Threshold Voltage Control in Organic Field-Effect Transistors by

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Threshold Voltage Control in Organic Field-Effect Transistors by Surface Doping with a Fluorinated Alkylsilane Jakob Zessin, Zheng Xu, Nara Shin, Mike Hambsch, and Stefan C. B. Mannsfeld*

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Center for Advancing Electronics Dresden (cfaed) and Faculty of Electrical and Computer Engineering, Technische Universität Dresden, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: Doping is a powerful tool to control the majority charge carrier density in organic field-effect transistors and the threshold voltage of these devices. Here, a surface doping approach is shown, where the dopant is deposited on the prefabricated polycrystalline semiconducting layer. In this study, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS), a fluorinated alkylsilane is used as a dopant, which is solution processable and much cheaper than conventional p-type dopants, such as 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ). In this work, the depositions from the gas phase and from solution are compared. Both deposition approaches led to an increased conductivity and to a shift in the threshold voltage to more positive values, both of which indicate a p-type doping effect. The magnitude of the threshold voltage shift could be controlled by the FTCS deposition time (from vapor) or FTCS concentration (from solution); for short deposition times and low concentrations, the off current stayed constant and the mobility decreased only slightly. In the low doping concentration regime, both approaches resulted in similar transistor characteristics, i.e., similar values of shift in the threshold and turn-on voltage as well as mobility, ION/IOFF ratio and amount of introduced free charge carriers. In comparison with vapor deposition, the solution-based approach can be conducted with less material and in a shorter time, which is critical for industrial applications. KEYWORDS: organic field-effect transistors, p-type doping, surface doping, fluorinated alkylsilanes, self-assembled monolayers

1. INTRODUCTION Doping of semiconductors in organic field-effect transistors (OFET) is a key technology to tune and control device parameters, and thus an important ingredient for implementing these devices in circuits. Particularly, the threshold and turn-on voltage can be modulated by doping, but improved mobility1−3 and a reduction in gate bias stress4 have also been reported. For organic semiconductors, however, doping efficiency is relatively low compared to inorganics and so fairly high concentrations of dopants are necessary, often reaching a few mole percent.5,6 Introducing dopants in such high concentrations usually leads to an increased off current and a possibly decreased mobility due to structural and electronic disorder in the semiconducting film by the dopant molecules. Thus, doping of OFETs is so far mainly realized as contact doping to lower contact resistance and improve charge carrier injection,7,8 as bulk doping with low or ultralow doping concentrations,2,9−11 as well-defined doping layers in the active channel region,12 or as interfacial doping at the interface between the gate dielectric13,14 and semiconductor film.15 A promising and widely studied approach of interface channel doping is the application of self-assembled monolayers (SAMs) to the gate dielectric surface. The improved device performance is mainly attributed to an enhanced growth mode of organic semiconductors by a lower surface energy and the © XXXX American Chemical Society

passivation of trap states on the oxide−semiconductor interface.16−18 The SAM molecules usually consist of an alkyl tail and a head group to anchor to the oxide surface. The most common anchoring groups are silanes (e.g., octadecyltrichlorosilane) and phosphonic acids (e.g., octadecylphosphonic acid) to bind on silicon oxide and aluminum oxide surfaces, respectively. Hence, the anchoring group is attached to the oxide dielectric, whereas the organic tail is oriented to the semiconductor. However, doping effects induced by interfacial SAMs were also observed and found to vary in strength and type when SAM molecules with modified alkyl chains, such as fluorinated (p-type doping) and amino-terminated (n-type doping) alkyl chains, were applied.19−21 Although the SAM interfacial doping approach is well studied, it has some inherent drawbacks, such as the maximum shift in threshold voltage is limited by the type of the SAM used and the density of the SAM, which cannot easily be increased over a monolayer when grown on the dielectric surface. A modulation of the threshold voltage by a partial SAM coverage of the gate dielectric is only possible when different SAM molecules could be mixed because an Received: July 21, 2018 Accepted: December 14, 2018

A

DOI: 10.1021/acsami.8b12346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Molecular structure of FTCS, (b) experimental setup and the standard device architecture for the vapor deposition of FTCS, and (c) experimental setup and standard device architecture for solution deposition of FTCS; pristine devices contain no FTCS layer.

conjunction with their orientation, with the silane group anchoring to oxygen defect sites on the organic semiconductor. This was speculated to lead to a charge transfer between the SAM and the organic single crystal. The method was further applied to polymer films30 and fibers,31 carbon nanotube meshes,32 graphene sheets, and highly ordered pyrolytic graphite.33 The formation of a SAM layer was not observed for all systems, but a stable doping effect was still achieved.30,31 The most effective and most often used dopant molecule was FTCS in all the mentioned studies. These approaches can be also classified as surface or sequential doping, where the doping layer is deposited on the prefabricated semiconductor. The term surface doping itself does not imply that all the dopant molecules remain at the surface of the semiconductor after deposition. The advantage of surface doping, in contrast to bulk doping, is that the structure of the organic semiconductor remains undisturbed, which is beneficial for the characteristics of the doped layer. It was shown that surface-doped polymers result in a higher conductivity than bulk-doped films.34,35 In top- and bottomgate device architectures, the application of surface dopants led also to improved device characteristics by trap passivation. 3,36,37 However, in most of these surface-doping approaches, the dopant is deposited from the gas phase and only few works showed solution-based surface-doping approaches.34,36 In this work, we demonstrate how a molecular surface treatment of the semiconductor can be utilized to control the threshold voltage in conventional small-molecule OFETs from both gas and solution phases. The dopant is a potential SAM molecule and can be even applied in cases when another SAMmolecule functionalization is used prior to the deposition of the semiconductor to modulate the latter’s growth on the gate dielectric surface. Specifically, we show the p-type doping approach by a surface treatment of the semiconductor with FTCS, in bottom-gate, top-contact devices that use silicon dioxide as gate dielectric. Vacuum-deposited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), an air stable derivate of pentacene, was used as an organic semiconductor. FTCS was deposited on top of the semi-

