Role of Sub-Nanometer Dielectric Roughness on Microstructure and

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Role of Sub-Nanometer Dielectric Roughness on Microstructure and Charge Carrier Transport in #,#-Dihexylsexithiophene Field-Effect Transistors Mengmeng Li, Tomasz Marszalek, Klaus Müllen, and Wojciech Pisula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03233 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Role of Sub-Nanometer Dielectric Roughness on Microstructure and Charge Carrier Transport in α,ωDihexylsexithiophene Field-Effect Transistors Mengmeng Li‡, Tomasz Marszalek‡, Klaus Müllen,*,‡ Wojciech Pisula*,‡,# ‡

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

#

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

Zeromskiego 116, 90-924 Lodz, Poland *[email protected]; [email protected] Keywords: organic field-effect transistors; microstructure; π-stacking; charge carrier transport; dielectric surface roughness

Abstract The effect of dielectric roughness on the microstructure evolution of thermally evaporated α,ωdihexylsexithiophene (α,ω-DH6T) thin films from a single molecular layer to tens of monolayers (ML) is studied. Thereby, the surface roughness of dielectrics is controlled within a subnanometer range. It is found that the grain size of a α,ω-DH6T monolayer is affected by dielectric roughness, especially for 1.5 ML, whereby the transistor performance is barely

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

influenced. This can be attributed to a domain interconnection in the second layer over a long range formed on the rough surface. With deposition of more layers, both microstructure and charge

carrier

transport

exhibit

a

roughness-independent

behavior.

The

structural

characterization of α,ω-DH6T 10 ML by grazing-incidence wide-angle X-ray scattering (GIWAXS) reveals that the interlayer distance is slightly decreased from 3.30 to 3.15 nm due to a higher roughness, while an unchanged π–stacking distance is in excellent agreement with the roughness-independent hole mobility. This study excludes the influence of molecular-solvent interaction and pre-aggregation taking place during solution deposition, and provides further evidences that the microstructure of the interfacial layer of organic semiconductors has only minor impact on the bulk charger carrier transport in thicker films.

Introduction Enormous advances have been achieved in the device performance of organic field-effect transistors (OFET) in the last two decades.1-5 However, the fundamental mechanism of charge carrier transport in transistors is not yet fully understood. To answer this critical question, much effort has been made to investigate the contribution of different molecular layers on the charge carrier transport.6-7 As well-known crystalline organic semiconductors with good charge carrier mobility and stability, oligothiophenes and their derivatives were able to form high-quality thin films with only a few single molecular layers, providing an excellent opportunity to explore the relation between film thickness and mobility in a monolayer precision.1, 8-11 A number of detailed studies revealed that only a few layers adjacent to the dielectric were mainly responsible for the charge carrier transport, and that the contribution from other additional layers is negligible,9, 11-12 where the molecular ordering in each layer was unchanged within the whole film. Although the monolayer transistors based on conjugated polymers were also reported, their mobilities were


2

Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orders of magnitude lower than those of the bulk film.13-14 In spite of this impressive progress, it is still challenging to precisely control the evolution of the microstructure and subsequent device performance of the organic monolayer transistors. It is generally accepted that the interface between semiconducting layer and dielectrics plays a critical role in the self-assembly of organic semiconductors,15 in which the dielectric roughness is one of the most important factors. Many reports demonstrated that the self-assembly and molecular organization in organic semiconductors were significantly hampered by a rougher surface, leading to low transistor performances.16-18 Nevertheless, the dielectric roughness used in most studies was on a nanometer scale, affecting the microstructure of the entire semiconductor film. Previously, we have developed a strategy to precisely control the dielectric surface roughness ranging from 0.15 to 0.39 nm.19 Importantly, the roughness in this defined range influences the microstructure of only the interfacial layers close to the surface, maintaining the high order in the bulk film. For solution processed organic semiconductors, it was proven that the interfacial microstructure has only a minor influence on the charge carrier transport in thicker films and that the charge carriers can by-pass these poorly ordered areas over the better ordered ones in the bulk film.19 In solution processing, the nature of the solvents is of vital importance because the interaction between solvent and molecule strongly influences the self-assembly of organic semiconductors. More importantly, strong π-interactions between conjugated molecules exist so that aggregation in solution can take place before and during processing.20-22 Both factors limit the exploration of the intrinsic role of dielectric surface roughness on the self-assembly of organic semiconductors. On the contrary, thermal evaporation in vacuum excludes these external influences and allows the deposition of single molecules in a more precise fashion. Therefore, in


