Printed organic transistors and complementary ring oscillators operatable at 200 mV

Applications of organic thin-film transistors (OTFTs) include wearable health monitors and next-generation Internet-of-Things systems driven by a small energy-harvesting power supply. Such applications require low voltage and low power consumption organic ICs. In this paper, we demonstrate complementary ICs based on printed p-type and n-type OTFTs operatable at an ultralow supply voltage of 200 mV. For that purpose, threshold voltages were finely tuned by dual-gate structure and self-assembled monolayer. Complementary inverter-based ring oscillators operated at small supply voltages down to 200 mV and exhibited a power consumption as small as 6 pW per stage.

][3] OTFTs can be fabricated using printing technology such as an inkjet at low cost, low temperature, and low energy onto flexible substrates such as polyethylene-naphthalate (PEN) films and a-few-μm-thick Parylene films. 4,5)Applications of the OTFTs include next-generation Internet-of-Things (IoT) devices and organic RF identification (RFID) tags.The power sources of these devices are usually small batteries or energy harvesting units. 6,7)A variety of organic energy harvesting materials and devices have been reported, such as organic photovoltaics, 8) organic thermo-generators, 9) piezoelectric nanogenerators, 10) triboelectric nanogenerators, 11) and biofuel cells. 12)Many of the energy harvesting devices typically output voltages below 1 V and small powers below 1 μW.Therefore, low voltage and low power operation of organic Ics are important.
Low-voltage operation of OTFTs has been demonstrated employing ultra-thin polymers, 13,14) metal-oxide/self-assembled-monolayer hybrid gate dielectrics, 15) organic electrochemical transistors with ionic liquids, 16,17) interface engineering at semiconductor/gate dielectric interfaces. 18)iang et al. reported printed OTFTs employing Schottky contacts for low voltage and high gain operation. 19)Kitahara et al. reported printed OTFTs using fluoropolymer and organic semiconductor single crystals with meniscus-controlled printing to minimize the trap density at the semiconductor/gate insulator interface. 20)Shiwaku et al. reported unipolar organic inverters operatable at an ultralow voltage of 0.5 V by tuning turn-on voltages of OTFTs. 21) complementary configuration composed of p-type and n-type OTFTs is reliable for low power operation, since through current is blocked by ether p-type or n-type OTFTs.To realize low voltage and low power consumption of complementary organic ICs, p-type and n-type OTFTs need to exhibit symmetric characteristics and small turn-on voltages.However, there are few reports of organic complementary ICs capable of operating below 1 V.
In this paper, we demonstrate printed p-type and n-type OTFTs and complementary ICs operatable at an ultra-low voltage of 0.2 V.For fine-tuning of turn-on voltage, we introduced dual-gate electrodes to n-type OTFTs and a selfassembled monolayer on the gate electrode to p-type OTFTs.Dual gate electrodes shorted with each other are known to shift turn-on voltage and improve the subthreshold slope in our previous study. 22)The work function shift of the gate electrode is able to shift the turn-on voltage by changing the flat band voltage of the metal-insulator-semiconductor capacitor in the OTFTs. 23)Printed OTFTs exhibited turn-on voltage near 0 V and balanced characteristics between nand p-type OTFTs.Printed organic ring oscillators were fabricated on a flexible substrate and exhibited low voltage operation down to 0.2 V and low power consumption of 6 pW per stage.
