Doped Organic Crystals with High Efficiency, Color-Tunable Emission toward Laser Application

Abstract

Doping an organic crystal such as an inorganic semiconductor without having a bad influence on crystalline quality is a very difficult task because weak intermolecular interactions and lattice mismatches exist in organic condensed states. We report here the successful growth of tetracene and pentacene-doped trans-1,4-distyrylbenzene (trans-DSB) crystals with high crystalline quality, large size, and excellent optical properties. The doped concentration up to 10% can be achieved by controlling the temperature of the crystal growth zone. The first key point for the crystals with a high doping ratio is the choice of the host (trans-DSB) and guest (tetracene or pentacene) molecules with comparable crystal lattice structures, which ensure less lattice mismatch. The second key point is crystal growth at relative high temperatures by the physical vapor transport (PVT) method, which gives the guest molecules high kinetic energy to incorporate into the crystal lattice of the host. These doped crystals with slice shape and large size (millimeter scale) maintain ordered layer structures and crystal surface continuities, which are verified by X-ray diffraction (XRD) and atomic force microscopy (AFM) analysis. Efficient energy transfer from the host to the guest and the suppressing of the interaction among the guest molecules lead to color-tunable emission and high luminescent efficiencies (blue for undoped trans-DSB, η = 65 ( 4%; green for tetracene-doped trans-DSB, η = 74 ( 4%; red for pentacenedoped trans-DSB, η = 28 ( 4%). Steady-state and time-resolved fluorescence spectroscopy of undoped and doped crystals, and their amplified spontaneous emissions, have been investigated. These doped crystals are expected to be of interest for lightemitting transistors, diodes, and electrically pumped lasers.