A Complex Interrelationship between Temperature-Dependent

Article pubs.acs.org/JPCC

A Complex Interrelationship between Temperature-Dependent Polyquaterthiophene (PQT) Structural and Electrical Properties S. Grigorian,*,†,‡ S. Escoubas,‡ D. Ksenzov,† D. Duche,‡ M. Aliouat,‡ J.-J. Simon,‡ B. Bat-Erdene,† S. Allard,§ U. Scherf,§ U. Pietsch,† and O. Thomas‡ †

Department of Physics, University of Siegen, Walter-Flex-Strasse 3, D-57072 Siegen, Germany IM2NP, Faculté des Sciences et Techniques, Aix-Marseille Université, Avenue Escadrille Normandie Niemen, Case 142, F-13397 Marseille, France § Macromolecular Chemistry and Institute for Polymer Technology, Bergische Universität Wuppertal, Gauss-Strasse 20, 42119 Wuppertal, Germany ‡

S Supporting Information *

ABSTRACT: The influence of annealing temperature on the structural and electrical properties of conjugated poly(dodecylquaterthiophene) (PQT-12) polymer films is exploited. The temperature induced changes of structural parameters are monitored by in situ grazing incident X-ray diffraction (GIXD) and the conductivity. They are complemented by studies of the dielectric properties using variable angle spectroscopic ellipsometry (VASE). An increase of the scattered intensity, the size of the crystalline domains, and the current response is observed for a first thermal cycle with stepwise heating up to 90 °C, which revealed two polymorphs with different degrees of interdigitation in PQT-12. Irreversible changes are observed for the second cycle with a higher thermal budget up to 140 °C and are connected with a transition from the highly ordered to powder-like disordered phase for the main PQT-12 form whereas the second polymorph with stronger interdigitation completely vanished. In agreement with these observations high-temperature VASE studies demonstrated a blue shift of the transitions with a reduction in the conjugation length caused by an increase in the twist and torsion of the backbone. Combined GIXD, VASE, and electrical characterizations show that PQT-12 exhibits a complex interplay between two polymorphs with a strong influence on the charge carrier transport depending on the thermal budget employed.

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conditions9 which can be further improved by substrate surface treatment and thermal annealing.4 Typically, the crystallites from the PQT-12 molecules form two-dimensional molecular sheets due to interdigitation of the side chains shown in Figure 1c. This allows a good charge transport in the direction along the conjugated backbone, but a very poor charge transport in the perpendicular direction within molecular sheets caused by the insulation effect of the alkyl side chains.4,9 The molecular sheets, often referred to as lamellae, are connected together via overlapping of π molecular orbitals of the interfacing molecules in the adjacent molecular sheets. This short-range overlapping of π molecular orbitals is referred to as π−π stacking, and it enables charge transport between the molecular sheets. Multiple PQT-12 phases with a small variation of the interplanar distances4 or two different polymorphs have been reported in previous studies.10,11 In particular, the PQT-12 polymer has two phase transitions at about 100−120 °C and

INTRODUCTION Thin film organic semiconductors are the subject of intense research due to their wide range of low cost and environmentally friendly applications. Among them, the most important ones are the organic light emitting diodes (OLEDs), organic solar cells, and organic field-effect transistors (OFETs). One of the important representatives of organic semiconductors is the poly(3-alkylthiophene) linear polymers. They are p-type conducting polymers with high charge-carrier mobility and are frequently used as active layer in organic solar cells as well as for organic field-effect transistors.1,2 The changes of semicrystalline morphology and the structure of regioregular poly(3-alkylthiophene) thin films can be controlled by thermal annealing.3−6 Another favorable class of thiophene-based conducting polymers is the poly(dodecyl-quaterthiophene) (PQT-12) providing relatively higher environmental stability in comparison to the widely used poly-3-hexylthiophene (P3HT).7 PQT-12 is one of the promising solution-processable organic polymers required for high-performance organic semiconductor material and designed to provide excellent solution processability as well as the ability to self-assemble.8,9 Moreover, PQT-12 provides a reasonable high field-effect mobility and stability under ambient © XXXX American Chemical Society

Received: March 16, 2017 Revised: September 27, 2017 Published: September 29, 2017 A

DOI: 10.1021/acs.jpcc.7b02489 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) 2D GIXD pattern of PQT-12 drop cast film with high molecular weight; (b) out-of-plane and in-plane line profiles of the 2D image; (c) 2D molecular sheets.