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

this study α,ω-dihexylsexithiophene (α,ω-DH6T) is thermally evaporated on the dielectric of a controlled surface roughness within a sub-nanometer scale to precisely monitor the evolution of the microstructure and charge carrier transport. The reason why α,ω-DH6T has been chosen as model compound is because of its layer–by-layer growth following the Frank-van der Merwe mode up to elevated thicknesses. This behavior is rather exceptional and represents an important requirement for these investigations in comparison to other rod-like small molecules, such as pentacene, which show a film roughening upon thickening due to the Stranski-Krastanov 3D growth mode. It is demonstrated for α,ω-DH6T that the interfacial microstructure has basically no impact on the charge carrier transport for thicker films in a good agreement with our previous findings.19 Experimental Section Preparation of dielectric with sub-nanometer surface roughness. Dielectrics of S1-S5 were prepared accordingly to the reported procedure.19 2 mL of tetramethyl orthosilicate (TMOS) was diluted with 2 mL of ethanol, subsequently hydrolyzed by a solution of H2O (0.8 mL) and HCl (16 µL, 2 M) and then heated to 70 oC over 30 min. After vigorous stirring, heating was stopped and the solution was aged for 24 h. To precisely tune the resultant surface roughness, the aged solution was diluted by 10 times using a mixed solvent of ethanol/H2O with different volume ratios. These precursor solutions were spin-coated onto the silicon wafers with 300-nm-thick thermally grown SiO2 under the speed of 2000 rpm for 1 min. To remove the organic residue on the surface, the annealing process was carried out at 700 oC for 1 h in a nitrogen atmosphere. Finally, dielectrics of S1-S5 were obtained with the surface roughness ranging from 0.15 to 0.39 nm. The thickness of all spin-coated SiO2 layers is ~10 nm.


4

Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Organic semiconductor deposition and device fabrication. S1-S5 were cleaned by 10 min ultrasonication in acetone and subsequent 10 min ultrasonication in isoproponal. No difference of dielectric roughness can be observed before and after cleaning substrates. α,ωdihexylsexithiophene (α,ω-DH6T, Sigma) thin films were then deposited using vacuum thermal evaporation at a rate of 1.5 Å/s and a pressure of 10-6 Torr. During the deposition, the substrates were kept at room temperature. All samples with the same film thickness were thermally evaporated in the same batch. OFET devices were fabricated with top-contact bottom-gate architecture. The source and drain electrodes with 60 nm in thickness were deposited by Au evaporation. The channel length and width are 20 and 400 µm, respectively. Characterization. Morphologies of organic semiconductor layers were characterized by a Digital Instruments Nanoscope IIIa Atomic Force Microscopy (AFM) in tapping mode. GIWAXS measurements were performed by means of a solid anode X-ray tube (Siemens Kristalloflex X-ray source, copper anode X-ray tube operated at 35kV and 40mA), Osmic confocal MaxFlux optics, X-ray beam with pinhole collimation and a MAR345 image plate detector. The beam size was 0.5 × 0.5 mm and samples were irradiated just below the critical angle for total external reflection for X-ray beam (~0.18°). Data analysis was carried out with Datasqueeze software. A Keithley 4200-SCS was used for all standard electrical measurements in a glovebox under nitrogen atmosphere. Results and Discussion Silicon wafers with 300 nm-thick thermally grown SiO2 are firstly used as the substrates for the deposition of organic semiconductors without any further surface passivation. The rootmean-square surface roughness (Rms) of the dielectric is accurately modulated by a simple but efficient solution method which was proposed in our previous report.19 Tetramethyl orthosilicate


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

(TMOS) is hydrolyzed by hydrochloric acid in H2O/ethanol, and then directly spin-coated on the silicon wafer. After annealing at 700 oC under nitrogen atmosphere, an additional SiO2 layer is created on the wafer with the surface roughness ranging from 0.15 to 0.39 nm (Figure 1 and Table S1). It has to be emphasized that the increase in SiO2 thickness (~10 nm) and in surface area due to a higher roughness can be neglected for the capacitance calculation.19

Figure 1. Schematic illustration of the transistor configuration. α,ω-DH6T thin films from mono- to multilayers are deposited by vacuum thermal evaporation, on dielectric surfaces with modified surface roughness on the sub-nanometer scale. The chemical structure of α,ω-DH6T is shown on the top.

