2018, Vol. 29 
,
Xiangke Wangd,
Wenping Hub,c
b Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China;
c Collaborative Innovation Center of Chemical Science and Engineering(Tianjin) & Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China;
d College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China;
e Department of Physics & Astronautics Shanghai Jiao Tong University, Shanghai 200240, China
Organic field effect transistors (OFETs) are the basic component of organic circuits and have demonstrated potential applications in flexible and printable organic electronics. Over the past decades, it has witnessed the significant advances with the improvements of charge carrier mobilities by several orders magnitude higher than the initial for OFETs [1-5]. In addition, recently, bi/multi-functional OFET-based devices have also received increasing attentions due to their scientific and technological significance in organic integrated devices. For example, organic phototransistors (OPTs) are a kind of typical integrated optoelectronic devices, which combine light detection and signal magnification properties in a single device with much higher sensitivity and lower noise than photodiodes [6-8]. To achieve high device performance, both for OFETs and OPTs, efficient charge transport in the active layer is crucial, which is significantly affected by the intrinsic property of semiconducting materials, molecular orders and packing models, etc. Of course, besides excellent charge transport ability, effective light absorption with harvest incident photons for generating excitons and exciton dissociation are also of great importance for improving the performance of OPTs, which are further affected by the optical properties of materials including their solid state structures [9-11].
In contrast to vacuum-deposited organic thin films where highly crystalline solid state structures could be well-controlled and obtained by precisely controlling the deposition conditions. Increasing molecular orders with high and or even single crystallinity in solution-processed thin films, especially for conjugated polymers is significantly challenging. The reasons are ⅰ) conjugated polymers are naturally disordered with complex molecular structures and polydispersity and tend to form irregular structures at nanometer to micrometer length scales in solid state; 2) complex interchain entanglements of polymer chains make it difficult to control the conjugated polymers to form highly ordered packing structures, not to mention single crystalline films [12]. To form the highly ordered structures, the strong assembly ability of conjugated polymers themselves along with enough space and energy for their following complete assembly during the solution process are both important factors, which may remarkably affect the formation of crystal nucleus and further growth for forming highly crystalline solid state structures [13].
Spin-coating is a general solution method to obtain thin films of conjugated polymers, the simplicity of which also ensures its potential applications in low-cost devices. However, amorphous and or low-degree ordered thin films of conjugated polymers were usually obtained through the rapid spin-coating process. Various strategies including solution-shearing process, post-solvent treatment, off-center spin-coating process, drop-casting etc., have been developed with the aims of adding external shearing force and or increasing the assembly times for organic molecules to form the crystalline polymer thin films. However, most of these processing cases usually result in thick films and need a large amount of material wastes [14, 15]. Moreover, it is widely accepted that the conducting channel in both OFETs and OPTs is close to the dielectric layer, that is, ultrathin films with high-degree molecular orders in the few molecular layers near the semiconductor/ dielectric interfaces are sufficient for efficient charge transport. Therefore, developing facile, efficient spin-coating process meanwhile with a small material waste to fabricate highly crystalline ultrathin conjugated polymer films is highly desirable for polymer optoelectronic devices.
In this article, with diketopyrrolopyrrole-quaterthiophene (PDQT) (Fig. 1a)asanexample, it is found thatby simply reducing the solution concentration for spin-coating meanwhile with the assistance of post-annealing, significantly enhanced crystallinity of PDQT thin films with formation of typical single crystalline domains could be achieved. A significant enhancement in the charge transporting property may attributed to enough space and energy for better molecular assembly in thin films, which were characterized by atomic force microscopy (AFM) selected area electron diffraction (SAED), and grazing incidence X-ray diffraction (GIXD) systematically show High performance polymer transistors and phototransistors were finally constructed based on the optimallow-concentration (2 mg/mL) spincoated ~12 nm thick PDQT films, which giving a high charge carrier mobility of 2.28 cm2 V-1 s-1 and a photoresponse on/off ratio of 2.1×107 at VG = 0 V under white light irradiation of 6mW/cm2. These results demonstrate a bright future of PDQT crystalliene films for large-areaflexible integrated optoelectronic devices via the potential application of effective low-concentration processing approach with reduced material waste.