incomplete monolayer would create a high electrical disorder and thus a reduced mobility.22 If complete layers of a single type of SAM molecule are formed on the dielectric, the threshold and turn-on voltages depend rather discretely on the specific SAM molecule.20,23 Even though there are indications of a connection between dipole moment and shift in the threshold voltage, there is no evidence for a dependable correlation between the two and a desired shift in the device characteristics of a given semiconductor would have to be generated by a trial-and-error approach using many different SAM molecules.24,25 For phosphonic acids, it has been shown that this problem can be partially mitigated by mixing alkyland fluorinated SAMs, and the threshold voltage can be controlled on AlOx dielectrics. However, this technology has not yet been demonstrated for other types of SAM/dielectrics combinations.26,27 Furthermore, the dielectric surface treatment with the SAM molecules, which creates the desired threshold voltage shift, does not necessarily result in the best mobility because the surface energy of the SAM-coated dielectric surface largely governs the thermodynamics of the semiconductor growth (wetting vs dewetting surface). This approach therefore likely requires a trade-off between achieving high mobility and controlling the threshold voltage. All the aforementioned approaches use the bottom-gate device architecture. Boudinet et al. demonstrated the use of SAMdoping layers in the top-gate architecture by depositing a SAM on a glass substrate and depositing the semiconductor on the SAM.28 However, in all these cases, the interfacial doping approaches are limited to certain oxide surfaces, which poses challenges when flexible substrates are to be used. Calhoun et al. introduced a different approach for interfacial doping and deposited SAM molecules such as (tridecafluoro1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS) directly on organic semiconductor single crystals. In the case of FTCS, they observed a strong increase in conductivity and the formation of a FTCS monolayer on top of the single crystals.29 In top-gate transistors, the SAM monolayer was also directly located at the semiconductor−dielectric interface and showed a huge onset voltage over 2.75 kV. This strong doping effect was explained by the SAM molecules’ strong dipole moment in B


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Figure 2. Transfer characteristics of pristine devices (blue curves) and doped devices (red curves) for different deposition times of the dopant, (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 6 h, and (f) 17 h. The OFETs were characterized with a Keysight B1500A semiconductor device analyzer. The transfer characteristics were acquired by applying a forward and reverse gate voltage sweep between 40 and −100 V at VDS = 0, −50, and −100 V. The transfer curves in this paper are shown for a drain voltage at −100 V. All discussed characteristics were obtained from the forward scans. The transistor characteristics, mobility (μsat) and threshold voltage (VTH) were extracted in the saturation regime by eq 1

conducting layer (Figure 1a) after the semiconductor had been grown on the octadecyltrimethoxysilane-modified (ODTMS) SiO2 dielectric, which yielded the best semiconductor thin film morphology in our experiments. FTCS was deposited in two ways: by a simple vapor deposition and by a spin-coating protocol for easier and faster deposition. By simply varying the FTCS deposition time or the FTCS concentration, we were able to tune the threshold voltage. The use of a soluble semiconductor and dopant generates the possibility of fully solution-processed devices, including dopant layers.

ISD =

WCox μsat(VG − VTH)2 2L

(1)

with W and L being the channel width and length and Cox (11.5 nF cm−2) the gate dielectric capacitance per unit area. The turn-on voltage (VON) was also extracted from transfer characteristics and is defined as the gate voltage at which the drain current starts to exceed the noise level. The ION/IOFF ratio is defined as the ratio of the maximum current (at VG = −100 V) and the average off-current (at VG > VON). Each data point represents one sample and 3 to 5 devices were measured for every sample. For the conductivity measurements, TIPS-pentacene was deposited on ODTMS-treated glass slides. On top gold electrodes were deposited as described above. The resistance was then measured between two electrodes, with length between 100 and 200 μm and width of 4.5 mm and then used to calculate the conductivity. Atomic force microscopy (AFM) images were measured with a Flex-Axiom (Nanosurf) in tapping mode with TAP190Al-G tips from Budget Sensors. Kelvin probe force microscopy (KPFM) measurements were done in air with the same AFM system using electrically conductive tips (ElectriMulti75-G from Budget Sensors). In our setup, the sample is grounded and the voltage is applied to the tip. The measured signal is the contact potential difference (cdp) between the surface and the conductive tip. All the measurements are referenced to undoped TIPS-pentacene, which has therefore a cpd of 0 V. As the signal is strongly influenced by adsorbed molecules, like water, the measurement is discussed qualitatively but not quantitatively. For a quantitative analysis, the samples would need to be measured in vacuum or a protection gas atmosphere. To achieve comparable values, all the samples are from one batch and measured with the same tip, within 1 d after fabrication. The KPFM signal is obtained in the same scan as the topographic image.

