1 Introduction

Organic field effect transistors (OFETs) have great attention in future electronic applications including sensors, radio-frequency identification tags, and flexible displays due to their low-cost, large area, light weight, printable, and mechanical flexibility [1,2,3,4,5]. The properties of dielectric/semiconductor interface play an important role for charge transfer between dielectric and semiconductor, which effects electrical parameter of OFETs. Hence, treatment, using self-assembly molecules, at this interface can have dramatically improve device performance [6].

To date, several metal oxides have been investigated as dielectric layer in OFETs. Among these materials, many studies have noticed that Al2O3 is an effective dielectric layer in many studies due to its controllable oxide thickness and formed high-quality self-assembled monolayers (SAMs) [7]. However, charge transport occurs in the a few nanometres between the organic semiconductor and insulator layer. It is limited because of the roughness value of Al2O3 and it could be increased with controlling oxidation process that is affected on device performance [8, 9]. Therefore, SAMs molecules have been examined for their ability to modify the surface properties of the Al2O3 insulator layer to obtain a good morphology and to form better ohmic contact. Jang et al. studied the various phosphonic acid SAMs modifying to hydrophilic surface of Al2O3 and compared the properties of the SAMs according to their functional end groups and alkyl chain length [10]. The result showed that greater contact angle characteristics are produced by a surface that is more hydrophobic due to a longer alkyl chain length. The effect of gate dialectic and thickness on threshold voltage modifying with phosphonic acid-based SAMs to the Al2O3 surface was performed by Aghamohammadi et al. [11]. Kawanago et al. reported that the mobility and On/Off ratio increased while the trap density between Al2O3 gate dielectric and channel decreased after SAMs treatment [12]. These results show that the modification of SAM molecules into the Al2O3 dielectric layer contributes to improve the performance parameters of OFET.

Boron-based materials have been dramatically playing important role in recent years as a remarkable candidate in optoelectronic devices due to its high stability, low toxicity, easy handling, and less reactivity as well as having high quantum yield [13,14,15,16]. Organic molecules known as boronic acids are composed of a trivalent boron atom with two hydroxyl groups attached to the boron atom and one C–B bond. Boronic acids have been utilized in separation, sensing, medical therapeutics, and materials science [17,18,19,20]. In addition, the optoelectronic application of boronic acid including organic solar cells (OSCs), perovskite solar cells (PCSs), and OFET has been great attention over the last decade [21,22,23,24,25,26,27,28]. The use of boric acid derivatives in OFETs for saccharide sensing and biomolecule detection has been studied [17, 29,30,31,32,33]. Given that they include hydroxyl, silane, sulfonic acid, phosphonic acid, and carboxyl acid-based SAMs are the most often utilized to treat metal oxide surfaces [10, 34,35,36,37]. Addition of the potential to non-covalently mutually influence with hydrogen bonds, self-assembly of boronic acids form onto metal oxide surface shows a covalent bond forming properties. A range of supramolecular self-assembly can develop in the solid state of boronic acids thanks to the hydroxyl groups of hydrogen bond donors [38]. Alic et al. reported that the electrical characteristics of OFETs were enhanced by using boronic acid, and its SAM molecules derivatives to treat the SiO2 surface [39, 40]. There has been few research on alkyl-boronic acid SAM (BA-SAM) molecules, even though numerous SAM molecule types have been employed to treatment OFETs [23,24,25, 41]. Herein, the impact of different chain length alkyl-BA SAMs on the device performances of OFETs based on DNTT was investigated.