Figure 1 shows the schematic structure, materials, and optical microscope image of the printed OTFT circuits.n-type OTFTs were fabricated in a dual-gate bottom-contact structure for improved switching performances, whereas p-type OTFTs were fabricated in a bottom-gate bottom-contact structure for ease of fabrication.125 μm-thick PEN films were used as flexible substrates.Parylene (diX-SR, KISCO) was deposited by CVD using SCS Labcoater 2 (PDS 2010, Specialty Coating Systems) as a base layer to control the wettability of silver nanoparticle ink.Silver nanoparticle ink (NPS-L, Harima Chemical) was printed onto the Parylene layer using an inkjet printer (DMP-2800 series, Fujifilm) as an n-type bottom electrode and then sintered at 120 °C for 30 min in air.Parylene was coated again as a bottom gate dielectric layer of n-type OTFTs.Silver source and drain electrodes for n-type OTFTs and bottom gate electrodes for p-type OTFTs were printed onto the Parylene gate dielectric by the inkjet.A 1 wt% solution of fluoropolymer (Teflon AF1600, Dupont) in fluorinert (FC-43, 3 M) was printed using a dispenser system (Image Master 350 PC, MUSASHI Engineering) to define the semiconductor region and channel width.The surface of the silver electrodes was covered with a self-assembly monolayer, 4-methylbenzenethiol (4-MBT, Sigma-Aldrich) by immersing the substrates under 10 mM solution in isopropanol.A benzobisthiadiazole derivative, TU-3 (Tokyo Chemical Industry, 0.06 wt%) and poly(α-methylstyrene) (PαMS, M w ∼30000, Sigma-Aldrich, 0.02 wt%) were dissolved in 1-methylnaphthalene [Fig.1(b)].The TU-3:PαMS solution was printed using the dispenser system and annealed at 120 °C for 30 min in an inert atmosphere to form the active layer.Parylene was coated again as a top-gate dielectric for n-type OTFTs and a bottom-gate dielectric for p-type OTFTs.Then, silver source and drain electrodes covered with pentafluorobenzenethiol and bank layer were printed.A p-type organic semiconductor, 2,7-dioctyl [1]benzothieno [3,2-b][1]benzothiophene (C 8 -BTBT, Sigma-Aldrich, 0.3 wt %), and polystyrene (PS, M w ∼280000, Sigma-Aldrich, 0.1 wt%) were dissolved in mesitylene and printed using the dispenser system and annealed at 60 °C for 30 min in an inert atmosphere.The design of printed OTFTs and circuits was based on a previous report. 24)The maximum process temperature was 120 °C, enabling the fabrication of various flexible films such as PEN films and ultra-thin Parylene films.
Figure 2(a) shows the typical transfer characteristics of the printed n-and p-type OTFTs at drain voltages of V D = 5 V and −5 V, respectively.Channel lengths were 61 and 63 μm, channel widths were 985 and 971 μm, and capacitances per unit area including top and bottom gates were 26 and 13 nF cm −2 for the n-and p-type OTFTs, respectively.Saturation mobility of the n-and p-type OTFTs were 0.041 ± 0.0072 cm 2 V −1 s −1 and 0.095 ± 0.081 cm 2 V −1 s −1 , respectively.The threshold voltage of the n-and p-type OTFTs were 0.53 ± 0.097 V and −0.30 ± 0.20 V, respectively, and the turn-on voltage of both transistors exhibited nearly 0 V.The subthreshold swing (SS) of the n-and p-type OTFTs exhibited 187 ± 33 mV dec −1 and 172 ± 61 mV dec −1 , respectively.The on/off ratio of the OTFTs was 10 4 −10 5 .Figure 2(b) shows that the output characteristics of the n-and    The voltage transfer characteristics in Fig. 3(c) show that the complementary inverters exhibit full-swing operation at supply voltages V DD down to 0.5 V. Voltage gain was 9.7 at V DD = 0.5 V. Stand-by power consumption (V IN = 0 V or V DD ) was lower than 10 pW and peak-power consumption (V OUT = V DD /2) was 30 pW when V DD = 0.5 V.These values are comparable to those in the previous work of low-voltage organic complementary inverters with vacuum-deposited electrodes.Our complementary inverters exhibit lower standby power consumption at V IN = 0 V than unipolar and pseudo-CMOS inverters, 21) because the current from VDD to GND is blocked by either p-or n-type OTFTs.CMOS configuration current flows only around the transition voltage.
In order to evaluate the dynamic characteristics of the printed organic complementary inverters, we fabricated

