Synthesis of Cyclopentadithiophene–Diketopyrrolopyrrole Donor–Acceptor Copolymers for High-Performance Nonvolatile Floating-Gate Memory Transistors with Long Retention Time

Organic flash memories that employ solution-processed polymer semiconductors preferentially require internal stability of their active channel layers. In this paper, a series of new donor–acceptor copolymers based on cyclopentadithiophene (CDT) and diketopyrrolopyrrole (DPP) are synthesized to obtain high performance and operational stability of nonvolatile floating-gate memory transistors with various additional donor units including thiophene, thiophene–vinylene–thiophene (CDT–DPP–TVT), selenophene, and selenophene–vinylene–selenophene. Detailed analyses on the photophysical, two-dimensional grazing incident X-ray diffraction, and bias stress stability are discussed, which reveal that the CDT–DPP–TVT exhibits excellent bias stress stability over 105 s. To utilize the robust nature of CDT–DPP–TVT, floating-gate transistors are fabricated by embedding Au nanoparticles between Cytop layers as a charge storage site. The resulting memory devices reveal bistable current states with high on/off current ratio larger than 104 and each state can be distinguished for more than 1 year, indicating a long retention time. Moreover, repetitive writing–reading–erasing–reading test clearly supports the reproducible memory operation with reversible and reliable electrical responses. All these results suggest that the internal stability of CDT–DPP–TVT makes this copolymer a promising material for application in reliable organic flash memory.