Grégory Lalet

Hiroki Kurita
email: h.kurita0319@gmail.com

Takamichi Miyazaki

Akira Kawasaki

Jean-François Silvain

Gre ´gory Lalet

Jean-Franc ¸ois

J.-F Silvain

Á A Kawasaki

Microstructure of a carbon fiber-reinforced aluminum matrix composite fabricated by spark plasma sintering in various pulse conditions

est destinée au dépôt

Introduction

The success of spark plasma sintering (SPS) is usually related to the fact that this method can provide higher heating rates, shorter holding times, and properties that are at least as good as those measured on samples generated by hot pressing [START_REF] Munir | The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method[END_REF][START_REF] Ullbrand | Thermomechanical properties of copper-carbon nanofiber composites prepared by spark plasma sintering and hot pressing[END_REF][START_REF] Daffos | Spark plasma sintered carbon electrodes for electrical double layer capacitor applications[END_REF][START_REF] Jain | Achieving uniform microstructure and superior mechanical properties in ultrafine grained TiB 2 -TiSi 2 composites using innovative multistage spark plasma sintering[END_REF][START_REF] Mizuuchi | Processing of Al/SiC composites in continuous solid-liquid coexistent state by SPS and their thermal properties[END_REF][START_REF] Mizuuchi | Processing of diamond particle dispersed aluminium matrix composites in continuous solid-liquid co-existent state by SPS and their thermal properties[END_REF][START_REF] Mizuuchi | Thermal conductivity of diamond particle dispersed aluminium matrix composites fabricated in solid-liquid co-existent state by SPS[END_REF][START_REF] Tan | Fabrication of diamond/aluminium composites by vacuum hot pressing: process optimization and thermal properties[END_REF][START_REF] Chen | Thermal properties of aluminiumgraphite composites by powder metallurgy[END_REF]. This method is very promising, especially for ceramics, due to its ability to create nanoscale size grain [START_REF] Hungrı ´a | Spark plasma sintering as a useful technique to the nanostructuration of piezo-ferroelectric materials[END_REF][START_REF] Yar | Chemically produced nanostructured ODS-lanthanum oxide-tungsten composites sintered by spark plasma[END_REF] and to obtain original properties [START_REF] Jain | Achieving uniform microstructure and superior mechanical properties in ultrafine grained TiB 2 -TiSi 2 composites using innovative multistage spark plasma sintering[END_REF][START_REF] Dumont-Botto | Synthesis by Spark Plasma Sintering: a new way to obtain electrode materials for lithium ion batteries[END_REF][START_REF] Okinaka | Thermoelectric properties of Rare Earth-doped SrTiO3 using combination of Combustion Synthesis (CS) and Spark Plasma Sintering (SPS)[END_REF]. Via application to metal-matrix composites, several recent studies have shown that the SPS method allows many properties to be improved. Kurita et al. [START_REF] Kurita | Multi-walled carbon nanotube-aluminium matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion[END_REF] produced multi-walled carbon nanotube-reinforced aluminum matrix composites with an ultimate tensile strength that was 4 times higher than that of pure aluminum (Al) with the conservation of remarkable fracture elongation of aluminum. Ullbrand et al. [START_REF] Ullbrand | Thermomechanical properties of copper-carbon nanofiber composites prepared by spark plasma sintering and hot pressing[END_REF] showed that, by means of SPS instead of hot pressing, a stronger adhesion can be obtained between matrix and reinforcement. Long et al. [START_REF] Long | Spark plasma sintering of mechanically alloyed in situ copper-niobium carbide composite[END_REF] produced composites with an electrical conductivity that is almost 4 times higher and micro hardness that is 3.5 times higher than those measured on samples generated using with a classic sintering method.

It is well known that an alumina (Al 2 O 3 ) layer, which can prevent the sintering of Al, usually covers Al particles. In order to remove this layer, magnesium may be added to the Al powder, but this addition may not be desirable if high purity is required [START_REF] Lumley | Surface oxide and the role of magnesium during the sintering of aluminium[END_REF]. Another study has shown that the SPS method is able to partially fracture the Al 2 O 3 layer, whereas classical hot pressing cannot [START_REF] Xie | Frequency effect on pulse electric current sintering process of pure aluminium powder[END_REF][START_REF] Omori | Sintering, reaction and crystal growth by the spark plasma system (SPS)[END_REF]. It can be underlined that in the case of the induction sintering method, the Al matrix has never been observed in direct contact with the carbon fiber because of a continuous layer of Al 2 O 3 at the interface. On the contrary, this Al 2 O 3 layer may be fractured if flash sintering is used. From those areas where Al 2 O 3 is fractured, a direct contact between the matrix and reinforcement is feasible. Moreover, those contact areas allow an inter-diffusion process between Al and carbon that can lead to the growth of aluminum carbide (Al 4 C 3 ). This difference leads to a local chemical link to the mechanical interface and then to a better transfer of mechanical properties in the case of flash sintering compared to the hot pressing process.

However, the fundamental mechanisms of the SPS method are still unclear and different mechanisms have been proposed, such as the formation of plasma, which leads to cleaning of the particle surfaces and their subsequent activation [START_REF] Omori | Sintering, reaction and crystal growth by the spark plasma system (SPS)[END_REF]; however, this has not yet been proven and is even contested by some authors [START_REF] Hulbert | The absence of plasma in ''spark plasma sintering[END_REF]. Alternative explanations to the plasma effect have been proposed, for example rapid Joule heating [START_REF] Anselmi-Tamburini | Fundamental investigations on the spark plasma sintering/synthesis process II. Modeling of current and temperature distributions[END_REF][START_REF] Khor | Effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of randomly packed hollow sphere (RHS) cell wall[END_REF], local melting [START_REF] Kurita | Multi-walled carbon nanotube-aluminium matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion[END_REF][START_REF] Ji | The mechanisms of microstructure formation in a nanostructured oxide dispersion strengthened FeAl alloy obtained by spark plasma sintering[END_REF][START_REF] Kwon | Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminium matrix composites[END_REF][START_REF] Kwon | Carbon nanofiber reinforced aluminium matrix composite fabricated by combined process of spark plasma sintering and hot extrusion[END_REF], or particle deformation [START_REF] Eriksson | Fast densification and deformation of titanium powder[END_REF].

In this study, we present the processing of carbon fiberreinforced aluminum matrix (Al-CF) composites using SPS with various pulse conditions. We focused on the effect of pulse conditions of SPS on the microstructure of Al-CF composites and demonstrated the control of the quantity of Al 4 C 3 at the Al/CF interface during SPS using the carefully observed transmission electron microscopy (TEM) images for Al particle boundaries and Al/CF interfaces. Also, based on local Joule heating, the mechanism of local temperature increases in the SPS process was proposed.

Materials and methods

Materials

Al powder (F3731, Poudres Hermillon, 99.97 %) was prepared as a matrix powder. Auger electron spectroscopy (AES) revealed that Al particles were covered with a thin continuous Al 2 O 3 layer of less than 40 nm as shown in Fig. 1. As a reinforced fiber, carbon fibers (CN80C, Nippon Graphite Corporation) were prepared and chopped after annealing at 400 °C for 1 h in air to remove their polymer coating. Al powder and chopped carbon fibers were mixed for 5 min under argon gas to obtain the Al ? CF 30 vol.% composite powder.

