Research On Organic Materials And OLED Devices Based On Heavy Atom Effect

Thermally activated delayed fluorescence（TADF）materials and exciplexes with TADF properties have become the current research hotspots for luminescent materials.Both materials have a tiny energy gap between the singlet and triplet state(ΔE_ST)by separating the highest occupied molecular orbital（HOMO）from the lowest unoccupied molecular orbital（LUMO）,allowing triplet excitons to be transformed into singlet excitons with the help of thermal power,and therefore enabling a theoretical 100%internal quantum efficiency（IQE）to be achieved.Nowadays,there has been much work demonstrating the significant advantages and promising prospects of TADF materials and exciplexes as luminescent materials.In addition,according to the formation process and structural properties of TADF materials and exciplexes,it can be found that they can also be used as the host for energy transmission.Due to the presence of a donor fragment that transmits holes and an acceptor fragment that transmits electrons,TADF materials and exciplexes are naturally bipolar.This property grants them excellent bipolar carrier transport properties and helps to create a wide excitons recombination zone.Moreover,the TADF materials and exciplexes can convert triplet excitons to singlet excitons through the additional reverse inter-systemic crossover（RISC）channel,which makes the primary energy transfer path when sensitizing dopants in TADF or exciplexes host materials the F?rster energy transfer between singlet excitons.This allows for more efficient utilization of triplet excitons,thus improving device efficiency,especially for traditional fluorescent dopants.In this thesis,we constructed novel TADF and exciplex host materials with heavy atom effect（HAE）,respectively,and fabricated several groups of organic light emitting diode（OLED）devices with high efficiency.The details of the study are as follows:（1）A novel TADF body material,Br-DMIC-TRZ,was designed and synthesized by substituting a bromine atom for hydrogen on the benzene ring.Our study shows that due to the introduction of a bromine atom,the bromine-substituted compound exhibits a faster RISC rate,increasing from 1.05×10⁶ per second to 2.27×10⁶ per second.We fabricated several sets of OLED devices using Br-DMIC-TRZ and DMIC-TRZ without introducing bromine and the traditional host material CBP,respectively.The results show that the performance of the Br-DMIC-TRZ-based OLED devices is significantly better than that of the DMIC-TRZ and CBP-based OLED devices.These results indicate that introducing the HAE in the TADF host can improve OLED devices’electroluminescence（EL）performance.（2）A series of exciplex host materials based on HAE were constructed using the commonly used donor material TCTA and its brominated derivative Br-TCTA.The results show that Br-TCTA-based OLED devices have higher EL performance.We observed that the RISC rate increased from 1.48×10⁶ per second to 3.04×10⁶ per second in the exciplex films formed with the acceptor B4PYMPM due to the introduction a bromine atom.Our study demonstrates that introducing the HAE in the exciplex host can also accelerate the RISC process,improve the utilization of triplet excitons,and ultimately improve the electroluminescence（EL）performance of OLED devices.