Percolation of Carbon Nanoparticles in Poly ( 3-Hexylthiophene ) Enhancing Carrier Mobility in Organic Thin Film Transistors

To improve the field-effect mobility of all-inkjet-printed organic thin film transistors (OTFTs), a composite material consisted of carbon nanoparticles (CNPs) and poly(3-hexylthiophene) (P3HT) was reported by using homemade inkjet-printing system.These all-inkjet-printed composite OTFTs represented superior characteristics compared to the all-inkjet-printed pristine P3HT OTFTs. To investigate the enhancement mechanism of the blended materials, the percolation model was established and experimentally verified to illustrate the enhancement of the electrical properties with different blending concentrations. In addition, experimental results of OTFT contact resistances showed that both contact resistance and channel resistance were halved. At the same time, X-ray diffraction measurements, Fourier transform infrared spectra, ultraviolet-visible light, and photoluminescence spectra were also accomplished to clarify thematerial blending effects.Therefore, this study demonstrates the potential and guideline of carbon-based nanocomposite materials in all-inkjet-printed organic electronics.


Introduction
The extensive researches of organic semiconductors have attracted considerable attention due to their potential in printed and flexible electronics.Because of the advantages of organic electronics, such as low cost, light weight, large-area manufacturing, and mechanical flexibility, these researches promote applications of organic electronics.For instance, organic thin film transistors (OTFTs) [1], solar cells [2], organic light-emitting diodes [3], and sensors [4] were proposed and implemented.In spite of these promising developments, some crucial problems have blocked the path to commercialization, such as the mobility of organic semiconductor and the feasibility of manufacturing.To address manufacturing technologies, solution processes are one of the candidates.These processes include spin coating, screen printing, gravure printing, roll-to-roll printing, and inkjet printing.Among these processes, inkjet printing has advantages of mask-less deposition, etch-free fabrication, and low material consumption [1,5].
Although the inkjet printing technique can offer many advantages in applications, the electrical properties of OTFTs still need to be improved from the aspect of materials.To enhance the effective mobility of the OTFT, commonly, the self-assembled monolayers (SAM) have been used to modify the interfacial properties between the organic semiconductor, dielectric layer, and the contact electrodes [6][7][8].However, the SAM technique is not adapted for the all-inkjet-printing process, because the solvent used in SAM may easily dissolve the inkjet-printed layer.On the other hand, the O 2 plasma treatment can also improve the surface characteristics of the dielectric layer [9].But the plasma would break the bonds at the dielectric surface which would drive the drain current unsaturated [10].To address these issues, some researchers have directly printed blending composite materials to improve the electrical properties of OTFTs [11].Blending carbon nanotubes (CNTs) and organic semiconductive polymer as an active layer can enhance the carrier mobility of OTFTs [12][13][14][15].However, forming a homogenous suspension of the CNT-blended material needs additional treatments such as ultrasonication or functionalized CNT.
Compared with the CNTs/organic semiconductor in the previous reports, carbon nanoparticles (CNPs) could be a proper choice for inkjet printing because the CNP-blended ink has higher uniformity than CNT-blended ink does.In this study, the OTFTs based on composite organic materials, that is, poly(3-hexylthiophene) (P3HT) blended with CNP, were fabricated to study the mobility enhancement phenomenon.In addition, a series of material analyses were accomplished to verify its characteristics.For instance, Xray diffraction (XRD) measurements were used to study the crystalline ordering in the composite film; Fourier transform infrared (FTIR) spectra were used to confirm the bonding formation between the CNP and P3HT; ultraviolet-visible light (UV-Vis) and photoluminescence (PL) spectra were conducted to explore the formation of the charge-transfer complexes (CTC).The contact resistance measurements are achieved to explore the interfaces between the electrodes and the active layer, further to study the carrier injection barrier.Based on the above analysis and the local electric field enhancement model [13,16,17], the percolation theory interpreted the electrical properties of the OTFTs at the different blended CNP concentrations.