SPS conditions

Columnar Al-CF 30 vol.% composites (/15 9 10 mm 3 ) were consolidated for 10 and 40 min under vacuum, under compressive stress below 50 MPa and at a temperature of 600 °C, by SPS (Dr. Sinter LAB (SPS-515), SPS Syntex Inc.). The temperature was monitored by a K-type thermocouple in direct contact with the thin carbon foil containing the powder. The pulse conditions correspond to the time during the electrical pulses (each pulse corresponding to 3 ms) which were applied to the sample (ON time) and the time when no current was sent (OFF time). The pulses used in this study were 24:1, 12:2; 6:4, 3:3, and 4:6; the first number corresponds to the time ON and the second to the time OFF. The 12:2 pulse is the pulse condition recommended by SPS Syntex Inc.

Characterizations

The relative densities of Al-CF 30 vol.% composites were measured by the Archimedes principle. For TEM observations, Al-CF 30 vol.% composites were mechanically polished to a thickness of less than 50 lm using SiC papers and thinned down by focused ion beam (FIB) apparatus (SMI2050, Seiko Instruments). The microstructures were observed by a high-resolution transmission electrons microscope (HR-TEM, Hitachi) equipped with selectedarea diffraction (SAD) patterns.

Results and discussion

Effect of pulse conditions

Figure 2 shows the evolution of voltage, current, and temperature before reaching the dwell temperature in SPS for each pulse condition. It was noted that the momentary current intensity was high and the numbers of high current intensities were increased when the ON:OFF ratio was decreasing (the pulses of SPS were shifted from 24:1 to 4:6). This is because a very high current is applied over a short time when the ON time is short, and it becomes zero to avoid any excess temperature increase after that. Indeed, the temperature was periodically changed between 595 and 605 °C during the holding time at dwell temperature (600 °C) in a ratio of 3:3 as a result of this current cycle. The consolidation at 4:6 was mandatorily stopped due to the melting of Al and its leakage from the space between the carbon punch and the mold. Then, the temperature monitored by the K-type thermocouple was 500 °C, which Fig. 1 Atomic percent of carbon, oxygen, and aluminum elements on Al particle as a function of etching depth but there were some voids at the tip ends of carbon fibers. It has been reported that Al can instantaneously transform to liquid phase in SPS [START_REF] Kurita | Multi-walled carbon nanotube-aluminium matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion[END_REF][START_REF] Kwon | Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminium matrix composites[END_REF][START_REF] Kwon | Carbon nanofiber reinforced aluminium matrix composite fabricated by combined process of spark plasma sintering and hot extrusion[END_REF] and that the SPS method is able to partially fracture the alumina layer; this is in contrast to classical hot pressing [START_REF] Xie | Frequency effect on pulse electric current sintering process of pure aluminium powder[END_REF][START_REF] Omori | Sintering, reaction and crystal growth by the spark plasma system (SPS)[END_REF]. Therefore, it can be inferred that this tight interface was achieved by the infiltration of Al liquid through the broken Al 2 O 3 layer.

Figure 5 shows the Al particle boundaries in Al-CF 30 vol.% composites fabricated by SPS during 40 min. Failures of the Al 2 O 3 layer were observed in each pulse condition [START_REF] Lalet | Thermal expansion coefficient and thermal fatigue of discontinuous carbon fiber-reinforced copper and aluminum matrix composites without interfacial chemical bond[END_REF]. However, the failures of the Al 2 O 3 layer were limited in 24:1 (see Fig. 5a), while the Al 2 O 3 layer was fractured throughout the layer in 12:2, 6:4, and 3:3 (see Fig. 5b-d). In addition, the observation of the filling of Al in the triple point of Al particle boundaries (see Fig. 5c) clearly shows that Al can infiltrate into slight spaces through the broken Al 2 O 3 layer. These TEM micrographs support the hypothesis mentioned above.

Figure 6 shows the Al/CF interface in Al-CF 30 vol.% composites fabricated by SPS during 40 min. The void was mainly observed at the Al/CF interface in the Al-CF 30 vol.% composite fabricated in 24:1. It seems that this void was achieved due to the limited diffusion of Al, which provided weak interfacial bonding. Moreover, no chemical compound was observed at a few-contacted Al/CF interfaces due to the quasi-continuous thin Al 2 O 3 layer. Therefore, this seems to correspond to a mechanical contact rather than a chemical bonding in the Al-CF 30 vol.% The average relative densities of Al-CF 30 vol.% composites were 98.1 and 99.0 % for sintering times of 10 and 40 min, respectively, regardless of the pulse conditions. This result shows that the final densities of Al-CF 30 vol.% composites are very high for this sintering temperature (600 °C), while there is only a slight difference between sintering times, and the densification level is not dependent on the pulse conditions.

Figure 3 shows the Al/CF interfacial microstructure in Al-CF 30 vol.% composites fabricated by SPS over 10 min. In the Al-CF 30 vol.% composite fabricated in the 24:1 pulse, there was a void between the Al matrix and the carbon fiber and Al did not make contact with the surface of the carbon fiber (see Fig. 3a). Further TEM observations revealed that Al particles were still covered with a thin continuous Al 2 O 3 layer on Al particles identified before SPS (see Fig. 1) and that this continuous Al 2 O 3 layer prohibited direct contact between the Al and carbon fiber, as shown in Fig. 4. Also, the SAD pattern revealed that the structure of Al 2 O 3 layer was amorphous. This Al/CF interfacial microstructure was close to that in the Al-CF composite fabricated by the hot press method. In 12:2, 6:4, and 3:3, the tight Al/CF interface in the Al-CF 30 vol.% composite fabricated by SPS during 10 min was observed, composite fabricated in 24:1. In 12:2, 6:4, and 3:3, the needle-like crystals were observed at the directly contacted Al/CF interface and the tip end of the carbon fiber. SAD patterns revealed that these crystals were Al 4 C 3 (see Fig. 6c,d). The formation of Al 4 C 3 crystals indicates that there was interfacial chemical bonding, which can be attributed to the interface reinforcement. However, the quantity of Al 4 C 3 in the Al-CF 30 vol.% composite fabricated by SPS during 40 min in 12:2 was smaller than that in 6:4 and 3:3, and Al 4 C 3 was mainly observed at the tip end of the carbon fiber (i.e., the more reactive area than the side wall of carbon fiber), as shown in Fig. 6b. Concerning 6:4 and 3:3, the observed microstructures were similar and Al 4 C 3 crystals were easily observed at the sidewall and the tip ends of carbon fiber. Furthermore, not only were needle-like Al 4 C 3 crystals observed (see Fig. 6d), but broad Al 4 C 3 crystals (see Fig. 6c) were also seen in 6:4 and 3:3. composite, it is possible to observe the crystallization of alumina near the fractured areas, as shown in Fig. 7. Since this crystallization has never been observed using hot pressing, it cannot be related to the average holding temperature. Therefore, local heating, specific to the SPS method, has to be considered. -In 4:6 (i.e., a higher maximum current intensity than other pulse conditions), Al melted and leaked from the space between the carbon punch and mold during processing when the temperature was around 500 °C (150 °C under the melting temperature of aluminum) while the temperature was monitored by the thermocouple in direct contact with the sample, as mentioned above. This indicates that the real temperature within the sample was locally higher than the melting point of aluminum (660 °C).