2 Experimental

2.1 Fabrication of OFET

As seen in Fig. 1, OFETs with a bottom-gate, top-contact configuration was fabricated on glass substrates. Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT), an organic semiconductor based on thienoacene, was selected as the active layer because to its excellent hole mobility and air stability [42,43,44]. Figure 1 also displays the molecular structures of boronic acid-based SAM molecules for dielectric surface treatments for C8-BA (octylboronic acid), C10-BA (decylboronic acid), C12-BA (dodecylboronic acid), and C14-BA (tetradecylboronic acid). After being cleaned with organic solvents in an ultrasonic bath, the substrates were left in the UV-ozone for 10 min. Aluminum (Al) was deposited a thickness of 100 nm for the gate electrodes through a shadow mask. Thanks to the anodization process, a thin film of aluminum oxide (Al2O3) with a thickness of 10 nm was formed for the gate dielectric. The anodization process detail was given in reference [45]. In short, an electrolyte solution containing citric acid was used to apply a constant current between the Pt counter electrode and the Al contact. The Al2O3 surface was modified by immersing the films in alkyl-boronic acids SAM molecules dissolved in isopropanol (IPA) (99.9%, Sigma Aldrich) at a concentration 10–3 M for 24 h at ambient conditions. The substrates were rinsed, dried, and then transferred in the nitrogen glove box. Afterward, a semiconducting layer of 30 nm of DNTT was deposited via thermal evaporation at a vacuum base pressure of approximately 10−6 mbar. A quartz crystal microbalance was used to measure the film thickness while the deposition rate was around 0.1 Å/s. MoO3/Ag source and drain contacts with thickness of 10 nm/60 nm were deposited using thermal evaporation, resulting in a device that had a channel length (L) of 50 μm, and a channel width (W) of 1 mm, which was utilized by the shadow mask.

Fig. 1
figure 1

Chemical structure of DNTT; schematic structure of bottom gate, top contact OFET device; chemical structures of alkyl-BA SAM molecules

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2.2 Thin film and transistor characterization

The performance of organic field effect transistor fabricated is strongly integrated with the morphological, electrical, and surface characteristics of both treated and non-treated surfaces. Atomic force microscopy (AFM) is one of the most widely used methods in research for examining surface morphology. The AFM was created in response to a need to get over the constraint of Scanning Tunneling Microscopy, which can only examine conductive samples. It can examine nearly any kind of sample in a variety of environmental settings. These days, the AFM is utilized for purposes other than topographical imaging, highlighting several kinds of surface characteristics, such as the surface potential. Hence, the morphological study of both treated and non-treated substrates was performed in ambient air using an AFM (NT-MDT AFM NTEGRA Solaris) in “tapping” mode at room temperature. On the other hand, the Kelvin Probe Force Microscope (KPFM) (NT-MDT AFM NTEGRA Solaris) was used to measure the electrostatic potential of both treated and non-treated surfaces. Contact angle measurement technique is a quick and non-destructive technique to find out the hydrophobicity properties of surfaces. Since the polarity of the surface determines the contact angle, doping or adsorbing materials, such as self-assembled monolayers (SAMs), can induce modifications. Thus, to determine the hydrophobicity of the surfaces, contact angle measurement was performed. DSA 100 KRÜSS was used to measure contact angles. The most significant characteristics of an organic field-effect transistor—charge carrier mobility, threshold voltage, subthreshold voltage, and on/off ratio—provide information on the device’s performance. To measure these properties, the OFETs were subjected to current–voltage (I–V) measurements in a nitrogen environment glovebox utilizing a 4200 Keithley semiconductor parameter analyser and a micromanipulator probe station.

3 Results and discussions

3.1 Characterization of thin films

Contact angle of water drop values were calculated onto non-treated and treated surfaces to investigate that the different chain length alkyl-BA SAMs were formed onto Al2O3 surfaces, As shown in Fig. 2 and As summarized in Table 1, contact angles of water were found as 63.43° on non-treated and 65.85° on C8-BA, 51.11° on C10-BA, 55.78° on C12-BA and 56.46° on C14-BA SAMs treated surface.. These results are in an agreement with former studies, therein after SAMs treatment hydrophobicity decreases [46]. The C10-BA (45.9 dyn/cm), C12-BA (44.5 dyn/cm) and C14-BA (43.9 dyn/cm) have the highest surface energy and C8-BA has the lowest (36.2 dyn/cm), as a comparison the surface energy of non-treated surface (38.2 dyn/cm), which indicates that more stronger surface than non-treated surface hydration occurs at the SAMs-treated surfaces [47, 48]. It also showed that could be an evidence for the dependence of the contact angle on the surface morphology [49].