Materials and Methods
The all-inkjet-printed OTFTs were fabricated by using a homemade inkjet printing system.The fabricated OTFTs was bottom-gate, bottom-contact structure.We used 0.3 wt.% P3HT (Uni-Onward Corp., Taiwan), dissolved in p-xylene, as an organic semiconductor.The P3HT solution was blended with various concentrations of CNPs (Qf-Nano Tech.Co. Ltd., Taiwan, model: QF-PHG-1P) including 1 wt.%, 5 wt.%, 10 wt.%, and 43 wt.% with respect to P3HT.The manufacturing process of the OTFTs is shown in detail in previous report [18].In this study, the channel length and width are typically 20 m and 400 m, respectively.However, to verify the contact resistance of OTFTs, a series of transistors was made in different channel lengths (20,60, and 100 m), and the channel width was expanded to 800 m.The electrical properties of the fabricated OTFTs were measured by using Agilent 4156C in a dark environment with humidity controlled (relative humidity <25%) in atmosphere.
To identify the material properties of the blended semiconductors, the fabricated OTFTs were analyzed by a series of methods.The solid-state UV-Vis absorption spectra were determined by using Jasco V-570 UV/V-s/NIR spectrophotometer.The PL spectra were obtained by using Jasco FP-6300 with an excitation wavelength of 453 nm.The FITR transmittance spectra were derived by using Thermo Nicolet NEXUS470.The XRD were recorded by using PANalytical, X' Pert PRO.The field emission scanning electron microscope (SEM) images were performed by using JOEL JSM-6700F.

Results and Discussion
Figure 1 shows the transfer characteristics of the all-inkjetprinted OTFTs with various CNP concentrations.The threshold voltages (  ) of the OTFTs were obtained from the plot of the  GS - DS 1/2 at saturation region by extrapolating the fitting line and locating the -intercept, as shown in Figure 1(b).However, the switching behavior of the transistor degenerated, while 43 wt.%CNP blended, as shown in Figure 1(a).Because the CNP dominated the channel conductance, the   of OTFT with 43 wt.%CNP could not be defined.
To obtain the field-effect mobility, the output characteristics of the OTFTs are shown in Figure 2. Three kinds of methods were approached to calculate the average field-effect mobility accurately for different OTFTs.Because the OTFTs blended with 0 wt.%, 1 wt.%, and 5 wt.%CNP exhibited saturation behavior (as shown in Figures 2(a)-2(c)), the fieldeffect mobility of such OTFTs was calculated in saturation region ( DS = −70 V) by plotting the  GS - DS 1/2 and fitting the data to the equation [12,14] where the capacitance   = 11.0 × 10 −9 F cm −2 .The OTFT with 10 wt.% CNP did not present saturation behavior (as shown in Figure 2(d)) but was still modulated by the gate bias.Consequently, the average field-effect mobility was estimated in linear region ( DS = −60 V) by using the equation [19] As shown in Figure 2(e), at the same time, the OTFT with 43 wt.%CNP had neither saturation behavior nor gatemodulation effect.First, the conductivity was derived from the channel length and cross-sectional area ( =  × ) by where the thickness  = 100 nm.Then the mobility could be estimated by [20]  =   .
The electrical properties of the OTFTs including mobility, on/off ratio, and   are summarized in Table 1.The effective mobility ( eff ) of OTFT ranges from around 10 −4 to 10 −2 cm 2 /V-s, which is comparable to those of inkjet-printed P3HT OTFTs reported previously [11,21].The  eff can be modeled by using percolation theory as described by the following equation [22,23]: where the  pristine-P3HT was the intrinsic mobility of pristine P3HT; the maximum mobility ( blend-max ) was obtained from the OTFT with highest CNP blending concentration (i.e., 43 wt.%);  is the blending concentration of CNP; the   is percolation threshold; and  is a fitting parameter.After fitting the effective mobility to (5), the following parameters were extracted:   = 7.5 wt.% and  = 1 wt.%.The fitting result is plotted in Figure 3  of P3HT can be increased [13] and the effective mobility of the OTFTs was also increased.When the CNP concentration is near or equal to the   , the percolation path is gradually formed, which is caused by the aggregation of CNPs.While the CNP concentration is above the   , the charge carrier would travel directly through the percolation path, and the CNPs will dominate the conductivity.It should be noted that the local electric field, which was induced by CNP, would also affect the threshold voltage of OTFTs.The threshold voltage was shifted from −12.5 V (pristine P3HT) to 3 V (CNP/P3HT = 10 wt.%), which increased with the CNP concentration.The similar phenomena and the field enhancement factor have also been reported [16,17].
To investigate the mechanism of the improved performance, the contact resistance between the P3HT and the electrodes, poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT : PSS), was studied.In order to exam the electronic properties at the interface, the OTFTs were operated in linear region ( DS ≪  GT ;  DS = −0.1 V;  GT =  GS −   = −5, −15, −25, −35, −45, and −55 V) with different channel lengths ( = 20, 60, and 100 m) and constant channel width ( = 800 m).Figure 4 shows the on-resistance ( on ) as a function of channel length at fixed  GT for pristine P3HT and CNP-blended OTFT.The  on can be represented by  on =  ch +   , where  ch represents the channel resistance and   symbolizes the contact resistance.The contact resistance and channel resistance of OTFT with  = 20 m are summarized in Table 2. Figure 4 also shows that incorporating CNPs with P3HT reduced the contact resistance and channel resistance by an average factor of two.These results suggested that incorporating CNPs could modulate the hole-injection barrier between the PEDOT and P3HT, which could reduce the contact resistance and result in a higher effective carrier injection [14,24].
The morphology of the CNP/P3HT nanocomposites was observed by an optical microscope and SEM. Figure 5(a) shows the optical images in the channel region of OTFTs with 0 wt.%CNP, 1 wt.%CNP, 5 wt.%CNP, and 10 wt.% CNP.These images indicated that the CNPs aggregated when the concentration of CNP increased.For the detailed dispersion