2. EXPERIMENTAL SECTION TIPS-pentacene was purchased from Ossila and FTCS from Sigma Aldrich and ABCR. All chemicals were used as received without further purification. Pristine OFET devices were prepared in bottom-gate, top-contact architecture. As substrate, strongly n-doped silicon wafers with a 300 nm silicon dioxide layer were used. The doped silicon was used as gate electrode and the silicon dioxide as the gate dielectric (Cox = 11.5 nF cm−2). The wafers were cleaned by subsequent sonication in acetone and 2-propanol, followed by 20 min treatment in a UV/O3 chamber. On top of the gate dielectric, a SAM of ODTMS was spincoated based on a protocol reported by Ito et al.38 The semiconductor was deposited by vacuum deposition from a RADAK source. The final thickness of the TIPS-pentacene layer was 20 to 25 nm and the deposition rate was around 0.2 Å s−1. Top gold electrodes were evaporated through a shadow mask with an evaporation rate of 1.5 Å s−1 and a final thickness of ∼50 nm. The channel length and width of the transistors were about 100 μm and 4.5 mm, respectively. For the vapor deposition of FTCS, completely fabricated OFETs were placed together with a vial containing FTCS in a small glass desiccator. The desiccator was connected to a vacuum pump and then continuously evacuated for times between 0.5 and 17 h to deposit FTCS on the substrates (Figure 1b). For the solution deposition of FTCS, the same silicon substrates with an ODTMS layer and deposited TIPS-pentacene (but without top gold electrodes) were used. The FTCS solution deposition was carried out by spin-coating (Figure 1c). Solutions of FTCS in methoxyperfluorobutane (MFB) with different concentrations (1−20 mM) were freshly prepared and filtered before using. After casting the FTCS solution on the substrate, it was left for 10 s before spin-coating at 3000 rpm for 30 s. To remove any residual solvent, the samples were kept in vacuum for ca. 5 min. As the last step, the top gold electrodes were deposited as described previously.

3. RESULTS AND DISCUSSION 3.1. FTCS Vapor Doping. The FTCS surface doping of organic semiconductors was examined in organic thin film transistors with a bottom-gate, top-contact architecture, with C


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ACS Applied Materials & Interfaces 300 nm oxide highly n-doped silicon wafers serving as both gate dielectric and gate electrode. The gate dielectric was modified by spin-coating a layer of ODTMS for improving the quality of the insulator−semiconductor interface.38 The soobtained SAMs are smooth with a roughness below rms < 0.2 nm2 and water contact angles about 104−108°. The setup for the vapor-doping process is depicted in Figure 1b. The substrates were placed in a thoroughly cleaned glass desiccator with an FTCS filled vial and vacuum was applied for durations between 0.5 and 17 h. The low pressure leads to the evaporation of FTCS molecules, and after reaching a certain pressure, an equilibrium is established between the rate of FTCS evaporation, the deposition of FTCS on all surfaces, and the pump rate. Transfer (ISD(VG)) and output curves (ISD(VSD)) of the devices were measured before and after doping process. To be able to properly compare the results among the samples, the devices used for vapor deposition of FTCS were only selected among those pristine devices with similar threshold voltage (−41 to −58 V), off-state (IOFF ≈ 10−9 A), and hole mobility (μsat > 0.1 cm2 V−1 s−1). For the selected I(V) curves that are shown here (Figure 2, blue curves), we chose from the pristine samples with the lowest variance because the higher the variance of the pristine devices, the higher the variance of the doped devices became. After FTCS deposition, the I(V) curves of the same devices were measured again for a direct comparison of the change in the transistor behavior (Figure 2, red curves). The pristine devices showed a typical transistor behavior with well-defined accumulation and depletion regimes. Samples treated with FTCS vapor exhibited, already after 30 min of treatment, a clear shift in the transfer characteristics to more positive gate values, as it is commonly observed by interfacial doping with fluorinated silanes.14,19,20,39,40 By increasing the FTCS vapor exposure time, these shifts become larger and at deposition times larger than 2 h, the turn-on voltage shifted to outside the probed voltage range. This means for deposition times longer than 2 h, the transistor channel could not be depleted of mobile charges (switched off) anymore even if the most positive voltage (40 V) was applied. The observed shifts of the threshold voltage were 12, 28, 40, and 52 V for 0.5, 1, 2, and 4 h, respectively (Figure 3). After 6 h of deposition time, the threshold voltage shift saturated between 70 and 85 V. We would like to point out that for deposition times longer than 4 h, the curves start showing artefacts such as kinks at high gate voltages and a second regime with a shallower slope at smaller gate voltages, so that the estimation of the threshold voltage and mobility would become quite inaccurate. The ION/IOFF ratio showed a similar trend as the threshold voltage: whereas pristine devices showed an ION/IOFF ratio of 105, for brief deposition times (0.5 and 1 h), the ratio slightly increased because of a more positive threshold voltage and just a slightly decreased mobility. For longer treatment times, the ION/IOFF ratio started decreasing because the turn-on voltage shifted outside the gate voltage sweep range and saturated at around 102 after 6 h treatment time. The strong decrease in ION/IOFF is consistent with bulk doping of organic semiconductors when high doping concentrations are used.5,12 Besides changes in the ION/IOFF ratio, we also observed a decrease in mobility with increased deposition time. Doping of TIPS-pentacene devices for short times (t < 1 h) resulted in a mobility drop of 40% and for long deposition times, it dropped to about 10% of the pristine value.

Figure 3. FTCS vapor doping caused (a) shift of threshold voltage and turn-on voltage and (b) change of mobility of single batches for different treatment times extracted from measured transfer curves; error bars show standard deviations.

