Low surface energy interface-derived low-temperature recrystallization behavior of organic thin films for boosting carrier mobility

Shuya Wang , Zhan Wei , Yahan Yang , Xiaoli Zhao *, Qingxin Tang *, Yanhong Tong * and Yichun Liu
Centre for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China. E-mail: zhaoxl326@nenu.edu.cn; tangqx@nenu.edu.cn; tongyh@nenu.edu.cn; Fax: +86-431-85099873; Tel: +86-431-85099873

Received 12th September 2019 , Accepted 26th September 2019

First published on 30th September 2019


Due to low-temperature processing properties and high carrier mobilities, solution-processed small molecule organic thin-film transistors (OTFTs) are promising candidates for enabling low-cost flexible electronic circuits and displays. Much progress has been made in terms of material performance, however, there remain significant concerns about well understanding and controlling the morphology and polymorphism of soluble small molecule organic semiconductor thin films. Here, we investigated the physical mechanisms of the dielectric interfacial properties on the recrystallization of soluble small molecule organic semiconductor thin films, and demonstrated an effective route to improve the thin-film morphology and device performance. We found that a low surface energy interface can derive low-temperature recrystallization behavior of solution-processed organic semiconductor thin films, which has an important impact on boosting the carrier mobility of OTFTs. After low-temperature recrystallization, the carrier mobility of OTFTs is dramatically enhanced by about one order of magnitude. These attractive results suggest that this work allows a better understanding about the role of dielectric interfacial properties in assisting the thin-film recrystallization towards the desirable thin-film morphology, indicating promising potential for future low-cost high-performance solution-processed organic electronic devices.


Introduction

Organic thin-film transistors (OTFTs) have attracted tremendous attention for their potential applications in wearable and implantable electronics,1–6 wherein soluble small molecule organic semiconductors (OSCs) are especially interesting owing to their high carrier mobilities and compatibility with solution-processed techniques, which promises low-cost manufacturing of high-performance flexible and conformal OTFTs.7,8 Efforts in the past decade to carefully engineer the molecular structure of small molecule OSCs and control their microstructure and morphology have led to carrier mobilities now far surpassing those of amorphous silicon – the modern industrial standard TFT material.9–11 Even higher mobilities of over 10 cm2 V−1 s−1 have been reported in polycrystalline or single-crystal thin films.12–16

Although many successes have been achieved in this area, thorny obstacles still exist in well understanding and controlling the morphology and polymorphism of soluble small molecule OSC thin films. Because of the weak van der Waals interactions between organic molecules, the charge transport properties of solution-processed OSC thin films are strictly related to their final morphologies, molecular orientations and microscopic structures, which strongly influence the carrier mobility of OTFTs.17–20 During solution processing, small molecule OSCs generally suffer from uncontrolled nucleation and growth, which results in thin films with poor reproducibility and performance limiting their functionality in electronic devices.21,22 To overcome this hurdle, considerable efforts have been devoted to tuning the molecular packing and thin-film morphology by improving the fabrication conditions or by interface engineering, such as deposition technique,23,24 solvent evaporation rate,15,25 liquid surface tension force,16,25 solvent vapor annealing,26,27 thermal treatment28–30 and the surface energy of gate dielectric layer.31 However, in spite of considerable progress during recent years, the understanding of the correlation between the fabrication conditions and the final morphologies in terms of the crystallization mechanism of solution-processed OSC thin films is still deficient.

In this article, on a low-surface-energy octadecyltrichlorosilane (OTS) modified dielectric layer, we demonstrated high-performance small molecule-based OTFTs via simple spin coating, wherein the solution-processed thin films could controllably recrystallize via low-temperature post-annealing treatments. We found that the surface energy of the dielectric layer has a key role in the thermodynamics and kinetics of the recrystallization process. The low surface energy interface effectively derives the low-temperature recrystallization behavior of OSC thin films, which boosts the rearrangement of organic molecules towards the desirable smooth thin-film morphology. As a result, due to the low surface energy of the dielectric surface, the carrier mobility of OTFTs is dramatically enhanced by about one order of magnitude after near-to-room temperature post-annealing treatment (≤40 °C). This methodology can be used to well control and improve the recrystallization behavior of solution-processed small molecule OSC thin films for high-performance electronic devices, allowing a better understanding about the role of dielectric interfacial properties in assisting the recrystallization of small molecule OSC thin films.