-Chen et al. and Cincotti et al. [START_REF] Chen | Fundamental investigations on the spark plasma sintering/ synthesis process I. Effect of dc pulsing on reactivity[END_REF][START_REF] Cincotti | Modeling of SPS apparatus: temperature, current and strain distribution with no powders[END_REF] have reported that decreasing the ON time and increasing the OFF time increase the maximum current intensity. In this The mechanism of the local temperature increase in SPS

Although it has been proven that the SPS method leads to improved properties, the fundamentals mechanisms are not well established since the historical hypothesis concerning a spark plasma phenomenon [START_REF] Omori | Sintering, reaction and crystal growth by the spark plasma system (SPS)[END_REF][START_REF] Anselmi-Tamburini | Fundamental investigations on the spark plasma sintering/synthesis process II. Modeling of current and temperature distributions[END_REF] has never been directly proven and has also been impugned [START_REF] Hulbert | The absence of plasma in ''spark plasma sintering[END_REF]. Now, the presence of large temperature gradients has been identified in many samples, which may also result in a large increase of the temperature in some places and the formation of a local liquid phase [START_REF] Kurita | Multi-walled carbon nanotube-aluminium matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion[END_REF][START_REF] Anselmi-Tamburini | Fundamental investigations on the spark plasma sintering/synthesis process II. Modeling of current and temperature distributions[END_REF][START_REF] Khor | Effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of randomly packed hollow sphere (RHS) cell wall[END_REF][START_REF] Ji | The mechanisms of microstructure formation in a nanostructured oxide dispersion strengthened FeAl alloy obtained by spark plasma sintering[END_REF][START_REF] Kwon | Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminium matrix composites[END_REF][START_REF] Kwon | Carbon nanofiber reinforced aluminium matrix composite fabricated by combined process of spark plasma sintering and hot extrusion[END_REF].

Our experiments agree quite well with mechanisms associated with a local increase of temperature and the presence of a liquid phase. As evidence, we can put forward the following points:

-Alumina is initially amorphous at the surface of aluminum powder and remains amorphous when a hot pressing method is used. In our Al-CF 30 vol.% study, the momentary current intensity was higher and the number of high current intensities was increased in the following sequence 24:1, 12:2, 6:4, 3:3, and 4:6. Since Joule effects are directly related to intensity, it can be inferred that the local increase in temperature will be more present when pulses shift from 24:1 to 4:6, while there is no significant difference in microstructure among 12:2, 6:4, and 3:3. It is also reasonable to consider that the alumina layer can be broken because of internal stresses. If the Joule effect is strong enough, local melting of the aluminum occurs. Due to the presence of the liquid phase and the subsequent expansion, the stress on the alumina layer is increased and may result in its fracture [START_REF] Kurita | Multi-walled carbon nanotube-aluminium matrix composites prepared by combination of hetero-agglomeration method, spark plasma sintering and hot extrusion[END_REF]. This hypothesis corresponds well with the observed microstructures, where the fracture rate is increasing for 24:1, 12:2, 3:3, and 6:4, which are also the conditions where local increases in temperature are more commonly present. -Aluminum carbide is commonly observed in Al-CF 30 vol.% composites fabricated in 6:4 and 3:3 (Fig. 6c,d), whereas it is quasi-absent in the microstructures of the Al-CF 30 vol.% composites fabricated in 24:1 and 12:2 (Fig. 6a,b). The thermal energy needed for the growth of aluminum carbide can be caused by the strong local Joule effect occurring with 3:3 and 6:4. -The two different observed Al 4 C 3 microstructures displayed in Fig. 6c and d can be linked to those already observed and discussed by Yang et al. [START_REF] Yang | Interface microstructure and reaction in Gr/Al metal matrix composite[END_REF]. In their study, the samples were manufactured using a casting method and the aluminum carbide was grown as broad and separated crystals or as a continuous layer composed of slim crystals depending on the liquid phase temperature. Consequently, it can be inferred that a temperature close to 710 °C or below will enable needle-like crystals to be observed (Fig. 6d) and that a temperature around 750 °C or higher is reached in areas where broad crystals are observed (Fig. 6c).

crystals to be formed at the Al/CF interface. Furthermore, it was demonstrated that 6:4 and 3:3 provided larger quantities of Al 4 C 3 than 12:2. Based on local Joule heating, the mechanism of local temperature increases and association with the liquid phase of aluminum was proposed in the spark plasma sintering process.

Conclusions

Al-CF 30 vol.% composites were fabricated by SPS using various pulse conditions. It was revealed that the different pulse conditions resulted in the various microstructures seen in Al-CF 30 vol.% composites. Pulse conditions of 24:1 achieved a similar microstructure to that of composites fabricated by conventional hot pressing: a quasi-continuous oxide layer covering metallic grains prevents the diffusion process between matrix particles and also between the matrix and reinforcements. On the contrary, the shorter ON times and longer OFF times allowed the oxide layer to be fractured and Al 4 C 3

Fig. 2

 2 Fig. 2 The evolutions of current, voltage, and temperature before reaching dwell temperature in spark plasma sintering in a 24:1, b 12:2, c 6:4, d 3:3, and e 4:6 pulse conditions

Fig. 3 Fig. 4

 34 Fig. 3 TEM micrographs of Al/Cf interfaces in Al-CF 30 vol.% composite fabricated during 10 min in a 24:1, b 12:2, c 6:4, d 3:3, and e 4:6 pulse conditions

Fig. 5

 5 Fig. 5 TEM micrographs of Al particle boundaries in Al-CF 30 vol.% composite fabricated during 40 min in a 24:1, b 12:2, c 6:4, d 3:3, and e 4:6 pulse conditions

Fig. 6

 6 Fig. 6 TEM micrographs of Al/CF interfaces in Al-CF 30 vol.% composite fabricated during 40 min in a 24:1, b 12:2, c 6:4, d 3:3, and e 4:6 pulse conditions

Acknowledgements The authors would like to thank the French ''De ´lagation Ge ´ne ´rale pour l'Armement,'' the ''Japan Society for the Promotion of Science,'' and the ''Global COE Program ''Materials Intergration (International Center of Education and Research), Tohoku University,'' MEXT, Japan'' for financial support.


Adara Babuji

Inés Temiño

Ana Pérez-Rodríguez

Olga Solomeshch

Nir Tessler

Maria Vila

Jinghai Li

Marta Mas-Torrent

Carmen Ocal
email: cocal@icmab.es

Esther Barrena
email: ebarrena@icmab.es

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance

Keywords: OFETs, Contact doping, organic semiconductors, interfaces, C60F48, KPFM 2