α,ω-Dihexylsexithiophene (α,ω-DH6T, Figure 1) is a good candidate for high-performance monolayer-based organic electronics due to its two key advantages. On the one hand, charge carriers can be shielded by the alkyl chains from the dielectric interface reducing the density of the trapping sites at the interface.9 On the other hand, the strength of intermolecular interactions between the conjugated cores can be enhanced by the alkyl chains promoting the crystallinity of the semiconducting layers.9 It must be emphasized that the growth of α,ω-DH6T is in a layer-bylayer manner, facilitating the investigation on the interfacial microstructure and subsequent


6

Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


evolution. In the first step, α,ω-DH6T ultrathin films with a single molecular layer (monolayer, ML) are fabricated by vacuum thermal evaporation. The microstructure is characterized by AFM in tapping mode, as shown in Figure 2a. The monolayer coverage is ~70%, defined as 0.7 ML. This monolayer consists of isolated disk-like grains, and the height profile exhibits a thickness of ~2.9 nm (Figure S1). Grains are defined as crystallites which are separated by structural defects, grain boundaries, in the polycrystalline thin layers. Grains can comprise one or more domains. The determined thickness is in agreement with the d-spacing of α,ω-DH6T films found by GIWAXS, confirming a lamellar organization of the rod-like molecules.23 The roughness of the deposited films is gradually increased at higher Rms due to the rougher dielectric surface, and the molecular self-assembly is limited (Figure S2). Furthermore, it is obvious from Figure 2c that the dielectric roughness critically affects the grain size (D) of α,ω-DH6T. With a slight increase in Rms from 0.15 nm (S1) to 0.19 nm (S2), a reduction in grain size is clearly observed from 211±34 to 162±24 nm. When deposited on S5 (Rms=0.39 nm), the D value of the disc-like islands continuously drops to 125±17 nm. However, these isolated grains are not connected over a long range so that there is no sufficient conduction channel established allowing charge carrier transport between source and drain electrodes.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 21

Figure 2. AFM images of the α,ω-DH6T 0.7 ML (a) and 1.5 ML (b) deposited on S1-S5. All images in (a) and (b) have the same scale bars. c) Grain size of 0.7 ML and 1.5 ML as a function of Rms. d) Second-layer coverage of α,ω-DH6T 1.5 ML as a function of Rms. e) Transfer characteristics of α,ω-DH6T 1.5 ML on S1 and S5 at VDS=-80 V.

Deposition of further α,ω-DH6T molecules on the dielectric surface leads to a slow coalescence of the isolated circular grains and finally to a fully covered single molecular layer. Based on the first monolayer, the second layer begins to grow, as shown in Figure 2b. The coverage of the second layer is approximately 50-65% (Figure 2d), defined as 1.5 ML. This increase in coverage can be related to the slight variation of the roughness of the first layer (Figure S2). In comparison with 0.7 ML, the microstructure evolution of α,ω-DH6T 1.5 ML is


