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Xiaotong Liu, Yunlong Guo,* Yongqiang Ma,* Huajie Chen, Zupan Mao, Hanlin Wang, Gui Yu,* and Yunqi Liu* Organic/polymer thin-film transistors (O/PTFTs) are of great interest for practical applications in active-matrix displays, radiofrequency identification tags, biosensors, and integrated circuits owing to their advantages of low cost, light weight, and mechanical flexibility.[1–6] Recently, great progress has been made in OTFTs; donor (D)−acceptor (A) conjugated copolymers have demonstrated high mobilities above 1 cm2 V−1 s−1, and a superb mobility of 10.5 cm2 V−1 s−1 has been achieved in PTFTs,[7–10] which is much higher than that of amorphous silicon devices. However, these high-mobility PTFTs still need a large operating voltage (usually exceeding 60 V) and were fabricated on glass or silicon wafers, which limits the low-power applications and the intrinsic flexibility of polymer semiconductors.[10,11] For some vacuum-evaporated small molecules, the operating voltage of OTFTs has been successfully reduced to –3 V,[12] for example for pentacene,[13–17] C60,[18] dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b] thiophene,[19] and 2-tridecyl[1]benzothieno[3,2-b][1]-benzothiophene.[20] However, these processes for small molecules cannot be well reconciled with the requirement of solution-processable polymer semiconductors. For polymer semiconductors, flexible and low-power-consumption TFTs are still limited. Therefore, exploring flexible, high-performance PTFTs with low operating voltage is very important, and it is highly desirable for low-cost, flexible, organic electronics. Typically, there are two routes to achieving low-operatingvoltage O/PTFTs: 1) reducing the thickness of the insulating layer and 2) using high-permittivity (dielectric constant, κ) dielectric materials. However, the shift to an ultrathin insulating layer (option 1) is often accompanied by a decrease in uniformity and an increase in leakage current. Therefore, high-κ materials will be good candidates for the gate insulators of low-voltage O/PTFTs. Several dielectric materials, such

X. T. Liu, Prof. Y. Q. Ma College of Science China Agricultural University Beijing 100193, P. R. China E-mail: [email protected] Dr. Y. L. Guo, Dr. H. J. Chen, Z. P. Mao, H. L. Wang, Prof. G. Yu, Prof. Y. Q. Liu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]; [email protected]; [email protected]

DOI: 10.1002/adma.201306084

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Flexible, Low-Voltage and High-Performance Polymer Thin-Film Transistors and Their Application in Photo/ Thermal Detectors

as inorganic high-κ materials,[21] polymers,[22,23] and electrolytes[24,25] have been applied to achieve low-voltage transistors. The high-κ inorganic materials zirconium oxide (ZrO2) and hafnium oxide (HfO2) have been used because of their high κ, but these high-κ materials often have highly hydrophilic surfaces, making them incompatible with molecule-ordered semiconductors.[26,27] Besides, a high temperature or a complicated procedure was needed in the fabrication, which limited the flexibility of the transistor. As for electrolytes, the low switching speed was a serious issue and it required electrode materials in contact with the electrolyte to be electrochemically stable and corrosion resistant.[28] Therefore, searching for alternative gate dielectrics is a key issue for low-power PTFTs. In the study reported here, we realized low-voltage and highperformance PTFT devices based on n-octadecyltrichlorosilane (OTS)-modified poly(vinyl alcohol) (PVA) as a gate dielectric layer. The water-soluble PVA has many advantages, such as low processing temperature, compatibility with flexible substrates, and good resistance to solvents after the cross-linking process.[29,30] Two diketopyrrolopyrrole (DPP)-based copolymers (PDQT and PDVT-10, structures are shown in Figure 1a) were chosen as semiconductors. PDQT and PDVT-10 have been reported as high-mobility polymer semiconductors previously,[11,31] but high operating voltage (above –60 V) and Si/ SiO2 substrate were needed. We successfully achieved low operating voltage (∼3 V) and flexible PTFTs with these two polymers and all fabricating processes were below 150 °C. The PDVT-10based device showed a high mobility of 11.0 cm2 V−1 s−1, an on/ off current ratio of 1.2 × 104, and a threshold voltage of –1.66 V, while the PDQT-based device exhibited a moderate mobility of 1.7 cm2 V−1 s−1, an on/off current ratio of 8.0 × 103, and a threshold voltage of –1.30 V. It is worth mentioning that the mobility of 11.0 cm2 V−1 s−1 for the PDVT-10-based devices reported here is the highest reported value for solution-processed and low-voltage PTFTs. For the first time, we have fabricated flexible and low-voltage photo/thermal detectors using the DPP-based polymers PDQT and PDVT-10 as active materials. The PDVT-10-based photodetector exhibited a photoresponsivity of 433 mA W–1 and a photocurrent/dark current ratio of 176. In addition, thermal sensors based on PDQT/PDVT-10 showed good thermal responsiveness. Therefore, these flexible and low-voltage photo/thermal detectors may have potential applications for electronic eyes and electronic skins. A top-contact configuration was adopted for the device, and its structure is shown in Figure 1b. Aluminum was used as the gate electrode, deposited on a poly(ethylene terephthalate)