130−150 °C, which correspond to the switch from the crystalline-solid to the liquid-crystalline phase (melting/ disordering of alkyl side chains), and from the anisotropic liquid-crystalline to the isotropic liquid phase (melting/disordering of facial π−π stacking; the absence of interdigitation of the side chains), respectively. In the current work, we present in situ studies of PQT-12 films during thermal annealing. Two cycles with different thermal budget were applied: during the first one PQT-12 film was stepwise heated up to 90 °C and cooled back to room temperature, revealing rather reversible changes. For the second cycle with a higher thermal budget up to 140 °C, irreversible changes were observed. These irreversible changes are related to modifications of two polymorphs existing in PQT-12. The behaviors of polymorphs are directly correlated to the electrical and to the optical properties measured on the same conducting channel as a function of the temperature. Sample Preparation. Highly regioregular PQT-12 polymer with high molecular weight (the weight and average molecular weights of the polymer are 28 kg/mol (Mn) and 120 kg/mol (Mw); polydispersity index (PDI) = 4.29) synthesized at the Macromolecular Chemistry group, University of Wuppertal, was dissolved in chloroform at 5 mg/mL concentration. For X-ray and electrical investigations, the PQT-12 films were fabricated by drop-casting on glass wafers of 18 × 18 mm2. Gold electrodes (30 nm thickness) were thermally evaporated on the substrate using a shadow mask separated by channels of 1 and 2 mm widths for the electrical measurements. The optical constants of the materials have been investigated as a function of temperature using variable angle spectroscopic ellipsometry (VASE). For the VASE measurements, homogeneous PQT-12 films were spin coated with a rotation speed of 1000 rpm on Si substrates covered by a 295 nm thick SiO2 layer. The film thicknesses (59 ± 4 nm) have been determined using a BrukerDektak mechanical profilometer.

during 3 min. For the second, high thermal budget cycle the polymer films were heated from 90 to 140 °C with the same steps and the same heating rate as used in the first cycle. After measurements, the samples were cooled back to RT with several temperature steps. For both thermal cycles, the microstructure of the PQT-12 films was studied by GIXD at every heating step during the annealing process. To minimize oxidative doping by atmospheric oxygen, both in situ thermal cycles were conducted in the dark. All in situ X-ray studies presented in this work were carried out at beamline BL9 of DELTA synchrotron radiation facility at TU Dortmund, Germany. This beamline is dedicated to small-angle X-ray scattering and GIXD experiments using image plate 2D MAR345 (3450 × 3450 pixels, resolution of 100 μm per pixel). The detector to sample distance was 400 mm, and the angular scale was calibrated measuring the diffraction pattern of a silicon powder. A point detector was used for aligning the sample within the X-ray beam and for the reflectivity measurements. The photon energy was 15 keV, corresponding to a wavelength of 0.83 Å. The incident angle was chosen at αi = 0.13°, which allows for a high sensitivity to the microstructure of film. The gold electrodes were connected to a Keithley2400 SourceMeter, and a constant voltage of 5 V was applied in order to measure electric current flowing through the film. Variable angle spectroscopic ellipsometry (VASE) measurements were performed using a rotating-polarizer ellipsometer (Semilab GES5).13−15 In order to ensure high reflection coefficients at the Si/SiO2/organic interfaces, the SE measurement was performed on PQT-12 layers coated onto Si substrates with a 295 nm thick SiO2 layer.14 Spectroscopic ellipsometry analysis (SEA) software (Semilab company)16 was used to fit the SE measurements of tan(Ψ) and cos(Δ) and extract the dielectric functions ε(λ) of the materials. This software uses the Levenberg−Marquardt algorithm to minimize the mean squared error (MSE) between the measured and the calculated ellipsometric data. Data were measured from 400 to 1600 nm at incident angles 55°, 60°, and 65°. After initial measurements at room temperature, the dielectric functions of PQT-12 were measured for the two thermal cycles at every heating step during the annealing process controlled by the HFS350X-GI LINKAM heating stage. For VASE data analysis, the optical properties of the films are considered as anisotropic with separation on two in- and out-ofplane components.21 The in-plane (εIn) and out-of-plane (εOut) parts are the ordinary and extraordinary components of the dielectric functions (εIn,Out = εIn,Out1 + i*εIn,Out2). VASE is an indirect technique because an appropriate optical model is required reflecting the structure of the sample under

