Nonvolatile organic field-effect transistor memory devices using polymer electrets with different thiophene chain lengths

Abstract

We report the synthesis of poly(5-hexyl-2-vinylthiophene) (PVT) and poly(5-hexyl-5′′-vinyl-2,2′:5,2′′-terthiophene) (PVTT) as charge storage electrets for nonvolatile organic field effect transistor (OFET) memory devices of n-type semiconducting N,N′-bis(2-phenylethyl)perylene-3,4,9,10-bis(dicarboximide) (BPE-PTCDI). The effects of the conjugated thiophene chain length on the morphology, OFET mobility and memory characteristics are explored and compared to those of the styrene or fluorene side chain. The mobility of the OFET memory device using PVTT as an electret is significantly smaller compared with that of PVT because its large torsional angle hinders the molecular packing of BPE-PTCDI. However, the OFET memory device using the PVTT electret has the largest hysteresis window of 81 V, compared to PVT, polystyrene (PS), and poly(styrene) para-substituted with fluorene (PSt-Fl). The highest HOMO energy level of PVTT facilitates the charge transfer from BPE-PTCDI and leads to the largest memory window. The backbone non-coplanarity prevents the back transfer of the charge for the nonvolatile memory characteristics. The device shows excellent nonvolatile behavior for bistable switching and the write–read–erase–read (WRER) cycles are operated over 100 cycles. The shifted threshold voltages of the OFET memory devices using PVTT are stable over 10⁴ s, and the ON and OFF states could maintain 10⁴ s with the I_on/I_off current ratios of 10³. This study suggests that the pendent conjugation length and the backbone coplanarity of polymer electrets significantly affect the charge mobility and electrical characteristics of OFET memory devices.