Using triplet-triplet up-conversion to make CW organic solid state lasers

We use semiconductors throughout our lives, and most are made from inorganic crystals like silicon and gallium arsenide. In contrast, this project explores organic semiconductors which are carbon-based materials that combine simple fabrication, tuneable properties, efficient light emission and the possibility of being deposited on a wide range of materials to make devices that can be flexible. The best-known examples are displays made from OLEDs (organic light-emitting diodes). Organic semiconductors are also attractive for lasers, but so far all organic lasers can only operate in a pulsed mode. They cannot operate continuously because the light-emitting excited states turn into "triplets". "Triplets" do not emit light, so do not contribute to laser action, and worse they actually inhibit lasing by absorbing light emission at the laser wavelength. Attempts to overcome this problem have used triplet scavengers that remove and deactivate the triplets. This leads to some improvement, but is wasteful as the triplet energy is lost as heat without contributing to light emission. We will explore a new approach to the problem of triplet management in lasers. Instead of deactivating the triplets we will combine pairs of them to make a light-emitting excitation. Hence instead of wasting them, we will turn them into useful excitations that help achieve lasing. The process we will use is triplet-triplet up-conversion. This process has been observed in approximately 20 materials, but its potential to help lasers has not been explored. Using this approach, we aim to make the first continuous solid state organic laser. This will be a new type of laser emitting light in the visible, with wavelengths that are currently difficult to generate, and could see future applications integrated into . new laser-based sensors and diagnostic instruments that support a cleaner environment or improved healthcare. We will begin by measuring the efficiency of triplet-triplet up-conversion in candidate materials - both sourced commercially, and from our project partners who are leading researchers in the USA and Japan. We will build a rate equation model to describe light emission and triplet formation, and use it to identify the most promising approach to continuous lasing. Then we will build and test the laser. Hence our objectives are to: 1. Develop reliable measurements of triplet-triplet up-conversion (TTU) efficiency and rate constants to advance current understanding of TTU mechanism and to identify the most suitable materials for triplet recycling. 2. Build a photophysical model to describe population dynamics and photon flux for the range of pump light intensities using measured parameters; use the model to identify the most feasible regimes for continuous operation. 3. Show that TTU enables longer than 100 ms laser pulse generation as a step to continuous operation. 4. Demonstrate the first continuous operation of a solid-state organic laser and identify the most suitable configurations for future electrical driving. We expect our results to be important for laser users, laser researchers and organic semiconductor researchers. Organic lasers can be lightweight and flexible and so have potential for wearable physical and biomedical sensors. Improved understanding of triplet up-conversion materials will also help applications in solar cells, photocatalysis and bio-imaging.