Junko
Aimi
*a,
Takeshi
Yasuda
a,
Chih-Feng
Huang
b,
Masafumi
Yoshio
a and
Wen-Chang
Chen
c
aResearch Center for Functional Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. E-mail: AIMI.Junko@nims.go.jp
bDepartment of Chemical Engineering, i-Center for Advanced Science and Technology (iCAST), National Chung Hsing University, Taichung 40227, Taiwan
cDepartment of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
First published on 4th February 2022
Solution-processed organic field-effect transistor (OFET) memory devices are fabricated using a blend film of 6,13-bis(triisopropylsilylethynyl)pentacene and phthalocyanine-cored star-shaped polystyrene. A highly crystalline organic semiconductor thin film was obtained on the star-shaped polymer with charge-trapping sites via a one-pot spin-coating process through vertical phase separation, which is advantageous for OFET memory device applications. The resultant OFET device demonstrated a charge carrier mobility of 0.10 cm2 V−1 s−1 and an on/off current ratio of 106. Upon application of a gate bias, a substantial reversible threshold shift was observed, along with long charge-retention ability, thereby confirming the memory characteristics of the device.
Solution-processable organic semiconductors themselves have been extensively investigated and incorporated into OFET devices. For instance, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) is a typical small-molecule semiconductor with solubilizing side groups. The charge-carrier mobility in OFET devices is strongly influenced by the crystalline structure and alignment of the organic semiconductors.15 To achieve high device performance using a solution process, researchers have studied various methods of preparing thin films of organic semiconductors. Polymer-matrix-assisted phase separation is one of the most promising methods to prepare OFETs with high charge carrier mobility.16–21 The mixed polymer increases the wettability of the substrates and decreases the evaporation speed of the solvents to induce crystallization of the organic semiconductor. For instance, an OFET prepared using a spin-coated blend film of TIPS-pentacene and poly(α-methylstyrene) exhibited a charge carrier mobility as high as 0.54 cm2 V−1 s−1.17 The selection of an appropriate polymer insulator,22–24 polymer molecular weight,17,25,26 and processing solvent27–30 is important for controlling the crystal growth of organic semiconductors in the polymer matrix. Further advances in the coating process, such as the development of the blade coating technique,25,31,32 have led to additional enhancements in OFET device performance.
Here we envision that the method of using a polymer matrix to construct high-performance OFET devices via a solution process can also be applied to OFET memory devices when the matrix polymer is capable of charge storage. Recently, we prepared metallophthalocyanine (MPc)-cored star-shaped polystyrene (MSP) and used it as a charge-trapping material in a pentacene-based OFET memory device.33 The star polymer with an MPc core is readily soluble in organic solvents, enabling the fabrication of polymer films with a smooth surface via simple spin-coating onto a Si substrate. The MSP films exhibited a unique three-dimensional morphology, where the MPc cores were distributed and isolated by the surrounding arm-polymer matrix. A bottom-gate top-contact OFET memory device was fabricated via vacuum deposition of pentacene onto the polymer film, followed by deposition of Au electrodes. When a negative gate voltage was applied to the device, hole charges that accumulated in pentacene migrated to the MPc core, whereas polystyrene arms restrained leakage of the trapped charges to achieve nonvolatile memory similar to that based on nano-floating-gate transistors. In addition, the density of the MPc core—specifically, charge-trapping sites, can be controlled by the length of the arm polymer, enabling control of the resultant memory device performance.34
As a continuation of our studies on the OFET memory devices, we herein attempt to develop a simple and easy method of preparing OFET memory devices using polystyrene-based nano-floating gates. We used our unique design of memory materials—star-shaped polystyrene with specific charge-trapping sites—to prepare vertically phase-separated organic semiconductor/memory layers via a one-pot solution process. More specifically, the MPc-cored star-shaped polystyrene promotes the crystallization of small molecular semiconductors of TIPS-pentacene via polymer-matrix-assisted phase-separation and simultaneously provides organic semiconductor/memory layers on the substrates. The morphology and structure of the blend films of the star polymer and TIPS-pentacene prepared by a simple spin-coating process from various organic solvents were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), polarized optical microscopy, UV-vis absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS). We obtained highly crystalline and vertically phase-separated TIPS-pentacene on star polymer blend films from an ortho-dichlorobenzene (ODCB) solution, which is advantageous for OFET memory device applications. We finally demonstrated an OFET device with a bottom-gate top-contact configuration (Fig. 1a). The resultant device showed a hole mobility of 0.1 cm2 V−1 s−1 and a high on/off current ratio greater than 106. In addition, a memory window of 19.7 V was probed under an applied gate voltage in the range ±50 V and the device demonstrated long charge-retention ability. Thus, the present one-pot solution process would be a useful approach for preparing non-volatile organic memories without cumbersome fabrication and costly vacuum-deposition processes of organic semiconductors.
