Design And Synthesis Of Tert-butylcarbazole Based Organic Functional Materials For Organic Light-emitting Diodes

Organic light-emitting diodes（OLEDs）have received considerable attention in both the academic and industrial communities.OLEDs have been widely utilized as display screens in commercial electronics and exhibit a high potential for organic solid-state lighting due to the advantages such as self-illuminating,light weight,low power consumption and flexibility.The emitting layer is critical to OLEDs where the host material plays an important role in balancing electron/hole transport and improving device efficiency.On the other hand,thermally activated delayed fluorescent（TADF）materials simultaneously utilize the singlet and triplet excitons and thus break the 25%internal quantum efficiency limit of traditional fluorescent materials.A series of 3,6-bis（t-butyl）carbazolyl-based small-molecular host materials/TADF materials are designed and characterized for organic light-emitting diodes.（1）By combination of both the benzimidazolyl and 3,6-bis（t-butyl）carbazolyl units,small-molecular host materials t-BuCz-m-NPBI and t-BuCz-m-2NPBI were obtained.They possessed simple synthesis and purification,a high T_d of 352 and 426^oC（at 1%weight loss）and T_g of 137 and 186 ^oC,respectively.Due to the better solution processability,t-BuCz-m-NPBI selected as the host and Ir（MDQ）₂（acac）as the guest phosphorescent dopant were introduced as the emitting layer via inkjet printing into the OLED.We studied different hole-transporting materials on the OLED performance.In case of the PVK（poly（vinyl carbazole））layer,the maximum current efficiency（CE）and brightness of the resulting device were 2.5 cd/A and 1262cd/m²,respectively.By contrast,replacing PVK with the TFB layer,the maximum current efficiency and brightness increased respectively to 10.8 cd/A and 3277 cd/m²since TFB has improved hole mobility as well as a higher HOMO level leading to a smaller hole-injection barrier at the PEDOT:PSS interface.（2）A 3,6-bis（t-butyl）carbazole-based thermally activated delayed fluorescent material DTC-2CN-BINOL with two cyano groups was designed and synthesized.The analytically pure product was obtained by simple purification with a T_g of 174 ^oC and T_d of 391 ^oC at a 1%weight loss.In addition,due to introducing the bulky tert-butylcarbazolyl moieties,the HOMO and LUMO energy levels of the molecule are effectively separated,leading to a smallΔE_ST of only 0.03 eV.DTC-2CN-BINOL was doped into t-BuCz-m-NPBI in green TADF devices.At a 10%doping concentration,the resulting OLED provided a largest current efficiency of 14.7 cd/A（corresponding to 4.6%external quantum efficiency）and power efficiency of 12.4lm/W.With increasing the doping concentration from 5 to 30%,the electroluminescence spectrum was redshifted,resembling the photoluminescence of the undoped DTC-2CN-BINOL film.Furthermore,it should be noted that with increasing the doping concentration,the device efficiency exhibited smaller efficiency roll-off.For instance,at a brightness of 100 cd/m²,the EQE（5%doping concentration）was 3.9%vs.4.5%（30%doping concentration）.While the brightness increased to1000 cd/m²,the EQE was 3.0%（5%doping concentration）vs.4.0%（30%doping concentration）,indicating that the compound DTC-2CN-BINOL may possess a good dispersity in the host t-BuCz-m-NPBI and moreover balanced hole and electron in the emitting layer.