In absolute values, this means an average mobility of 0.1 cm2 V−1 s−1 for 1 h FTCS deposition time and down to 0.01 cm2 V−1 s−1 for long deposition times (t > 12 h). The decreased mobility indicates either an increased density of shallow trap defects in the TIPS-pentacene grains, an increase in trap states at grain boundaries, or an increase in contact resistance, which can lower the measured (effective) mobility.7,8 At least for high doping concentrations, an increased contact resistance can be assumed to be the cause because of a pronounced S-shape of the output characteristics (Supporting Information, Figure S1) that is not observed for short deposition times. An elevated contact resistance is initially somewhat counterintuitive because the dopant molecules are deposited on top of the devices with already formed top electrodes but can be explained by the diffusion of FTCS molecules into the area between the semiconductor and the top electrodes during the FTCS deposition. The conductivity of the doped films was investigated by 2point measurements on glass slides and the values are shown in Table 1. Pristine films had a too small conductivity to be measured with our setup. After 30 min vapor deposition of FTCS, the conductivity increased and reached its maximum after 1 h with 1.7 × 10−3 S cm−1. For longer treatment times, the conductivity started decreasing again, which is often observed for higher doping concentration and can be attributed to a decreased mobility due to a high trap density.9,41 Because of the decreasing mobility and the lowered ION/IOFF ratio, doping experiments should be performed with deposition times below 2 h to keep a reasonably high mobility and a slightly improved ION/IOFF ratio. In addition to the electrical characterization of the doped TIPS-pentacene OFETs, AFM was used to monitor the growth and morphology of the FTCS layer at various stages and to test how the FTCS vapor treatment influences the film morphology. As already pointed out above, one potential D


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ACS Applied Materials & Interfaces Table 1. Conductivity Values Obtained by 2-Point Measurements and Average cpd Valuesa t (h)

c (mmol l−1)

σ (S cm−1)c

Δcpd (V)b

μ (cm2 V−1 s−1)

Vapor Doping −1.06 −1.27 −1.40 −1.49 −1.45

0.5 1 2 4 6

5.7 1.7 1.5 1.1 6.4

× × × × ×

10−4 10−3 10−3 10−3 10−4

(7.6 × 10−4) (8 × 10−4) (1.2 × 10−4) (2.4 × 10−4) (1.1 × 10−4)

0.25 0.1 0.14 0.08 0.09

Solution Doping 0 1 4 10

0.001 −0.65 −1.25 −1.19

na 3.6 × 10−5 (4.7 × 10−6) 4.1 × 10−5 (5.2 × 10−6) 8.7 × 10−5 (4.2 × 10−5)

0.21 0.13 0.11 0.023

a The average pristine mobility of all batches is about 0.26 (± 0.103) cm2 V−1 s−1. bcpd values are obtained by KPFM measurements; with reference to undoped TIPS-pentacene. cThe values in brackets are the standard deviation.

Figure 4. AFM graphs of FTCS vapor treated TIPS-pentacene layers for different deposition times. (a) Untreated TIPS-pentacene, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 6 h, and (f) 17 h; (g) profile pristine sample, (h) profile 0.5 h, no indication of FTCS deposition, (i) profile 1 h, FTCS grains becoming observable, higher than an FTCS monolayer; image size is 5 × 5 μm2. False color height scale: (a) 72.3 nm, (b) 74 nm, (c, d) 63.6 (e) 103.8 nm, and (f) 44 nm.

disadvantage of the dielectric interface, SAM-doping approach is that the SAM alters the surface energy of the growth

substrate significantly, which couples with the electric impact of the SAMs to cause possibly undesirable changes in the E


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deposited on top of the first layers, they have no or only a small effect on the electrical characteristics of the device. To further evaluate the deposition mechanism, KPFM images were taken. With this technique, the contact potential difference (cpd) is measured relative to the AFM tip. Changes in the local surface potential are usually related to a change in the work function or surface dipoles. Therefore, it is a useful technique to visualize deposited molecules with a large dipole moment on TIPS-pentacene and has been utilized in related works for evaluation of the surface potential of SAMs on oxide and semiconductor surfaces.13,14,44 Within the probed area of an individual sample, we cannot detect areas with shifts in the cpd that could be attributed to FTCS domains (Supporting Information, Figure S4). However, by comparing the average cpd among FTCS-doped samples with undoped TIPSpentacene samples, a strong shift of −1.06 V can already be observed after 0.5 h of doping (Table 1). The negative shift is in agreement with previous publications where a negative shift by fluorinated SAMs relative to an organic semiconductor was reported.13,44 This shift in the sample’s average cpd proves that there are FTCS molecules deposited on the TIPS-pentacene even though they cannot be detected or resolved by AFM. For longer FTCS deposition times, the cdp shift is just slightly stronger and reaches its maximum after 2 h of FTCS deposition. On the basis of the findings of AFM and KPFM measurements, we can conclude that the FTCS molecules are deposited uniformly across the whole TIPS-pentacene layer without any apparent preference for pores or TIPS-pentacene islands. After 2 h, the initial stages of a surface layer formation of FTCS can be seen, coinciding with the saturation of the cpd signal. A formation of a closed SAM could not be observed. 3.2. Discussion of the Doping Mechanism. In the following, we want to discuss the mechanism of the FTCS doping and the corresponding effects in the I(V) characteristics. The effect of the shifted threshold voltage to more positive values corresponds to p-type doping by the deposited FTCS molecules, i.e., additional positive mobile charges are introduced into the organic semiconductor, thus resulting in a shift of the Fermi level toward the highest occupied molecular orbital (HOMO) of the semiconductor. This results in the observed shift of the transfer characteristics to more positive gate voltages. The generation of free charge carriers can be assumed from the observed shift of the threshold voltage as well as the increased conductivity. The shift of threshold- and turn-on voltage is in agreement with p-type doping by interfacial SAMs,13,19−21,23,39 as well as p-type doping by molecular1,12 and surface doping.36,45,46 To discuss potential doping mechanism through FTCS molecules, various points need to be considered: (i) the byproduct of the hydrolysis of the FTCS molecules, (ii) the binding of the FTCS molecules to the semiconductor and (iii) the influence of the device architecture and the position of the doping layer. As for the first point, the formation of a SAM by trichloroalkylsilanes is well understood and starts with the hydrolysis of the highly reactive silicon−chlorine bond with the consequence of releasing hydrochloric acid (HCl).47,48 The small amount of water necessary for the reaction is residual water in the reaction vessel or adsorbed on the surface of the substrate. The trihydroxylalkylsilane would then bind to hydroxyl groups or oxygen defects on the surface and crosslink with adjacent SAM molecules to form a SAM by twodimensional polymerization. The hydrolysis of FTCS molecules leads to the release of HCl, which is a strong acid and