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| Fig. 1. (a) Molecular structure of PDQT. (b) Absorption spectra of PDQT in solution and thin film. (c, d) LUMO and HOMO diagrams of PDQT repeat unit calculated by the gaussian software (Gaussian 09, Frisch MJ, Trucks GW, Schlegel HB, Gaussian, Inc., Wallingford CT, 2009) at B3LYP/6-31G(d) level. | |
Current-voltage (I-V) characteristics were accomplished by a Keithley 4200 SCS with a Micromanipulator 6150 probe station in a clean shielded box under the ambient atmosphere at the room temperature. The morphologies of PDQT films were analyzed in air using atomic force microscopy (AFM, Digital Instrument, Nanoscopy Ⅲa). The microstructures of PDQT thin films were performed in SAED (JEOL 1011 operated at 120 kV), GIXD measurements were performed on Beamline 7.3.3 at the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated by the gaussian software (Gaussian 09, Frisch MJ, Trucks GW, Schlegel HB, Gaussian, Inc., Wallingford CT, 2009) at B3LYP/6-31G(d) level. For UV-vis spectra, solution of PDQT in chlorobenzene solvent and the prepared PDQT thin-films on quartz substrates were recorded on a Jasco V-570 spectrophotometer, respectively.
PDQT thin films were prepared through simply spin-coated method from different solution concentrations in same solution volume (20 μL) at the speed of 2000 rpm for 30 s onto the octadecyltrichlorosilane (OTS) modified Si/SiO2 (300 nm) substrates. Subsequently, some of the pre-prepared PDQT films were further annealed at 50 ℃ and 100 ℃ for 60 min in vacuum condition for contrast experiments. For device fabrication, gold electrodes with thickness of 25 nm were thermally deposited on top of the PDQT films through a shadow mast with width/ length = 8 (channel width is 240 mm and channel length is 30 μm, respectively). The field-effect mobility was calculated from the standard equation for saturation region in semiconductor field effect transistors: IDS = (W/2 L) μ Ci (VG-VT)2. Where IDS is drainsource current, μ is field-effect mobility, W and L are the channel width and length, Ci is the capacitance per unit area of the gate insulator (Ci = 7.5 nF/cm2) VG is the gate voltage, and VT is the threshold voltage. For PDQT-based OPT devices, a white-light lamp with a power density ranging from 0.37 mW/cm2 to 6.0 mW/cm2 was employed as the illumination source. The parameters of phototransistors photo-responsivity (R) and photocurrent on/off ratio (P) are defined by the following equations:
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Where IDS, d and IDS, i are the drain current in dark and under illumination, respectively. S is the effective area of devices and Pi is the power of incident optical in unit area
To date, various conjugated polymers have been successfully designed and synthesized under the efforts of chemistry scientists [16]. Especially, the donor-acceptor (D-A) copolymers with the attractive advantages of high charge carrier mobility and easily modulated absorption region have recently attracted increasing interests in functional optoelectronic devices, leading to the boost significant improvement in polymer optoelectronic device performance [17, 18]. The reasons are that the excellent D-A conjugated molecular structures not only facilitate low-energy charge-transfer transitions but also broaden the absorption spectra, which are beneficial for effective light harvest and high performance optoelectronic properties. PDQT as one of the most representative D-A copolymers, which was first reported by Li et al. in 2011 [19], and exhibits efficient charge transport property with the available carrier mobility of 0.97 cm2 V-1 s-1 under ambient temperature based on 30 nm PDQT films. Fig. 1b shows the UV-vis spectra of PDQT in chloroform solution and thin films, respectively, displaying absorption maxima, λmax, at 792 nm both in solution and ultrathin films with a wide range absorption from 400 nm to 1000 nm in the whole UV-vis-IR range. From the absorption edge, the energy bandgap (Eg) of PDQT is calculated to be around 1.2 eV. The wide range optical absorption characteristics with the narrow gap of PDQT suggest the easy charge transition from its HOMO to the LUMO levels and would be easily excited by light, further making it to be best candidate for potential applications in photocontrolled devices. In addition, the subtle changes between the absorption spectra of PDQT solution and thin films to some extent reflect the good molecular assembly ability of PDQT for ordered aggregates, which is beneficial for the subsequent modulation of their packing structures via the appropriate solution processing with enhanced crystallinity. Moreover, highly extended electron clouds are distributed along the conjugated polymer backbone (Figs. 1c and d), suggesting the efficient charge transport of polymer chains and its potential great contribution to the improvement of device performance.