8

Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


far more distinct. Due to the layer-by-layer growth (Frank-van der Merwe) of α,ω-DH6T, the interfacial adhesion between the first layer and substrate is stronger than the interlayer cohesion between the first and the second layer,28,29 leading to such differences in microstructure. On S1, the isolated grains with 389±73 nm in diameter are deposited, and few aggregates appear on their top. The dark background in the AFM height images represents the fully covered first monolayer (Figure 2b and S3). It is obvious from Figure 2c that the grain size on S2 is dramatically decreased to 223±62 nm. Dielectrics with higher Rms values generate compact plate-like grains with the size reduction from 214±49 nm at Rms=0.27 nm (S3) to 127±37 nm at Rms=0.39 nm (S5). The decrease in grain size reaches 67 % for the second layer of 1.5 ML, compared with the value of 41 % for 0.7 ML. To gain information about the charge carrier transport in the 1.5 ML, OFET devices are fabricated in a top-contact bottom-gate configuration that is effective to minimize the contact resistance without affecting the thin film microstructure of the semiconducting layer. Source and drain electrodes are deposited by Au evaporation with 60 nm in thickness. The transfer and output characteristics depict a typical linear/saturation behavior for all transistors (Figure S4). Since the second layer of 1.5 ML does not form a long-range connection, it is believed that the charge carrier transport is primarily determined by the first monolayer. The transistors on S1 show an average hole mobility of 2.04×10-3 cm2V-1s-1 with the maximum value of 2.25×10-3 cm2V-1s-1. This value is identical to the previous report for α,ω-DH6T on non-functionalized SiO2.9 It seems to be reasonable to expect a lower transistor performance on S5 because of smaller grain size, higher density of trapping sites at the semiconductor/dielectric interface and stronger surface scattering effects on charge carriers.19 Surprisingly, transistors on S5 exhibit similar mobilities to S1 with an average value of 1.47×10-3 cm2V-1s-1 (Figure 2e). This behavior


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

can be related to the size and linkage between domains in the second layer. The islands of the second layer on the rough dielectric S5 are smaller than on S1, but interconnected over a relatively longer range due to a higher film coverage (Figures 2 b and d), which is beneficial for bridging over structural defects of the first monolayer creating additional pathways for the charge carriers.10 The on/off ratio is also independent of the dielectric roughness, with a value of 104 (Table S2).

Figure 3. a-e) AFM images of α,ω-DH6T 3 ML deposited on S1-S5. All images have the same scale bar. The dash circles indicate the grain size. f) Transfer characteristics of α,ω-DH6T 3 ML on S1 and S5 at VDS=-80 V.

The topographies of α,ω-DH6T 3 monolayers (3 ML) are present in Figure 3. The dark isolated spots with tens of nanometers in size represent the completely covered second layer. A higher dielectric roughness causes a smaller grain size in the third layer that is indicated by dash circles in Figure 3. Additionally, few aggregates grow on the top of the third layer, suggesting the nucleation of the forth layer, whereby the density of these nucleation points becomes higher


10

Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the dielectric with a higher Rms value. The additional layers build more pathways for charge carriers and therefore the field-effect mobility of α,ω-DH6T 3 ML is doubled compared with 1.5 ML. Figure 3f shows the transfer plots of 3 ML on S1 and S5. At VDS = -80 V and VGS = -80 V, the value of drain current (-IDS) exhibits a gradual degradation from 1.45 to 0.75 µA with increasing Rms from 0.15 to 0.39 nm (Figures 3f and S5), which is ascribed to the smaller grain size (dash circles) and higher density of grain boundaries. At the same time, the turn-on voltage for the transistor on S5 is increased due to induced trapping sites and the effect of surface scattering.17,

24

As the dielectric roughness increases, the average value of hole mobility is

reduced from 4.48×10-3 cm2V-1s-1 at Rms=0.15 nm (S1) to 2.86×10-3 cm2V-1s-1 at Rms=0.39 nm (S5). In addition, all transistors of α,ω-DH6T 3ML show similar on/off ratio, with the value of 104-105 (Table S2).

Figure 4. a-e) AFM images of the α,ω-DH6T 10 ML deposited on S1-S5. All images have the same scale bar. f) Transfer characteristics of α,ω-DH6T 10 ML on S1 and S5 at VDS=-80 V.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

Finally, α,ω-DH6T 10 ML is fabricated with ~30 nm in thickness. AFM images in Figure 4 exhibit identical topographies with only slight differences in film coverage of the top layer, indicating the independence of molecular self-assembly on the dielectric roughness for thicker films. Interestingly, the monolayer thickness of 3.9 nm on the top of 10 ML is higher (Figure S6) than that in the first interfacial molecular layer (~2.9 nm, Figure S1), which is consistent with the report.9 Such a behavior can be ascribed to the FW growth mode of α,ω-DH6T, where the interfacial adhesion between the first layer and the dielectric is stronger than that between the first layer and upper ones.28,29 In comparison with the ultrathin films such as 1.5 ML and 3 ML,

