Microstructure-Dependent Charge Carrier Transport of Poly(3

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Microstructure Dependent Charge Carrier Transport of Poly(3hexylthiophene) Ultrathin Films with Different Thicknesses Lukasz Janasz, Marzena Gradzka, Dorota Chlebosz, Wojciech Zajaczkowski, Tomasz Marszalek, Adam Kiersnowski, Jacek Ulanski, and Wojciech Pisula Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00563 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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• Microstructure Dependent Charge Carrier Transport of Poly(3-hexylthiophene) Ultrathin Films with Different Thicknesses Lukasz Janasz1, Marzena Gradzka2, Dorota Chlebosz2, Wojciech Zajaczkowski3, Tomasz Marszalek4, 5, Adam Kiersnowski2, Jacek Ulanski1,*, Wojciech Pisula1,3,* 1

Lodz University of Technology, Faculty of Chemistry, Department of Molecular Physics,

Zeromskiego 116, 90-924 Lodz, Poland. E-mail: [email protected] 2

Polymer Engineering & Technology Division, Wroclaw University of Technology,

Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland 3

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

E-mail: [email protected] 4

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg,

Germany 5

InnovationLab, Speyererstr. 4, 69115 Heidelberg, Germany

Keywords: organic field-effect transistors, poly(3-hexylthiophene), microstructure, charge carrier transport, monolayers

Abstract: Since the interfacial order of conjugated polymers plays an essential role for the performance of field-effect transistors, comprehensive understanding on the charge carrier 1 ACS Paragon Plus Environment

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transport in ultrathin semiconducting films below thicknesses of 10 nm is required for the development of transparent and flexible organic electronics. In this study, ultrathin films based on poly(3-hexylthiophene) as conjugated polymer model system with a thickness range from single monolayer up to several multilayers are investigated in terms of microstructure evolution and electrical properties of different molecular weights. Interestingly, a characteristic leap in field-effect mobility is observed for films with thickness greater than four layers. This threshold mobility regarding film thickness is attributed to the transition from 2D to 3D charge carrier transport along with an increased size of the P3HT aggregates in the upper layers of the film. These results disclose key aspects on the role of the film interlayer on the charge carrier transport through conjugated polymers in transistors.

Introduction

Organic electronics have been in the interest of both science and industry for the last two decades.1 Due to potential applications in large area and flexible electronic devices, like displays or solar cells, rapid development of new organic semiconductors, circuit architectures and processing methods is still an important issue.2,3 Among various organic semiconductors, conjugated polymers are investigated because of their favourable mechanical and electrical properties.4,5 Employment of soluble conjugated polymers in organic electronics allows solution processing-by e.g. spin-coating, dip-coating or even large area printing of the active films.6 Poly(3-hexylotiophene) (P3HT) is widely used as working-horse due to its stable and suitable semiconducting properties as hole-transporting (p-type) semiconductor.7 P3HT has been intensively studied in organic photovoltaic cells (OPVs)8-9 and in organic field-effect transistors (OFETs).10,11 One of the valid issues concerning organic electronics is the development of ultrathin semiconducting layers.12 Mechanical and optical properties of such thin films, are beneficial 2 ACS Paragon Plus Environment

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for flexible and transparent devices.13 Therefore, a systematic study on active films with thicknesses below 10 nm is required for their implementation in future devices. Especially wet techniques are in major interest for obtaining ultrathin films, due to low cost and possibility of large area deposition. Films containing just few monolayers are also important in terms of basic scientific research on the phenomena of charge transport itself. Experimental12,14 and theoretical15 studies show that the conducting pathway is confined to maximum several monolayers of the organic semiconductor deposited on the dielectric material. Since the charge transport in the active films occurs in the first monolayers at the semiconductor/dielectric interface, understanding the role of the microstructure and molecular organization in these layers on the conduction is essential to directly correlate film structure and electrical properties. For the investigation of ultrathin films, P3HT is chosen as a model system due to its common application in various organic electronic devices. The film thickness is precisely controlled from approx. 2 nm, which corresponds well to edge-on oriented polymer chains in a P3HT monolayer16, up to approx. 12 nm thick multilayers. This approach allowed to study the charge carrier transport in distinct layers of the ultrathin films and to gain understanding on the mobility jump at a particular number of layers. By applying polymers with different molecular weights also the role of chain length on the microstructure and transport evolution in ultrathin films was investigated.