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METHODS The effects of temperature on the microstructure of PQT-12 films, their electric response, and optical properties are directly correlated by means of in situ grazing incidence X-ray diffraction (GIXD), electric measurements, and VASE during the thermal annealing of the polymer films. The drop cast films on glass substrates with two gold electrodes were placed on a temperature controlled HFS350X-GI LINKAM heating stage.12 Two thermal cycles were applied: for the first one with the low thermal budget the samples were heated from 25 to 50 °C and then up to 90 °C raised in steps of 10 °C and a heating rate of 10°/min. After a stabilization time of 2 min, the GIXD patterns were acquired B


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Figure 2. 2D GIXD patterns of the drop-cast PQT-12 film at different annealing stages: (a) at RT (initial state), (b) at 90 °C in the first cycle, (c) at RT (after the first cycle), (d) at 90 °C, (e) at 140 °C in the second cycle, and (f) at RT (final state).

Figure 3. Temperature dependence of the lattice spacing (a), crystallite size (b), and intensity (c) of the 100 peak in the out-of-plane directions during the first and the second thermal annealing cycle.

investigation. The εIn,Out dielectric functions of PQT-12 films have been fitted with the sums of Gaussian oscillators which are suitable to describe strong electron−phonon coupling in πconjugated molecules.15 To evaluate a dispersion model, the root-mean-square error (RMSE) between measured and calculated data has been minimized. The optical model was developed in the SEA (WinElli3) software.16 The results show a good agreement between the measurements and the calculations for all incident angles, which indicates that the dispersion model is robust and appropriately fits the data. The RMSE obtained from the fits of the measurements is 0.05 for all incident angles. A comparison between measured and calculated ellipsometric data for the PQT-12 at room temperature is given in Figure S1.17 Two thermal cycles have been applied to the films. The dispersion model obtained at room temperature has been gradually adjusted to evaluate the PQT-12 dielectric functions at every heating step during the annealing process.

Direct Observations of Changes in Crystalline Morphology by in Situ GIXD. The initial 2D-GIXD pattern of the drop cast PQT-12 film measured at room temperature is shown in Figure 1. The GIXD pattern reveals an oriented PQT-12 film with the strongest 100 peak along the out-of-plane direction (qz).18 Moreover, two polymorphs marked with h00 and h00′ indices are already visible in this initial stage. The difference of these polymorphs is clearly noticeable in the out-of- and in-plane line profiles (Figure 1b). Based on the GIXD analyses, the crystalline phases of both polymorphs can be distinguished (Figure 1c): the first hkl phase (first or main form) with less interdigitated side chains and the second hkl′ form with shorter side-chain interval and stronger interdigitation called further on first and second polymorph, respectively. Figure 1c shows the main difference between both polymers depicted for the preferential edge-on configuration where the backbones and π−π stacking are directed parallel (in-plane) whereas the lamellas oriented perpendicular (out-of-plane) to the substrate, respecC


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Figure 4. Temperature dependence of the lattice spacing (a), crystallite size (b), and intensity (c) of the 100′ peak in the out-of-plane directions during the first and the second thermal annealing cycle.