α,ω-DH6T 10 ML exhibits a significantly improved charge carrier transport. At VDS=-80 V and VGS=-80 V, the value of -IDS for 10 ML is one order of magnitude higher than that for 3 ML (Figure S7). Moreover, the drain current at low VGS in the transfer characteristics becomes smoother (Figures 4f and S7). α,ω-DH6T 10 ML devices on S1 and S5 show identical transfer curves, as presented in Figure 4f. The mobility values range from 0.06 to 0.07 cm2V-1s-1 independent of the dielectric roughness. These results are in good agreement with our previous report.19 The maximum mobility reaches 7.23×10-2 cm2V-1s-1, which is, to the best of our knowledge, the highest mobility for α,ω-DH6T on non-functionalized SiO2 dielectric without annealing treatment.9 To identify the influence of dielectric roughness on the molecular organization, GIWAXS is performed. Thin films with the thickness below 10 nm do not yield reasonable X-ray scattering. Therefore, films of α,ω-DH6T 10 ML on S1 and S5 are characterized by GIWAXS, as shown in Figures 5a and S8. In both cases, the GIWAXS pattern exhibits a well-defined organization which is confirmed by reflections up to the second order appearing on the meridional plane (qz). For 10 ML on S1 the first order peak is localized at qz = 0.19 Å-1 and qxy= 0 Å-1 indicating an


12

Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interlayer distance of 3.30 nm (Figure 5a). An additional reflection on the off-equatorial at qz=0.44 Å-1 and qxy=1.29 Å-1 (labeled as peak “B” in Figure 5a) corresponds to a d-spacing of 0.445 nm and is assigned to the hexyl side chains. Furthermore, an off-equatorial π-stacking peak (peak “A” in Figure 5a) appears at qz=0.80 Å-1 and qxy=1.37 Å-1 which is related to a d-spacing of 0.38 nm and indicates a tilting of edge-on arranged molecules in respect to the substrate.

Figure 5. a) GIWAXS pattern of α,ω-DH6T 10 ML on S1. Reflections indicated as A and B are assigned to π-stacking and alkyl chains, respectively. Schematic illustration of the surface organization of α,ω-DH6T: b) molecular conformation with alkyl chain tilting with respect to the

α,ω-DH6T core; c) α,ω-DH6T tilting by an angle θ towards the substrate.

The precise conformation of the α,ω-DH6T molecules is evaluated on the basis of the following considerations (Figures 5b and 5c). According to literature, lengths of the conjugated thiophene core of 2.02 nm and each hexyl group of 0.93 nm are assumed.25 An angle between molecular core and substituents of ϕ = 32° is estimated according to cos ϕ = 0.38 nm / 0.45 nm, with 0.38 nm as the closest π-stacking distance and 0.45 nm as the intermolecular distance


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

between alkyl chains (Figures 5a and 5b). Taking the calculated angle ϕ into account, the length of the α,ω-DH6T molecule (Lo) can be determined: Lo= 2.02 + 2×0.93×cos ϕ = 2.02 + 2×0.93×cos 32° = 3.59 nm (Figure 5b). Based on results obtained for unsubstituted sexithiophene (6T) which is typically tilted by θ=111.3° on the surface a monolayer thickness of