come

INTRODUCTION

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of thin-film organic field-effect transistors (OFETs). The investigation of structure-property relationships has underlined the strong implications of crystallinity, morphology and the importance of the two crucial interfaces: the electrode/semiconductor interface and the dielectric/semiconductor interface. 1,[START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF][START_REF] Lüssem | Doped Organic Transistors[END_REF][START_REF] Minari | Highly Enhanced Charge Injection in Thienoacene-Based Organic Field-Effect Transistors with Chemically Doped Contact[END_REF][START_REF] Natali | Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods[END_REF][START_REF] Paterson | Recent Progress in High-Mobility Organic Transistors: A Reality Check[END_REF][START_REF] Paterson | Addition of the Lewis Acid Zn(C6F5)2 Enables Organic Transistors with a Maximum Hole Mobility in Excess of 20 cm 2 V -1 s -1[END_REF][START_REF] Xu | Doping: A Key Enabler for Organic Transistors[END_REF][START_REF] Pfattner | Tuning Crystal Ordering, Electronic Structure, and Morphology in Organic Semiconductors: Tetrathiafulvalenes as a Model Case[END_REF][START_REF] Bittle | Dependence of Electrical Performance on Structural Organization in Polymer Field Effect Transistors[END_REF] A key strategy for enhancement of device properties is contact doping, where a dopant thin layer is incorporated at the source and drain metal/semiconductor interface. [START_REF] Lüssem | Doped Organic Transistors[END_REF][START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF][START_REF] Ante | Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors[END_REF] Selective electrical doping of the organic surface in contact with the electrode leads to improved charge injection by reduction of the contact resistance, where inefficient contacts represent often a serious bottleneck in OFETs. [START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF][START_REF] Ante | Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors[END_REF][START_REF] Olthof | Ultralow Doping in Organic Semiconductors: Evidence of Trap Filling[END_REF][START_REF] Paterson | Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm 2 V -1 s -1[END_REF][START_REF] Borchert | Small Contact Resistance and High-Frequency Operation of Flexible Low-Voltage Inverted Coplanar Organic Transistors[END_REF] The structural integrity of doped films is a major concern because low molecular weight and planar dopant molecules may diffuse into the organic semiconductor (OSC) film and intercalate within the π-stacking, [START_REF] Méndez | Doping of Organic Semiconductors: Impact of Dopant Strength and Electronic Coupling[END_REF][START_REF] Méndez | Charge-Transfer Crystallites as Molecular Electrical Dopants[END_REF][START_REF] Pfattner | Organic Metal Engineering for Enhanced Field-Effect Transistor Performance[END_REF][START_REF] Opitz | Energy Level Alignment at Planar Organic Heterojunctions: Influence of Contact Doping and Molecular Orientation[END_REF] compromising the performance and lifetime of devices. [START_REF] Li | Measurement of Small Molecular Dopant F4TCNQ and C60F36 Diffusion in Organic Bilayer Architectures[END_REF][START_REF] Jacobs | Comparison of Solution-Mixed and Sequentially Processed P3HT:F4TCNQ Films: Effect of Doping-Induced Aggregation on Film Morphology[END_REF][START_REF] Bruder | A Novel P-Dopant with Low Diffusion Tendency and Its Application to Organic Light-Emitting Diodes[END_REF] The use of bulkier molecules as p-type dopants, as fluorinated fullerene derivatives can surmount such issues. [START_REF] Strobel | Surface Conductivity Induced by Fullerenes on Diamond: Passivation and Thermal Stability[END_REF][START_REF] Smets | Doping Efficiency and Energy-Level Scheme in C60F48-Doped Zinc-Tetraphenylporphyrin Films[END_REF] Along with the different strategies developed to improve device properties, there have been numerous advances in the synthesis of high mobility OSCs as it is the case of the benzothieno [3,2-b][1]benzothiophene (BTBT) derivatives, [START_REF] Ruzié | Synthesis, Chemical Stability, Packing, Cyclic Voltammetry[END_REF][START_REF] Takimiya | Organic Semiconductors Based on[END_REF][START_REF] He | Asymmetric Conjugated Molecules Based on[END_REF]28 affording OFETs with field effect mobilities higher than 1.0 cm 2 /Vs and using diverse fabrication processes. [START_REF] Yuan | Ultra-High Mobility Transparent Organic Thin Film Transistors Grown by an off-Centre Spin-Coating Method[END_REF][START_REF] Schweicher | Bulky End-Capped [1]Benzothieno[3,2b]Benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Wei | High Mobility Solution-Processed C8-BTBT Organic Thin-Film Transistors: Via UV-Ozone Interface Modification[END_REF][START_REF] Deng | Channel-Restricted Meniscus Self-Assembly for Uniformly Aligned Growth of Single-Crystal Arrays of Organic Semiconductors[END_REF][START_REF] Zhou | Inch-Scale Grain Boundary Free Organic Crystals Developed by Nucleation Seed-Controlled Shearing Method[END_REF][START_REF] Yao | Direct Photolithography on Molecular Crystals for High Performance Organic Optoelectronic Devices[END_REF] Their low HOMO energy level (EHOMO) has allowed air-stable electrical performance and alkyl side chain engineering has been widely applied to improve the materials' solubility for solution processability. [START_REF] Takimiya | Organic Semiconductors Based on[END_REF] With the synthesis of OSCs with superior intrinsic charge transport properties and deep low-lying HOMO energy level (EHOMO ≈ 5.4-5.8), [START_REF] Takimiya | Organic Semiconductors Based on[END_REF][START_REF] He | Asymmetric Conjugated Molecules Based on[END_REF][START_REF] Temiño | A Rapid, Low-Cost, and Scalable Technique for Printing State-of-the-Art Organic Field-Effect Transistors[END_REF] ensuring efficient charge carrier injection by tailoring the electrode/semiconductor interface has become a stringent need. [START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF][START_REF] Minari | Highly Enhanced Charge Injection in Thienoacene-Based Organic Field-Effect Transistors with Chemically Doped Contact[END_REF] Besides, interfacial engineering may also reduce degradation with time due to film dewetting, a phenomenon strongly affecting the long-term stability of devices based on BTBT and related organic semiconductors. [START_REF] Pérez-Rodríguez | Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties[END_REF][START_REF] Breuer | Exceptional Dewetting of Organic Semiconductor Films: The Case of Dinaphthothienothiophene (DNTT) at Dielectric Interfaces[END_REF][START_REF] Yamamoto | Experimental and Numerical Investigation of Contact Doping Effects in Dinaphthothienothiphene Thin-Film Transistors[END_REF][START_REF] Dohr | Reversibility of Temperature Driven Discrete Layer-by-Layer Formation of Dioctyl-Benzothieno-Benzothiophene Films[END_REF] Despite the importance of contact doping, very few studies have focused on specific properties of the dopant/OSC interface. Fundamental questions remain about how the physicochemical properties of the OSC surface influence the dopant/OSC interface characteristics and thereby the injection properties. Certainly, stability is a crucial aspect as well. To address these questions, we have investigated the nanoscale properties of the dopant/OSC interface and the electrical properties in the respective OFETs. As they constitute one of the most promising families of ptype OSCs used in OFETs, [START_REF] Yuan | Ultra-High Mobility Transparent Organic Thin Film Transistors Grown by an off-Centre Spin-Coating Method[END_REF][START_REF] Schweicher | Bulky End-Capped [1]Benzothieno[3,2b]Benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order[END_REF][START_REF] Wei | High Mobility Solution-Processed C8-BTBT Organic Thin-Film Transistors: Via UV-Ozone Interface Modification[END_REF][START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2-b]Benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF] we have chosen two [1]benzothieno[3,2-b]- [1]benzothiophene (BTBT) molecules, the asymmetrically (BTBT-C8) and symmetrically (C8-BTBT-C8) alkylated derivatives, i.e., with one or two octyl side groups respectively. For efficient p-type doping, a molecule with larger electron affinity than the ionization energy of the OSC is required. Hence, as the p-type dopant, we employ here a bulky isotropic and thermally stable molecule, C60F48, the fluorofullerene with maximum fluorine content. [START_REF] Zhang | Effect of Curvature on C-F Bonding in Fluorinated Carbons: From Fullerene and Derivatives to Graphite[END_REF] The choice is made because the increase of fluorine entails an increase of the electron affinity up to ~5.5 eV, making this material a very strong electron acceptor. [START_REF] Smets | Doping Efficiency and Energy-Level Scheme in C60F48-Doped Zinc-Tetraphenylporphyrin Films[END_REF][START_REF] Nell | Influence of Dopant-Host Energy Level Offset on Thermoelectric Properties of Doped Organic Semiconductors[END_REF] We demonstrate a nanostructured morphology of the C60F48/OSC hetero-interface caused by the nucleation of C60F48 crystallites at the step edges of the underlying OSC thin film. We show that, in addition to the beneficial effect of the dopant in reducing the contact resistance of the BTBT-based OFETs, the specific morphology of the C60F48/OSC interface causes an outstanding dewetting inhibition and, therefore, improvement of the stability of the films and the respective devices.

