| Abstract [eng] |
Photophysical Properties of Isophthalonitrile-Based TADF Emitters and Their Application for Blue OLEDs Today phosphorescent emitters are widely used in organic light emitting diode (OLED) industry. Phosphorescent OLEDs exhibit high efficiency and color purity, however, phosphorescent emitters contain heavy metals therefore they are expensive and cannot realize sufficiently stable blue light emission. Moreover, OLEDs today are mainly fabricated using thermal evaporation in a vacuum. This technology can easily ensure the fabrication of high quality multiple layers with excellent device performance. However this OLED production process is energy-intensive and makes the utilization of the expensive OLED materials very low (~20%). Consequently efficient new generation OLED emitters which do not contain heavy metals and are more cost effective solution-processing technologies have recently attracted much attention. In this work, two new generation blue organic emitters exhibiting thermally activated delayed fluorescence (TADF) are investigated. TADF emitters also can theoretically achieve 100 % PL quantum efficiency, however, they do not contain heavy metals. TADF emitters studied in this work are – DCzIPN and DCzIPNMe (modified DCzIPN molecule with additionally attached methyl groups). The aim of this work is to quantitatively evaluate impact of methyl groups on isophtalonitrile TADF emitter PL properties as well as to fabricate solution-processed blue OLEDs with studied TADF emitters and optimize device structure. To achieve this goal, the photophysical properties of the studied emitters in solutions and thin films were investigated – UV absorbance and PL spectra, PL quantum yields, PL transients. Thereafter, three series of spin-coated OLED prototypes with these emitters were fabricated. OLED structure was modified by inserting hole transport layer as well as by varying emitter concentration in the emissive layer. In addition, OLED optical simulation was performed to optimize electron injection layer thickness. It was found that the wider spatial separation of donor moieties caused by the additional methyl groups attached to the DCzIPN molecule enhances the charge transfer (CT) properties of the TADF molecule and improves its photoluminescence properties (~1.5 higher quantum efficiency and ~15 faster delayed fluorescence rate). However, such structural change reduces the S1 excitation energy (by 72 meV) and thus makes it harder to form OLED exhibiting deep blue emission. In addition, fabricated OLED prototypes with modified emitter (DCzIPNMe) exhibited much higher external quantum efficiency (EQEmax = 21.7%) than OLED with DCzIPN emitter of the same structure (EQEmax = 9.5%) as well as significantly lower EQE roll-off. Importantly, up to now OLED with DCzIPNMe introduced in this work is one of the most efficient solution processed OLED emitting in sky blue spectral range. |