Fig. 1 (a) Schematic representation of the fabrication of OFET memory devices. (b) Chemical structures of TIPS-pentacene and polymer dielectrics of CuSP and PS4. |
Ids = WCtotμeff(Vg − Vt)2/(2L) | (1) |
The relations between the capacitance (Ctot) of the device, SiO2 wafer (CSiO2), polymers (Cpoly), and polymer dielectric constant (ε) are defined as
(2) |
(3) |
The spin-coated blend films were analyzed by X-ray diffraction (XRD) to characterize the structure of the TIPS-pentacene in the blend film. As shown in Fig. 2a, the out-of-plane XRD patterns of the mixed film obtained from ODCB exhibited strong and sharp diffraction peaks corresponding to the crystal lattice of the (00l) planes of TIPS-pentacene, indicating a crystalline structure parallel to the substrate, with an interplanar d-spacing of 16.3 Å estimated from the peak at 2θ = 5.36°. This result indicates that the triisopropylsilylethynyl groups of the TIPS-pentacene molecules align in the edge-on orientation with respect to the substrates, consistent with a single crystal of TIPS-pentacene with a domain spacing of 16.8 Å.35,36 The XRD pattern of the blend film spin-coated from toluene solution showed an additional peak at a d-spacing of 6.92 Å, indicating that the crystals oriented in directions other than (00l) were included in the mixture. The XRD pattern of the film prepared from CHCl3 did not show clear peaks; instead, the amorphous halo originating from the polymer chains was observed at 2θ ≈ 20°. The surface morphology analyzed using atomic force microscopy (AFM) supported these results (Fig. 2b). The large continuous crystal grains of TIPS-pentacene were observed in the blend films spin-coated from ODCB, whereas small needle-like crystals with a size of ∼0.5 μm and a random orientation were observed in the blend film spin-coated from toluene. The film from CHCl3 exhibited a relatively smooth surface without large crystal domains. These results reveal that the crystallization of the TIPS-pentacene in the star-polymer matrix was highly dependent on the solvent, mainly because of the difference in the solvents’ evaporation rate. Slow evaporation of an ODCB solution induced phase-separation of the TIPS-pentacene and star-shaped polymer, promoting crystallization of the TIPS-pentacene, whereas fast evaporation inhibited the formation of crystals in the case of the film spin-coated from CHCl3 solution. Approximately the same XRD patterns and AFM images were obtained for the blend films prepared using PS4 as the polymer matrix, implying that the CuPc core in CuSP did not disturb the crystallization of the TIPS-pentacene (Fig. S1 and S2, ESI†).
Fig. 2 (a) XRD profiles of TIPS-pentacene/CuSP blend films spin-coated from various solvents. (b) AFM height images of the TIPS-pentacene/CuSP blend films. |
The blend films on quartz plates were also analyzed by polarized optical microscopy (POM). As shown in Fig. 3a, large spherulite textures were observed for the blend films spin-coated from ODCB. The blend film spin-coated from a toluene solution showed smaller spherulite textures; the film spin-coated from CHCl3 did not show large crystals. The different morphologies of the TIPS-pentacene in the blend films spin-coated from the various solvents were also examined by UV-vis absorption spectroscopy (Fig. 3b). The absorption spectrum of TIPS-pentacene in a CHCl3 solution showed three characteristic absorption bands in the 500–700 nm region; these bands originated from the first excited singlet state. A similar spectrum was observed for a film spin-coated from a CHCl3 solution of TIPS-pentacene/CuSP, indicating that, like the TIPS-pentacene in the solution state, the most of the TIPS-pentacene molecules in the polymer film existed in monomeric form. The fast evaporation of CHCl3 solution during the spin-coating process might disperse the TIPS-pentacene molecules in the polymer matrix before crystallization proceeded. An additional shoulder band at ∼700 nm was observed in the spectrum of the film obtained from toluene solution, which originated from the aggregated structure of TIPS-pentacene. The spectrum of the blend film spin-coated from ODCB solution showed broader absorption bands at approximately 600, 650, and 700 nm. These results reveal that most of the TIPS-pentacene molecules assembled to form crystals even in the presence of the polymer matrix. Notably, the absorbance from the CuPc core at ∼630 nm in the spectra of the blend films was overlapped by the strong absorption of the TIPS-pentacene because of the small amount of CuPc unit in the polymer matrix.
