔV

where C is the capacitance, A is the area of device, and ΔV is the peak-to-peak value of the modulating gate field. Hence Figure 6. (a) Absorption spectra of nanoribbon PBTTT film (blue line) and nanoribbon PBTTT film doped with FeCl3 solution (red line) and CMS of FETs with PVP dielectric with increasing VG (+10 V to −80 V); (b) CMS of nanoribbon PBTTT FETs with PS for gate dielectric at different VG.

(ΔT /T )/(ΔT⊥/T⊥) = (σ /σ⊥)

of polymer alignment in the bulk of uniaxially aligned films deposited by zone-casting is indeed very similar to that on the surface of these films. This insight is important for the interpretation of mobility anisotropies measured in transport experiments on these films.17 When the polarized CMS spectra are normalized to the largest peak of the charge-induced absorption region of the spectrum at 1.8−1.9 eV, all three curves overlap very well in the region of the charge induced absorption with only a slight red shift for the CMS spectrum polarized in ribbon direction and a blue shift for one polarized in the polymer chain direction with respect to the CMS spectrum measured without a polarizer (Figure 5b). This red and blue shift is more evident when looking at the bleaching peak at 2.0−2.7 eV and the same tendency in shift of peaks is found in the polarized normal absorption spectra (Figure 5d). In previous work, we have investigated the morphology of the nanoribbon phase and found that, due to the distribution of polymer chain lengths, unaligned polymer chains or chain segments reside mainly in the disordered domain boundary regions between the crystalline nanoribbons. These unaligned polymer chain segments in the domain boundary regions, which are often called as rigid amorphous fraction,29 possibly experience a weaker interchain interaction that may result in less pronounced H-aggregate broadening and blue shift of the optical absorption. It is also possible in these domain boundaries that there are J-aggregate-like conformations between chain ends emanating from adjacent nanoribbons that could explain the red-shift in the absorption spectrum of these less well oriented chains that contribute to the absorption

increased, the charge concentration in the channel increases and a new charge induced absorption peak is observed around 1.5 eV between the two peaks at ∼1.2 eV and ∼1.8 eV which are known to be caused by polarons.18 It is interesting to note that this peak is also present in absorption spectra of chemically doped PBTTT films as shown in red in Figure 6a. Doped PBTTT films are prepared by dropping a solution of FeCl3 in isopropanol on top of the film while the substrate is slowly spinning. FeCl3 is a strong p-type dopant. After dropping the FeCl3 solution the color change of the PBTTT film is immediate and obvious. In addition, the absorption spectrum changes dramaticallythe neutral absorption decreases and a large peak at low energy, which is a signature of a chemically doped state, emerges in PBTTT CMS spectra. When CMS spectra taken at increasing gate bias in Figure 6a are normalized to the maximum value of the bleaching peak centered at 2.2 eV, the rise of the intermediate peak becomes more distinct (Supporting Information Figure 2). The chargeinduced absorption peak around 1.5 eV also corresponds to the peak observed in polymer−electrolyte gated PBTTT transistors when higher bias is applied in the electrochemical doping regime30 and also often in other chemically doped polymers, such as poly(3-hexylthienylene).31−33 This gives a clue on the origin of the peak which can be related with excess charge carriers induced in PBTTT nanoribbon film by large gate bias. 2079

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One possible explanation of this peak is that as the gate bias is increased and more charges are induced in the channel, some polarons combine to form bipolarons. Horowitz et al. used optical spectroscopy to demonstrate that bipolarons are prone to be created in long oligomers (n > 10) in comparison to short thiophene molecules.34 It has also been reported that a similar peak is observed in the spectrum of PBTTT upon application of a high voltage across a polyelectrolyte dielectric. This peak has been attributed to electrochemical doping by ion diffusion into the polymer film.35 An alternative explanation is provided by considering that in semicrystalline films of PBTTT there will be a certain concentration of chain segments at the interface with low energy chain conformations and long conjugation length. These will be populated first by a small gate voltage. Once these are all occupied a further increase in gate voltage necessitates the population of higher energy sites with shorter conjugation length. So, less ordered regions, such as domain boundaries between ribbons, will begin to accommodate charges at high gate voltages. These may still exist as singly charged polarons, but the new peak in charge-induced absorption would reflect the change in the chain conformation and intermolecular environment in which these charges are formed. In this respect it is also worth noting that the chargeinduced absorption peak at 1.9 eV becomes smaller as the applied gate bias increases while the peak around 1.25 eV maintains almost the same intensity except at +30 V when the channel is almost at complete depletion. The charge induced absorption peak at 1.9 eV in PBTTT is attributed to interchain delocalization in the two-dimensional plane of the channel,7 and a decrease of this peak suggests that the charged species that starts to be induced at higher voltages may be more localized than the charge induced at lower voltage. This trend is again observed for both dielectric layers, PVP (Figure 6a) or PS (Figure 6b). We also measured CMS of terraced PBTTT transistors in high gate voltages up to −60 V, and it turns out that terraced phase PBTTT devices do not show the peak at 1.5 eV at high gate voltages (Supporting Information Figure 3). It could be speculated that bipolarons can only be formed in the nanoribbon phase where a larger grain boundary area is available rather than in the terraced phase. This larger grain boundary area can lead to a higher chance to initiate the generation of a second type of charged species in the disordered regions. To provide further insight into the charge species formed at high gate voltages we have performed polarized CMS measurements on PBTTT devices with PVP and PS gate dielectrics at high gate voltages (Figure 7). The previously discussed red shift in the bleaching peak of the spectrum for light polarized along the nanoribbon direction and the blue shift for light polarized in the polymer chain direction are consistently observed in these devices as well. We find that the additional charge-induced absorption peak around 1.5 eV, which appears as a rising background between the two lowvoltage polaronic peaks, is more prominent when the light is polarized in the direction of the nanoribbons, while it is much less pronounced when the light is polarized perpendicular to the ribbon direction. We observed similar behavior for both dielectric layers, PVP (Figure 7a) and PS (Figure 7b), which means it is not an effect of the specific dielectric layer. We also emphasize that at low gate voltage of −20 V the shape of the charge-induced absorption spectrum was very similar for both polarization directions (as shown in inset of Figure 7b).

