3.1 Solution processed PDPP2T-TT-OD/pentacene composite thin film
Pentacene is one of the most widely used materials in small molecule organic semiconductors with the exceptional thermal stability, whereas it is essentially insoluble in conventional organic solvents at room temperature. Therefore, vacuum deposition methods are frequently used to fabricate pentacene films. Additionally, solution-processing is a desirable choice for organic semiconductors because of the low cost and simple operation by solution deposition of thin films. Hence, soluble Pre-PENT was selected as the nucleating agent, which was able to be dissolved in conventional organic solvents and reverted to pentacene in one-step at a certain temperature (Fig. 1a).
The chemical structures of the Pre-PENT after and before heating were characterized by UV-Vis absorption spectra (Fig. S1) and FTIR (Fig. 1b). For the sake of the noticeable comparison of characteristic peaks in Fig. 1b, the peak intensity of the pentacene precursor before heating was subjected to an overall weakening treatment (that is, the peak intensity×0.4). The characteristic absorption peak at 1706 cm− 1 of the Pre-PENT before thermal decomposition attributed to the stretching vibration of the carbonyl group (C = O), the multiple absorption peaks in the range of 1000 and 1300 cm− 1 assigned to the stretching vibration of N-sulfonyl carbamate group, and the stretching vibration peak at 2978 cm− 1 corresponding to the unsaturated -CH bond were able to be apparently observed. Nevertheless, the above absorption peaks were evidently weakened after the precursor was heated at 150°C for 1 h, and new absorption peaks appeared at 905 cm− 1 and 728 cm− 1, confirming the precursor was decomposed to form pentacene after heat treatment as expected.
The Pre-PENT and polymeric semiconductor (PDPP2T-TT-OD) thin film was spin-coated from chloroform solution on the silicon substrate. Later, the Pre-PENT was thermally decomposed into pentacene and then crystallized, the polymer crystal structure was going to be further adjusted during annealing at the same time (Fig. 1c). In fact, the interactions between solute and substrate resulted in phase separation due to diverse adhesion of polymer/small molecule semiconductor to the substrate [33, 34]. As PDPP2T-TT-OD and pentacene existed in a thin film state, an electrical potential developed during phase separation bringing about remarkable changes in the microstructures of thin film [35, 36]. Physicochemical characterizations of the composite semiconductor films were carried out via manifold test methods, and the influence of small molecule crystal nuclei on the crystallization behavior of polymers was investigated in the following sections.
3.2 Characterization of structure and surface morphology
In order to more intuitively observe the accumulation of polymer chain segments near the pentacene crystals, a specific area was selected for AFM microscopic morphology. There were two-phase substances presented in Fig. 2a, one (A area) was the small molecular phase tending to nucleate uniformly, the other (B area) was a large-area polymer phase with a flat surface. As illustrated in Fig. 2b, the height of the monolayer molecular chain was about 3 nm (red line area) demonstrated by the cross-sectional line profile analysis, which was similar to the monolayer height of PDPP2T-TT-OD [37]. Analogously, the average height of the steps was 1.4 nm (blue line area) in the island-like phase as shown in Fig. 2d, which was consistent to the van der Waals size of pentacene in the long axis direction [38]. The above corresponding molecular dimensions are revealed by ball-and-stick model in Fig. 2c. Since pentacene dispersed in the polymer matrix in the form of particles, the phase separation phenomenon of blend was able to be explained by the nucleation growth mechanism [39]. In this way, an island-like structure where one phase distributed in another phase was formed, namely, “sea-island” structure.
The influence of the introduction of pentacene on the crystallization of PDPP2T-TT-OD for hybrid thin films was examined by XRD profiles as shown in Fig. 2d. The pure pentacene (0:1) film displayed three plane diffraction peaks at 2θ = 6.10°, 12.24° and 19.26°, which separately belonged to (001), (002) and (003) lattice planes. Moreover, the pure PDPP2T-TT-OD (1:0) film had a sharp (100) lattice plane diffraction peak at 2θ = 4.44°. With the increase of the pentacene in the composite thin film (5:1, 2:1, 1:1), the peak intensity of polymer moiety (100) lattice plane was clearly faded, and the characteristic peak of pentacene appeared. The weakening of the absorption band in polymer moiety was owing to the large amount of pentacene, blocking the arrangement in polymer molecular chains, so then suppressing the crystallization. However, when the pentacene was lessening in the composite thin film (10:1, 15:1, 20:1), the diffraction peak intensity of polymer moiety (100) lattice plane became stronger, which was higher than that of the pure polymer indicating a better crystallinity [40]. In addition, a new weak (200) lattice plane diffraction peak arose in the composite thin film, suggesting the crystallinity had improved through π-π stacking interaction between polymer chains and small molecule moieties due to the addition of pentacene as well [41].
