Spiro[fluorene-9,9′-thioxanthene] core based host materials for thermally activated delayed fluorescence devices

Four novel spiro[fluorene-9,9'-thioxanthene] core based wide-energy-gap compounds Cz-S, 2Cz-S, Cz-SO2, and 2Cz-SO2 were developed as host materials for a general thermally activated delayed fluorescence (TADF) emitter (4s, 6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). The excited states of the sulfane-containing Cz-S and 2Cz-S are in local emission (LE) states while those of the sulfonyl-containing Cz-SO2 and 2Cz-SO2 are in charge transfer (CT) states. The device based on 2Cz-S exhibits the highest efficiency among them with a maximal current efficiency (CE) of 52.1 cd/A and a maximal external quantum efficiency (EQE) of 16.5% due to the largest radiative triplet exciton utilization capacity through intersystem crossing from T-1 to S-1. In addition, the devices based on Cz-SO2 and 2Cz-SO2 exhibit obviously reduced efficiency roll-off due to their improved carrier balance in a wide current density range, and their J(0) (the current density at the half maximum of the EQE) values reach as high as 93 and 115 mA/cm(2). It is also of interest their EL spectra are independent on the host dipole moment due to the spiro-conformation that may reduce the interaction between host and dopant.