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Figure 1. a) Chemical structures of PDQT, PDVT-10, and PVA. b) Photograph of the flexible PTFTs and schematic view of a top contact device using Al as the gate electrode and OTS-modified PVA as the gate dielectric. c) Typical output and transfer characteristics of FETs based on PDQT and OTSmodified PVA, exhibiting a mobility of 1.7 cm2 V−1 s−1, W/L = 8800/80. d) Typical output and transfer characteristics of FETs based on PDVT-10 and OTS-modified PVA, exhibiting a mobility of 11.0 cm2 V−1 s−1, W/L = 8800/80. The red line indicates the gate leakage current of the films.

(PET) substrate at a pressure of 6 × 10−4 Pa. Then the film was treated with UV-ozone for 20 min. Finally, PVA was spin-coated on the substrate. For a low-voltage transistor, the dielectric constant is crucial. PVA was selected as the gate insulator because of its solution processability and high κ. The accurate dielectric constant was calculated from Ci = κε / d

(1)

where Ci is the capacitance value, κ the dielectric constant, ε the vacuum permittivity, and d the insulator thickness. 3632

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Further, an OTS self-assembled monolayer (SAM) was formed between the polymer insulator and semiconductor layer in order to increase the mobility value of the OTFTs. The capacitance measurements with frequency from 103 to 5 × 105 Hz at ±1 V were performed to study the capacitance properties of PVA and OTS-modified PVA (Figure S1, Supporting Information). The sandwich structures Al/PVA/Au and Al/PVA-OTS/ Au were used. The thickness of PVA film was about 230 nm measured by an Ambios Technology XP-2 surface profilometer. The capacitance value for OTS-modified PVA is 28.0 nF cm−2 (at 104 Hz). According to Equation (1), the dielectric constant of

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I DS = (W / 2L)C i μ (VGS −Vth )2

(2)

where W and L are the channel width and length, respectively. Ci is the capacitance of the polymer insulator. From Equation (2), effective field-effect mobilities of 1.7 cm2 V−1 s−1 and 11.0 cm2 V−1 s−1 were obtained from PFETs of PDQT and PDVT-10, respectively. From the transfer characteristics of PDQT- and PDVT-10-based devices, on/off current ratios of 8.0 × 103 and 1.2 × 104 and threshold voltages of –1.30 V and –1.66 V were obtained, respectively. However, large hysteresis and a serious leakage current were observed from the transfer curves of FETs without OTS-modified PVA (see Figure S2a, Supporting Information). PDVT-10-based devices show a low mobility of 0.39 cm2 V−1 s−1. In comparison, the film with the OTS-SAM modification provides higher mobility, less hysteresis, and lower charge leakage, indicating that the surface modification of OTS on PVA is very important if PDVT-10 films are to achieve high performance. The distribution of mobilities and the gate leakage currents of PDQT and PDVT-10 films are listed in Figure S3 (Supporting Information). To test the stability of the devices, we performed bias-stressing measurements (Figure S4, Supporting Information). Although the IDS showed slight decays (for PDQT-based devices about 16% and for PDVT-10-based devices ∼25% at VDS = –2 V and VGS = –2 V after 2900 s under 20% relative humidity), the mobility and on/off ratio of the devices displayed little change. Further, the effect of humidity on devices was also studied. Although the mobilities for PDVT-10 (or PDQT)-based devices decreased by about 56% (or 38%) under humidity ranging from 20% to 75% (Figure S5, Supporting Information),[33] our devices showed good stability below 20% humidity. The threshold voltage versus relative humidity is also shown in Figure S5. The surface morphologies and roughness between gate dielectric and semiconductor are critical to OTFT device performance. In order to detail the process, we investigated the surface of the Al electrode, the PVA insulator, and PDVT-10 film by atomic force microscopy (AFM); the AFM images are shown in Figure S6 (Supporting Information). The root-meansquared (RMS) roughness of the Al electrode was estimated to be 1.60 nm, with the value dropping to 1.11 nm after treatment with UV-ozone. A very smooth surface was obtained by covering the surface with PVA (see Figure S6c), and the RMS values were further reduced to 0.384 nm after OTS treatment (see Figure S6d). The changes of surface morphology based on PDQT and PDVT-10 films with OTS-modified surface are shown in Figures S6e and f, respectively. Directly after spincoating, the surfaces of the polymeric semiconductors were still very smooth. After further annealing at 150 °C, the RMS roughness of PDQT and PDVT-10 increased to 0.914 nm

