Purely Organic Phosphor Sensitization Based Highly Efficient Electrofluorescence Material

Abstract

Pure organic room temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal-based complexes to fabricate highly efficient organic light emitting devices (OLEDs). However, for reported RTP materials, low photoluminescence quantum yields (PLQYs) in thin film state seriously impede their applications in OLEDs. On the other hand, how using normal organic fluorescence materials to fabricate OLEDs with an internal quantum efficiency (IQE) over 25% remains a great unaddressed issue beyond the thermally activated delayed fluorescence (TADF) sensitization approach. Here, we establish a strategy to construct highly efficient OLEDs based on pure organic RTP material sensitized fluorescence emitter. The key point for our strategy is that benzimidazole-triazine molecules (PIM-TRZ), 2,6-di(phenothiazinyl)naphthalene (β-DPTZN) and 5,6,11,12-tetraphenylnaphthacene (rubrene) were screened as host, phosphor sensitizer and fluorescent emitter, respectively. Detail photophysical characterizations demonstrate that the host material PIMTRZ with unique RTP nature is critical for achieving phosphor sensitizing process. As an organic RTP compound, the singlet and triplet state energy levels of β-DPTZN perfectly match with those of PIMTRZ, resulting in the formation and lasting existence of phosphor’s excitons in emitting layer. The large overlap between the absorption spectrum of rubrene and PL spectrum of PIM-TRZ:10% β-DPTZN film can facilitate the Förster energy transfer from the triplet β-DPTZN to the singlet rubrene and the finally displayed fluorescence is derived from singlet excited states of rubrene. The perfect collocation of host, phosphorescent sensitizer and fluorescent emitter in the emitting layer promise the predominant performance of the devices with external quantum efficiency (EQE) of 15.7%. The PLQY of emitting layer is 60.3%, and therefore about 90% carrier injection induced excitons are harvested for light emission. We present a new strategy to fabricate efficient fluorescent devices by employing ingenious combination of host, phosphorescent sensitizer and fluorescent emitter, which is significant to the development of OLEDs.<br>