[21p-W521-1~18] 12.4 Organic light-emitting devices and organic transistors

△ [21p-W521-4] Control of major carriers in an ambipolar semiconductor by self-assembled monolayers

〇Masahiro Nakano¹, Itaru Osaka¹, Kazuo Takimiya¹ (1.RIKEN)

Keywords:ambipolar,self-assembled monolayer

Ambipolar semiconducting polymers and oligomers are promising candidates for applications in solution-processed complementary-like inverters, which could differentiate the current silicon-based integrated circuit (IC) technology requiring substantial energy consumptions. However complementary-like inverters based on ambipolar semiconducting oligomers and polymers have several drawbacks: small noise margins, small voltage gain, and requirement to drive under inert atmosphere. In addition, in the inverter operation, neither of the pull-down or pull-up transistor can not be complete off-state, originating in the ambipolar nature of the semiconducting layer, which causes substantial leakage current between the V_DD and GND termini of the inverter. The leakage currents increase the power dissipation and decrease the noise margins, and as a result, the performances of ambipolar complementary-like inverters are generally poor compared to organic CMOS inverters based on both p- and n-channel unipolar organic semiconductors.During the course of our materials research in organic electronics, we have found naphtho[2,3-b:6,7-b']dithiophene-1,4,5,8-tetracarboxylicdiimide (NDTI)-based semiconducting polymer, PNDTI-BT which has suitable energy levels of HOMO and LUMO, -5.6 and -4.1 eV, respectively, showed air-stable/balanced p-channel/n-channel behavior in the ambipolar OFET devices. Using this PNDTI-BT, we found the interesting behavior in the OFET devices, which can solve the above problem, that is to say, OFETs fabricated on the 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDTS)-SAM-modified Si/SiO₂ substrate showed unipolar p-channel characteristics clearly, and there were no electron transport in n-channel operation. To envisage the effect from the p-channel unipolarization assisted by the FDTS-SAM, we fabricated complementary-like inverters on a substrate with two distinct regions modified with different SAM. On a Si/SiO₂ substrate with the FDTS- (for the pull-up transistor) and ODTS-modified regions (for the pull-down transistor), PNDTI-BT thin film was spin-coated, and then Au electrodes with symmetrical channels were patterned by vapor deposition through the shadow mask. Compared with the inverter fabricated on the ODTS-modified substrate, the switching characteristics are greatly improved. We also found that 3-(N,N-dimethylamino)propyltriethoxysilane (MAPS)-SAM can induce almost perfect n-channel unipolarization, which is completely the opposite behavior to the OFETs fabricated on the FDTS-modified substrate. With the unipolarization effects caused by two different SAMs on the substrate surface, we fabricated complementary-like inverters on substrates with two distinct regions modified with FDTS and MAPS for the pull-up and pull-down transistors, respectively. The voltage transfer curve, where almost ideal inversion with high voltage gain (~180) is clearly observed. Furthermore, as expected, the maximum penetration current, I_DD, at the inversion point is as low as 10^–6 A.