organic semiconductor thin film growth. Indeed, in several interface SAM-doping studies, it was found that pronounced different growth modes of the semiconductor occur between untreated and SAM-treated dielectric samples.13,20 This makes it somewhat difficult to delineate changes in the electrical behavior from changes due to a different growth mode. Still, in such studies, it was found that the main reason behind the observed threshold voltage shifts is the choice of SAM molecules, much less so the resulting growth mode.13,20 In contrast to these studies, our films were always grown on the same ODTMS-treated surfaces that are fabricated with the same method. This means that the changes in OFET characteristics we observe are exclusively related to the deposition of the FTCS molecules. AFM images of TIPS-pentacene samples, which were vapordoped for varying times (Figure 4), were taken. In Figure 4a, a 20 nm thick undoped TIPS-pentacene film is shown. The TIPS-pentacene layer consists of connected islands leaving pores in between the layer. The islands show small terraces with steps of about 1.5−1.7 nm, which is in the range of the caxis of the TIPS-pentacene unit cell (c = 1.68 nm) and indicates upright standing TIPS-pentacene molecules.42,43 The pores in between the TIPS-pentacene islands likely directly expose the dielectric surface. However, the area fraction of exposed dielectric surface is less than 0.05%. Still, this means that the FTCS molecules can directly reach the dielectric surface. After 0.5 h of FTCS deposition (Figure 4b), there were no obvious changes in the film morphology, even though a clear shift was already present in the OFET transfer characteristics. From the absence of any resolvable FTCS layer on the TIPS-pentacene film, one could speculate that for short deposition times, the FTCS molecules are either too mobile to be imaged or have already diffused into the TIPS-pentacene film. However, in the phase contrast of the AFM scans (Supporting Information, Figure S2), a change in the phase angle can be observed, indicating that FTCS molecules also remain on the surface of the TIPS-pentacene layer. In samples doped for 1 h (Figure 4c), however, small islands are visible, which are very likely FTCS islands. As can be seen in Figure 4i, these islands remain small in size, the height is not uniform, and usually bigger than the length of an FTCS molecule (≈ 1.3 nm).44 In addition, the pores in the TIPS-pentacene film started to become filled by FTCS, an effect that is more obvious for longer treatment times (Figure 4d−f). Further, an additional shift of the phase angle can be seen (Supporting Information, Figure S2). After 6 h treatment time, almost all pores are filled, and in samples that were doped for 17 h, a film of FTCS is visible and the underlying film structure of TIPSpentacene can barely be seen anymore. The average film thickness after 17 h was at least 37 nm compared to 20 nm for pristine TIPS-pentacene films. It should be mentioned that although the AFM image shown here for 1 h treatment does show some FTCS grains, in other samples, no grains were visible after the same treatment time. Even if there are no large differences in the doping strength, for some samples, we could not observe FTCS grains even after 2 h deposition time (Supporting Information, Figure S3b). On the other hand, for some samples, especially for long deposition times, we observe growth of a high density of large agglomerates of FTCS with close to 100 nm in height (see Supporting information, Figure S3a). For treatment times longer than 6 h, no further evolution of the doping efficiency is observed in the electric characteristics, which show that even when large amounts of FTCS are F