Considering the typical spin-coated thick conjugated polymer thin films, it is difficult to realize the complete molecular assembly especially for the molecules at the bottom of films due to the limited space (Fig. 2). To solve this problem and achieve increased molecular crystallinity in solid state, herein, we redesign the spincoating process, that is consciously reducing the solution concentration for spin-coating. On one hand, through the simple low-concentration spin-coating process, it is easy to effectively reduce the film thickness, and moreover possibly to obtain ultrathin conjugated polymer films, which possess many unique advantages in organic electronic applications [20]. On the other hand, it can be seen that as a conceptual extension of bottom-up supramolecular self-assembly approaches [13, 21, 22] in which the molecules could completely organize from bottom to up due to enough space provided. In addition, because as little as possible amount of organic materials is used during this low- concentration spin-coating, we can effectively reduce the amount of material waste, which is important for potential applications. Fig. 2 shows the schematic of low-concentration spin-coating process for preparing high quality crystalline ultrathin conjugated polymer thin films with three typical steps: ⅰ) dropping dilute solution onto the already spin-operating OTS-modified SiO2 substrate where the PDQT polymer chains could be extended to a large extent with the reduced entanglements; ⅱ) the formation of partially ordered ultrathin conjugated polymer films on the substrate; ⅲ) further enhanced molecular assembly of PDQT chains to form the resulting high crystalline films due to the enough space and energy provided with the assistance of post-annealing process. For comparison, the schematic process of molecular assembly in the thick films is also demonstrated, where the molecules, especially at the bottom of films are highly hindered due to the limited space to some extent. Actually, in the previous studies, reducing the solution concentration and extending the assembly time appropriately, which are crucial for the better molecular assembly of conjugated polymers to form high crystalline, and even single crystal micro/nano-wires [14, 15, 23, 24]. That is to say, complete molecular extension and assembly process are both important factors for the formation of high quality crystal nucleation and their further growth in solid state.
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| Fig. 2. Schematic concept of low-concentration (LC) spin-coating solution process toward high crystalline conjugated polymer films, especially with highly extended polymer chains at the bottom of the films close to the interface. For comparison, the schematic of molecular assembly via high-concentration (HC) solution processing is also shown together here. | |
To test our idea, here, three different solution concentrations of 3 mg/mL, 2 mg/mL and 1 mg/mL have been systematically investigated in the experiment. Fig. 3 shows the AFM height morphologies of PDQT films obtained under different spin-coating processes and annealing treatment conditions. As expected, by varying the solution concentration from 3 mg/mL to 1 mg/mL, the thickness of obtained PDQT films could be well-controlled and reduced from 25.2 nm for 3 mg/mL down to ~7.9 nm for 1 mg/mL (Figs. S1 and S2 in Supporting information). Comparing the films under different temperatures, the thickness fluctuation for the thinner films from low-concentration spin-coating process is much smaller than that thick films spin-coated from 3 mg/mL, indicating the better pre-molecular assembly in the films under room temperature. In addition, the fiber network microstructures in films are becoming more and more clear as evidenced by the distinct domain boundary, also suggesting the enhancement of molecular assembly with formation of ordered structures in the thinner films. And after the post-annealing process with elevated temperatures of 50 ℃ and 100 ℃, respectively, the ordered structures are further improved for all films due to the external energy provided, as evidenced by the gradually consistent AFM morphologies. All the PDQT films exhibit very smooth surface with low root mean square (RMS) roughness of below 0.5 nm, which is comparable to that of bare SiO2. This result clearly demonstrates that this technique provides a simple and efficient pathway to control the molecular assembly, with obvious enhancement of ordered aggregates in the thinner films prepared from lowconcentration solution spin-coating process.
