About the Project
The purpose of this project is to bring together materials expertise in St Andrews and Karlsruhe to make a new generation of organic semiconductors lasers. These lasers will use a new class of light-emitting material. The project will involve studying the photophysical and lasing properties of the new materials and then develop lightweight flexible lasers made from them.
The first laser used a crystal of ruby to amplify light. We aim to make lasers from much more readily available materials. We will work with organic semiconductors, remarkable plastic-like materials that combine the simple fabrication of plastics with the electrical and optical properties of semiconductors. When a voltage is applied to a thin layer of these materials they give out light, and the resulting device is called an organic light-emitting diode (OLED).
The most recent generation of OLEDs emit by a process called thermally activated delayed fluorescence (TADF). This project is to explore this new generation of light-emitting materials for use in lasers, thereby making a new class of laser. So far TADF materials have been developed and studied almost entirely for applications in OLEDs and displays. The requirements for lasers are related (e.g. light emission is required) but not identical. They are interesting to explore because they are a whole new class of laser materials and also because they provide a possible route to overcome a key problem in organic lasers. In organic lasers there is a tendency to accumulate triplets which can stop the laser action, leading to pulsed operation. TADF materials can convert the undesired triplets into useful light emission providing a very interesting route to overcoming this problem.
The first major task is to develop a rate equation model for lasing from TADF materials. This will differ from previous models because of the ready interconversion of singlets and triplets in TADF materials – especially the possibility of triplets converting to singlets. As application of TADF materials to lasing is new, many of the input parameters will need to be measured. In particular gain coefficients, triplet absorption and the dynamics of both singlets and triplets need to be measured. This will be achieved by combination of transient absorption and transient luminescence measurements. There is an attractive complementarity of capability as Karlsruhe has excellent facilities for long-time transient absorption measurements, and St Andrews has the most extensive facilities for transient luminescence. It is expected the student would initially be based in Karlsruhe, making short visits to St Andrews for transient luminescence measurements.
Subsequently will be use of the rate equation model to screen candidate lasing materials from the wide range of TADF OLED materials made by Profs Braese and Zysman Colman in the chemistry departments in Karlsruhe and St Andrews respectively. The most promising materials will be selected for further study for lasing, and then to design candidate lasers. It is expected that distributed feedback will be used to make lasers because this gives low threshold and the required gratings can be readily replicated. An attractive feature of the proposed collaboration is that the master grating structures can be made over larger area than is possible in St Andrews using the Karlsruhe Nano Micro Facility, and the gratings can then be replicated using expertise in nanoimprint lithography in St Andrews. KIT has outstanding expertise in two-photon direct laser writing which not only gives a similar resolution to electron beam lithography but also enables 3D structures to be made. We expect laser fabrication and testing to occur mainly in St Andrews. These studies will provide valuable information to test and refine the rate equation model of lasing which will then inform further refinements in TADF lasing materials and laser design. Because TADF materials help deal with the problem of triplets in organic semiconductor lasers, our work will also help define a pathway to addressing the grand challenge of electrical pumping of organic semiconductor lasers. At the end of the project we aim not only to have demonstrated high performance lasers made from TADF materials, but to have identified the key materials design considerations for TADF lasing and a validated rate equation model of TADF lasing that many groups will find useful for the development of these new devices.
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