Figure 5. Angular distribution at the 100 and 100′ peaks (left and right, respectively) for the drop-cast PQT-12 film at different annealing stages: (1) at RT (initial state), (2) at 90 °C in the first cycle, (3) at RT (after the first cycle), (4) at 140 °C in the second cycle, and (5) at RT (final state).

tively. In the out-of-plane profile, the h00 diffraction peaks of the first polymorph correspond to the stacking of the thiophene backbones separated by alkyl side chains. The well pronounced 100 and 200 peaks are positioned at qz = 0.37 Å−1 and 0.74 Å−1, respectively, and related to the interplanar spacing d100 ∼ 17.0 Å. There is a weak, wide peak centered around qxy ≈ 1.71 Å−1 in the in-plane profile (see Figure 1b). This peak corresponds to π−π stacking distance of ∼3.7 Å, between two cofacial backbones, and is thus indexed as 020 (note that d020 = d020′.). We have also indexed peaks at positions for the second polymorph at qxy = 0.5 Å−1 and 1.0 Å−1 indexed by 100′ and 200′, respectively. The corresponding spacing of layered polymer backbone d100′ is 11.8 Å. The wide peak centered around qxy ≈ 1.5 Å−1 visible in the outof-plane and in-plane (Figure 1b) line profiles corresponds to the 300′ reflection of the second polymorph. The obtained lattice parameters are in good agreement with the literature.10,11 Interestingly, for these studies for as-dropped and further posttreated PQT-12 films, the second polymorph was observed after the samples were heated above 90 °C. In order to separate the different types of thermal transitions, we have applied two temperature cycles to the PQT-12 films. The 2D GIXD patterns of the first cycle are shown in Figure 2; initial RT (a), at 90 °C (b), and back to RT (c). A pronounced improvement in the structural quality was found at 90 °C which

mostly preserves after cooling back to RT. The second annealing cycle has been applied to the sample right after the first one when the sample reached RT. Here the PQT-12 film was fast heated until 90 °C and then further annealed with isochronal steps every 10 °C up to the melting temperature of 140 °C. After keeping the sample at this temperature for 8 min, it was cooled back to RT. Interestingly, for this second cycle, the fast annealed film at 90 °C shows again two distinguishable polymorphs with features similar to those of the first cycle. The temperature dependent microstructural changes of both polymorphs of the PQT-12 film during the first and the second thermal cycles are shown in Figures 3 and 4. During the first cycle upon heating to 90 °C we found a linear expansion of the d100 distance (see Figure 3a; 4a). For the main PQT-12 form this expansion is reversible, i.e., after cooling down to RT all the 100 distances coincide with the initial value. The thermal expansion coefficient, αT, is estimated to be ∼9.3 × 10−4 K−1. This value is of the same order of magnitude as the thermal expansion coefficient found in the literature for the thiophene-based conjugated polymers such as P3HT5 and its derivatives.19 Interestingly, the π−π conjugation network is rather stable (1% change) upon the first annealing cycle and exhibits a negative thermal expansion, ∼−1.5 × 10−4 K−1. D


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Figure 6. Temperature dependence of the 020 peak (left) and electrical conductivity (right) of the PQT-12 film at the first and second annealing cycles.

Figure 7. PQT-12 in-plane (εIn2(λ)) and out of plane (εOut2(λ)) components of the imaginary part of the PQT-12 dielectric functions at different annealing stages: εIn2(λ) at the first (a) and the second annealing cycle (c) and εOut2(λ) at the first (b) and the second annealing cycle (d). It has to be noticed that, in panel b, εOut2(λ) at initial RT and back to RT exactly coincide.

However, for the second cycle, the situation is different: further annealing caused an irreversible compression of the 100 distances. During the heating process for both cycles, the sizes of

crystallites (so-called coherence length in the out-of-plane direction calculated by the Scherrer equation) increases, while during the cooling, the sizes remain unchanged (Figure 3b). In E