The effect of introducing pentacene on the optical absorption properties of PDPP2T-TT-OD for hybrid thin films was studied via UV − vis absorption spectra in Fig. 2e. There were two absorption bands named Band Ⅰ and Band Ⅱ. Band Ⅰ was the absorption peak subdivided into (0–0), (0–1) and (0–2) absorption peaks for charge transfer within the molecule, and Band Ⅱ was the weak π-π transition peak with the summit at 400–500 nm. The PDPP2T-TT-OD film had a dominant peak at 740 nm (0–1) and another two shoulder peaks at 675 nm (0–2) and 820 nm (0–0), indicating the molecular aggregation resulting in the intermolecular interactions. The PDPP2T-TT-OD/pentacene composite thin films also exhibited a dominant peak and two shoulder peaks. Accordingly, the intensity of the shoulder peak (0–2) weakened with the increase of pentacene, which was assigned to the enhancement of chain order and aggregation in polymer molecules.
Notably, the crystallinity of PDPP2T-TT-OD had a trend of change with the pentacene content, which was represented by XRD profiles (Fig. 2d). The inherent driving force for this change was that the pentacene crystal nuclei induced the ordered arrangement of the polymer backbones through π-π interactions, and this induction could also be manifested via the absorption peaks of electrons transitioning between two kinds of molecular energy levels, as shown in UV-vis spectra (Fig. 2e).
So as to explore the surface morphology and phase structure of PDPP2T-TT-OD/pentacene composite thin films, the AFM measurement was implemented in tapping mode as illustrated in Fig. 3a-d. The results show micropores on the surface of the polymer film (1:0), which was potential to be the traps during the charge transfer process and thus decrease the device performance [42, 43]. The composite thin films in the mass blending ratios of 20:1 and 10:1 had a few large holes without the orientation of polymer chain packing. But the composite thin film (15:1) possessed the distinctive fibrous structure assigned to the more uniform dispersion of pentacene in polymer matrix. The similar micromorphology could also be observed by POM (Fig. 3e-h). And yet the chain packing existed the evident orientation induced via π-π interaction revealing in Fig. 3i, which was beneficial to the transportation of the carriers to achieve a better performance. Briefly, the oriented film with the low surface roughness means high planarity and structural uniformity, contributing to the preparation of organic electronic devices.
3.3 Analysis of carrier transport properties
OTFTs with a bottom-gate/top-contact (BGTC) configuration (Fig. 4a) were subsequently fabricated to study the charge transport properties of the composite thin films. The optical image of the devices with the channel length and width of 50 and 1200 µm, respectively, is presented in Fig. 4b. The complete device fabrication and characterization methods were included in the experimental section. The transfer and output characteristics of OTFTs devices based on PDPP2T-TT-OD/pentacene composite thin films are depicted separately in Fig. 4c, 4e and Fig. S4. The performance parameters of the devices were summarized in Table 1. The data demonstrate that the device based upon the composite thin film in a mass blending ratio of 15:1 exhibited peak value (6.56 × 10− 3 cm2 V− 1 s− 1), which was 16 times higher than that of the pure polymer-based (1:0) OTFT (4.06 × 10− 4 cm2 V− 1 s− 1), attributed to the improvement of crystallinity in composite. It is suggested that the introduction of pentacene facilitated the orderly accumulation of polymer chains, so then enhancing the interaction between molecular chains and achieving the purpose of optimizing the charge transport path [44, 45].