011010-3
© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd oscillation at 0.2 V indicates that all the inverters operated at 0.2 V.Among the five ring oscillators we measured, five exhibited oscillation at 0.3 V and four exhibited oscillation at 0.2 V, as shown in Fig. S1 in the supporting information.This operation voltage is the lowest among organic ICs to our knowledge.Considering that the dual-gate structure in TU-3 OTFTs improved the performance of operational amplifiers significantly in our previous study, 22) the dual-gate structure should be essential to obtain the low voltage operation.Signal delay per stage (t d ) was calculated as t d = 1/(2nf osc ), where n is the number of the inverter stages, f osc is the measured oscillation frequency.The delay t d was 25 ms at V DD = 5 V and 410 ms at V DD = 0.2 V. Signal delay per stage of our devices was compared with previous reports including vacuum process and unipolar-based ring oscillators, 21,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] as summarized in Fig. 5. Since the delay t d depends on the operation voltage V DD in general, we plotted t d against V DD . Bue and orange symbols denote the ring oscillators whose semiconductor and electrode layers were fabricated in solution processes.Red and orange symbols denote the ring oscillators using vacuum-deposited electrodes.Our ring oscillators can operate at the lowest voltage, and exhibit the top-level signal delay among the ring oscillators operatable below 1 V. Time-averaged current consumption I ave of our ring oscillators was measured at the GND terminal using a current amplifier (LI-76, NF corporation).The measured current and output voltage are shown in Fig. S2 in the supporting information.Power consumption estimated as I ave V DD was found to be as low as 300 nW and 30 pW with supply voltages of 5 V and 0.2 V, respectively (Fig. S3).Since this power consumption corresponds to a load resistance of 16 MΩ between VDD and GND, the low voltage and low power printed organic ring oscillators can be readily operated by energy harvesting devices, such as 13.56 MHz rectifiers in our previous study.45) In summary, we fabricated printed organic complementary inverters and ring oscillators operating at 0.2 V.By introducing the dual gate electrodes and work function shift of gate electrodes for n-type and p-type semiconductors, respectively, organic complementary inverters exhibited the operation at a low voltage of 0.2 V and low power consumption below 10 pW.Printed organic ring oscillators composed of five stages and a single buffer operated at 0.2 V.These results indicated that organic complementary ICs are applicable to IoT ecosystems using energy-harvesting and passive organic RFID systems without external batteries in printed electronics.

Fig. 1 .
Fig. 1.(a) Cross-section diagram of printed OTFTs.(b) Chemical structures of the materials used in the printed OTFTs.(c) Photograph of the organic ICs based on n-and p-type printed OTFTs.
5-stage ring oscillators with a single-stage voltage buffer.The circuit diagram and optical microscope image are shown in Figs.4(a) and 4(b).
Figure 4(c) shows the oscillation of the output voltages at V DD = 5, 0.5, 0.3, and 0.2 V.The

Fig. 3 .
Fig. 3. (a) Circuit diagram and (b) optical microscope image of the printed organic complementary inverter.(c) Voltage transfer characteristics of the printed organic complementary inverter.Output voltage (V out ), power consumption, and small-signal gain as a function of input voltage (V in ).

Fig. 4 .
Fig. 4. (a) Circuit diagram and (b) optical microscope image of the printed organic ring oscillator with five-stage complementary inverters and a single-stage buffer.(c) Output voltage of the ring oscillator at V DD = 5, 0.5, 0.3, 0.2 V.

Fig. 5 . 4 ©
Fig. 5. Signal delay per stage of the printed organic complementary ring oscillator as a function of V DD , compared with previous reports.Green: vacuum deposition electrode and solution-processed semiconductor; Blue: printed electrode and semiconductor; Red: vacuum deposited electrode and semiconductor; Orange: all printed or solution-processed.