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dielectric functions.21 The spectra of the material imaginary parts reflect their absorption properties and, thus, are strongly correlated to the morphology of the films.22 Furthermore, these spectra provide information decorrelated from the layer thicknesses. Figure 7 shows the εIn2(λ) and εout2(λ) components of the imaginary parts of the PQT-12 dielectric functions during both cycles. The εIn1(λ) and εout1(λ) components of the real parts of the PQT-12 dielectric functions are given in Figure S2.17 The evolution of εIn2(λ) and εOut2(λ) during the first annealing cycle is given in Figures 7a and 7b, respectively. It has to be noticed that, in Figure 7b, εOut2(λ) at initial RT and back to RT exactly coincides. The in-plane component εIn2(λ) at initial RT (Figure 7a) displays two shoulder peaks and a strong peak resulting from π−π* transitions. While, in previous studies, the two shoulder peaks at 500 and 590 nm have been attributed to side-chain ordering, the strong peak appearing at 548 nm has been attributed to π−π stacking.22 The peak positions at 590, 548, and 500 nm are assigned as A0−0, A0−1, and A0−2 vibronic transitions, respectively.22 At the initial room temperature, the out-of-plane component εOut2(λ) (Figure 7b) displays only one shoulder peak at 496 nm assigned as A0−2 and a strong peak at 552 nm assigned as A0−1. The ratio A0−0 to A0−1 was extracted from the ellipsometric models for qualitatively evaluating the in-plane crystallinity of the films. In our model, A0−0 and A0−1 correspond to the amplitudes of the Gaussian oscillators assigned to the A0−0 and A0−1 vibronic peaks (see Figure S317). After the first cycle, the in-plane crystallinity ratio A0−0 to A0−1 increases to 1.19 after cooling back to the RT, which reveals an improvement of the in-plane crystallinity. This in-plane crystallinity improvement is in accordance with the 2D GIXD patterns of the drop-cast PQT12 films after the first cycle. However, no temperature-induced change is observed for the εOut2(λ) component. It can be due to the fact that spectroscopic ellipsometry is sensitive to both crystalline and noncrystalline domains resulting in averaged measurements while the GIXDis mainly probing the crystalline domains. Figures 7c and 7d show that strong changes in the in-plane εIn2(λ) and out-of-plane εOut2(λ) imaginary components of the film dielectric functions are induced by the second annealing cycle. During the second cycle, the in-plane crystallinity of the films decreases from 1.19 to 0.22 when the temperature increases from the initial room temperature to 140 °C, which reveals a strong decrease of crystallinity along the in-plane directions. Then, it increases to 0.62 after cooling back to the RT. These results reveal a strong degradation of the in-plane crystallinity during the second annealing cycle in accordance with GIXD analyses. The strong change in the out-of-plane component of the imaginary parts of the dielectric function εOut2(λ) (Figure 7b) also reveals a strong change in the out-of-plane crystallinity during the second cycle. However, VASE provides averaged features of the temperature induced order.

contrast, for the second polymorph, there is rather a moderate growth of the crystallites followed by substantial broadening at higher temperatures (Figure 4b, open circles). The oscillation in Figures 4a and 4b in comparison to Figures 3a and 3b are related to less accuracy of the measurements due to lower intensity for the second polymorph. Correspondingly, the error is two times higher in the case of the second polymorph in comparison to the main form and can be reached for the lattice spacing up to 4% and for the crystalline size up to 10%. For both polymorphs, the outof-plane peak intensity rises up during the first annealing cycle (Figures 3c and 4c). For the main form, further improvement of intensity is observable with increasing temperature up to 115 °C and intensity dramatically decreases for higher temperatures (Figure 3c). The first crystalline form is initially highly oriented, however, due to the second annealing cycle; the 100 peak shape is broadened because of a higher degree of orientational disorder. The angular redistribution of the scattering intensity of the 100 and 100′ peaks at the different stages of the thermal treatment is shown in Figure 5. The angular line profiles are taken from 2D GIXD patterns presented in Figure 2 and transferred into polar coordinates with the azimuthal angle φ = 90° for the out-of-plane direction. During the first cycle, the widths of the peaks for both polymorphs are not increased, indicating that both forms are in a highly ordered state. At high temperatures during the second cycle, the misorientation and disordering of both forms take place, and after cooling down to RT, the structure quality of the PQT film gets irreversibly more disordered. The d020 interplanar spacing is weakly dependent on the temperature (the corresponding value of the Δd shift is less than 0.2 Å) and shows a nonlinear behavior (above all, for temperatures higher than 100 °C). Figure 6 shows the integrated intensity of the π−π stacking (left) and a direct current response (right) of the PQT-12 film during the thermal annealing process for both cycles. In order to enhance scattered signal the integrated intensity of the π−π stacking was extracted from a rectangular area (Δqxy= 0.2 Å−1; Δqz = 0.1 Å−1) centered at qxy = 1.7 Å−1 of the 020 peak. In the first annealing cycle, the 020 integrated intensity associated with the π−π stacking and the electric current rise up with increasing temperature (see Figure 6, solid red circles). Whereas the integrated intensity of the π−π stacking shows a monotonic increase up to 90 °C, the current quickly increases up to 60 °C and saturates reaching a maximum at around 80 °C. During the cooling it decreases linearly when reducing the temperature. After the first annealing cycle, we found an improvement for the current and the 020 integrated intensity. In the second annealing cycle, for the first step of annealing up to 90 °C the current and the π−π stacking reach similar values as for the first cycle (Figure 6, open red circles). However, further annealing caused a dramatical decrease of the current, and the structure becomes more disordered. From 90 to 120 °C the current dropped down by almost 1 order of magnitude. Interestingly, there is a further enhancement of the 020 integrated intensity up to 110 °C. Such delay between a maximum of the current and the intensity was monitored during the solidification process of the drop-cast P3HT films.20 At temperatures higher than 120 °C, the X-ray intensity further decreased and reached near to zero for the film almost melted at 140 °C. These features are irreversible, and the π−π stacking and the current did not recover after annealing. Anisotropy of Thin Films by in Situ VASE. The anisotropy of the thin films of the semiconducting polymers can be investigated through the imaginary parts of their anisotropic

