Synergistic trap engineering via bromine functionalization and steric conformational twisting in carbazole–fluorene copolymers for high-retention organic memory
摘要
To address the critical issue of low charge-trapping efficiency in organic field-effect transistor memory (OFETM), this study introduces bromine functional groups onto previously synthesized carbazole-based polymers. Poly(carbazole dibromofluorenone) (PBrFlCz) and poly(phenylcarbazole dibromofluorenone) (PBrFlPhCz) were synthesized via Friedel–Crafts polycondensation. Structural analysis and physicochemical characterization revealed substantial modulation of the materials’ energy levels: the HOMO/LUMO energies of PBrFlCz and PBrFlPhCz shift to −5.69/−3.99 eV and −6.40/−4.01 eV. The incorporation of bromine, coupled with phenyl-induced backbone twisting, synergistically deepens the frontier molecular orbitals, creating deep trap states for robust charge confinement. Specifically, the high polarizability and strong electron-withdrawing nature of bromine induce deep trap states, while the deepened HOMO level suppresses charge back-injection, thereby enhancing memory retention. Atomic force microscopy (AFM) indicates both carbazole-based polymer thin films are hydrophobic, exhibiting contact angles exceeding 101° and root-mean-square (RMS) roughness values as low as 0.82 and 1.22 nm, promoting optimized crystallization behavior of the pentacene semiconductor layer. When employed as charge trapping layers in OFETM devices, the devices based on these polymers exhibit exceptional memory performance: Following bromination, the non-phenylated polymer exhibited a notable enhancement in its storage window, expanding from 28.53 V (PFlCz) to 46.93 V (PBrFlCz), representing a 65% increase. In contrast, the phenyl-substituted analog demonstrated a slight decrease in its storage window, from 52.21 V (PFlPhCz) to 51.85 V (PBrFlPhCz). This saturation effect is attributed to the steric hindrance imparted by the phenyl substituent, which disrupts molecular packing and impedes further increases in trap density, while effectively localizing trapped charges for enhanced retention stability. This trade-off between phenyl substitution and bromination requires further optimization. Additionally, the devices exhibited excellent retention, maintaining stable on/off ratios of 8.34 × 103 and 2.10 × 105 after 104 s, which are 28 and 53 times higher than those of their unbrominated counterparts, respectively. This performance enhancement originates from the synergistic interplay of bromination: reduced charge transfer resistance via electron-withdrawing induction, and optimized pentacene crystallization via hydrophobic interface engineering. This research provides critical molecular design insights into the development of high-performance, non-volatile organic memory materials.