Fig. 2
figure 2

Water contact angles and calculated surface tension: the surfaces of Al2O3 (non-treated) and treated with alkyl-BA SAM molecules

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Table 1 Value of contact angle, roughness, and electrostatic potential for non-treated and boronic acids with four different chain lengths grafted Al2O3
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In order to deprive of the surface morphology of Al2O3 surface and after treatment by alkyl-BA SAMs, AFM images were taken and analyzed using the WsXM software [50]. Table 1 shows measured roughness values. The surface morphology of Al2O3 is seen in Fig. 3 both before and after treatment. As summarized Table 1, The average roughness of surfaces increases from 1.05 nm for non-treated surface to 1.47 nm, 3.26 nm, 3.73 nm, and 2.72 nm for C8-BA, C10-BA, C12-BA, and C14-BA, respectively. The findings show that the formation of alkyl-BA SAMs onto Al2O3 surfaces was effective to increase the average roughness of surfaces. An increase in chain length may result in a surge in all treated surfaces [51].

Fig. 3
figure 3

Atomic force microscopy (AFM) height images of the surfaces of Al2O3 (non-treated) and treated with alkyl-BA SAM molecules

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As shown Table 1, average roughness of DNTT layer onto non-treated and treated surface increased from 10.48 to 11.55 nm for C8-BA, 10.50 nm for C10-BA, 13.63 nm for C12-BA, and 10.59 nm for C14-BA. Furthermore, exceptionally thin DNTT layer (3 nm) was evaporated to understand behind mechanism dielectric and semiconductor layer, as shown Fig. 4. Grain sizes of DNTT is showed clearly differences which lead to effect electrical parameter of OFETs thanks to decreasing the density of charge traps [6, 52]. As seen Fig. 4, grain size of DNTT on the C8-BA and C10-BA have a grain size of ~ 0.6 μm, which is clearly bigger than on non-treated, C12-BA and C14-BA SAMs.

Fig. 4
figure 4

Atomic force microscopy (AFM) height images of very thin layer (3 nm) of DNTT active layer on bare Al2O3 (non-treated) and treated surfaces. Line profiles in random direction are provided on the left of each image

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The alkyl chain length and the different SAMs have been reported for a various study such as phosphonic acid on Al2O3 [53], phosphonic acid on SiO2 [54], carboxylic acid on Al2O3 [55], and phosphates on TiO2 [56]. There are no studies on the relationship between the alkyl chain length and SAM structure of boronic acids on aluminum oxide. The binding mechanism between SAMs and metal oxide surface may exhibit different behavior due to SAMs microstructure. Hence, we may argue that alkyl-BA SAMs of less about ten carbon atoms could be form disordered and liquid-like monolayers which are affected the surface morphology [57].

To measure the electrostatic potential of surfaces and understand electronic interaction the non-treated and treated surfaces and the DNTT, KPFM measurements was performed [58]. As summarized in Table 1, non-treated surface shows an electrostatic potential value of 0.992 eV and treated surfaces have an electrostatic potential value of 1.033 eV, 1.025 eV, 1.03 eV, and 1.155 eV for C8-BA, C10-BA, C12-BA, and C14-BA SAMs, respectively. Alkyl-BA SAMs do not show a wide range surface potential value for Al2O3 compared to phosphonic acids [11].

The charge layer, introduced by charge mobility, between non-treated/treated surface and semiconductor, is affected on electrical parameter of device [53]. In addition, the electrical properties of surfaces could also influence on morphology of following layer.

3.2 Electrical parameters of OFETs

One of main aim of this study was to investigate the effect of alkyl-BA SAMs with different chain length on OFETs performance. Figure 5 display the typical transfer and output curves of non-treated and treated with alkyl-BA SAM OFETs. Current On/Off ratio was calculated almost 105 in gate voltage sweeps. Within drain voltage sweeps, proper linear and saturation behaviors were observed. The transfer curves of non-treated and treated with alkyl-BA SAM devices showed a significant difference with forward and backward scans. All the devices presented a big hysteresis effect because of impurities, defect of active layer morphology or electron trap in the insulating layer. Box charts of the electrical parameters of non-treated and treated with alkyl-BA SAM OFETs are represented comparatively (Fig. 6).