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DISCUSSION Detailed GIXD studies of the 100 and 100′ peaks allow us to differentiate the behaviors of both polymorphs upon thermal treatment. For the first thermal cycle, the main polymorph 100 shows an enhanced intensity until 90 °C, which partly remains after cooling down to room temperature. This enhancement is accompanied by an increase of the crystallite domain size. For the second polymorph 100′ annealing caused a pronounced improvement of the structure which is further enhanced upon F


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Figure 8. Schematic representation of the phase transitions for the first (orange) and second (blue) polymorphs after first (T1) and second (T2) annealing cycles.

current is due to the overall good interconnecting network and soft grain boundaries between crystalline domains and reached before the highly crystalline solid phase.20 Indeed, at the 90 °C, a maximum of the current can be associated with a substantial softening of the second polymorph whereas the main form does not reach its highest crystallinity. Further annealing up to 110− 115 °C causes an improvement of the main polymorph together with a formation of the hard grain boundaries whereas the second polymorph is substantially melted. This fact is responsible for the reduction of current despite the structural improvement of the first polymorph. These findings support the intragrain charge transport studies in solid P3HT by pulse-radiolysis microwave conductivity.26 Moreover, the macroscopic transport is determined by the local motion of charges within ordered grains of polythiophenes as well as by the transport through disordered material surrounding these grain domains.26 These disordered regions can essentially hinder the current response. Further annealing causes a strong misorientation and decrease of the main form as well as 2 orders of magnitude reduction of the current. These findings support that enhancement of the current response is closely correlated with the preferential edge-on orientation of the both polymorphs as well as with the improvement of the second polymorph with a stronger side chain interdigitation.

