Organic light-emitting diodes made from small molecular weight organic semiconductors are composed of multiple functional layers. These layers are in most cases amorphous assemblies of the molecular building blocks. In our joint paper with our collaborators of the Group of Nanomaterials and Microsystems (GNaM) at the Universitat Autònoma de Barcelona entitled ‘High-performance organic light-emitting diodes comprising ultrastable glass layers‘, we shed some light on the growth of these layers. The work is published in Science Advances. The molecules making up the amorphous layers do not necessarily fall into place perfectly so that over time (but these are very long times beyond the lifespan of an OLED) the molecules wiggle into a more compact assembly. This settling can be accelerated greatly if the molecules on the surface are given some extra energy to migrate. Providing excess thermal energy through an elevated substrate temperature, much more stable morphologies – called ultrastable glasses – are formed. The optimum condition for this growth is around 85% of the materials glass transition temperature.
In our study we have tested this growth condition for four different phosphorescent emitters in one common device stack and found that both the external quantum efficiencies and device lifetimes significantly increased. The illustration below summarizes our work graphically.
The full citation is: J. Ràfols-Ribé, P.-A. Will, C. Hänisch, M. González-Silveira, S. Lenk, J. Rodríguez-Viejo, S. Reineke, High-performance organic light-emitting diodes comprising ultrastable glass layers. Sci. Adv. 4, eaar8332 (2018). DOI: 10.1126/sciadv.aar8332.
In our recent publication entitled ‘Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters‘ published in the Journal of Physical Chemistry C, we discuss how the population of triplet excitons in emitters which sport efficient phosphorescence at room temperature influence the overall luminescence properties. An important emphasis here is on the exciton dynamics of the fast fluorescence (nanoseconds) and the slow phosphorescence (milliseconds), which span over six orders of magnitude in excited state lifetimes, depending on the respective sample composition. All of these results are obtained at room temperature.
We acknowledge the funding from the German excellence cluster cfaed (TU Dresden) and from European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 679213).
Ramon Springer joined the group of Prof. Jang Hyuk Kwon (Department of Information Display, Kyung Hee University, South Korea) to carry out a Master thesis topic within the international Masters course Organic and Molecular Electronics (OME) at the TU Dresden. His thesis task was to develop a white-light emitting, multiple OLED stack based on blue and yellow units to be used in AMOLED displays. Here, aside from the optimization of device efficiency, the color quality and angular stability were parameters to be optimized. His work led to a recent publication in Optics Express entitled “Cool white light-emitting three stack OLED structures for AMOLED display applications“. Congratulations to a very successful research stay abroad.
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.
A new paper was just published in Applied Physics Letters entitled ‘High efficiency, dual emission from an organic semiconductor‘. The work describes the observation of highly efficient luminescence of both singlet and triplet states of a purely organic semiconductor, namely N,N’-bis(4-benzoyl-phenyl)-N,N’-diphenyl-benzidine, at room temperature. This unusual observation is the result of a very effective suppression of non-radiative modes within the triplet manifold of the molecule, enabling high efficiency phosphorescence. Together with efficient fluorescence, this molecule transforms in a dual state emitter, a phenomenon we term ‘biluminescence’. As neither singlet nor triplet states are a loss channel in the emitter molecule, the mixing between the two states through intersystem crossing (ISC) and back (reverse ISC) only dictates the relative intensities of fluorescence and phosphorescence. An biluminescence emitter may find future applications as ultra broadband emitters, exciton probes, various types of sensors, and spin independent energy transfer intermediates.