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dopant is applied on the surface of the semiconducting film and can diffuse into the film to potentially dope the entire bulk of the semiconductor, additional mechanisms have to be taken into account for the broadening of the subthreshold region. The first mechanism is the creation of new deep trap states, which can be located at the semiconductor−dielectric interface or in the bulk.53−55 The second mechanism that has to be considered, is the introduction of bulk charge carriers, which have to be depleted to turn the device off. Meijer et al. found that in oxygen-doped poly(2,5-thienylenevinylene) and poly(3hexylthiophen-2,5-diyl),52 the OFET can be thought of as a composite of a normal accumulation interface transistor with a conductive bulk channel on top, producing transfer characteristics that can be interpreted as a superposition of the individual characteristics of the two separate channels. Above the threshold voltage, the bulk channel can be treated as a depletion-type transistor with a different mobility than the accumulation interface transistor.4,52 To achieve complete depletion, i.e., to fully turn the device off, a high positive voltage has to be applied. This is similar to the behavior that we observed in our devices after doping with FTCS. For long FTCS deposition times, it would be not possible to deplete the created bulk channel. In such a scenario, the subthreshold regime of the accumulation interface channel is determined by the depletion of the bulk channel. In the present case, it is difficult to discriminate between the two possible causes (additional bulk traps and bulk charges) for the broadening, and they could in fact very well both be contributing. We do not find a clear signature of an additional depletion-type bulk transistor as was observed by Meijer et al.52 in our transistors, but still for some of our doped devices, a nonlinear behavior is visible in the subthreshold region (Figure 2c,d). The exact mechanism of the doping of organic semiconductors by SAM molecules has so far, despite a considerable number of published studies, not been conclusively explained.13,20,29,56 For a successful charge transfer from the HOMO of the semiconductor (5.3 eV for TIPSpentacene57) to the lowest unoccupied molecular orbital (LUMO) of the dopant, these two levels should energetically at least be closed or aligned. However, the LUMO energies of the hydrolyzed silanes have been calculated to about 1.8 (± 1) eV,22 resulting in a large energy offset to the HOMO of most organic semiconductors and thus making a direct charge transfer unlikely in this standard picture of molecular doping. Previous reports that utilize polar SAM molecules to achieve a doping effect commonly relate this effect to the formation of dense and ordered monolayers that create strong dipole electric fields. Supporting this conjecture, alkylsilane-based SAMs without functional groups show almost no doping effect.13,20,29,31 In contrast, in our experiments, it is very likely that the SAM molecules diffuse throughout the TIPSpentacene film, making it difficult to imagine the formation of an ordered layer of dipoles as being the cause of the observed doping effect. To work out the detailed molecular doping mechanism thus remains a future scientific challenge. 3.3. FTCS Solution Doping. So far, we demonstrated that FTCS surface doping, via vapor-phase deposition in ambient conditions with a simple setup, works on polycrystalline thin films. However, this method requires long reaction times, which might limit its practical utility for a scaled-up, industrial fabrication. For such a scenario, solution-based doping methods are more favorable because they are applicable in continuous roll-to-roll processes. For a solution-based doping

could dope the semiconductor itself. Therefore, the vapor doping was also tested with (tridecafluoro-1,1,2,2tetrahydrooctyl)triethoxysilane (FTES), a molecule in which the chlorosilane group is replaced by an ethoxysilane group. During the hydrolysis of FTES, ethanol is produced instead of HCl. An FTES deposition time of 1 h led to similar results as vapor doping with FTCS (Supporting Information, Figure S5). The shift in the threshold voltage is less pronounced for FTES, which is in good agreement with the lower reactivity and the lower doping efficiency observed in other work.29 This result shows that the released HCl is not the active doping species. Furthermore, it was shown in other works that nearly no chlorine species remained in the doped substrates.30,49 Regarding the second point raised above, the binding and formation of an alkylsilane SAM requires oxygen sites, either in the form of defects on organic semiconductors like rubrene29,44,49 or surface oxides as in the case of Si.19,20 However, TIPS-pentacene is not easily oxidized due to the passivation of the 6, 13 position with the TIPS groups.50,51 Still, the highly reactive FTCS molecules can undergo selfpolymerization, which could stabilize the dopant molecules on the TIPS-pentacene film. The missing bonding sites can be one explanation that the formation of a SAM could not be detected. However, in similar studies reported in the literature, where no potential substrate or film surface bonding sites are existent and where no SAM was found, a strong doping effect by FTCS could be observed.30,31 The final point, which has to be addressed, is the interplay between the FTCS deposition process and the OFET layer architecture. Independent of whether the SAM is deposited on the dielectric surface or on the semiconductor surface, in the resulting OFET devices, the SAM is usually located near or at the semiconductor−dielectric interface.13,19,20,29 Doping approaches with comparable device architecture in which the dopant layer (SAM or molecular dopant) is spatially separated from the OFET channel (e.g., on the side/surface of the semiconductor opposite the dielectric) still resulted in a pronounced threshold voltage shift, a broadened subthreshold slope, and an increased off current4,28,46similar to our findings and those of bulk doping experiments.1,2,52 For example, Boudinet et al. use top-gate (top-channel) devices with anchored bottom-layer SAMs. The use of a relative high semiconductor thickness (100 nm) led still to a pronounced doping effect. This was explained by the spatial separation of the SAM from the active channel, so the SAM-induced field and the gate electric field did not overlap, leading to a conducting layer adjacent to the SAM. In conclusion, we showed that the byproduct HCl does not cause the doping effect and that even in the case that no SAM was formed on the semiconductor film, still a pronounced doping effect was observed. Another situation worth illuminating are effects that could broaden the subthreshold region, as observed in our experiments. In undoped devices, the subthreshold region is linked to the deep traps, which are assumed to be located at the semiconductor−dielectric interface. A common estimation for the interfacial deep trap density (Ntrap) is eq 2, where e is the elementary charge.20,36 Ntrap =

|VTH − VON|·Cox e

(2)

For our pristine devices, this results in an interfacial deep trap density of 1 to 4 × 1011 cm−2. In our experiment, where the G


