Electrical parameter extraction of organic transport layers and study of electron and hole-only devices

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Name of the Speaker: K Chitra Sai Srivatsava (EE19S032)
Name of the Guide: Dr. Debdutta Ray
Date/Time: 25th July 2022, 5:30 pm – 6:30 pm

Organic Light Emitting Diodes (OLED) based displays in smartphones and wearables have seen a steady rise over the last few years. The next (3rd) generation Thermally Activated Delayed Fluorescence (TADF) Organic Light Emitting Diodes (OLEDs) avoid heavy metal atoms used in the current 2nd generation OLEDs. However, the major challenge in TADF OLEDs is the roll-off of external quantum efficiency (EQE) at high intensities. In our work, we used a TADF host 2,6-Bis(9,9-diphenylacridin-10(9H)-yl)pyrazine (PrDPhAc). The efficiency roll-off is presumed to be caused by Triplet-Triplet Annihilation (TTA) and charge imbalance. To minimize EQE roll-off, the recombination zone needs to be in the middle of the emissive layer. The recombination zone deviates from the center of the emissive layer at high voltages due to the difference in electron and hole mobility in the respective transport layers in OLED. A detailed study of electron and hole-only devices with different thicknesses of organic layers and different injection layers are fabricated and analysed. J-V characteristics show that electron injection is less than hole injection, which could be the reason for charge imbalance in OLED. 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN) and Poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) PEDOT: PSS were used as the hole injection layers. Single-layer devices were fabricated to understand the electrical properties of organic transport layers. The layers act as hole/electron transporting layers in the 3rd generation OLED stack. Temperature-dependent electrical characterization of these devices were carried out. The activation energies associated with carrier transport and charge carrier mobility were extracted using a surface fitting with an appropriate charge injection and transport models. In our study we used 1,3,5-Tris(3-pyridyl-3-phenyl)benzene (TmPyPB) and 4,4-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) as electron and hole transport layer respectively. The extracted activation energy values in TmPyPB(89meV) and TAPC(68meV) devices shows that the reason behind charge imbalance is the low electron injection efficiency.