α,ω-DH6T is given by the projection on the Lo sin θ axis, leading to 3.59 nm × sin 111.3° = 3.34 nm (Figure 5c). The calculated theoretical value is in agreement with the result obtained from GIWAXS (3.30 nm) for the film on S1. The interlayer distance of 3.15 nm for α,ω-DH6T 10 ML on S5 is slightly smaller. It has to be noted that these values of interlayer distance are well consistent with the monolayer thickness determined by AFM (Figure S6). A slight decrease in dspacing is also found for the alkyl chains with 0.45 nm and π-stacking with 0.39 nm. Since the angle (φ) between the α,ω-DH6T core and the alkyl chain remains unchanged (cos ϕ = 0.39 nm / 0.45 nm, Figure S7), the decrease in the interlayer distance on S5 might be attributed to a larger molecular tilting (larger angle θ) with respect to the surface. In addition, the interlayer reflection for 10 ML on S5 shows a minor reduction in full width at half maximum (FWHM = 3.7×10-3) compared with S1 (FWHM = 5.4×10-3) indicating a larger coherence length (CLIL =36 nm) for 10 ML on S5 than S1 (CLIL =25 nm). At the same time, both α,ω-DH6T 10 MLs on S1 and S5 exhibit almost the same in-plane coherence length in the π-stacking direction of CLπ = 13 nm. Since the main charge carrier transport takes place in-plane of the film, an identical transistor performance as discussed above is reasonable (Figure 4f). Figure 6 summarizes the OFET relevant parameters, including the saturation hole mobility (µh) and threshold voltage (VT), as a function of dielectric roughness for α,ω-DH6T thin films from mono- to multilayers. It is evident that the dielectric roughness on a sub-nanometer scale has a significant influence on the self-assembly of α,ω-DH6T monolayers. It is worth pointing


14

Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


out that the field-effect mobility of thermally evaporated α,ω-DH6T 1.5 ML seems also barely dependent on the Rms value, which is attributed to the fact that the interconnection of domains in the second layer compensates the effect of dielectric roughness on charge carrier transport. The deposition of more layers obviously increases the hole mobility and effectively decreases the threshold voltage due to the generation of more pathways for the charge carriers, which reveals the transport of charge carriers in upper layers.14, 19, 26 Figure 6b depicts a slight increase in |VT| with increasing dielectric roughness for 3 ML and 10 ML, which is ascribed to higher density of trapping sites induced by higher Rms and scattering effects.17,

24

More importantly, both

microstructure and transistor performance in multilayers seem to be independent on the sub-nm dielectric roughness originating from sufficient pathways for charge carriers created in upper layers. These results reveal the minor impact of interfacial microstructure on the charge carrier transport in multilayers.19

Figure 6. Hole mobility (a) and threshold voltage (b) with corresponding error bars of α,ωDH6T thin films from mono- to multilayers as a function of dielectric roughness.

There are clear evidences that in solution the strong π-interactions between conjugated molecules have critical influence on molecular self-assembly so that molecular aggregates


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

already are formed before solid film deposition.20-22,

27

Page 16 of 21

When layer growth is initiated, not

individual molecules but rather aggregates contribute to the film formation. It has been proven that such self-assembly is significantly influenced by sub-nm dielectric roughness for the interfacial layer of organic semiconductors.19 On the contrary, during thermal evaporation individual molecules are deposited on the dielectric one by one, which have much more freedom than aggregates in solution and are able to compensate the effect of dielectric roughness on the microstructure. Therefore, the microstructure and charge carrier transport of thermally evaporated thin films are much less sensitive to the dielectric roughness as compared with solution processing. Conclusion The evolution of microstructure and charge carrier transport in α,ω-DH6T transistors has been investigated layer by layer by controlling the surface roughness of the dielectric on a subnm scale. Both AFM and GIWAXS reveal that such roughness has an impact on the microstructure of only the first monolayer (0.7 ML and 1.5 ML) without affecting upper layer (3ML and 10 ML). In spite of obvious decrease in domain size induced by the dielectric roughness, the interconnection of domains in the second layer for 1.5 ML ensures the charge carrier transport resulting in a roughness insensitive transistor performance (within the investigated roughness range). The difference between solution processed and thermally evaporated monolayers can be attributed to the fact that solution processing allows conjugated molecules to pre-aggregate in solution before and during film deposition,27 while thermally evaporated molecules with more freedom can compensate the structural defects caused by the dielectric roughness. On the other hand, thermal evaporation in vacuum excludes effects associated with molecule-solvent interactions. With the deposition of more layers, the influence


16

Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of dielectric roughness on the microstructure of organic semiconductors disappears. GIWAXS for α,ω-DH6T 10 ML exhibits similar molecular organization such as interlayer and π-stacking distances, independent of Rms. At the same time, the impact of the dielectric roughness on the transistor performance is also negligible for 10 ML. The results presented in this study are in good agreement with our previous study by solution processing providing more evidence that the interfacial microstructure has basically no impact on the charge carrier transport for thicker film.19 Therefore, a further progress of this finding has been taken on the general significance applicable to all organic semiconductors independent of the deposition technique. It is worth pointing out that the conjugated molecule used in this study, α,ω-DH6T, possesses a layered two-dimensional (2D) growth mode (Frankvan der Merwe), even for 10 ML. In comparison, in the cases of most rod-like small molecules such as pentacene, the growth of thin films rapidly steps from 2D mode to 3D mode (StranskiKrastanov) upon thickening.28-30 In order to generalize our findings, the future focus should be put on organic semiconductors with 3D growth mode.