cooling. In this case, the sizes of crystalline domains remain almost unchanged. Therefore, the structural improvement is assigned to the order within the crystalline domains where interdigitation plays a crucial role.23 Both PQT-12 polymorphs are observed to improve their structural quality and preferential orientation (edge-on) after the first annealing cycle. Moreover, the π−π stacking reveals a similar trend during the first thermal cycle confirming overall improved three-dimensional structure. A schematic representation of the structural evolution of both polymorphs after each thermal cycle is shown in Figure 8. At high temperatures for the second thermal cycle, most of the PQT-12 films exhibit a temperature induced molecular conformation with more twist or torsion of the backbone in the liquid crystalline state.4,10 Here the behaviors of both polymorphs are different: upon annealing the 100 main form continues to improve further up to 110−115 °C, whereas the second form is drastically decreased at these temperatures. Further annealing to 140 °C causes a dramatic decrease of the main polymorph which is partly recovered upon cooling. Such misorientation of the crystalline domains and reduction in the π−π stacking interactions are supported by observing the blue shift of the in-plane imaginary component of the PQT12 dielectric function. Previously a bimodal behavior of the structure and mobility has been found for the high molecular weight P3HT films5 with disfavor of the transistor behavior at the temperatures above 100 °C. Based on the Gaussian disorder model for the conjugated polymers the temperature dependence of mobility under quasiequilibrium can be described by non-Arrhenius behavior.25 This theoretical model is valid only in restricted temperature range and deviates for the regioregular P3HT at temperature higher than 120 °C.24 An improvement of the order and the field-effect mobility was found for low molecular weight PQT-12.25 However, in the case of the conductivity, the situation is rather complex: the temperature change can be associated both with the carrier concentration and with the charge transport (charge mobility). There is a direct correlation between the current response and the π−π stacking shown in Figure 6. The first thermal cycle improves both the conductivity and conjugation length. For the second thermal cycle at higher temperatures, the induced disorder can significantly reduce the current. In spite of the π−π conjugation improvement up to 110−115 °C the current drops down. Exemplary, for in situ solidification of P3HT films a delay mechanism between the structural development and the highest current response has been found in ref 20. The highest

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CONCLUSIONS

We demonstrate that the maximum temperature in an annealing cycle has a different influence on the structural, optical, and charge transport properties of PQT-12 films. For the first cycle with the moderate thermal budget, we found an improvement of the order and enhancement of the conjugation length supported by GIXD and VASE measurements, respectively. In particular, the second PQT-12 polymorph has been significantly improved after the first thermal cycle. This improvement of the order was accompanied by an enhancement of the conductivity. For the second cycle with the higher thermal budget, the electric measurement revealed that the current response decreases upon further annealing below the melting point, despite the fact that the intensity and the crystallite sizes of the films have been further improved in the respective temperature regimes. This improvement has been found for the first PQT-12 polymorph whereas the second polymorph quantity has gradually decreased. In spite of the further improvement in the sizes of crystallites, the first PQT-12 polymorph revealed more misorientation and powderlike structural features. Therefore, we suggest that the decrease in conductivity is caused by reduced orientational order and interconnections between the crystallites which are embedded G