Fig. 3 (a) POM images of the TIPS-pentacene/CuSP blend films. (b) Absorption spectra of TIPS-pentacene/CuSP blend films and TIPS-pentacene solutions. |
We used XPS to investigate the detailed structure in the blend films of TIPS-pentacene and CuSP spin-coated from ODCB and CHCl3 on the Si wafer substrates. The depth profiles of the film were collected by Ar+-ion sputtering with an etching rate of 2.7 nm min−1 for the film from ODCB and 3.2 nm min−1 for the film from a CHCl3 solution. All XPS peaks were referenced to the neutral C 1s peak at a binding energy of 284.8 eV. The peak intensities at binding energies corresponding to C 1s and Si 2p were plotted against the sputter time (Fig. 4). Notably, the Si 2p binding energies in the spectra of the TIPS-pentacene and SiO2 on the Si-wafer substrates were sufficiently different for each component to be identified (Fig. S3, ESI†). In addition, the peaks of N 1s or Cu 2p in the spectra of CuSP were difficult to observe because of the small atom%. The depth profiles of the blend film spin-coated from ODCB indicate that the TIPS-pentacene phase was located at the upper interface and mainly existed to a depth of ∼20 nm from the top interface in a blend film with a total thickness of ∼50 nm. This result is consistent with the aforementioned AFM analyses. On the other hand, the peak intensities of Si 2p corresponding to TIPS-pentacene in the CHCl3-processed blend film were relatively constant throughout the film thickness. This result indicated that the TIPS-pentacene molecules were distributed in the polymer film without clear phase segregation, which also agreed with the above results. We concluded that the TIPS-pentacene forms large crystals on the CuSP polymer layer via vertical phase segregation of the blend film deposited by a simple one-pot spin-coating process from ODCB solution.
Fig. 4 XPS depth profiles of a TIPS-P/CuSP blend films coated from (a) ODCB and (b) CHCl3 solutions on a Si substrate. |
The electrical transfer and output characteristics of the fabricated OFET devices are shown in Fig. S4 (ESI†). The reliability factors (r) for μeff of the OFET devices with CuSP and PS4 were also estimated (Fig. S4, ESI†, inset).37 When the gate bias was scanned between +10 and −50 V, the typical p-type accumulation mode was observed for the blend films spin-coated from ODCB. The devices with the blend films of TIPS-pentacene and CuSP or PS4 spin-coated from ODCB showed average μeff values of 0.10 ± 0.05 and 0.11 ± 0.09 cm2 V−1 s−1, respectively. Compared to the reported μeff values of 0.21 cm2 V−1 s−1 for vacuum-deposited pentacene-embedded OFET,34 these values are sufficiently high for solution-processed OFET devices. The on/off ratios of the drain current (Ion/Ioff) were greater than 106, which is beneficial for the memory device to clearly distinguish between the “0” and “1” digital states. As previously discussed, large crystal grains of TIPS-pentacene on the polymer dielectrics provided efficient charge transport pathways in the OFET devices. By contrast, the OFET device with the blend film spin-coated from toluene showed a low hole mobility of 1.0 × 10−3 cm2 V−1 s−1 at the maximum (Fig. S5a and b, ESI†). In addition, the OFET devices with embedded blend films coated from CHCl3 did not demonstrate transistor characteristics (Fig. S5c and d, ESI†). The lack of large TIPS-pentacene crystals or the polymer insulating layer on the surface disturbed the efficient charge transport in the OFET devices.
Fig. 5 Transfer characteristics of OFET memory devices fabricated using (a) TIPS-P/CuSP blend films and (b) TIPS-P/PS4 blend films monitored at Vd = −50 V. |
Fig. 6 WRER test (a) and endurance characteristics (b) of the OFET memory device with TIPS-pentacene/CuSP blend films. |
Charge retention is also important when evaluating the nonvolatility of a memory device. To evaluate the memory retention characteristics, we monitored the drain current at Vg = −5 V after a writing operation at Vg = −50 V for 1 s and an erasing operation at Vg = +50 V for 1 s (Fig. 7). The difference in the drain on/off current was maintained at 102 for 12000 s. High stability of the stored charge was achieved because of the unique architecture of the star-shaped polymer, where the arm polymer chains surrounding the CuPc core assemblies restrain the leakage of the trapped charges.
Fig. 7 Retention time of the Id of the OFET memory device with TIPS-pentacene/CuSP blend films, as monitored at Vg = −5 V after writing (black squares) and erasing (white circles) operations. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ma01081f |
This journal is © The Royal Society of Chemistry 2022 |