Results and Discussion

Charge carrier transport For the first set of measurements, P3HT with a molecular weight of 94 kDa (referred further in the text as P3HT94) was applied as active material in OFETs. Polythiophenes with

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relatively high Mw are known to exhibit improved semiconducting performance in comparison to polymers with shorter chains.17 Therefore, P3HT94 was the first candidate to investigate the influence of film thickness on the microstructure and charge carrier transport. Spin-coating was used for film deposition within an ultrathin range from mono- to few multilayers by varying the rotation speed. The solution concentration, solvent, and deposition conditions remained the same in order to obtain ultrathin films, by changing only one parameter which was the spin-coating velocity. Charge transport properties of the ultrathin films were investigated in bottom gate, top contact OFET configuration. To study the microstructure of the P3HT films atomic force microscopy was used. The molecular arrangement was investigated with grazing incidence wide-angle X-ray scattering (GIWAXS). By varying the spin-coating velocity, set of films with thickness ranging from approx. 2.3 nm (one monolayer) up to 12 nm (6 ML) thick multilayers was successfully obtained and applied in top-contact OFETs. Figure 1 shows the representative set of OFETs characteristics and Figure 2b presents the field-effect mobility as a function of film thickness for P3HT94. A significant increase of the hole mobility from 1.3x10-3 cm²/Vs to 2.1x10-2 cm²/Vs for P3HT94 films thicker than 8.1 nm is evident in Figure 2b. Interestingly, films thinner than 8.1 nm revealed an almost thickness independent transistor performance. Devices with 2.3 nm thick layer exhibited a mobility of 1.3x10-3 cm2/Vs, while 8.1 nm thick films yield an almost identical mobility of 3x10-3 cm2/Vs. Above the thickness of 8.1 nm, the value raised sharply up to 2.1x10-2 cm2/Vs. Similar mobility leap was noticed by Joshi et al. for ultrathin P3HT films obtained by dip-coating.18 However, in that case, OFETs with 6 nm thick films exhibited very low mobility of 1.5x10-7 cm2/Vs due to the very low molecular weight of the investigated polymers (3.5 kDa). This value increased for approx. 9 nm thick films the to only 8x10-7 cm2/Vs and remained almost constant to thicknesses up to 70 nm.18 In comparison to


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that report, we not only obtained OFETs with 5 order of magnitude higher mobility, but also managed to investigate films thinner than 6 nm. Fabiano et al. have observed for n-type poly{[N,N-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5’(2,2’bithiophene)} (P(NDI2OD-T2) a saturation of the field-effect mobility for more than 3 edgeon oriented monolayers, although their report was not focused on explaining this phenomena.19 These results for (P(NDI2OD-T2) stay in agreement with our observations on P3HT ultrathin film discussed in this work. This similarity of serve mobility increase at a film thickness of 3 to 4 monolayers indicates certain general trend possibly valid for a wide range of semiconducting polymers. To gain insight into the behaviour of P3HTs of different molecular weights, two additional polymers with Mw = 34 kDa and = 170kDa, referred as P3HT34 and P3HT170, have been also studied. Figure 2 shows the mobility/thickness dependence for all three P3HT polymers. In the thickness range from approx. 2.0 nm up to approx. 7.0 nm all devices exhibited a comparable hole mobility of 2.0x10-3 cm2/Vs regardless the molecular weight. Moreover, all transistors showed a sharp mobility increase above a film thickness of approx. 8.0 nm. The sharp mobility increase for P3HT34 was observed above a film thickness of 7.2 nm, however, due to uncertainty of the thickness measurements, we recognize the value of 8.0 nm as the “threshold” thickness, which also corresponds to 4 monolayers of P3HT. The highest average mobility of 2.4x10-2 cm2/Vs was obtained for P3HT170, in comparison to P3HT94 with 2.1x102