Experimental section

OTFT fabrication

Highly doped silicon substrates with 300 nm SiO2 were cleaned by sonication in ethanol and deionized water for 10 min, and subsequently were dried with nitrogen gas on filter paper. These clean substrates were hydroxylated with oxygen plasma treatment at 100 W for 30 s. Then, these clean substrates were placed into a vacuum oven for the OTS (95%, Alfa Aesar) vapor treatment. The treatment time of OTS is 30 min at a temperature of 60 °C. Hereafter, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene, >99%, Sigma-Aldrich) was dissolved in hexane with a concentration of 10 mg mL−1. TIPS-pentacene thin films were deposited on the OTS/SiO2 dielectric layer by the spin-coating method. The spin coating was carried out at 7000 rpm for 30 s, followed by annealing at different temperatures and times in the Petri dish at ambient pressure. For comparison, TIPS-pentacene thin films were also spin-coated on the SiO2 dielectric layer, and then were annealed at 40 °C for 15 min. In addition, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT, 99%, Sigma-Aldrich) was also dissolved in hexane (3 mg mL−1), and then was spin-coated on the OTS/SiO2 dielectric layer using the optimal annealing temperature and time to verify the universality. Finally, 30 nm Au was deposited onto the TIPS-pentacene and C8-BTBT thin films by vacuum thermal evaporation through a shadow mask as a source and drain electrodes to construct bottom-gate top-contact OTFTs.

Characterization

Contact angles were measured by drop shape analysis using a drop shape analyzer. AFM measurements were carried out using the Dimension ICON instrument (Bruker, Germany). Coverage rates of TIPS-pentacene thin films were calculated by NanoScope Analysis. Electrical properties of TIPS-pentacene and C8-BTBT OTFTs were recorded with a Keithley 4200-SCS and the Cascade M150 probe station in a clean and shielded box at room temperature in ambient conditions. Field-effect mobilities (μ) were calculated in the saturation regime using the following standard equation:
 
image file: c9tc05043d-t1.tif(1)
where W and L stand for the width and length of the channel, respectively, Ci for capacitance per unit area of the dielectric layer, ≈10 nF cm−2 at 1 kHz measured using the Keithley 4200-SCS-CUV (Fig. S1, ESI), IDS for source–drain current, and VG for the gate voltage.

Results and discussion

Recrystallization process of the organic thin film

Fig. 1 clearly shows the recrystallization process of the TIPS-pentacene thin film. When TIPS-pentacene/hexane solution was spin-coated on the OTS/SiO2 dielectric layer, the OSC thin film recrystallized and its morphology changed drastically after a low-temperature post-annealing treatment in ambient conditions. The post-annealing temperature and time were fixed at 40 °C for 15 min. Quite different from the previous reports, our post-treated temperature is much lower than the reported annealing temperature (>100 °C),28–30 and is also much lower than the phase transition temperature of TIPS-pentacene (124 °C).32 For this phenomenon, the key reason can be attributed to use of the OTS molecule to modify the dielectric surface. As shown in Fig. 1a, a smooth OTS self-assembled monolayer (SAM) with the root-mean-square (RMS) of 0.23 nm is modified on the SiO2 surface, and shows a hydrophobic nature with the contact angle of 108°. By selecting hexane as a preferred solvent to ensure the good wettability of TIPS-pentacene solution on the hydrophobic surface, the continuous TIPS-pentacene thin film was successfully fabricated (Fig. S2, ESI). In our experiment, OTS was selected as the modification layer of the dielectric layer because of two main reasons. One reason is the decrease of the interfacial defect to ensure interface quality for further device preparation.33 The other key reason is its low surface energy, which facilitates the OSC molecule–molecule interactions rather than the molecule–substrate interaction, i.e., enables OSC molecules to move more easily on the solid dielectric surface.34,35 In Fig. 1b, the directly spin-coated thin film without any treatments obviously shows a disordered microstructure and rough surface with an RMS of 18.7 nm. After the low-temperature post-annealing, the thin film becomes uniform and smooth, and its roughness is reduced by about one order of magnitude (2.37 nm) as shown in Fig. 1c. This result manifests that the low surface energy interface allows the solution-coated thin film to recrystallize at a low post-annealing temperature.
image file: c9tc05043d-f1.tif
Fig. 1 Recrystallization behavior of the organic semiconductor thin film. Schematic diagrams and the corresponding AFM images of OTS/SiO2 (a), pristine thin film (b) and recrystallized thin film (c). The inset of (a) is the contact angle of deionized (DI) water on OTS/SiO2. Recrystallization behavior is achieved successfully by spin coating the TIPS-pentacene thin film on a low surface energy OTS/SiO2 dielectric (a and b), and then treating by low-temperature post annealing at 40 °C for 15 min (b and c).

