sp³-Linked Dimeric Carbazole-Based p-Type Hosts for Exciplex-Driven Green TADF OLEDs

Abstract

In this study, we propose a rational molecular design strategy using sp³-linked dimeric p-type hosts to address the limitations of low-molecular-weight carbazole-based hosts in solution-processed exciplex-based organic light-emitting diodes (OLEDs). We synthesized two new p-type dimeric hosts: 9,9''-((9H-fluorene-9,9-diyl)bis(4,1-phenylene))bis((9H-3,9′-bicarbazole)) (FL-PP) and bis(4-(9H-[3,9′-bicarbazol]-9-yl)phenyl)diphenylsilane (Si-PP). We systematically compared their properties with the monomeric reference host, 9-phenyl-9H-3,9′-bicarbazole (CzCzPh). When blended with the common n-type host, 9,9'-(6-(3-(triphenylsilyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (SiTrzCz2), both dimeric hosts readily formed efficient exciplexes. These exciplexes exhibited small singlet–triplet energy gaps (ΔEST ≈ 90–110 meV) and pronounced thermally activated delayed fluorescence (TADF) characteristics. Compared to CzCzPh, FL-PP and Si-PP exhibited significantly improved thermal stability, reduced crystallinity, and enhanced film morphology due to their elevated molecular weights and nonplanar dimeric structures. These blended films also demonstrated elevated triplet energies (T₁ > 3.0 eV) and substantial photoluminescence quantum yields (PLQYs) (approximately 30%). In terms of device performance, green TADF-OLEDs utilizing CzCzPh:SiTrzCz2 and FL-PP:SiTrzCz2 exciplex hosts achieved external quantum efficiencies exceeding 20%. This high efficiency can be attributed to their high PLQYs and favorable charge balance. However, devices utilizing CzCzPh experience significant performance degradation after high-temperature drying due to the poor thermal stability of the emissive layer. In contrast, devices incorporating FL-PP and Si-PP hosts effectively overcome this issue, showing robust thermal tolerance and stable operation. This study demonstrates that sp³-linked dimerization of CzCzPh-type hosts effectively addresses processability and stability limitations, enabling high-performance solution-processed exciplex-type TADF OLEDs.