cm2/Vs and P3HT34 with 8.0x10-3 cm2/Vs. A large mobility difference was found between

low (P3HT34) and high molecular weight polymers (P3HT94 and P3HT170). However, this molecular weight dependence of the mobility was observed only for films above the threshold thickness of approx. 8.0 nm. To answer the question, why molecular weight does not affect the mobility below a thickness of 8.0 nm and to understand the reason for the mobility leap, the microstructure and crystal structure was investigated.


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Microstructure Atomic force microscopy (AFM) in tapping mode was applied to investigate the microstructure of the ultrathin films. Representative step-high diagrams of the thinnest and thickest films are shown in the Figure 3. The measurements were focused on film thickness just above and below the threshold value of approx. 8.0 nm in order to correlate the change in device performance to variations in microstructure. The height AFM images of all measured P3HT94 films exhibited nodular structures, which are typical for P3HT (Figure 4).6 To visualise the size difference of the aggregates, grain size analysis was performed of representative AFM images shown in Figure 5 for P3HT94 films with thicknesses of 8.1 nm and 9.1 nm. The aggregates are marked by red color at a height threshold of 45%. For film thicknesses of 9.1 nm and 11.0 nm the nodular aggregates were approx. 50% larger than in films with thicknesses 2.3 nm and 8.1 nm (Table 1). The aggregates sizes in 2.3 nm and 8.1 nm thick films were almost unchanged. Therefore, the charge carrier mobility and aggregate size followed an identical trend with film thickness. To find out the trend for polymers with different molecular weights, the film microstructure of P3HT34 (Figure 6) and P3HT170 (Figure 7) were also inspected by AFM. Ultrathin films obtained for P3HT34 yield a similar, nodular microstructure for all thicknesses. However, the difference in grain size between films below and above the threshold thickness is about 19% which was accompanied by a mobility from approx. 2.0x10-3 cm2/Vs to 7.0x10-3 cm2/Vs (Table 1). Thereby, P3HT94 and P3HT170 expose an identical trend where the microstructure significantly varies between films of different thicknesses. The distinct change of about 50% in grain size in P3HT170 films was observed for the films below and above the threshold thickness of approx. 8.0 nm (Table 1). Again, the AFM results were in agreement to the charge transport behaviour of the ultrathin films. The severe mobility increase above a threshold thickness can be explained by changes in the microstructure between the layers and nature of the charge transport. Only a two-


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dimensional charge carrier transport takes place in films consisting of a single monolayer.20 Such kind of transport is significantly affected by the density of trapping sites at the semiconductor / dielectric interface or by surface scattering effects that may be caused by the roughness of the dielectric surface.21 The quasi 2D type of charge carrier transport is a result of a balance between the gate field accumulating charge carriers at the interface, and repulsive Coulomb interactions between the carriers. However, it has been proven that with adding additional monolayers, the charge carriers are also transported in the upper layers.15 Three effects were found which affect the transport upon increasing the thickness of the active film: (i) decrease of charge density due to increased volume which charge carriers occupy, (ii) reduction of Coulomb interactions between carriers and (iii) availability of new pathways for the charges. Decreasing the charge density tends to worsen the transport, whereby for disordered films, additional monolayers lead to improvement of the charge carrier transport due to the dominating role of the effects (ii) and (iii).15 Although the majority of charges are transported in the first few monolayers, adding next few layers allows the charges to bypass the trapping sites at this semiconductor / dielectric interface.22 Therefore, by increasing the number of layers, the hole transporting path may take also place in the upper layers of the active film (Figure 8). This assumption is supported by the fact that films thicker than 8.0 nm show higher mobility as expressed by a higher slope in the transfer characteristics, while films below a thickness of approx. 8.0 nm reveal a poor performance and thus a low transfer slope. Figure 9 exhibits the square root of the current in the transfer characteristics for P3HT94 for film thickness 9.1 nm and 2.3 nm. Interestingly, in the case of the thicker film, two linear regions can be distinguished. The straight line fitted to the lower source-gate voltages is almost parallel to straight line fitted for the 2.3 nm thick OFET. Therefore, it can be assumed that the first linear region of the thicker OFET corresponds to the charge carrier transport in the first monolayers, while the second fit with the steeper slope corresponds to the transport