Influence of surface energy on the thin-film recrystallization

To further verify that the low-surface-energy interface derives the low-temperature recrystallization behavior of OSC thin films, and explore the effect of dielectric surface energy on its recrystallization behavior, we spin coated TIPS-pentacene thin films on SiO2 and OTS/SiO2 dielectric surfaces with different surface energy by the same preparation conditions (Fig. 2). The spin coating was carried out at 7000 rpm for 30 s. The post-annealing condition is fixed at 40 °C for 15 min. Moving forwards, we fabricated bottom-gate top-contact OTFTs to further evaluate the performance of TIPS-pentacene thin films on two different dielectrics. Fig. 2a1 and a2 show the corresponding device structure, respectively. AFM images reveal the morphologies of TIPS-pentacene thin films before and after post-annealing under different surface-energy dielectrics (Fig. 2b1 and b2). The corresponding transfer characteristics of TIPS-pentacene OTFTs are shown in Fig. 2c1 and c2. It can be obviously observed that TIPS-pentacene thin films on different surface-energy dielectrics present a rather different morphology after low-temperature post-annealing. On the SiO2 dielectric layer, the morphology and field-effect mobility of the TIPS-pentacene thin film are almost the same after the post-annealing treatment, while the morphology and field-effect mobility of the OTS/SiO2 dielectric layer are improved significantly. As shown in Table 1, mobility increases by more than one order of magnitude due to the recrystallization behavior of the TIPS-pentacene thin film.
image file: c9tc05043d-f2.tif
Fig. 2 Low surface energy induced the thin-film recrystallization. (a1 and a2) Schematic device configuration of TIPS-pentacene OTFTs with and without the OTS-modified layer. AFM images (20 μm × 10 μm) and height curves of TIPS-pentacene thin films before and after post-annealing treatment and the corresponding typical transfer characteristics of TIPS-pentacene OTFTs with (b1 and c1) and without an OTS-modified layer (b2 and c2).
Table 1 Surface energy, adhesion energy, and the corresponding mobility of TIPS-pentacene OTFTs before and after the post-annealing treatment on different dielectrics
Dielectrics Surface energy (mJ m−2) Adhesion energy (mJ m−2) Post annealing Mobility (cm2 V−1 s−1)
SiO2 66.1 ± 0.4 78.3 ± 0.5 Before 9.4 × 10−5
After 5.5 × 10−5
OTS/SiO2 17.9 ± 0.2 50.6 ± 0.3 Before 0.12
After 1.45


The previously reported recrystallization behavior mainly applied high-temperature annealing to enable phase transition of OSC thin films, which limits the scope of application of the OSC materials,28 because most of the small molecule OSC thin films are extremely fragile, and they inevitably suffer from severe thermal radiation damage under high-temperature annealing resulting in the decrease of device electrical performance.36,37 In contrast, we applied near-to-room temperature annealing to enable the recrystallization of OSC thin films. In our experiments, low-temperature recrystallization behavior of TIPS-pentacene thin film is mainly related to the low surface energy of the OTS/SiO2 dielectric layer.