Supporting Information. Details of the surface roughness of S1-S5, electrical characterizations and GIWAXS. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *[email protected] (W. P.); [email protected] (K. M.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

References (1) Smits, E. C. P.; Mathijssen, S. G. J.; van Hal, P. A.; Setayesh, S.; Geuns, T. C. T.; Mutsaers, K. A. H. A.; Cantatore, E.; Wondergem, H. J.; Werzer, O.; Resel, R.; Kemerink, M.; Kirchmeyer, S.; Muzafarov, A. M.; Ponomarenko, S. A.; de Boer, B.; Blom, P. W. M.; de Leeuw, D. M. Bottom-up organic integrated circuits. Nature 2008, 455 (7215), 956-959. (2) Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching disorder-free transport in high-mobility conjugated polymers. Nature 2014, 515 (7527), 384-388. (3) Liu, J.; Zhang, H.; Dong, H.; Meng, L.; Jiang, L.; Jiang, L.; Wang, Y.; Yu, J.; Sun, Y.; Hu, W.; Heeger, A. J. High mobility emissive organic semiconductor. Nat. Commun. 2015, 6, 10032. (4) Luo, C.; Kyaw, A. K. K.; Perez, L. A.; Patel, S.; Wang, M.; Grimm, B.; Bazan, G. C.; Kramer, E. J.; Heeger, A. J. General strategy for self-assembly of highly oriented nanocrystalline semiconducting polymers with high mobility. Nano Lett. 2014, 14 (5), 2764-2771. (5) Sirringhaus, H. Organic field-effect transistors: The path beyond amorphous silicon. Adv. Mater. 2014, 26 (9), 1319-1335. (6) Horowitz, G. Organic thin film transistors: From theory to real devices. J. Mater. Res. 2004, 19 (07), 1946-1962. (7) Shehu, A.; Quiroga, S. D.; D’Angelo, P.; Albonetti, C.; Borgatti, F.; Murgia, M.; Scorzoni, A.; Stoliar, P.; Biscarini, F. Layered distribution of charge carriers in organic thin film transistors. Phys. Rev. Lett. 2010, 104 (24), 246602. (8) Dodabalapur, A.; Torsi, L.; Katz, H. E. Organic transistors: two-dimensional transport and improved electrical characteristics. Science 1995, 268 (5208), 270-271. (9) Mannebach, E. M.; Spalenka, J. W.; Johnson, P. S.; Cai, Z.; Himpsel, F. J.; Evans, P. G. High hole mobility and thickness-dependent crystal structure in α,ω-dihexylsexithiophene singlemonolayer field-effect transistors. Adv. Funct. Mater. 2013, 23 (5), 554-564. (10) Huang, J.; Sun, J.; Katz, H. E. Monolayer-dimensional 5,5′-bis(4-hexylphenyl)-2,2′bithiophene transistors and chemically responsive heterostructures. Adv. Mater. 2008, 20 (13), 2567-2572.