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occurring also in the upper layers. We assume that transition between quasi-2D to quasi-3D transport is responsible for the sharp mobility increase observed for all three molecular weights of the polymer. This transition is mainly favored by the larger grains in the upper layers in comparison to the ones close at the interface. It has been recently shown that the top film microstructure can significantly differ from the buried interfacial layers.22 Thereby, it was found that the upper layers exhibited a microstructure which supported the charge transport in contrast to the poorly ordered interfacial layers. Having this in mind, it can be assumed that in the case of P3HT top layers with larger grains mainly determined the overall charge carrier transport in enhanced the OFET performance.

Polymer organization To gain understanding on the polymer packing GIWAXS measurements were performed and the corresponding patterns for the samples are given in Figure 10 and 11. The main feature of the GIWAXS patterns recorded for P3HT94 and P3HT34 films is the h00 series located clearly on the meridian (Figure 10 and 11). A narrow intensity distribution of the h00 series over the azimuthal angle (χ) clearly suggests a high degree of edge-on orientation of P3HT chains in the crystalline units, regardless of the film thickness. It was proven that molecular orientation (edge-on / face-on) is thickness dependent in the case of P3HT for a range of 38 – 80 nm and of poly(3-hydroxybutyrate) for a range of 20 – 185 nm.23-24 However, such dependence was not observed in the case of this work, most probably due to the small film thickness up to maximum 11.3 nm. Such an orientation of P3HT crystalline units is believed to be favorable for the charge carrier transport in the active OFET layers.17 The main problem in the quantitative analysis of ultrathin polymer films, i.e. with thickness in the range of few up to ten nanometers is their limited capability to coherently scatter X-rays. Hence, only very distinct peaks originating from diffraction on the ordered units of the polymer could be recorded with a sufficient resolution (Figure 10 and 11 insets). Therefore, in our analysis we have focused on the geometrical parameters of the most prominent diffraction feature of the films which is the 100 peak resulting from X-ray diffraction on the stacked P3HT chains (see insets in Figure 10 and 11). Only in the case of these peaks, signal-to-noise ratios were found high enough while adverse scattering effects were negligible enough to perform a reliable 8 ACS Paragon Plus Environment

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quantitative analysis which is presented in Table 2. Increasing the P3HT34 film thickness from 3.2 to 7.2 nm causes in the GIWAXS patterns a change in the shape of the 100 intensity spot from more circular to more horizontally-extended elliptical (Figure 10 insets). This is basically a result of increasing thickness of crystalline domains in the layer and hence narrowing of the peak along the qz axis from 0.1 Å-1 for the 3.2 nm layer to 0.06 Å-1 for 7.2 nm thin films. In addition, we observed a broader intensity distribution along the azimuthal angle (χ) suggesting an increased (yet still low) degree of orientational disorder of the P3HT34 crystalline domains in the film. In the case of P3HT94 films, the azimuthal intensity distribution was practically unrelated to the layer thickness, suggesting that the domain orientation in the P3HT94 film did not depend on its thickness. The intensity coherently scattered by (100) planes of P3HT34 and P3HT94 crystals was found proportional to the film thickness (Table 2). The ratio of the intensity scattered by the (100) planes of the P3HT (I100) and the thickness of the film is directly proportional to the coverage of the surface by crystalline domains of the polymer. Based on these ratios we can conclude that the crystalline domains of P3HT34 cover a larger area of the film than in the case of P3HT94. In other words, the crystallinity degree of P3HT34 in the investigated films is higher than the crystallinity of P3HT94. The reason why we compared here the I100 intensity with tAFM and not ℓ is that the latter reflects the thickness of the crystalline phase but is also sensitive to the perfection of the crystals in the film. These two factors (i.e. crystal thickness and crystal perfection) cannot be resolved in a reliable manner on the basis of the Scherrer approach. Despite a relatively good agreement between tAFM and ℓ . (Table 2), the latter is rather to be expected proportional but not necessarily equal to the film thicknesses measured directly by AFM (Table 2). Hence, as the aim was to compare the intensities scattered by the unit volume of the film, the film thicknesses determined by AFM is taken into account. In contrast to the mobility and microstructure investigation, data from X-ray measurements did not exhibit a severe change near the threshold film thickness, where the charge carrier transport is shifted. In the case of P3HT34 the intensity of the 100 spot rises from 0.45 a.u. (for the film with thickness below the threshold) up to 0.63 a.u. (for the film with thickness above the threshold thickness). However, for the polymer with higher molecular weight, no such transition was observed, despite the fact, that in the case of P3HT94, the rise of charge carrier mobility was more pronounced than in the P3HT34 films. Furthermore, higher crystallinity 9 ACS Paragon Plus Environment