EXPERIMENTAL SECTION

BTBT-C8 and C8-BTBT-C8 were purchased from Sigma-Aldrich and used without further purification except in-situ degasification in vacuum at 120 o C for 15 min. Fluorinated fullerene C60F48 was synthesized as per the method developed at the Josef Stefan Institute (Slovenia). [START_REF] Kepman | Novel Method of Synthesis of C60F48 with Improved Yield and Selectivity[END_REF] The product was characterized by chemical analysis, electron-ionization mass spectrometry and IR-spectroscopy. The estimated purity was 95%. Thin films of few monolayers (ML) of one or the other BTBT molecules were grown on native oxide silicon surfaces via physical vapour deposition under an ultra-high-vacuum (UHV) of ~10 -8 mbar at room temperature (RT) at a rate of 5-9 Å/min as determined from a quartz crystal microbalance (QCM). The native oxide covered Si (100) substrates were sonicated, in acetone and ethanol for 10 min, and then annealed up to ~300 o C in UHV just prior to their use. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) measurements were carried out under ambient conditions using a commercial head and control unit from Nanotec Electrónica S.L. KPFM was measured in the frequency modulation mode (FM-KPFM), in which the tip is excited by an ac voltage (∼0.5 V) at a frequency (fAC ∼0.7 kHz) while a feedback loop adjusts the dc bias needed to nullify the frequency shift (Δf) at fAC. In our setup, the voltage is applied to the tip so that the higher the surface potential the lower the local effective work function. The surface potential maps are obtained simultaneously with the topography in a single pass and confirmed in the lift mode (tens of nm), ensuring no crosstalk between signals. CrPt-coated Si tips mounted in cantilevers with nominal k = 3 Nm -1 from Budget Sensors were used for dynamic measurements while Silicon Nitride tips mounted in soft cantilevers (k ≈ 0.01-0.5 Nm -1 , Veeco) for contact mode (normal and lateral force) AFM. Data were analyzed by using the WSxM freeware. [START_REF] Horcas | A Software for Scanning Probe Microscopy and a Tool for Nanotechnology[END_REF] The contact resistance was measured by KPFM in operando devices from the potential drop at the channel/contact edges. The ohm's law gives the contact resistance at the electrode by dividing the voltage drop at the electrodes by the current flowing through the channel simultaneously collected using Keithley 2450.

Out-of-plane X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXD) were carried out on the six-circle diffractometer at the SpLine beamline [START_REF] Castro | Optical Design of the General-Purpose Spanish X-Ray Beamline for Absorption and Diffraction[END_REF][START_REF] Rubio-Zuazo | The Multipurpose X-Ray Diffraction End-Station of the BM25B-SpLine Synchrotron Beamline at the ESRF[END_REF] (BM25B at ESRF, Grenoble, France) with λ=1.2 Å. XRD measurements were performed on in-situ grown films before and after evaporating C60F48 on their top. The experiments were performed in a multipurpose portable UHV-compatible chamber mounted in a vertical geometry on the sixcircle multipurpose diffractometer. [START_REF] Ferrer | Multi-Use High/Low-Temperature and Pressure Compatible Portable Chamber for in Situ Grazing-Incidence X-Ray Scattering Studies[END_REF] The specular diffraction data presented in Figure 2a were measured in-situ for C8-BTBT-C8 while those in Figure 2b correspond to two different samples for BTBT-C8.

OFETs fabrication: Films of symmetric C8-BTBT-C8 (~25 nm) were grown as mentioned above on Si (100) substrates with 200 nm-thick silicon oxide. Top electrodes were fabricated by thermal evaporation of 30 nm gold through a mask (OSSILA) with 6 different channel lengths (L= 30, 40, 50, 60, 80 and 100 µm) and fixed channel width (W = 4 mm). Devices with C60F48 under the gold contacts (contact doping) were fabricated by patterning with the same shadow mask. In addition, devices with C60F48 deposited over the entire OSC surface (including the channel) were also investigated. The electrical characterization of OFETs was performed in ambient conditions using an Agilent B1500A semiconductor device analyzer connected to the samples with a Karl Suss probe station. The field effect mobility (µ), Von, Ion/Ioff ratios were calculated from the transfer curves in the saturation regime.

RESULTS AND DISCUSSION

We first present a study of the morphology and structure of pristine films of BTBT-C8 and C8-BTBT-C8 on native oxide/Si substrates. As derived from AFM and XRD the two molecules form high quality crystalline multilayers (Figure 1 and Figure 2). In the case of the symmetric C8-BTBT-C8 molecule, the heights measured by AFM for individual layers are multiples of 3 nm (Figure 1c and1e) that coincides with the length of a single molecule and the reported standing up orientation on SiO2 substrates. [START_REF] Gbabode | X-Ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1] Benzothieno [3,2-b] Benzothiophene Derivatives in Bulk and Thin Films[END_REF][START_REF] Grigoriadis | Viscoelastic, and Dielectric Properties of Symmetric and Asymmetric Alkyl[END_REF] In full agreement with the AFM data, an interlayer spacing of 2.91 nm is obtained from the Bragg peaks observed in out-of-plane XRD data (Figure 2a). We note that (00n) Bragg peaks up to the seventh order of diffraction are visible, which is the manifestation of a (001)-oriented film with a well-ordered layered structure.