Figure 7. Normalized CMS at 1.8 eV of accumulation region (a) with PVP dielectric without polarizer (black line) and polarized along the polymer backbone (blue) and ribbon (red). (b) Same as (a) but with PS dielectric. The gate voltage was −80 V for both devices and −20 V for inset in (b).

This observation clearly suggests that the charged species that gives rise to the charge-induced absorption peak at 1.5 eV at higher gate voltage is initially formed primarily on chain segments that are not aligned along the main polymer chain alignment direction, that is, chain segments that are located in the more amorphous domain boundaries in between the crystalline nanoribbons. However, it is important to remember that a charged species with similar absorption shape is observed as the main charged species in chemically doped films and in electrolyte-gated FETs, in which much higher charge concentrations are present than in our CMS measurements. In these experiments it appears unlikely that there is a sufficient number of sites available in the domain boundaries to accommodate the large concentration of charges. In other words the charge signature observed in chemical doping or electrolyte-gated devices is likely to reflect the nature of charges that are present in these experiments throughout the bulk of the polymer film, not just at domain boundaries. Of course, in these experiments one also has to consider the presence of a negatively charged counterion, which is not present in the CMS experiments and introduces an additional source of charge localization. When taking together our CMS observations at high gate voltages and the shape of the charge-induced observation that is observed in chemical doping experiments one might conclude that although the more localized charge species that we are beginning to induce at high gate voltages are initially formed preferentially in the domain boundaries, there must be similar high energy sites with short conjugation length present even in the crystalline domains, which must eventually be populated when one approaches the charge concentrations that are typical for chemical doping. This might provide an indication that the crystalline domains in the nanoribbon phase of PBTTT are in fact not perfectly ordered but that there is a significant concentration of conformational defects incorporated into these domains. If they were perfectly ordered, one might argue that it would not be necessary to induce charges into the less ordered domain boundaries at higher gate voltage, since there would be a sufficient concentration of low-energy sites with long conjugation length available to accommodate all charges in the FET channel.

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CONCLUSION CMS experiments on nanoribbon PBTTT FETs have been performed in the UV−visible spectral range, in order to obtain spectroscopic insight into the polaronic charge carriers that are formed in the accumulation layer of these devices. The basic 2080

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Notes

spectral features such as the bleaching and charge induced absorption peaks are similar to those of terraced PBTTT. The bleaching signal in the CMS spectra is carefully compared to UV−visible absorption spectra. By superimposing the two spectra we have found evidence for a broad charge induced absorption background that extends into the bleaching region of the CMS spectrum. Electroabsorption is observed in the depletion regime, and its features become more pronounced at low temperatures. The electroabsorption signal matches the first derivative of the neutral absorption spectrum which is characteristic of the quadratic Stark shift associated with induced dipole moments. In uniaxially aligned nanoribbon PBTTT, the CMS spectrum ΔT/T has a higher intensity when the incident light is polarized along the polymer backbone. The anisotropy of the magnitude of the bleaching peaks in CMS spectra of aligned nanoribbon PBTTT films measured with polarized light is compared to the dichroic ratio of the bulk polymer film as deduced from thin film polarized optical absorption spectra. Since the two anisotropies are similar and CMS probes only the interface between the semiconducting and insulating layer, while the neutral absorption probes the bulk of the semiconducting film, we conclude that the PBTTT nanoribbons in the bulk and at the interface have similar degrees of structural alignment. An interesting feature in the CMS spectrum which appears at high gate voltages is an emerging charge induced absorption at 1.5 eV between two main charge induced absorption peaks at 1.2 and 1.8 eV, which is observed regardless of what polymer gate dielectric layer is used. This new species can be explained either as a bipolaron or a polaron in a more localized environment. It is interesting that the charge-induced absorption spectrum of this species is similar to that observed in chemical doping experiments, which suggests that in FET devices at high gate voltages, there is an insufficient concentration of low-energy sites to accommodate all charges and that similar chain configurations are beginning to be charged that are also being observed in chemical doping experiments at much higher charge concentrations.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the ERC program of NRF/MEST (R11-2005-048-00000-0), by the Ministry of Knowledge Economy (MKE) in 2012 (2012-434-10043822), and in part by the new faculty research program 2011 of Kookmin University in Korea.

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ASSOCIATED CONTENT

S Supporting Information *

Normalized CMS spectra of nanoribbon PBTTT FET with PVP dielectric layer at bleaching peak and of terraced PBTTT FET with PS dielectric layer. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(H.S.) E-mail: [email protected]. Present Addresses

§ (M.J.L.) School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136− 702, Korea. ∥ (Z.C.) Laboratoire Physique et Etude des Matériaux, ESPCI/ CNRS/UPMC UMR 8213, 10 rue Vauquelin, 75005 Paris, France. ⊥ (R.d.P.) Institut für Physik and Astronomie, Physik weicher Materie, Universität Potsdam, Karl-Liebknecht-Str. 24−25, Haus 28, 14476 Potsdam-Golm, Germany.

Author Contributions

All authors have given approval to the final version of the manuscript. 2081

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