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observed for P3HT34 may seem surprising, since this polymer exhibited worse mobility than P3HT94. It is already known that polythiophenes with lower molecular weight exhibit worse charge carrier transport, despite higher level of crystallinity.17 Therefore, one could expect, that such behavior would also be observed in case of ultrathin films. Furthermore, recent reports suggest that in order to improve the charge carrier transport in thin film transistors, high crystalline order is not absolutely necessary.25-26It has been reported that short-range aggregation of conjugated polymer may determine the conduction, rather than long-range crystalline order.25 Another report revealed that a high mobility poorly crystalline films can be related to an increased amount of localized aggregates in the amorphous region of the conjugated polymer.26 Having these reports in mind, the absence of a significant change in crystallinity for P3HT94 at the threshold film thickness, despite of a severe mobility leap, is not surprising, since factors other than the crystalline structure may also influence the charge carrier transport. Therefore, as previously mentioned we address the threshold type of mobility changes to: i) increased size of polymer aggregates, ii) change from 2D to 3D type of charge carrier transport.

Conclusions P3HT films with thicknesses from 2.3 nm to 12 nm have been successfully applied as active layers in bottom gate, top contact OFETs. For three different molecular weights (34 kDa, 94kDa, 170 kDa) the hole mobility remained at the level of 2.0 x 10-3 cm2/Vs for film thickness between 2.0 nm to 7.0 / 8.0 nm. Above that thickness, a sharp and significant mobility increase was observed. This threshold thickness corresponds to approx. 4 polymer layers. We conclude that above this thickness the charge carriers are transported also in the upper layers of the film, in other words, a transition from 2D to 3D charge carrier transport occurs. Such transition allows the charge carriers to bypass the trap sites and reduces the


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influence of surface-scattering.21 This conversion from 2D to 3D transport was mostly studied theoretically (e.g. in Monte-Carlo simulations)27 or for small conjugated molecules.28-29 The mobility increase is related to larger grains in upper layers in contrast to the less distinct interfacial microstructure. The large grains in the upper layers favour the charge carrier transport due to reduced amount of boundaries as trapping sites.