Fig. 5
figure 5

Transfer (a) and output (b) characteristics of the devices on non-treated; those of the devices treated with alkyl-BA SAM molecules: C8-BA (c) and (d); C10-BA (e) and (f); C12-BA (g) and (h); C14-BA (i) and (j)

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Fig. 6
figure 6

Device performance parameters of OFETs with non-treated and alkyl-BA SAMs treated Al2O3 dielectric: linear mobility (µlin), saturation mobility(µsat), threshold voltage (Vth), hysteresis (ΔVth), subthreshold slope (SS), and current on/off ratio (Ion/Ioff)

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The influence of treating alkyl-BA SAMs was assessed for each configuration using at least 10 OFETs. For OFETs, the linear (µlin) and saturation (µsat) mobilities were calculated using the following equations:

$$\mu_{lin} = \frac{L}{{C_{i} WV_{ds} }}\left( {\frac{{\partial I_{ds} }}{{\partial V_{gs} }}} \right);\mu_{sat} = \frac{2L}{{C_{i} W}}\left( {\frac{{\partial \sqrt {I_{ds} } }}{{\partial V_{gs} }}} \right)^{2}$$
(1)

where L is the channel length, W is the channel width, and Ci is the dielectric capacitance. The maximum values were taken from the gate voltage dependent mobility curves. The electrical parameters of OFETs extracted from transfer curves are listed in Table 2. Although C10-BA shows a similar mobilities with non-treated samples, generally Alkyl-BA SAMs treated OFETs indicated higher mobility than non-treated samples for both regimes. Increasing the surface roughness could cause positive effect on decreasing interface trap intensity as well as negative effect on the mobilities of OFETs. Therefore, resulting in high mobilities for C8-BA SAM, C12-BA SAM, and C14-BA SAM were observed.

Table 2 Summary of device performance parameters of OFETs with non-treated and boronic acids with four different chain lengths grafted Al2O3 dielectric
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Threshold voltage (Vth) is another important parameter which is extracted from the square root of the drain current by extending the linear part to the zero current. The forward and backward scanned curves for Vth are also used to observed Hysteresis (ΔVth) effect. Herein, notable differences were observed for Alkyl-BA SAMs treated OFETs. Threshold voltage of devices presented a decrease from around − 0.52 V for non-treated device, to − 0.32 V for C8-BA SAM, 0.10 V for C10-BA SAM, 0.29 V for C12-BA SAM, and 0.24 V for C14-BA SAM treated devices. These results also demonstrated the successful formation of alkyl-BA SAM on Al2O3 surfaces. On the other hand, ΔVth values also reduced from around 0.57 V in non-treated device to about 0.28 V for C8-BA SAM, 0.39 V for C10-BA SAM, 0.22 V for C12-BA SAM, and 0.25 V for C14-BA SAM treated devices. Decreasing hysteresis effect is a powerful indication that Alkyl-BA SAM form onto the oxide surface BA SAM due to saturating charge trapping. Subthreshold slope (SS) which is associated with the interface trap densities [59], calculated as 0.18 V/dec for C8-BA SAM, 0.10 V/dec for C10-BA SAM, 0.23 V C12-BA SAM, and 0.24 V for C14-BA SAM treated devices in contrast to 0.24 V/dec for the non-treated device. After Alkyl-BA SAM treatment, SS values also confirmed the decreasing trap densities at the interface. There were no significant differences in current on/off ratio in devices, and this showed that good gate control on devices [60].

4 Conclusion

In summary, the Al2O3 dielectric surface treatment with the alkyl chain lengths BA SAM molecules could enhance the electrical performance of OFETs. Nonetheless, the surface energy shows a little difference proven by water contact angle measurements, significant improvements were observed in some device performance parameters such as mobility and threshold voltage. Since these electrical parameters are related to the trap densities of the interface layer, treating alkyl chain lengths boronic acid molecules the Al2O3 surface is proven to be a powerful approach to passivation of the Al2O3 surface. In addition, the chain length of the boronic acid SAM molecules has a significant effect on the surface morphology of semiconductor and OFET performance. We observe that the medium chain length treated OFETs have the highest mobility, the largest on/off ratio, and the lowest SS value. Further developments in optoelectronics will occur because to the numerous benefits of employing SAMs, as demonstrated by a variety of examples. The following are advantages of SAM-based optoelectronic devices for future research: (1) cheap cost; (2) flexibility of the device; (3) work-function modification; and (4) increase of device stability. Because SAMs are solution processable, the SAM-based device process may be completed with straightforward spin-coating or printing, which simplifies the complexity and consequently the cost of the fabrication process. Additionally, the SAM-based process’s low heat budget improves its compatibility with flexible substrates. These advantages make it very simple to deploy flexible devices such as communication devices and sensors. Therefore, this work can be pioneer a new application area associate with the boronic acid and its derivate molecules.