The observed Kiessig fringes and Laue oscillations arise from the smoothness and coherent order of the film (Figure S1 in Supporting Information). On the other hand, the thin film topography of the asymmetric BTBT-C8 consists of terraces separated by steps of heights multiples of 4 nm in most cases (Figure 1d and1f). As the length of BTBT-C8 is ~2 nm, [START_REF] Gbabode | X-Ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1] Benzothieno [3,2-b] Benzothiophene Derivatives in Bulk and Thin Films[END_REF][START_REF] Grigoriadis | Viscoelastic, and Dielectric Properties of Symmetric and Asymmetric Alkyl[END_REF] the AFM data point to a bilayer (two molecules thick) stacking, in full agreement with the reported bulk crystal structure consisting of stacked lamellae, each formed by two head-to-head (or tail-to-tail) molecules. However, single molecule steps of 2nm height were also found (Figure 1f). Out-of-plane XRD data in Figure 2b show the (001) Bragg peak (at qz= 1.47 nm -1 ) and higher order diffraction peaks which correspond to a multilayer structure with an interlayer spacing of ~4.12 nm. However, based on the reported BTBT-C8 crystal structure by Gbabode et al., [START_REF] Gbabode | X-Ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1] Benzothieno [3,2-b] Benzothiophene Derivatives in Bulk and Thin Films[END_REF] the relative intensity of the Bragg peaks is not the expected. Specifically, the intensity ratio measured for (001): (002) = 20:27 differs dramatically from the 20:1 expected value. The anomalous high intensity of the (002) Bragg peak can be explained by the presence of a crystalline structure with single layer periodicity in coexistence with the reported bilayer structure, a scenario that agrees with the AFM observations. It is plausible to suggest that this single layer stacking is a metastable structure formed in films grown at RT. The preferred orientation (00l) along the surface normal results from the upright orientation of the molecules. Whereas it is indisputable that C8-BTBT-C8 thin films expose an alkylterminated surface, it is not evident for BTBT-C8 due to the asymmetric structure of the molecule. We elucidate this issue employing friction force microscopy (FFM), a valuable tool to identify materials with a different chemical nature at their surface. [START_REF] Pérez-Rodríguez | Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties[END_REF] Combined topographic AFM and FFM imaging (Figure S2 in Supporting Information) shows no significant friction differences between single-layer and bilayer areas, indicating similar alkyl-termination. For a bilayer BTBT-C8 stacking, an alkyl-terminated surface can only arise from a head-to-head assembly. This conjecture is supported by the identical growth behaviour of C60F48 on C8-BTBT-C8 and BTBT-C8 films described next. The structure of C8-BTBT-C8 and BTBT-C8 films upon deposition of C60F48 (≈15 nm) has been characterized by out-of-plane XRD (Figure 2). Importantly, the respective (00n) reflections characteristic of C8-BTBT-C8 and BTBT-C8 remain unchanged, demonstrating that the dopant does not alter the structure of the underlying OSC films. The two new Bragg peaks emerging at qz= 6.27 nm -1 and qz= 12.52 nm -1 are ascribed to C60F48 in agreement with a closed packed fcc structure with the (111) crystal plane parallel to the substrate. The peak position at qz= 6.27 nm -1 corresponds to an inter-plane spacing of d111 = 1.00 nm. We note that the narrow C60F48 Bragg the full width half maximum (FWHM) of the (222) peak. To study the interface morphology for dopant thicknesses (few nanometers) relevant for contact doping, [START_REF] Ante | Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors[END_REF][START_REF] Günther | Contact Doping for Vertical Organic Field-Effect Transistors[END_REF] we investigate by AFM the first stages of the C60F48 growth on BTBT-C8 and C8-BTBT-C8. Figure 3 shows representative topographic and lateral force FFM images obtained for a nominal C60F48 coverage of ~2 nm. The growth mode is similar in both films: C60F48 nucleates forming islands, most of them decorating the BTBT step edges. These assemblies are clearly distinguished from the surrounding OSC surface by their lighter colour (i.e., higher friction) in the lateral force images. For the deposited C60F48 coverage, the island heights range between 5 and 12 nm (Figure 3c), i.e., corresponding to 5-12 individual C60F48 layers (~1 nm). The formation of extremely flat and hexagonal assemblies supports the crystalline fcc structure derived from XRD data. The origin of such equivalent results should be found in the same type of termination exhibited for both C8-BTBT-C8 and BTBT-C8 films. The low interaction of C60F48 with the alkyl-terminated surface gives rise to island growth (Volmer-Weber mode). From statistical AFM images analysis, we can confirm that the C60F48 islands decorating the steps sit on the lower terraces probably due to the interaction of C60F48 molecules with the aromatic core of BTBT molecules at the edges (Figure S3 in Supporting Information).

On the (001)-oriented BTBT films, C60F48 is separated from the conjugated BTBT core by the alkyl chains exposed at the surface. However, though this implies no π-orbital overlap between the dopant and the π-conjugated core, charge transfer between the HOMO of OSC molecules and the LUMO of C60F48 may occur across the alkyl chains by electron tunnelling. The electronic surface properties following the incorporation of the p-type dopant are revealed by KPFM, which provides a quantitative determination of the work function with nanoscale spatial resolution. The KPFM maps indicate a lower (higher) surface potential (work function) on the regions covered by C60F48 for both C8-BTBT-C8 and BTBT-C8 films (Figure S4 in Supporting Information).

Figure 4 displays typical topographic and KPFM images of a C8-BTBT-C8 film upon deposition of 2 nm of C60F48. To avoid possible charge transfer between the Si substrate and the acceptor molecules through the native SiO2, this particular heterostructure was grown on a 200 nm thick SiO2/Si substrate, the same used for the OFETs fabrication. The surface potential map entirely replicates the surface nanomorphology and reveals an important decrease of 0.5 ± 0.1 V on the areas covered by the C60F48 crystallites as can be seen in the surface potential (SP) and height profiles shown in Figure 4c-4d. In general, a critical aspect affecting the long-term stability of devices is the tendency of OSC films to dewet from SiO2. [START_REF] Breuer | Exceptional Dewetting of Organic Semiconductor Films: The Case of Dinaphthothienothiophene (DNTT) at Dielectric Interfaces[END_REF] , [START_REF] Yamamoto | Experimental and Numerical Investigation of Contact Doping Effects in Dinaphthothienothiphene Thin-Film Transistors[END_REF] In a previous work, we showed that C8-BTBT-C8 thin films suffer from a strong dewetting with time provoking notable degradation of the fabricated OFETs. [START_REF] Pérez-Rodríguez | Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties[END_REF] Figures 5a and5d show the topographic images of an as-grown BTBT-C8 film and the same sample measured after two years, respectively. A strong dewetting of BTBT-C8 films is observed. The initial laterally continuous films evolved to high islands that leave uncovered large substrate regions. Outstandingly, film dewetting is frustrated by the passivating influence of C60F48 revealed by the similar morphology observed many months after preparation of the C60F48-covered samples. The comparison between the topographic images of the C60F48/C8-BTBT-C8 and C60F48/BTBT-C8 films freshly prepared (Figures 5b and5c) and those measured more than one year later (Figure 5e and5f) illustrates the morphological stability of the dopant/OSC interface. In general, dewetting is more pronounced for thinner films but also occurs in thicker ones (see Figure S5 in Supporting Information). Since thin film dewetting is expected to start by a mass retraction from steps edges, where the molecules have less binding energy, the above results point to step pinning due to C60F48 nucleation and are a clear proof of dewetting inhibition leading to increased stability of the OSC films over time. This revealing fact implies that the deposition of C60F48 can be used as a strategy to stabilize the morphology of thin layers, having the effect of a capping protecting layer. The same mechanism is responsible for the enhanced thermal stability of the films, shown next for BTBT-C8 (see results for C8-BTBT-C8 in Figure S6 of the Supporting Information). As can be seen in the XRD data of Figure 6, annealing a pristine BTBT-C8 film at 80 ºC leads to film desorption (Figure 6a). In contrast, when a film has the dopant overlayer, the ordered lamellae structure stands even up to an annealing temperature of 100 °C (Figure 6b). The Bragg peaks position remains unaffected, ruling out intermixing of C60F48 and BTBT molecules. Note, however, the strong intensity reduction of the (002) reflection upon annealing. As commented above, this peak has the contribution of a single layer-structure. Therefore, the decrease of the (002) intensity can be explained by the disappearance at ≤100 °C of the single-layer phase formed at RT, indicating it is a structure which is less stable than the lamellae. The above XRD results support the beneficial effect of C60F48 in preserving the structure of OSC films by supplying them with a certain degree of thermal robustness. As we have already demonstrated for the stability of the films with time, the preservation effect is attributed to the particular morphology of the C60F48/OSC heterointerface, with closed-packed crystallites anchored at the step edges. It is therefore plausible to think that these crystallites act as pinning centres, hampering sublimation and migration of the less bound edge OSC molecules when the temperature rises and, therefore, causing higher thermal stability of the films.

