Executive Summary : | Currently all the commercial OLED devices employ triplet emitters known as phosphorescent emitters, having heavy metal atom (iridium or platinum). Phosphorescent emitters utilize the spin orbit coupling and spin mixing due to presence of heavy metal atom. Spin mixing in phosphorescent emitters relaxes the forbidden triplet channel non radiative decay and turns the triplet channel to an allowed radiative transition. Therefore, triplet phosphorescent emitters have internal quantum efficiency (IQE) of 100%. However, phosphorescent emitters are expensive due to the presence of heavy metal atom in addition iridium and platinum are not abundant. The OLED devices having triplet phosphorescent emitters also exhibit severe efficiency roll-off and is attributed to high triplet density which results in triplet-triplet annihilation. The singlet-triplet energy gap which is most important for any OLED emitter material represented as ΔEST for phosphorescent emitters is usually above the thermal energy range. For new class of OLED emitters known as TADF emitters this ΔEST is small of the order of few kbT, hence at room temperature the triplet excitons via intersystem crossing (RISC) can easily move to singlet exciton state, due to their difference in relaxation time to ground state which is usually radiative. This delayed emission of triplets through singlet channel with delayed relaxation time is termed as thermally activated delayed fluorescence (TADF). In TADF emitters the theoretical maximum IQE can be as high as 100%. There are two classes of TADF emitters i. intermolecular ii. intramolecular TADF. In this project we would like to explore intermolecular exciplex concept which naturally has TADF due to separation of HOMO and LUMO orbitals of emitter. Compared with intramolecular TADF emitter, the intermolecular TADF concept has unique advantages: (a) large number of hetero molecular exciplex candidates can be easily screened by combining already available (suitable) commercial electron and hole transport donor-acceptor materials; there is no need for complex synthesis of specific emitters. Synthetic approach is tedious and intensive in addition device related materials usually need 99.99% purity. Number of high purity materials are already commercially available which are not yet explored for exciplex intermolecular TADF emission. (b) The light emission in these devices can be from simple device similar to inorganic P-N junction and potentially need only two layers. (c) The bipolarity due to D-A interface of the emission layer offers us additional handle to tune mobility, as the host materials used for OLEDs usually have issue with poor balance of electron and hole mobilities thereby suffering from charge balance in the OLED device. intermolecular exciplex can be used as an emitter and host simultaneously. Bipolar hosts composed of electron and hole transport materials can solve charge balance and improve the external quantum efficiency of the OLED device. |