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| Fig. 3. AFM images (5 μm × 5 μm) of PDQT films prepared from different solution conditions with the assistance of subsequent annealing treatments. | |
To investigate the effectof spin-coating solution concentrationon the crystallinity of PDQT films, XRD, GIXD and TEM structure measurements were further carried out. Specially, XRD results show the crystalline nature of PDQT molecules in films (Fig. S3 in Supporting information) with an out-of-plane interlayer d distance of ≈19.9 Å, which is consistent with the previous reported results, suggesting the side chains of PDQT perpendicular to the substrate, with the preferential edge-on molecular packing [19]. The GIXD results were in good agreement with the XRD observations (Fig. 4 and Fig. S4 in Supporting information), as evidenced by the appearance of (100) interlayer diffractions in the out-of-plane GIXD patterns. A broad π-π stacking with a distance of 3.92 Å in all directions of films is also detected, which is slightly larger than that of the traditional spin-coated films (3.75 Å) [19]. The broad π-π distance probably was ascribed to the effect of high-degree wriggling of PDQT polymer chains in the thin films. The strong assembly ability of PDQT conjugated polymers themselves together with enough space and energy resulted in the excellent crystallinity of PDQT films, even though formed by low solution concentration (1 mg/mL). Additional, Comparing the FWHM values under different concentrations, the values for the thinner films is smaller than that thick films spin-coated from 3 mg/mL, indicating the better crystallinity in the thinner films under room temperature (Fig. S5 in Supporting information). TEM measurements are also performed on the PDQT films obtained from different concentrations. It is found that the crystallinity of PDQT aggregate domains in films increased with the reduce of solute concentration. Surprisingly typical single crystal with strong and sharp diffraction spots were obviously observed for the crystalline domains prepared from PDQT thin films with low solute concentration of 1 mg/mL.
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| Fig. 4. (a–c) Out-of-plane GIXD diffractions of PDQT films prepared under different conditions: r.t. 50 ℃ and 100 ℃ annealing treatment. (d–f) SAED patterns obtained based on PDQT films prepared from 1 mg/mL, 2 mg/mL and 3 mg/mL solution, respectively with the assistance of post-annealing at 100 ℃. | |
After a deep understanding of the effect of solution-processing conditions on molecular assembly and crystallinity of PDQT films, the charge transport investigations were further carried out. Bottom-gate top-contact (BGTC) OFETs based on PDQT thin films obtained by different solution concentration were fabricated on OTS-modified Si/SiO2 (300 nm) substrate with gold as the source and drain electrodes. For each condition, around 20 devices have been measured for statistical studies. Representative transfer and output curves of PDQT-based thin film OFETs are shown in Figs. 5a and b. Obviously, in comparison, the PDQT films obtained from 2 mg/mL solution demonstrate relatively higher transporting property compared to the films prepared from 1 mg/mL and 3 mg/mL, probably due to both its high crystallinity and good continuity in films. Moreover, the thin PDQT film-based OFETs exhibit very ideal output curves with typical linear and saturation regions and ohmic contact characteristics. Fig. 5c and Table S1 (Supporting information) summarize the field-effect performance of all the devices constructed based on different PDQT films. The relationship between the performance and the concentration was summarized in Fig. S6 (Supporting information). All the devices exhibit very high on/off ratio up to 106. The highest mobility value achieved is up to 2.28 cm2 V-1 s-1 with an average mobility of 2.08 cm2 V-1 s-1 for 2 mg/mL spin-coated PDQT thin films under 100 ℃ annealing process, w hich meanwhile give a threshold voltage as small as -1.41 V. The mobility achieved in this condition is two times of magnitude higher than the films obtained from 3 mg/mL. While, further decrease of solution concentration to 1 mg/mL, the carrier mobility is decreased probably due to the presence the discontinuous regions in such thin films (Fig. 2). It is worth mentioning that the transfer characteristics of devices display outstanding stability without obvious degradation under the air condition (Fig. S7 in Supporting information). Additional, negligible hysteresis phenomenon was observed for our prepared PDQT-thin film transistors (Fig. S8 in Supporting information), indicating the high quality interface contact between the PDQT thin films and OTS-modified dielectric surface. All the results demonstrate that appropriately optimizing the solution processing, it is crucial for improving the device performance, especially based on the thin films with small amount of material waste.