As reported in previous studies, [START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF][START_REF] Minari | Highly Enhanced Charge Injection in Thienoacene-Based Organic Field-Effect Transistors with Chemically Doped Contact[END_REF][START_REF] Ante | Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors[END_REF][START_REF] Opitz | Energy Level Alignment at Planar Organic Heterojunctions: Influence of Contact Doping and Molecular Orientation[END_REF][START_REF] Pérez-Rodríguez | Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties[END_REF][START_REF] Günther | Contact Doping for Vertical Organic Field-Effect Transistors[END_REF][START_REF] Hou | Reduced Contact Resistance in Top-Contact Organic Field-Effect Transistors by Interface Contact Doping[END_REF] the contact resistance of OFETs can be reduced by contact doping. The significant increase of the local work function measured by KFPM on the areas covered by C60F48 (Figure 4) indicates the formation of an interface dipole. However, the observed discontinuity of the C60F48 layer is certainly not the optimal nanoscale morphology to improve charge injection. Within this context, we have evaluated the effect of contact doping for C8-BTBT-C8 OFETs employing an inverted staggered (bottom-gate, top-contact) device architecture with varying channel length between 30 μm and 100 μm and two dopant coverages, namely 0.5 nm and 1 nm. Using a shadow mask, first the C60F48 dopant and then the metallic Au drain and source contacts were deposited on top of 25 nm-thick films of C8-BTBT-C8. As shown in Figure 7a, the channel topography consists of flat C8-BTBT-C8 terraces, similar to that of Figure 1a. The corresponding transfer curve for 1 nm of C60F48 is plotted in Figure 7b for a channel length of 40 μm along with that of the undoped reference device (orange line). Although the devices still present a non-ideal behaviour, the beneficial effect of contact doping is manifested through an increase of the current by one order of magnitude and a positive shift of the turn-on voltage (Von). The Von, as well as the Ion and saturation mobility, are found to increase with the amount of dopant (see Figure S7 in the Supporting Information). We suggest that these observations are due to reduced contact resistance by contact doping. To extract the contact resistances, the potential drop across the electrode/channel was measured by KPFM during the linear operation of the devices (Figure S8a in Supporting Information). The contact resistance decreases with contact doping at both electrodes, from 17.68 kΩ•cm to 12.24 kΩ•cm and from 8.88 kΩ•cm to 4.24 kΩ•cm at source and drain, respectively. For this particular device, the field effect mobility extracted from the current-voltage characteristics in the saturation regime increased from 1.03 cm 2 /Vs (in control device) to 3.34 cm 2 /Vs, being 5.00 cm 2 /Vs, the best value measured for contact-doped devices (Figure S7a in Supporting Information). The reduced contact resistance may reflect combined effects that improve charge injection at the contact/semiconductor interface and transport from the contacts to the channel. [START_REF] Minari | Charge Injection Process in Organic Field-Effect Transistors[END_REF] Among the invoked mechanisms for reducing contact resistance are decreasing the depletion-layer thickness, filling traps states at the organic semiconductor near the electrode interface or tuning the work function, all related to electron charge transfer at the dopant/OSC interface. [START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF][START_REF] Waldrip | Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier[END_REF][START_REF] Liu | Critical Impact of Gate Dielectric Interfaces on the Contact Resistance of High-Performance Organic Field-Effect Transistors[END_REF][START_REF] Zhao | Remote Doping of a Pentacene Transistor: Control of Charge Transfer by Molecular-Level Engineering[END_REF] To benefit from the stabilizing effect that C60F48 has on the organic film, we have also evaluated the case of its deposition in the absence of a mask, so that there are dopant molecules at the OSC in the channel region as well. Figure 7d shows the data for 1 nm of C60F48. The topographic image obtained within the channel displays the discontinuous dopant layer with the characteristic step decoration of C8-BTBT-C8 described above (Figures 3, 4 and5). The electrical characteristics of the devices exhibit the same behaviour as for only contact-doping: a positive shift of Von, accompanied by an increase of Ion and field effect mobility. Figure 7e shows the transfer curve for the L = 40µm OFETs. As in the case of contact doping, the increase in field effect mobility is attributed to a decrease of source and drain contact resistances, as supported by KPFM (Table S1 in Supporting Information). An important result is that adding C60F48 over the entire BTBT film does not cause any significant change in the Ion/Ioff current ratio (≈ 10 6 ), demonstrating that the channel conductivity is not affected. In other words, charge systems. [START_REF] Ante | Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors[END_REF][START_REF] Zhao | Remote Doping of a Pentacene Transistor: Control of Charge Transfer by Molecular-Level Engineering[END_REF] In Figures S8b andS8c we present a summary of the changes in Von and µ obtained for the two types of devices, i.e., with dopant deposited: a) only under the contacts (using a mask) and b) under the contacts and over the channel (without mask), respectively. The C8-BTBT-C8 OFETs were characterized after one year of storage in air. We observed still a good performance of the devices having a comparable µ than the freshly prepared devices. The transfer characteristics for fresh and aged devices are shown in Figure 7c and 7f (output characteristics in Figure S9 in Supporting Information). Given that the device of Figure 7a-b was damaged during manipulation, results for OFETs with a different channel length (L = 80 µm) are shown. Despite the good performance of the aged devices, some changes are noticed. The most significant is the large shift to -30 V observed in Von for contact-doped devices. This shift is contrary to what would be expected if the channel became hole-doped by C60F48 but is otherwise indicative of immobile trap holes at the organic/dielectric interface. In contrast, we do not observe any significant shift in Von when there is dopant not only under the contacts but also at the channel region. Although the underlying reasons are unclear, we tentatively ascribe stability of Von over time to diverse effects. First, the long-lasting morphology of the continuous and compact OSC film that protects the OSC/dielectric interface. On the other hand, the hydrophobic character of fluorinated fullerenes (due to the high fluorine content) [START_REF] Kniaź | Fluorinated Fullerenes: Synthesis, Structure, and Properties[END_REF][START_REF] Feng | Two-Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications[END_REF] and the nucleation of close packed C60F48 islands the at step edges of the OSC film can act as kinetic barrier preventing interlayer and/or intergrain (through grain boundaries or structural defects) water penetration. [START_REF] Lee | Toward Environmentally Robust Organic Electronics: Approaches and Applications[END_REF] It is remarkable that the off current in the transistors is not affected by the passage of time, indicating that the conductive channel remains undoped, a proof of the demonstrated long-term stability.

Provided deposition without shadow mask simplifies processing and, in addition, has the double beneficial role of improving charge injection and stability of the BTBT devices, we believe the procedure will reduce efforts in device doping manufacture. This work is considered as a proof of concept study and superior device performance is expected after optimization via pretreatment of the dielectric surface and a systematic refinement of the dopant nominal thickness.

