Our work on highly efficient biluminescent organic emitters at room temperature is featured in Advances in Engineering. Find the appropriate direct link here. The work describes an organic molecule, namely (BzP)PB that shows highly efficient fluorescence and phosphorescence at room temperature. Here, the intermixing between singlet and triplet manifold only determines the relative shares of fluorescence and phosphorescence, turning this emitter into a dual state emitter, where intercombination from one spin manifold to another does not represent an internal loss channel.
In my last post, I highlighted our most recent publication in Scientific Reports, which discusses novel strategies to achieve room temperature phosphorescence of organic semiconductors by means of sample engineering and exciton management (see: ‘Room temperature triplet state spectroscopy of organic semiconductors‘). In today’s post, I’d like to give some very convincing evidence, how well these approaches work out in real time and space.
In the video below, you see a couple of glass slides that are covered with a thin film composed of the polymer PMMA [Poly(methyl 2-methylpropenoate)], into which 2 wt% of the well known organic material NPB [N,N′-di(naphtha-1-yl)-N,N′-diphenyl-benzidine] is embedded. The sample is optically excited with a 365 nm LED, giving rise to blue fluorescence of NPB. Whenever the LED is turned off, the sample shows a persistent emission of green/yellow color, which is the phosphorescence of NPB. Conditions: room temperature, nitrogen atmosphere.
In the world of organic electronics, many devices and applications make use of the excitonic properties of organic semiconductors, namely light-emitting diodes, solar cells, photo-detectors, lasers, sensors, luminescent solar concentrators, optical up- and down-converters, and more. Excitons in organic molecules are highly localized states, giving rise to singlet and triplet excitons that differ in almost every aspect. Triplets are ‘dark’ states, who’s transition to the ground state is quantum mechanically forbidden, therefore, they are long-lived states, in crystals they can diffuse like crazy, and are typically significantly lower in energy than the molecule’s singlet state, as a consequence of exchange interactions. Still, if incorporated in the right way, one can specifically make use of triplets, e.g. through singlet exciton fission, thermally activated delayed fluorescence, or spin-orbit coupling.
Without any doubt, it is essential to know the triplet state energy of a given organic molecule to be able to design excitonic devices. Given the vast of organic molecules known and possible to design, the task of experimentally determining the triplet state is time consuming and can get very frustrating. This is because the ‘dark’ triplet generally only unmasks it’s properties at cryogenic (< 77 K, often at ~ 10 K) temperatures, where non-radiative modes are frozen out. This task would be simplified to great extend, if spectroscopy could be carried out at room temperature.
In our new paper published in Scientific Reports today, ‘Room temperature triplet state spectroscopy of organic semiconductors‘, we report on our recent efforts to simplify the determination of triplet states through sample engineering that allows to carry out these experiments at room temperature. Key trick to unlock room temperature phosphorescence from random organic materials is the engineering of a rigid matrix-enviroment that suppresses many non-radiative modes, very similar to the cooling of the sample. We test this on a variety of materials well known in the field of organic electronics. This scheme is very effective and powerful. Beyond simple spectroscopy of triplet states, it also allowed the observation of biluminescence, i.e. efficient, simultaneous fluorescence and phosphorescence of organic molecules.