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ACS Applied Materials & Interfaces process, a solvent for FTCS is needed that is orthogonal to the semiconductor so that this solvent will not dissolve the already formed TIPS-pentacene layer during the doping process. TIPSpentacene, however, is attacked by most common solvents (e.g., dichloromethane, toluene) and attempts to use such solvents leads to effects ranging from just decreased mobility to the complete dissolution of the semiconductor layer. A class of suitable orthogonal solvents are highly fluorinated ethers that have previously been used for photolithographic structuring of organic semiconductors.58−62 An additional advantage of these solvents is that they are chemically inert and environmentally benign.60 From this group of materials, MFB was used as a solvent for solution-casting of FTCS because it can dissolve FTCS, but it should have no impact on the underlying TIPS-pentacene films. We chose spin-coating as simple and fast laboratory-scale process for solution deposition. TIPS-pentacene substrates were prepared in the same way as for vapor-phase experiment, except the doping layer was deposited before the deposition of the top gold electrodes. A spin-coating protocol was used that was inspired by the work of Ito et al. in which the formation of highly crystalline monolayers of ODTMS on silicon surfaces was described.38 Several concentrations (1−20 mM) of FTCS in MFB were spin-coated on TIPS-pentacene substrates and compared with pristine devices as reference and devices treated with pure solvent to rule out the effects of the solvent (Figure 5). The solutions were always freshly prepared and used within 1 h to avoid precipitation, even though the solutions stay clear for several days.

Figure 6. Extracted values of (a) threshold and turn-on voltage and (b) charge carrier mobility from the transfer curves of solution-doped TIPS-pentacene transistors. The values were extracted at VDS = −100 V, VTH from the steepest slope and μ from the average slope for VG ≫ VTH.

to 73% for 1 mM, 57% for 4 mM, and approximately 10% for higher concentrations of the mobility of the pristine devices. The morphology of the spin-coated FTCS layers was also determined by AFM (Figure 7). No additional surface layer or change in the TIPS-pentacene island morphology can be detected for pure-solvent-treated sample and FTCS solutions of concentrations up to 2 mM (Figure 7a−d). However, in the AFM graphs of higher concentrations, small agglomerates (white dots) are observed that become larger for higher FTCS concentrations. For 20 mM solutions, these agglomerates are covering nearly the entire measured area (AFM graphs of 5, 10, and 20 mM can be found in the Supporting information, Figure S9). These agglomerates likely form through hydrolysis during the spin-coating process because the spin-coating is done in ambient conditions. These agglomerates form no matter whether the solution was filtered or not. In addition, one can observe that starting with the 3 mM solution, the pores in the TIPS-pentacene film are becoming visibly filled, particularly near the larger agglomerates. The filling of the pores continues with increasing concentrations but appears to remain localized to the area of the already existing agglomerates. This localization of FTCS is better visible in the AFM phase images (Supporting Information, Figure S10). For the spin-coated samples, KPFM data were also taken: the single scans can be found in the Supporting Information (Figure S9) and averaged cpd values with reference to TIPSpentacene are listed in Table 1. The samples on which just pure MFB solvent was spin-coated did not show any change in the cdp, which is in agreement with the almost unchanged threshold voltage. The sample spin-coated from 1 mM showed a shift in cpd of −0.7 V, even though no changes were visible in the AFM images. The samples spin-coated from 4 mM and 10 mM solution had a similar cpd of about −1.2 V, which is in

Figure 5. Averaged transfer characteristics of FTCS solution-doped TIPS-pentacene films, for clarity just forward scans are shown, reverse scans and output curves can be found in the Supporting Information (Figures S6−S8).

Samples treated with pure MFB showed only a small shift in threshold and turn-on voltage as well as an almost unchanged mobility. These changes can be attributed to in-batch variations. Samples that were modified by spin-coated FTCS films showed, even for the lowest tested concentration of 1 mM, a shift in the transfer characteristics that was stronger than the shift induced by the pure solvent (Figure 6). For high concentrations (>4 mM), the shifts in the curves were saturating for the threshold voltage, whereas the turn-on voltage was further increased (shifted toward more positive values) until it could not be properly determined for concentrations higher than 10 mM. Quantitatively, this means for an FTCS concentration of 1 mM, the threshold and turn-on voltage were increased by 22 and 30 V. The shifts for 4 mM were 33 and 67 V for threshold voltage and turn-on voltage, respectively. The mobility drops H


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Figure 7. AFM images taken of solution-processed FTCS layers on a 25 nm TIPS-pentacene film, (a) pristine sample, (b) pure solvent, (c) 1 mM FTCS, (d) 2 mM FTCS, (e) 3 mM FTCS, and (f) 4 mM FTCS. Scan size is 5 × 5 μm2, false color scale bars are (a) 72.6 nm, (b) 74 nm, (c) 65.2 nm, (d) 82.5 nm, (e) 61.5 nm, and (f) 99.9 nm.

both samples is very low (25% of the pristine device for vapor and 7% for solution) compared to the undoped samples, which can be explained by an increased contact resistance and increased shallow trap density that is often observed for high doping concentrations.2,9 However, as the samples reaching the maximum shift of the threshold voltage are anyway doped to strong, regarding a degradation of the transfer characteristics, the discussion will be stronger focused on the short deposition times and low concentrations. One can see that in the 1 h and 3 mM samples, which have the same threshold voltage shift, the turn-on voltage is also identical. In addition, the mobility of the doped samples relative to the undoped sample remains similar for the solution-doped sample (70%) and vapor-treated ones (60%). The ION/IOFF ratio also remains in the same order of magnitude (105) for both doping approaches. The similarity of both the doping approaches for short FTCS vapor deposition times and small FTCS concentrations can be clearly seen in the scatter plot in Figure S12. Because of the different fabrication procedure of the devices, FTCS vapor deposition was applied after the top electrode deposition in contrast to solution deposition, where FTCS was deposited before the top electrodes. For a better comparison, we also tested samples fabricated by vapor-phase treatment with a duration of one 1 h, where the dopant was applied before the top electrode deposition. The transfer and output characteristics of these samples (Supporting Information, Figure S13) do not show distinct differences, indicating that the FTCS layer in between has no significant impact on the contact resistance, if it is thin. Two explanations can be given, first the layer is thin enough and charge carriers can tunnel through the