It can effectively weaken the adhesive energy between the TIPS-pentacene thin film and the dielectric layer, making it lower than the cohesive energy of the TIPS-pentacene thin film itself, thereby promoting the interaction between the OSC molecules. According to the measured surface energy of the dielectric (γpD, γdD) and the TIPS-pentacene thin film (γpTIPS, γdTIPS) (Fig. S3 and Table S1, ESI), the adhesive energy (Ead) was calculated using the following equation:38

 
image file: c9tc05043d-t2.tif(2)
According to this equation, the value of the dielectric surface energy is proportional to the value of the adhesion energy. Therefore, low surface energy of the OTS/SiO2 dielectric layer can weaken the adhesive energy between the TIPS-pentacene thin film and the dielectric layer. The cohesive energy of TIPS-pentacene thin film can be calculated via the surface energy of the TIPS-pentacene thin film (γpTIPS, γdTIPS) (Table S1, ESI), the calculated equation is:35
 
Ecoh = 2(γpTIPS + γdTIPS) = 73.0 ± 0.8 mJ m−2(3)

Table 1 shows the calculated adhesive energy and cohesive energy on different dielectrics according to two formulas described above. The adhesive energy of TIPS-pentacene thin film on the OTS/SiO2 dielectric layer is lower than the cohesive energy of TIPS-pentacene thin film, i.e., it is energetically more favorable for TIPS-pentacene molecules to stick to each other in a bulk-like fashion, leading to the formation of regular and highly crystalline grains (Fig. 2b1). On the contrary, the adhesive energy of TIPS-pentacene thin film on the SiO2 dielectric layer is higher than the cohesive energy of TIPS-pentacene thin film, which leads to a locally disordered TIPS-pentacene thin film on the SiO2 dielectric layer (Fig. 2b2). This result originates from the competition between molecule–molecule and molecule–substrate interactions. If the adhesive energy is smaller, the molecule–molecule interaction is stronger than the molecule–substrate interaction, which is favorable for the diffusion of TIPS-pentacene molecules on the OTS/SiO2 dielectric surface, leading to a large mean free path and drastic changes in the TIPS-pentacene thin-film morphology (Fig. 2b1). If the adhesive energy is higher, the molecule–molecule interaction is weaker than the molecule–substrate interaction, which suppresses the diffusion of TIPS-pentacene molecules on the SiO2 dielectric surface, leading to a small mean free path and almost unchanged TIPS-pentacene thin-film morphology (Fig. 2b2). Therefore, under a very low post-annealing temperature, the TIPS-pentacene thin film on the OTS/SiO2 dielectric layer can recrystallize, and becomes very smooth with an RMS of only 2.37 nm. Meanwhile, the mobility of TIPS-pentacene OTFT based on the post-annealed thin film significantly increases compared to the pristine thin film as shown in Fig. 2c1 and Table 1. These results robustly demonstrate that the surface energy plays a crucial role in the thin-film recrystallization behavior, improving the morphology of TIPS-pentacene thin-film and the corresponding field-effect performance.

Influence of post-annealing temperature and time on the thin-film recrystallization