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and 0.884 nm, respectively (see Figures S6g,h). The same increasing-roughness phenomenon was also observed for the morphologies of the polymers on PVA films before and after annealing at 150 °C. These changes probably arise from the strong crystallinity of the polymer film on the OTS-modified surface after the annealing process. Tighter π−π stacking was further investigated by mans of grazing incidence X-ray diffraction (GIXRD). The diffraction pattern and molecular ordering of PDQT and PDVT-10 on PVA and OTS-modified PVA are shown in Figure 2. The PVA and two semiconductor films were prepared by spin-coating. In Figure 2a, the PDQT thin films on PVA and OTS-modified PVA substrates showed the same diffraction peak (100) (outof-plane) at 2θ = 4.6°, which corresponds to a d-spacing of 19.19 Å. In Figure 2c, for PDVT-10 thin films, the peak was at 4.2° with a d-spacing of 21.01 Å. The in-plane patterns for these thin films on PVA substrates exhibited π−π stacking distances of 3.81 Å for PDQT and 3.88 Å for PDVT-10, as calculated from their (010) peaks at 2θ = 23.3° and 22.9°. This π−π stacking of PDQT and PDVT on OTS-modified PVA films decreased to 3.75 Å (2θ = 23.7°) and 3.69 Å (24.1°), respectively. The surface energy of OTS-modified PVA and PVA was calculated by the Owens–Wendt–Rabel–Kaelble (OWRK) method based on measurements of contact angles. The surface energy of OTSmodified PVA decreased to 43.54 mN m–1 as compared with 52.98 mN m–1 for PVA. The smaller π−π stacking distance and better charge transport interface of PFETs based on OTS modification might be the reason for the higher mobility as compared with non-OTS-modified ones. Recently, D−A conjugated polymers have displayed great potential applications in flexible functional organic devices, such as electronic skin (pressure or thermal detection) or eyes (light detection).[34,35] The high operating voltage and rigid substrate are the main obstacles to future applications. So the application of PDQT and PDVT-10 thin films in flexible photoand thermal detectors was tested. For photodetectors, illumination from the top was applied directly to the polymer thin films and I−V characteristics were shown in Figure 3. Figures 3a and d show the transfer characteristics of the PDQT and PDVT-10 photodetectors, respectively, in the dark and with light power of 0.85 mW cm–2. It is clear to see that Vth shifted in the positive direction upon illumination. Further, we calculated two important parameters for photodetectors: photoresponsivity (R) and the photocurrent/dark current ratio (P), using R = (I ill − I dark ) / Pill

(3)

P = (I ill − I dark ) / I dark

(4)

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OTS-modified PVA was about 7.3, which is consistent with the value reported previously.[32] Figures 1c and d show typical output and transfer characteristics of PDQT and PDVT-10 OTFTs, respectively, on OTSmodified PVA. With the increase of a negative VGS, more holes accumulated in the channel, resulting in the increase of IDS, and p-type characteristics were observed. The threshold voltage (Vth) and field-effect mobility (μ) in the saturation region were determined from

where Iill is the drain current under illumination, Idark is the drain current in darkness, and Pill is the incident illumination power on the channel. The R and P values of photodetectors based on PDQT and PDVT-10 are shown in Figures 3b and e, respectively. For PDQT photodetectors, the highest R and P were 186 mA W–1 and 448, respectively, at VGS = –1.46 V with light intensity of 0.85 mW cm–2. For PDVT-10 photodetectors, the highest R and P were 433 mA W–1 and 176, respectively, at VGS = –1.40 V under light illumination of 0.85 mW/cm2. Such

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Figure 2. a,c) Grazing incidence X-ray scattering pattern (out-of-plane) of PDQT (a) and PDVT-10 (c) annealed at 150 °C on PVA and OTS-modified PVA films. b,d) In-plane X-ray scattering pattern of PDQT (b) and PDVT-10 (d) annealed at 150 °C on PVA and OTS-modified PVA films.

a photosensitivity under low illumination power (

thermal detectors.

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