Experimental Materials P3HT polymers were purchased from Ossila and used as received. Three polymers were applied for the experiments: 1) P3HT34 with Mw = 34 kg/mol (polydispersity index, PDI=1.75) and regioregularity (RR) = 94.7%; 2) P3HT94 with Mw = 94.1 kg/mol (PDI= 1.90) and RR = 95.5 %, and 3) P3HT170 with Mw = 169.5 kg/mol (PDI=2.21) and RR = 98.5%. 1,2dichlorobenzene (HPLC grade) was purchased from Sigma-Aldrich and used as received. OFETs preparation and characterisation Silicone substrates with silicone dioxide layer (300 nm) were used as substrates. Firstly, they were washed in the ultrasonic bath in acetone and isopropanol for 15 minutes in each solvent. In the next step, the substrates were treated with oxygen plasma for 5 minutes and functionalized with HMDS vapor at 140oC for 6 hours. Solutions of P3HT and 1,2dichlorobenzene were prepared at concentrations of 1.5 mg/ml for P3HT94 and P3HT170 and 2.5 mg/ml for P3HT34. These solutions were stirred and annealed at 100oC for 2 hours. The active P3HT layers were deposited by spin-coating. The rotation speeds ranged from 400 rpm (for the thickest films) up to 6000 rpm (for the thinnest films). Gold source and drain electrodes were thermally evaporated in high vacuum using Edwards evaporator system. Each sample consisted of 20 separated OFETs. Channel length and width were 30 and 1000 µm respectively. The prepared devices were annealed before the measurements at 130oC for 2 hours in order to remove solvent residues. OFETs were measured using a micromanipulator probe station connected to a Keithley 2634b source meter. Output and transfer characteristics were measured in the voltage regime from 0 V to -80 V applied both to the gate electrode and


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between source and drain electrodes. Mobilities were calculated from the transfer characteristics in the saturation regime, using the following formula: =

( − )

where Id is the drain current, W the channel width, L the channel length, Ci the capacitance of gate dielectric, Vg the gate voltage, Vth the threshold voltage and µ is the charge carrier mobility. On each sample, 10 OFETs were measured and the average values are presented in the work. Measurement uncertainty was estimated as a standard deviation of performed measurements.

AFM measurements Veeco Dimension 3100 Atomic Force Microscope was used to inspect the microstructure and the thickness of the polymer films. All images were obtained in the tapping mode with Olympus silicone cantilevers at 320 kHz resonance frequency. Each sample was measured in three chosen areas (in the middle and near sample edges) to confirm uniformity of the films. Step high measurements were also performed in three different areas and each step was measured several times (3 – 5 times). The average values are presented in this work and the corresponding measurement uncertainty of the thickness was estimated according to small deviations for each measurement and roughness of the films. . Grain size analysis was performed with NanoScope Analysis 1.7 software by marking the grains above a specified threshold height. RMS roughness parameters were estimated with Gwyddion 2.47 software.

GIWAXS measurements Grazing-Incidence wide-angle X-ray scattering (GIWAXS) measurements were performed using 10 keV beam at the BL9 beamline at DELTA synchrotron facility in Dortmund (Germany). The beam was collimated by a system of adjustable X/Y slits to dimensions 50 µm normal to the sample plane and 1 mm in the sample plane. All the samples with 12 ACS Paragon Plus Environment

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dimensions 15×15 mm were irradiated at the incident angle (αi) of 0.11°. The irradiated areas (1.5 mm2) (beam width × sample length) were identical for all the samples. The camera length (336 mm) was calibrated using silver behenate standard. Diffraction patterns were recorded for 10 min on the MAR345 image plate detector. One-dimensional, out-of-plane intensity profiles were obtained by integration of 2D patterns using Datasqueeze (v 3.0.1) computer program. Further processing, including intensity and background correction, smoothing, Gaussian peak fitting and integration was performed using OriginPro 64. Fit2d was used for the purpose of 2D data presentation.

Figure 1. Output characteristics of OFETs with a) 8.1 nm and b) 9.1 nm thick P3HT94 films, c) square root of transistor transfer characteristics for P3HT94 films with various thicknesses.

Figure 2. Charge carrier mobility as function of film thickness for P3HT of different molecular weights: a) P3HT34, b) P3HT94, c) P3HT170.


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Figure 3. Step-height measurements for the a) thickest P3HT34 (11.3 nm), b) thinnest P3HT34 (2.6 nm), c) thickest P3HT94 (11.0 nm), d) thinnest P3HT94 (2.3 nm), e) thickest P3HT170 (13.1 nm), f) thinnest P3HT170 (2.6 nm) films. The “hill” visible before the step is a result of removing part of the film for the measurement.