18

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Dinelli, F.; Murgia, M.; Levy, P.; Cavallini, M.; Biscarini, F.; de Leeuw, D. M. Spatially correlated charge transport in organic thin film transistors. Phys. Rev. Lett. 2004, 92 (11), 116802. (12) Li, M.; An, C.; Pisula, W.; Müllen, K. Alignment of organic semiconductor microstripes by two-phase dip-coating. Small 2014, 10 (10), 1926-1931. (13) Scott, J. C.; Samuel, J. D. J.; Hou, J. H.; Rettner, C. T.; Miller, R. D. Monolayer transistor using a highly ordered conjugated polymer as the channel. Nano Lett. 2006, 6 (12), 2916-2919. (14) Fabiano, S.; Musumeci, C.; Chen, Z.; Scandurra, A.; Wang, H.; Loo, Y.-L.; Facchetti, A.; Pignataro, B. From monolayer to multilayer n-channel polymeric field-effect transistors with precise conformational order. Adv. Mater. 2012, 24 (7), 951-956. (15) Di, C.-A.; Liu, Y.; Yu, G.; Zhu, D. Interface engineering: An effective approach toward high-performance organic field-effect transistors. Acc. Chem. Res. 2009, 42 (10), 1573-1583. (16) Jung, Y.; Kline, R. J.; Fischer, D. A.; Lin, E. K.; Heeney, M.; McCulloch, I.; DeLongchamp, D. M. The effect of interfacial roughness on the thin film morphology and charge transport of high-performance polythiophenes. Adv. Funct. Mater. 2008, 18 (5), 742-750. (17) Steudel, S.; De Vusser, S.; De Jonge, S.; Janssen, D.; Verlaak, S.; Genoe, J.; Heremans, P. Influence of the dielectric roughness on the performance of pentacene transistors. Appl. Phys. Lett. 2004, 85 (19), 4400-4402. (18) Chabinyc, M. L.; Lujan, R.; Endicott, F.; Toney, M. F.; McCulloch, I.; Heeney, M. Effects of the surface roughness of plastic-compatible inorganic dielectrics on polymeric thin film transistors. Appl. Phys. Lett. 2007, 90 (23), 233508. (19) Li, M.; An, C.; Marszalek, T.; Baumgarten, M.; Müllen, K.; Pisula, W. Impact of interfacial microstructure on charge carrier transport in solution-processed conjugated polymer field-effect transistors. Adv. Mater. 2016, 28 (11), 2245-2252. (20) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. About supramolecular assemblies of π-conjugated systems. Chem. Rev. 2005, 105 (4), 1491-1546. (21) Wurthner, F. Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures. Chem. Commun. 2004, 14, 1564-1579. (22) Chen, Z.; Lohr, A.; Saha-Moller, C. R.; Wurthner, F. Self-assembled π-stacks of functional dyes in solution: structural and thermodynamic features. Chem. Soc. Rev. 2009, 38 (2), 564-584.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

(23) Seo, H.-S.; Zhang, Y.; Jang, Y.-S.; Choi, J.-H. Performance and transport characteristics of α,ω-dihexylsexithiophene- based transistors with a high room-temperature mobility of 0.16 cm2⁄Vs. Appl. Phys. Lett. 2008, 92 (22), 223310. (24) Völkel, A. R.; Street, R. A.; Knipp, D. Carrier transport and density of state distributions in pentacene transistors. Phys. Rev. B 2002, 66 (19), 195336. (25) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. Molecular engineering of organic semiconductors: design of self-assembly properties in conjugated thiophene oligomers. J. Am. Chemi. Soc. 1993, 115 (19), 8716-8721. (26) Wang, S.; Kiersnowski, A.; Pisula, W.; Müllen, K. Microstructure evolution and device performance in solution-processed polymeric field-effect transistors: The key role of the first monolayer. J. Am. Chemi. Soc. 2012, 134 (9), 4015-4018. (27) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Müllen, K. Influence of alkyl substituents on the solution- and surface-organization of hexa-peri-hexabenzocoronenes. J. Am. Chemi. Soc. 2005, 127 (12), 4286-4296. (28) Al-Mahboob, A; Fujikawa, Y; Sakurai, T; Sadowski, J. T. Real-time microscopy of reorientation driven nucleation and growth in pentacene thin films on silicon dioxide. Adv. Funct. Mater. 2013, 23, 2653-2660. (29) Frank, F. C.; van der Merwe, J. H. One-dimensional dislocations. I. Static theory. Proc. R. Soc. London 1949, A198, 205-216. (30) Yang, S. Y.; Shi, K.; Park, C. E. The effect of gate-dielectric surface energy on pentacene morphology and organic field-effect transistor characteristics. Adv. Funct. Mater. 2005, 15, 1806-1814.


20

Page 21 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic


21

Role of Sub-Nanometer Dielectric Roughness on Microstructure and

Recommend Documents