Further, the effect of anti-solvent on the charge transport property in composite thin films was investigated with the same configuration of OTFTs. The transfer and output characteristics of OTFT devices based on PDPP2T-TTOD/pentacene (15:1) composite thin films after anti-solvent treatment were illustrated in Fig. 4d, 4f and Fig. S5, respectively. The results indicate that the mobility of composite thin film OTFTs after the mixed solution of chloroform/ethanol (volume mixing ratio of 50/1) treatment, increased to 1.11 × 10− 2 cm2 V− 1 s− 1, which was 27 times than the pure polymer-based OTFT. It was ascribed that the addition of appropriate amount of anti-solvent can effectively promote the further aggregation and orderly intermolecular packing of polymer chains in solution. In other words, the crystallinity, surface morphology, roughness and grain size of the blending films were able to be optimized by regulating the type and ratio of the solvent and anti-solvent to acquire the optimum charge transport performance, as demonstrated by XRD profiles (Fig. S2) and AFM morphological images (Fig. S3). The detailed description and quantitative analysis of the relationship between grain size and the full width at half maxima (FWHM) were shown in the Supplementary Information (SI).
Table 1
Summary of the OTFTs performance for the PDPP2T-TT-OD/pentacene composite thin films
Sample
|
µave a (cm2 V− 1 s− 1)
|
µmax b (cm2 V− 1 s− 1)
|
VTh c (V)
|
Ion d (A)
|
Ion/Ioff e
|
SS f (V/dec)
|
1:0
|
3.74 (± 0.32) × 10− 4
|
4.06 × 10− 4
|
-1
|
1.06 (± 0.11) × 10− 7
|
~ 1.8 × 103
|
1.54 (± 0.96)
|
20:1
|
1.68 (± 0.76) × 10− 4
|
2.26 × 10− 4
|
-1.63 (± 0.48)
|
4.62 (± 1.91) × 10− 8
|
~ 3.44 × 103
|
0.82 (± 0.24)
|
15:1
|
5.04 (± 1.35) × 10− 3
|
6.56 × 10− 3
|
-1.17 (± 0.29)
|
1.69 (± 0.46) × 10− 6
|
~ 2.16 × 104
|
0.60 (± 0.20)
|
10:1
|
7.98 (± 1.39) × 10− 4
|
6.81 × 10− 4
|
-1.25 (± 0.50)
|
2.19 (± 0.35) × 10− 7
|
~ 1.09 × 104
|
1.18 (± 0.88)
|
Ethanol
|
6.70 (± 3.61) × 10− 3
|
1.11 × 10− 2
|
-1.33 (± 0.39)
|
16.8 (± 0.80) × 10− 6
|
~ 5 × 105
|
0.38 (± 0.06)
|
Methanol
|
2.10 × 10− 3
|
2.10 × 10− 3
|
-2
|
5.49 × 10− 7
|
~ 5 × 102
|
1.79
|
Cyclohexane
|
7.67 (± 2.63) × 10− 4
|
1.05 × 10− 3
|
-2.67 (± 0.57)
|
1.68 (± 0.61) × 10− 7
|
~ 3 × 104
|
0.81 (± 0.61)
|
a Average charge carrier mobility µave. b Maximum charge carrier mobility µmax. c Threshold voltage VTh, was estimated by linear extrapolation IDS1/2 with respect to VGS. d On state current Ion. e On/off ratio Ion/Ioff. f Subthreshold swing SS is the inverse of the maximum slope of the IDS–VGS plot. All the OTFT devices were tested with the same channel dimensions (W/L = 1200/50 µm), VDS = -20 V. The PDPP2T-TT-OD/pentacene composite thin films were prepared separately via the mass blending ratios of 1:0, 20:1, 15:1, 10:1 in chloroform (the first four lines). And the selected composite thin films blended in the mass ratio of 15:1 were further treated respectively in the volume mixing ratios of solvent/anti-solvent as follow: chloroform/ethanol = 50/1, chloroform/methanol = 20/1, chloroform/cyclohexane = 20/1 (the last three lines). |
Whereas, in contrast to the performance of PDPP2T-TT-OD based transistors in the previous reports [46–48], our field-effect mobility value of OTFTs was lower. On the one hand, perhaps due to the fact that the SiO2 surface of the substrate was not passivated with silane molecules, such as octadecyl trichlorosilane (OTS) and hexamethyl disilazane (HMDS) [49], resulting in high defect density and charge carrier trap states at the interface of the semiconductor/dielectric layer, hindering the effective transport of carriers. On the other hand, there was a nonlinear relationship at the low drain voltage (VD) from the output characteristics, signifying a higher electron injection barrier and a larger the interface contact resistance at the electrode/semiconductor layer interface, which affected the effective injection of electrons [50]. Thus, the interface of the dielectric/semiconductor layer and the electrode/semiconductor layer can be adjusted to optimize the effective carrier transport and electron injection, thereby further improving the field-effect mobility of OTFTs [51].