Temperature is known to influence the recrystallization of the OSC thin film, as well as the thin-film morphology and the corresponding electrical performance of OTFTs. Therefore, exploring the post-annealing temperature and time is critical for investigating and understanding the recrystallization behavior of solution-processed OSC thin films. To systematically carry out related research, we compare the morphology and field-effect performance of the TIPS-pentacene thin film after different post-annealing temperatures and times (Fig. 3 and 4). Fig. 3 shows the effect of post-annealing temperature on the thin-film morphology and the electrical performance. The post-annealing temperature ranges from 20 to 100 °C, maintaining a fixed post-annealing time for 15 min. In Fig. 3a, the morphology and RMS of TIPS-pentacene thin films vary greatly at different post-annealing temperatures. As the post-annealing temperature increases from 20 to 60 °C, the RMS of TIPS-pentacene thin films decreases gradually from 18.7 to 1.9 nm. However, when the post-annealing temperature further increases (>60 °C), the RMS of TIPS-pentacene thin films gradually increases. As the post-annealing temperature increases to 100 °C, the RMS increases to 16.4 nm. This can be attributed to the kinetic energy difference for the movement of TIPS-pentacene molecules under different post-annealing temperatures, leading to different mean free paths of TIPS-pentacene molecules and different morphologies of the TIPS-pentacene thin film. The corresponding transfer characteristics of TIPS-pentacene OTFTs at different post-annealing temperatures are shown in Fig. 3b. Fig. 3c exhibits dependence of mobility, roughness and coverage rate on the post-annealing temperature. All electrical tests were carried out in ambient conditions at room temperature. As the post-annealing temperature increases from 20 to 40 °C, the mobility of the TIPS-pentacene OTFTs gradually increases from 0.12 to 1.45 cm2 V−1 s−1 (Fig. 3b). To the best of our knowledge, this value (1.45 cm2 V−1 s−1) is even higher than the reported highest values of TIPS-pentacene OTFTs prepared by the spin-coating method in ambient conditions.39–46 When the post-annealing temperature further increases, the mobility of the TIPS-pentacene OTFTs decreases.
image file: c9tc05043d-f3.tif
Fig. 3 Effect of post-annealing temperature on the thin-film recrystallization. (a) AFM images (10 μm × 20 μm) and height curves of TIPS-pentacene thin films deposited from hexane solution under different post-annealing temperatures. (b) The corresponding transfer characteristics of TIPS-pentacene OTFTs with different post-annealing temperatures. (c) Dependence of mobility, roughness and coverage rate on the post-annealing temperature. The inserts are AFM images (5 μm × 5 μm) of the post-annealing temperature at 40 and 60 °C. The post-annealed time is fixed at 15 min.

image file: c9tc05043d-f4.tif
Fig. 4 Effect of post-annealing time on the thin-film recrystallization. (a) AFM images (10 μm × 20 μm) and height curves of TIPS-pentacene thin films deposited from hexane solution under different post-annealing times. (b) The corresponding transfer characteristics of TIPS-pentacene OTFTs with different post-annealing times. (c) Dependence of mobility, roughness and coverage rate on the post-annealing time. The post-annealed temperature is fixed at 40 °C.

The mobility decreases to only 0.037 cm2 V−1 s−1 when the post-annealing temperature increases to 100 °C (Fig. 3b). The increase of mobility can be ascribed to the fact that the semiconductor surface with a lower RMS can contribute to forming a good interface contact and reducing interface defects, hence facilitating carrier transport and boosting the mobility of top-contact OTFTs.47–49 Nevertheless, when the RMS of TIPS-pentacene thin film is minimum, the corresponding mobility is not the highest at the post-annealing temperature of 60 °C. At the post-annealing temperature of 40 °C, the RMS of TIPS-pentacene thin film is the second lowest, and the corresponding mobility is the highest. According to the magnified AFM images (the inserts in Fig. 3c), it can be more clearly observed that many small pin holes exist in the TIPS-pentacene thin film at the post-annealing temperature of 60 °C, which decreases the coverage rate of the thin film (Fig. 3c), resulting in the reduction of mobility. These comparative results indicate that the post-annealing temperature can significantly affect the recrystallization behavior of the solution-processed TIPS-pentacene thin film, and the recrystallized thin film with smooth and dense characteristics is beneficial to obtain high mobility. Additionally, post-annealing is not suitable for vacuum-deposited organic semiconductor thin films because the morphology of vacuum-deposited TIPS-pentacene thin film is stable, while the morphology of solution-processed TIPS-pentacene thin film is metastable (Fig. S4, ESI).

