Design and synthesis of fluorene-based block polymer host materials for efficient electroluminescence

Abstract

Solution-processing processes exhibit the advantages of simple fabrication, mass-producibility, high material utilization, and lower cost-effectiveness compared with vacuum vapor deposition techniques, but the development of efficient polymer host materials that can be used in solution-processing processes remains a major challenge. Here, we propose a fluorene-based block strategy to construct polymer host materials, P-Cz-F-BPN, with typical TADF characteristics, which avoids π–π conjugation and interaction between molecules through flexible alkyl bridges, thus giving solution-processable polymers with good solubility and exciton utilization. Theoretical simulations and photophysical tests show that the light-emitting units constructed by carbazole and benzophenone are blocked by a large volume of fluorenyl units, which can effectively increase the molecular distance between the light-emitting units, inhibit the aggregation-induced fluorescence quenching, and promote the exciton radiation transition. Among them, the P-Cz-F-BPN polymer has a high trilinear state (2.99 eV), a small single-trilinear energy level difference (ΔE_ST) and a high glass transition temperature (127 °C). Devices using the solution method with P-Cz-F-BPN doped with 5CzCN achieved maximum current efficiency (CE) and external quantum efficiency (EQE_max) of 24.8 cd A⁻¹ and 9.62%, respectively. This performance is 7 times higher than that of devices based on P-Cz-BPN. The significant improvement in device performance fully demonstrates the importance of block strategies for improving the photoelectric properties of polymer host materials. Therefore, the fluorene-based block strategy can effectively improve the electroluminescence efficiency of the material by ensuring a high triplet state and good film-forming property of the host, and provide a new way for the development of solution-processable host materials.