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Figure 4. AFM height images of P3HT94 films with thickness of a) 11.0 nm, b) 9.1 nm, c) 8.1 nm, and d) 2.3 nm. Insets are height images of the same films at higher magnification.


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Figure 5. Grain size analysis and corresponding AFM height images for P3HT94 films with thickness of a), b) 8.1 nm, and c), d) 9.1 nm. In the analysis in a) and c), aggregates are marked by red color at a height threshold 45%.


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Figure 6. AFM height images of P3HT34 films with thickness of a) 11.3 nm, b) 7.2 nm, c) 6.3 nm, and d) 2.6 nm. Insets are height images of the same films at higher magnification.

Figure 7. AFM height images of P3HT170 films with thickness of a) 8.4 nm and b) 4.5 nm. Insets are height images of the same films at higher magnification.


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Figure 8. Scheme of transition between a) 2D to b) 3D type of charge carrier transport in the transistor channel upon increasing the film thickness. Squares in the active layer symbolizes P3HT aggregates sizes in the lower and upper films with edge on orientation of the molecules.

Figure 9. Square root of the source-drain current in the transistor transfer characteristics for P3HT94 films with thicknesses of 9.1 nm and 2.3 nm with straight lines fitted to linear regions of the curves. 18 ACS Paragon Plus Environment

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Figure 10. 1D normalized GIWAXS out-of-plane profiles and experimental 2D GIWAXS patterns (insets) of a) 2.6, b) 6.3, and c) 7.2 nm thick P3HT34 films (thickness determined by AFM). Hollow circles show the averaged experimental intensity, lines represent the doubleGaussian fit of the experimental points.

Figure 11. 1D normalized GIWAXS out-of-plane profiles and experimental 2D GIWAXS patterns (insets) of a) 8.1, b) 9.1 and c) 11.0 nm thick P3HT94 films (thickness determined by AFM). Hollow circles show the averaged experimental intensity, lines represent doubleGaussian fit of the experimental points.


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Table 1. Thicknesses and mean grain sizes of P3HT films with different molecular weights. P3HT Mw [kDa]

Film thickness [±0.5 nm]

Mean grain size (in-plane)[nm]

34

11.3 7.2 6.3 2.6 11 9.1 8.1 2.3 8.4 4.5

49.9 51.0 43.4 39.7 57.4 58.7 40.0 37.2 55.5 37.1

94

170

Table 2. Variation in crystalline structure of ultra-thin P3HT films spin-coated from odichlorobenzene solution as revealed by analysis of the 100 reflection measured by GIWAXS (for GIWAXS results see Figures 10 and 11). Polymer

P3HT34

P3HT94

Mean layer thickness (AFM), tAFM

Scherrer coherence length for (100) planes, (tXRD)

d100

Normalized integral intensity, I100

[nm]

[nm]

[nm]

[a.u.]

2.6

6

1.80

0.42

0.14

6.3

7

1.70

0.45

0.09

7.2

9

1.70

0.63

0.09

8.1

8

1.73

0.42

0.06

9.1

9

1.70

0.44

0.05

11

9

1.70

0.48

0.05

I100/tAFM


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AUTHOR INFORMATION Corresponding Author * Wojciech Pisula, address: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, e-mail: [email protected] * Jacek Ulanski, address: Lodz University of Technology, Faculty of Chemistry, Department of

Molecular

Physics,

Zeromskiego

116,

90-924

Lodz,

Poland,

e-mail:

[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by National Science Centre, Poland through the grants DEC2013/08/M/ST5/00914 and UMO-2015/18/E/ST3/00322 as well as by Foundation for Polish Science through the grant MASTER/MISTRZ 9/2013. We gratefully acknowledge the DELTA electron storage ring in Dortmund (Germany) for a beamtime enabling preliminary X-ray diffraction measurements. REFERENCES 1.

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TOC:


Microstructure-Dependent Charge Carrier Transport of Poly(3

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