High-performance stretchable resistive memories using donor–acceptor block copolymers with fluorene rods and pendent isoindigo coils

Abstract

Diblock copolymers consisting of electron-donating poly[2,7-(9,9-dihexylfluorene)] (PF) rods and electron-withdrawing poly(pendent isoindigo) (Piso) coils were designed and synthesized through a click reaction. The electronic properties and interchain organization of the copolymers could be tuned by varying the PF/Piso ratio (PF14-b-Pison (n=10, 20, 60 and 100)). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the studied polymers were progressively reduced as the length of Piso increased, affecting the charge trapping and intramolecular charge transfer environment between PF and Piso domains. Thermally treated PF14-b-Pison thin films exhibited a clear nanofibrillar structure, and the d-spacing was enhanced systematically as the Piso chain length increased. Resistive memory characteristics were explored with a sandwich indium tin oxide/PF14-b-Pisons/Al device configuration. The enhanced conjugated PF conducting channels led to stable resistance switching behavior, exhibiting volatile SRAM (static random access memory) (PF14-b-Piso10) and nonvolatile WORM (write-once-read-many-times) (PF14-b-Piso20, PF14-b-Piso60, PF14-b-Piso100) characteristics with a large ON/OFF ratio (106) and a stable retention time (104 s). A more appealing feature is that such memory cells were integrated on a soft poly(dimethylsiloxane) substrate, allowing for the development of a stretchable data storage device. Reliable and reproducible electrical characteristics, including SRAM- and WORM-type memories, could be explored as the device was stretched under an applied tensile strain ranging from 0 to 50%. The studied donor–acceptor copolymers indeed showed great potential for stretchable electronic applications with controllable digital information storage characteristics. A copolymer whose electrical properties can be tuned by varying the ratio of its two blocks is promising for wearable data storage devices. Polymer–based memory devices have excited much interest because of their flexibility, low cost and printability, but so far no stretchable memory devices based on copolymers made up of electron–donating and –accepting blocks have been made. Now, Wen–Chang Chen of National Taiwan University and co–workers have fabricated a series of copolymers made of electron-donating rods and electron–accepting coils. The rods impart the copolymer with superior electrical properties, while the coils give it flexibility. Varying the ratio of the rods to coils allows the copolymer's properties to be varied. The researchers demonstrated that reproducible electrical characteristics can be obtained even when tensile strains of up to 50% are applied. Stretchable resistive memories were created using donor–acceptor rod-coil diblock copolymers, poly(polyfluorene)-b-poly(pendent isoindigo), with different block ratios. Reliable and reproducible electrical characteristics of volatile SRAM- and nonvolatile WORM-type memories were exhibited under applied tensile strains ranging from 0 to 50 %, demonstrating the potential to serve as promising candidates for high-performance wearable devices.