R
on (Ω)  As aforementioned, CNP improved the injection barrier and enhance the field-effect mobility at low blending ratio.When the blending concentration reaches a critical value, however, the CNPs form a percolation path, and the excess localized aggregations increase the off-current.As a consequence, the fabricated OTFT is turned into a resistor-like device and it might be further dominated by the electrical properties of CNPs.Based on the observations from Table 2 and Figure 5, small groups of CNP would aggregate, but the percolation path would not be connected when CNP/P3HT <10 wt.%.When the CNPs concentration was up to 10 wt.%, some localized aggregations started to connect to each other.While CNP/P3HT = 43 wt.%, most of CNPs were aggregated and the electrical properties of OTFT were dominated by CNPs.
In other words, the drain current of fabricated OTFT devices was not modulated by the gate bias.Previous investigations in XRD spectra show the intensity of the (100) reflection, which is caused by the lamellar layer structure, reduced with the increasing amount of CNP [18].This result indicated that adding CNP into P3HT will disturb the crystal ordering of the organic semiconductor.In general, the decreasing of crystal ordering suppresses the carrier transportation.However, adding CNP into P3HT improved the field-effect mobility of OTFT.To further explore the reason of this mobility enhancement, other analyses were executed.
The functional groups were identified by the absorption band of the FTIR spectra for P3HT and CNP/P3HT nanocomposite, as shown in Figure 6.The band at 721 cm −1 is associated with the methyl rock of P3HT.The peak at 819 cm −1 is contributed by the aromatic C-H, out-of-plane.The band at 1377 cm −1 is assigned to methyl deformation.The band at 1457 cm −1 is assigned to the symmetric C-C stretching mode, and the 1509 cm −1 is assigned the antisymmetric C=C stretching mode.We could mark the variation of the average conjugation length of P3HT from the ratio between the intensity at 1509 cm −1 and 1457 cm −1 [25,26].If the relative intensity increases at 1509 cm −1 , the average conjugation length will increase.From Figure 6, the intensity ratio is kept the same, so the average conjugation length of P3HT backbones did not change after adding CNPs, which means that the molecule structure of P3HT is maintained.Consequently, no new bonding was observed between CNPs and P3HT.
1.00 × 10   To study interactions among the P3HT chains in the CNP/P3HT, furthermore, the UV-Vis spectra of P3HT and CNP/P3HT nanocomposites are shown in Figure 7.The absorption peak which located at 453 nm represented to the electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the P3HT [27,28].Geng et al. reported that the  electrons of the blended CNTs would interact with the conjugated polymers, which represent as the UV-Vis absorption peaks broadening [29].The similar phenomenon could be observed in the spectra of CNP/P3HT.Figure 7 shows that the main absorption peak was broadened with the increasing amount of CNPs because of the - interaction between the conjugated chain of the P3HT and the  orbital of CNPs.
