| Abstract [eng] |
One of the most popular areas of organic optoelectronics - organic light emitting diodes (OLED). The newest, third-generation OLEDs are based on thermally activated delayed fluorescence (TADF) mechanism, which connects the first and the second generations: fluorescence lifetime and phosphorescence effectiveness. Although, OLEDs with TADF derivates are still not commercialized, scientists have big ambitions in creating metal-free blue emitters, which would lead to less energy consuming small screen devices. One of the biggest goals for manufacturers is to switch from the expensive physical vapor deposition method to the much cheaper solution-processed technology. The aim of this work was to investigate photophysical properties of new TADF derivatives and to check their potential to be employed in the third-generation solution-processed OLEDs. In this work an investigation of luminescence properties and OLED characteristics are presented. The derivates used in this work are 5tCzOMe and 5tCzPh, both consisting of phenyl-ketone acceptor and penta-carbazole donor moieties. DFT calculations and laboratory experiments showed that 5tCzPh features a faster reverse intersystem crossing rate (krISC). On the other hand, quantum yield of delayed fluorescence and its lifetime in 5tCzOMe showed greater values, which results in more efficient TADF. After making the reference TADF-OLED by known literature and making sure that the results are reliable, the OLEDs with new TADF materials were made. The OLEDs were both solution-processed and evaporated. All devices consisted of neat emitter layer because both derivates had high glass transition temperature and were able to form amorphous glasses. Solution-processed OLEDs achieved lower external quantum efficiency (1,8 - 5,5 %) than the evaporated ones (10,3 - 13,4 %) because of worse triplet exciton confinement in solution-processed emission layers. Also, solution-processed TADF-OLEDs consisting of triplet harvesting host 5tCzOMe doped with fluorescent emitter were fabricated. It has been proven, that this TADF derivative can be used as a host in TADF-OLEDs because external quantum efficiency reached 4,6 % and potentially could reach even higher values with an optimized device structure and more efficient singlet emitter. |