Conclusions

The interfaces between two BTBT derivatives, with one or two octyl side chains, and a fluorinated fullerene p-type dopant have been studied. Our results reveal that the asymmetric BTBT-C8 molecule adopts a packing sequence that defines an alkyl-terminated film surface as that of C8-BTBT-C8 films. In both cases, we find identical growth behaviour of C60F48 on their top, leading to the following key findings. The deposition of C60F48 molecules on the organic semiconductors surface at RT results in the formation of close-packed and oriented crystallites decorating the step edges of the film surface, leading to a nanostructured interface. The growth of C60F48 is likely determined by low interaction with the alkyl-terminated film surface and the cohesion energy between fullerene molecules. There is no incorporation by diffusion of C60F48 molecules into the underlying films that otherwise preserve their crystallinity, even after annealing. The particular nanostructuration of the interface has a beneficial effect on the longterm and thermal stability of the films. Steps pinning due to the nucleation of C60F48 inhibits upward mass transport and herewith undesired film dewetting and, in addition, it also hinders sublimation/detachment of less bounded molecules providing the films with increased thermal robustness, an endurance effect similar to film encapsulation.

The electrostatic landscape replicates the surface nanomorphology, with an important local increase of work function on the areas covered by C60F48 crystallites due to the formation of interfacial dipoles. Despite the discontinuity of the C60F48 overlayer, charge injection in OFETs is improved by reduction of the contact resistance, which is reflected in an increase of the effective field effect mobility and a lower Von (respect to control devices without C60F48). Thus, in inverted staggered devices, C60F48 can be deposited over the whole OSC film surface having the double role of improving the injection (contact doping) and the long-term time stability of the devices (channel endurance). The electrical stability of the OFETs in air was confirmed after one year.

For a given dopant/OSC system, the detailed morphology of their interface depends on the structural quality of the OSC layer as well as on its chemical termination and the interaction with the dopant molecules. By providing quantitative data correlating nanoscale properties and device characteristics, this work highlights the effect of the dopant/semiconductor interface on the assembly efficiency and stability and permits foreseeing that further improvement of device performances is conceivable by means of a deeper understanding of the interfacial properties.

ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge. X-ray diffraction patterns, AFM topographic images and profiles, lateral force images, surface potential images of C60F48 doped C8-BTBT-C8 and BTBT-C8 thin films; Transfer and output characteristics of the devices; OFET parameters obtained for the studied devices; KPFM map during OFETs operation as used for contact resistance measurements. Inset: The C 60 F 48 peak at q z =12.52 nm -1 decreases with temperature while the characteristic Bragg peak of C8-BTBT-C8 at q z =12.95 nm -1 remains unaltered.

Figure 1 .

 1 Figure 1. Scheme of the molecules, topographic AFM images and height profiles along with the

Figure 2 .

 2 Figure 2. Out-of-plane XRD for (a) C8-BTBT-C8 film (48nm) and (b) BTBT-C8 film (22nm)

Figure 3 .

 3 Figure 3. Topography and lateral force maps of C60F48 (~2 nm) deposited on top of (a, b) C8-

Figure 4 .

 4 Figure 4. (a) Topography and (b) simultaneously measured KPFM surface potential (SP) map

Figure 5 .

 5 Figure 5. Top: AFM topography of fresh samples of (a) pristine BTBT-C8 (16 nm), (b) C60F48 (2

Figure 6 .

 6 Figure 6. Out-of-plane XRD of (a) BTBT-C8 (22 nm) and (b) C60F48(15 nm)/ BTBT-C8 (22 nm). The black curves: as-deposited films. Blue curves: after annealing the films at 80 o C in (a) and 100 o C in (b).

Figure 7 .

 7 Figure 7. Schematics, AFM topography and transfer curves for devices consisting of C8-BTBT-

Figure S3 .

 S3 Figure S3. (a) Height profile along the line marked in the topographic image. (b) Topography of a thin film of C8-BTBT-C8 covered by a sub-monolayer of C 60 F 48 . The substrate is 200 nm thick SiO 2 on Si. . The contrast in the excitation frequency map (c) evidences the two different materials (C 60 F 48 and C8-BTBT-C8) and permits identifying the location of the dopant crystallites.

Figure S4 .

 S4 Figure S4. Topography (a, c) and surface potential maps (b, d) measured by KPFM for C 60 F 48 deposited on C8-BTBT-C8 (left) and BTBT-C8 (right) on Si.

Figure S5 .

 S5 Figure S5. Dewetting of a bare 30 nm-thick C8-BTBT-C8 film followed over 15 weeks. Though in general dewetting is more pronounced for thinner films, this set of images shows an incipient dewetting occurring in some local spots.

Figure S6 .

 S6 Figure S6. C 60 F 48 /C8-BTBT-C8 film annealed at different temperatures (from 20 °C to 120 °C).Inset: The C 60 F 48 peak at q z =12.52 nm -1 decreases with temperature while the characteristic Bragg peak of C8-BTBT-C8 at q z =12.95 nm -1 remains unaltered.

Figure S7 .

 S7 Figure S7. Saturation field effect mobility (a), Ion (b) and Von (c) for C8-BTBT-C8 (25 nm) OFETs with two different coverages of C 60 F 48 contact doping. Values measured for different channel lengths, from L = 30 µm to 100 µm, are shown. Orange squares, light green circles and dark green triangles correspond to bare C8-BTBT-C8, with 0.5 nm C 60 F 48 and 1nm C 60 F 48 , respectively.

Figure S8 .

 S8 Figure S8. (a) Line profile from the surface potential map measured by KPFM for a 40µm channel OFET of C8-BTBT-C8 with C 60 F 48 contact doping (device in Figure 7 in the manuscript), operated at V ds = -5V and V gs = -40V. The voltage drop at the source (ΔV s ) and drain (ΔV d ) contacts are measured from the surface potential profile. The source-drain current (I ds ) is simultaneously measured. The ratio between the V drop at each electrode and I ds gives the respective contact resistance (R c (s,d) = ΔV sd /I ds ). (b) The average shift in V on and (c) percentage increase in mobility with C 60 F 48 (1 nm) under the contacts (dark green) and deposited over the entire surface (light green).(d) Scheme of evaporation of C 60 F 48, with and without mask for dopant, under the contacts and deposited over the entire surface respectively.

framework of the doctoral program (PhD) of Material Science (Department of Physics) of the Universitat Autònoma de Barcelona (UAB). O.S. acknowledges the support of the Israel Ministry of Science, the Center for Absorption in Science of the Ministry of Immigrant Absorption, the Committee for Planning and budgeting of the Council for Higher Education under the framework of the KAMEA Program. J. Li. acknowledges the support of the Chinese Council for PhD fellowship. We acknowledge The European Synchrotron Radiation Facility (ESRF) and CSIC for the provision of measurements using the BM25-SpLine beamline.

AUTHOR INFORMATION

Corresponding Author

* cocal@icmab.es, * ebarrena@icmab.es Present Addresses † International Iberian Nanotechnology Laboratory (INL), 4715-330 Braga, Portugal. Current email: ana.perez@inl.int † † María Vila: IMDEA Materials Institute, 28906, Getafe, Madrid, Spain. Current email: maria.vila@imdea.org

Author Contributions

The manuscript was written through the contributions of all authors. All authors have given Adara Babuji, 1 Inés Temiño, 1 Ana. Pérez-Rodríguez, 1 † Olga. Solomeshch, [START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF] Nir Tessler, [START_REF] Liu | Contact Engineering in Organic Field-Effect Transistors[END_REF] Maria Vila, 3 † † Jinghai Li, 1 Marta Mas-Torrent, 1,[START_REF] Minari | Highly Enhanced Charge Injection in Thienoacene-Based Organic Field-Effect Transistors with Chemically Doped Contact[END_REF] Carmen Ocal 1* and Esther Barrena 1*