The LUMO-HOMO relaxation could be observed from the maximum emission peak ( max ) of the PL spectra [29], as shown in Figure 8.The spectra mark the  max at 589, 586, 572, and 566 nm for 0 wt.%, 1 wt.%, 5 wt.%, and 10 wt.% CNP/P3HT, respectively.The main emission peak appeared small blueshift with the increasing concentration of the blended CNP.This is because the ground state energy of the composite material was lower than that of P3HT.This difference results in the increase of the energy gap of relaxation [30,31].Another phenomenon that could be observed from the PL spectra is quenching effect.The PL quenching revealed that the CNPs may offer another decaying path which competing with the radiative relaxation and the quenching effect became more obvious with the CNPs concentration increased [30].Both the blue shift and quenching effects illustrated that the charge transfers appeared from the LUMO of P3HT to CNPs [29][30][31][32].Consequently, the CTC state formed in the CNP/P3HT composites and the energy level of the CTC state was between the work function of CNP and the HOMO of P3HT [33,34].
According to the experiments shown in this paper, the blending CNP/P3HT would not modify the molecular structures of P3HT or CNPs, either adjust the intrinsic mobility of P3HT or CNPs.Therefore, three reasons for effective mobility enhancement by incorporating CNP/P3HT nanocomposites could be concluded: (1) local electric field enhancement resulted from the CNPs that will induce higher carrier density of P3HT, (2) adding CNPs in P3HT matrix would reduce the carrier injection barrier and improve the injection efficiency, and (3) the formation of the CTC would enhance the carrier transportation between CNPs and P3HT.
The band diagrams of CNP/P3HT illustrate the effect of the local electric field, as shown in Figure 9.When the negative gate bias is applied, p-type organic semiconductor will be operated in accumulation mode, as shown in Figure 9(b).The addition of CNPs will induce local electric field to drive the P3HT near CNPs experience larger gate bias than that of external applied.Therefore, the P3HT will induce more carriers and further increase the effective mobility.Second, the work functions are 5.2-5.3,5.13, and 4.9 eV, respectively, for PEDOT, CNP, and P3HT [35][36][37][38].Adding CNPs into P3HT matrix can reduce the injection barrier which was verified by contact resistance measurement.Third, according to the material analyses in this study, we proposed that the formation of CTC at the interface between P3HT and CNPs could assist the carrier transportation in the channel region to enhance the effective mobility.

Conclusion
Adding CNPs in P3HT matrix enhanced the effective mobility by a factor of 10.The improvement of the contact resistance resulted from the injection barrier decreased at the interface between PEDOT and P3HT by cooperating CNPs into P3HT matrix.On the other hand, the improvement of the channel resistance is caused by the formation of CTC.According to the percolation model, local electric field is enhanced, while Advances in Materials Science and Engineering the blending concentration is less than the threshold concentration.This enhancement can induce more carriers of P3HT near CNPs to increase the effective mobility.If CNP concentration is near   , the percolation path is gradually formed.The local field enhancement will gradually diminish due to the CNPs aggregation.After the percolation path formed, charge will transport directly through the path.The percolation paths formed from the considerable aggregations of CNPs will dominate the electrical properties and degrade transistor characteristics of the OTFT.In this work, the basis of CNP/P3HT blended system has been analyzed for further applications in OTFT.

Figure 3 :
Figure 3: Percolation model for simulation and experimental data.

Figure 4 :
Figure 4: On resistance ( on ) measurement as a function of channel length with various  GT for (a) pristine P3HT and (b) CNP/P3HT = 5 wt.%OTFTs.

Table 2 :
Contact resistance and channel resistance analysis.