accordance with a saturation of the shift of the transfer characteristics for FTCS concentrations higher than 5 mM. 3.4. Comparison of the Vapor-Doping and SolutionDoping Approaches. To compare the vapor- and the solution-doping approaches, a graph can be found in the Supporting Information in which the relative mobility is plotted vs the shifted threshold voltage (Figure S12). A deposition time of 1 h for the vapor process and a 3 mM concentration for the solution process are particularly useful because both treatments result in the same threshold voltage shift. Also, the saturation time of the threshold voltage (6 h) is compared with the saturation concentration (5 mM) to discuss the maximum possible shift in the transfer characteristics (Table 2). It is obvious that the maximum shift of transfer Table 2. Comparison of FTCS Vapor-Doped (vd) and Solution-Doped (sd) TIPS-Pentacene Samples vd, 1 h vd, 6 h sd, 3 mM sd, 5 mM

ΔVTH (V)

ΔVON (V)

μ/μ(0)

IOFF (A)

28 84 28 36

42

0.58 0.25 0.68 0.07

10−9 10−6 10−9 10−9

42 69

characteristics is stronger for the vapor-doped samples than for solution-doped samples. This difference is also visible in the maximum shifts of the cpd (Table 1), which is larger for vapordoped samples (6 h, −1.45 V) than for solution-doped samples (10 mM, −1.2 V). The smaller shift could be explained by the formation of a less dense FTCS layer from spin-coating, which would lead to a less pronounced doping effect. The mobility of I


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ACS Applied Materials & Interfaces FTCS layer. A second explanation is the evaporation process itself. It is known from monolayer devices that gold deposited by thermal evaporation can damage these devices.63,64 It is therefore quite likely that the FTCS layers can also be damaged or at least be degraded by the thermally evaporated gold. To conclude the comparison of FTCS vapor and solution deposition, the doping process for low deposition times and low concentrations is very similar in the results. The solutiondoping approach that is not extensively optimized can save a lot of time in fabrication of doped devices. We pointed out above that both doping approaches lead to a similar shift in the threshold and turn-on voltage. The shift of the threshold voltage can be used to estimate the introduced free charge carriers as sheet carrier density (Nmobile) by eq 3,13,23 assuming that the charges are at the interface for gate voltages below the threshold voltage. Nmobile =

|VTH(FTCS) − VTH(0)|·Cox e

step to a completely solution-processed combination of organic semiconductor and doping layer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12346. Related output characteristics of vapor- and solutiondoped devices, additional AFM graphs, phase images related to the topographic AFM images, KPFM measurements of vapor- and solution-deposited FTCS, doping of TIPS-pentacene with FTES, hysteresis of solution-doped TIPS-pentacene films, comparison of doping before and after top electrode deposition, scatter plot: VTH vs μ (PDF)

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AUTHOR INFORMATION

Corresponding Author

(3)

*E-mail: [email protected].

For the previously compared deposition time of FTCS (1 h) and concentration of FTCS (3 mM), the induced charge carrier density is around 2 × 1012 cm−2. This value is in the same order of magnitude as the typically observed charge carrier density induced by interfacial SAMs of fluorinated and other polar SAM molecules determined either experimentally13,20,23 or by simulation.56 In comparison to the induced charge carrier density reported for an FTCS SAM on rubrene single crystals, the estimated value here is about one order of magnitude smaller (3 × 1013 cm−2).29 This can be attributed to the overall lower quality of the dopant layer and the lack of possible attachment sites in our devices in contrast to the SAMs on single-crystal rubrene. For longer deposition times, the charge carrier density is slightly increased; however, the degradation of the OFET characteristics is also elevated.

ORCID

Mike Hambsch: 0000-0002-8487-0972 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge support by the German Excellence Initiative via the Cluster of Excellence EXC 1056 “Center for Advancing Electronics Dresden” (cfaed).

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REFERENCES

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4. CONCLUSIONS In summary, we developed a surface-doping approach for polycrystalline films of vacuum-deposited TIPS-pentacene based on fluorinated alkylsilanes. The studied dopant, FTCS, is less expensive compared to commonly used dopants. In this work, we compare a simplified vapor deposition approach of FTCS to a novel solution-based process. In the solution-based approach, the dopant was spin-coated from an orthogonal solvent, a highly fluorinated ether, which has no significant effect on the semiconductor layer during the deposition process. In the more device-relevant low doping concentration regime, both deposition approaches lead to comparable results. However, the solution deposition process is much more efficient in terms of time and material than the vapor deposition method. The threshold and turn-on voltage could be modulated in the low-doping-concentration regime by varying the dopant deposition time (vapor) or dopant concentration (solution). High doping concentrations on the other hand, i.e., deposition longer than 2 h or concentrations above 4 mM, lead to a strong shift in the transistor characteristics resulting in transistors, that cannot be turned off anymore and exhibit a significantly decreased charge carrier mobility. In contrast to interfacial SAM doping, our approach can be applied independent of the substrate, gate dielectric, and device architecture. The potential combination of a soluble semiconductor (TIPS-pentacene) and dopant is an important J


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