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| Fig. 5. (a) Typical transfer curves of PDQT-films-based OTFTs via different solution-concentration processing. (b) The representative output curves of PDQT-OTFTs prepared from 2 mg/mL solution concentration. (c) Mobility distribution histogram for PDQT-OTFTs fabricated under different conditions. (d) Schematic device structure of PDQTbased phototransistor with top light illumination. (e) The transfer characteristics for PDQT thin film transistors operated under dark and light illumination (light intensity: 6.0 mW/cm2). (f) Light-modulated output characteristics for PDQT-based phototransistors under Vgs = 0 V and with light intensities of 1: dark, 2: 2.0 mW/cm2, 3: 2.8 mW/cm2, 4: 6.0 mW/cm2, respectively. | |
According to the investigation of high performance PDQT filmbased OFETs, furthermore, phototransistors based on the optimal high crystallineand continuousPDQTfilms (2 mg/mL) are fabricated with the same BGTC-OFET device configuration, by incorporating a top illumination on the devices (Fig. 5d). As we know that, with light irradiation, the PDQT-based devices exhibited strong photo dependenceon the illumination light intensities. As shown in Fig. 5e, under light illumination of 6 mW/cm2, the IDS current was significantly increased from ~10-12 A under dark to ~10-5 A under light at VG = 0 V. Moreover, with light illumination as the fourth electrode instead of the gate voltage, the devices also exhibited typical output characteristics similar to those of organic field-effect transistors, indicating the good modulation of light illumination on the fieldeffect transistor operation (Fig. 5f). According to the calculated equation, the R value of the phototransistors was up to 877 mA/Wat VG = 0 V. Surprisingly, the maximum on/off ratio of the phototransistor could be up 2.1 ×107 at VG = 0 V, which was much higher than that of the traditional polymer thin film-phototransistors [25, 26], due to synergistic effect of the high crystalline PDQT thin films, wide-range UV-vis absorption as well as effective absorption for ultrathin films.
In conclusion, from the aspect of increasing molecular crystallinity and reducing material waste, a low-concentration spin-coating process is introduced for the fabrication of high device polymer optoelectronic devices. With the typical PDQT D-A copolymer as an example, high performance transistors with mobility of up 2.28 cm2 V-1 s-1 and phototransistors with on/off ratio of 2.1 ×107 at VG = 0 are constructed based on the PDQT thin films prepared from 2 mg/mL solution process. The results suggest the potential applications of low-concentration processing with small amount of material waste for large-area low-cost high performance polymeric optoelectronic devices.
AcknowledgmentsThe authors acknowledge financial support from the Ministry of Science and Technology of China (Nos. 2017YFA0204503, 2016YFB0401100), the National Natural Science Foundation of China (Nos. 51725304, 91433115, 51633006, 51733004), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030300), and National program for support of top-notch young professionals.
Appendix A. Supplementary dataSupplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.03.034.
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