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Processed Photovoltaic Devices using PQT-12. Sol. Energy Mater. Sol. Cells 2010, 94, 2455−2458. (8) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. High-Performance Semiconducting Polythiophenes for Organic Thin-Film Transistors. J. Am. Chem. Soc. 2004, 126, 3378−3379. (9) Ong, B. S.; Wu, Y.; Li, Y.; Liu, P.; Pan, H. Thiophene Polymer Semiconductors for Organic Thin-Film Transistors. Chem. - Eur. J. 2008, 14, 4766−4778. (10) Jang, M.; Yang, H. Structural Control over Self-Assembled Crystals of pi-Conjugated Poly(3,3′’’-didodecyl-quaterthiophene) for Organic Field-Effect Transistor Applications. J. Nanosci. Nanotechnol. 2012, 12, 1220−1225. (11) Kang, S.-J.; Song, S.; Liu, C.; Kim, D.-Y.; Noh, Y.-Y. Evolution in Crystal Structure and Electrical Performance of Thiophene-based Polymer Field Effect Transistors: A Remarkable Difference between Thermal and Solvent Vapor Annealing. Org. Electron. 2014, 15, 1972− 1982. (12) Online available: www.linkam.co.uk/hfsxgi600-features. (13) Dkhil, S. B.; Duche, D.; Gaceur, M.; Thakur, A. K.; Aboura, F. B.; Escoubas, L.; Simon, J. J.; Guerrero, A.; Bisquert, J.; Garcia-Belmonte, G.; et al. Interplay of Optical, Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Efficient Polymer Solar Cells. Adv. Energy Mater. 2014, 4, 1400805. (14) Duche, D.; Bencheikh, F.; Dkhil, S. B.; Gaceur, M.; Berton, N.; Margeat, O.; Ackermann, J.; Simon, J.-J.; Escoubas, L. Optical Performance and Color Investigations of Hybrid Solar Cells based on P3HT:ZnO, PCPDTBT:ZnO, PTB7:ZnO and DTS(PTTh2)2:ZnO. Sol. Energy Mater. Sol. Cells 2014, 126, 197−204. (15) Aboura, F. B.; Duché, D.; Simon, J.-J.; Escoubas, L. Ellipsometric Study of the Optical Transitions of PC60BM and PC70BM Thin Films. Chem. Phys. 2015, 450, 102−108. (16) Software, Semilab Semiconductor Physics Laboratory Co. Ltd. (17) Supporting Information accompanies this paper. Figure S1 presents a comparison between measured and calculated ellipsometric data for the PQT-12 at room temperature. There is a good agreement between the measurements and the calculations for all incident angles, which indicates that the dispersion model is robust and appropriately fits the data. Then, the εIn1(λ), εout1(λ) components of the real parts of the PQT-12 dielectric functions are given in Figure S2. Finally, Figure S3 shows a comparison between the in-plane (εIn2(λ)) component of the imaginary parts of the PQT-12 dielectric functions at the initial and at the final RT states of the first annealing cycle. The two Gaussian oscillators corresponding to the A0−0 and A0−1 vibronic peaks have also been added in the graphics in order to show the evolution of the film morphology during the first annealing cycle. (18) Dosch, H. Critical Phenomena at Surfaces and Interfaces: Evanescent X-ray and Neutron Scattering; Springer-Verlag: Berlin; New York, 1992; 145 pp. (19) Pozdin, V. A.; Smilgies, D.-M.; Fong, H. H.; Sorensen, M.; He, M. Thermal Reorganization of Alkyl-Substituted Thienothiophene Semiconductors. J. Mater. Chem. C 2016, 4, 5255−5262. (20) Grodd, L.; Pietsch, U.; Grigorian, S. Direct Correlation Between Electric and Structural Properties During Solidification of Poly(3hexylthiophene) Drop-Cast Films. Macromol. Rapid Commun. 2012, 33, 1765−1769. (21) Zhokhavets, U.; Gobsch, G.; Hoppe, H.; Sariciftci, N. S. Anisotropic Optical Properties of Thin Poly(3-octylthiophene)-Films as a Function of Preparation Conditions. Synth. Met. 2004, 143, 113− 117. (22) Pandey, R. K.; Singh, A. K.; Prakash, R. Directed Self-Assembly of Poly(3,3‴-dialkylquarterthiophene) Polymer Thin Film: Effect of Annealing Temperature. J. Phys. Chem. C 2014, 118, 22943−22951. (23) Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Richter, L. J.; Chabinyc, M. L.; Toney, M. F.; Heeney, M.; McCulloch, I. Critical Role of Side-Chain Attachment Density on the Order and Device Performance of Polythiophenes. Macromolecules 2007, 40, 7960−7965.

within amorphous regions. At high temperatures, most of the poly(alkylthiophene) molecules exhibit a temperature induced molecular conformation that leads to more twist or torsion of the backbone, consequently resulting in a blue shift of in-plane components of the dielectric function with a reduction in the π−π stacking interactions. In situ studies revealed that the preferential edge-on orientation of both polymorphs together with the improvement of the second polymorph with stronger interdigitation is crucial for the enhanced current response and the moderate thermal budget is favorable for optimization of organic devices.

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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.7b02489. Ellipsometric data and PQT-12 dielectric functions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

S. Grigorian: 0000-0002-5495-979X D. Ksenzov: 0000-0002-0657-1678 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge BL9 beamline scientists at the DELTA synchrotron (Dortmund, Germany) for assistance during the experiment and are grateful for financial support from the DAAD-PROCOPE (Project No. 57211900), the BMBF (Project No. 05K13PS4), and CNRS-National Center for Scientific Research (France). S.G. thanks IM2NP, University of AixMarseille, for hosting the sabbatical leave.

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REFERENCES

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A Complex Interrelationship between Temperature-Dependent

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