Fig. 4 shows the effect of post-annealing time on the thin-film morphology and the electrical performance. The post-annealed temperature is fixed at 40 °C. As shown in Fig. 4a, the morphology and RMS of TIPS-pentacene thin films change dramatically at different post-annealing times. As the post-annealing time increases from 0 to 15 min, the mobility of the TIPS-pentacene OTFTs gradually increases from 0.12 to 1.45 cm2 V−1 s−1 due to the decrease of RMS (Fig. 4b). When the post-annealing time further increases from 15 to 60 min, the mobility of the TIPS-pentacene OTFTs gradually decreases from 1.45 to 0.77 cm2 V−1 s−1 due to the increase of RMS. Hence, when the RMS of the TIPS-pentacene thin film is minimum, the corresponding mobility at the post-annealing time of 15 min is the highest. This demonstrates that the suitable kinetic energy can move TIPS-pentacene molecules to the appropriate distance, leading to an efficient recrystallization of the TIPS-pentacene thin film, and facilitating the formation of high-quality smooth TIPS-pentacene thin films and the realization of high-performance OTFTs. However, the morphology of the spin-coated TIPS-pentacene thin film after long-term placement at room temperature changes obviously, but fails to become smooth. This can be attributed to the fact that TIPS-pentacene molecules can move at room temperature, but the small kinetic energy cannot move TIPS-pentacene molecules to the appropriate distance for realizing the smooth TIPS-pentacene thin film (Fig. S5, ESI). As discussed above, the post-annealing temperature of 40 °C and the post-annealing time of 15 min are the optimal post-annealing conditions to recrystallize the TIPS-pentacene thin film. It should be emphasized that the control of thin-film recrystallization is not only pivotal in OTFTs but also in all new emerging molecular-based electronic devices.

Universality of the low surface energy interface-derived low-temperature recrystallization behavior

To confirm the universality of our theory for the thin-film recrystallization behavior, we spin coated a C8-BTBT thin film on the OTS/SiO2 dielectric layer. C8-BTBT is selected as the active layer owing to its excellent electrical performance and good solubility in the organic solvent.13,16,50 Its molecular structure is shown in Fig. 5a. Fig. 5b shows the morphology of C8-BTBT thin films before and after post-annealing treatment. The post-annealing conditions are also fixed at 40 °C for 15 min. After low-temperature post-annealing treatment, the C8-BTBT thin film also presents recrystallization behavior with reduced roughness and increased grain size (Fig. 5b). The corresponding transfer characteristics of C8-BTBT OTFTs before and after post-annealing are shown in Fig. 5c. All C8-BTBT OTFTs were measured in ambient conditions, and all exhibited typical p-type characteristics. The mobility of post-annealing C8-BTBT thin film increases by forty times compared to the pristine thin film. These attractive results suggest that our theory is universal and provides a feasible route for achieving high-performance solution-processed organic electronic devices.
image file: c9tc05043d-f5.tif
Fig. 5 Universality of the thin-film recrystallization behavior. (a) Molecular structure of the C8-BTBT. (b) AFM images (20 μm × 10 μm) and height curves of C8-BTBT thin films on the OTS/SiO2 dielectric layer before and after post-annealing treatment. (c) The corresponding typical transfer characteristics of C8-BTBT OTFTs with an OTS-modified layer before and after post-annealing treatment.

Conclusions

In summary, we found that a low surface energy interface can derive low-temperature recrystallization behavior of solution-processed OSC thin films, which has an important impact on boosting the carrier mobility of OTFTs. In this case, surface energy of the dielectric layer, post-annealing temperature and time play key roles in thermodynamics and kinetics of the recrystallization process. By systematically adjusting these parameters, OSC molecules can be effectively rearranged to form the desired morphology and device performance. As a result, the mobility of TIPS-pentacene and C8-BTBT OTFTs is significantly enhanced by about one order of magnitude after the recrystallization of OSC thin films. The obtained highest mobility of TIPS-pentacene OTFTs is up to 1.45 cm2 V−1 s−1, which is higher than that of all the reported TIPS-pentacene OTFTs prepared by the spin-coating method in ambient conditions. This work provides a facile and controllable strategy to recrystallize OSC thin films for enhancing the carrier mobility of solution-processed OTFTs, allowing a better understanding about the role of dielectric interfacial properties in assisting the recrystallization of small molecule OSC thin films.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51703020, 51322305, 61574032, 51732003), 111 Project (B13013) and Fundamental Research Funds for the Central Universities (2412017QD008).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05043d

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