Properties and applications of microcrystalline organic thin films

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Abstract:Nontechnical: With some organic electronic devices now firmly established in the marketplace, it is becoming clear that organic electronics has the capacity to positively influence our technological ecosystem. However, the disordered films used in today's organic electronic devices hold the U.S. back in science and industry. Studies on single crystals of conjugated organic molecules have been critical in driving our understanding and testing the limits of various properties of organic semiconductors, such as mobility, exciton diffusion length, etc. Despite the very promising implications of such studies, highly disordered amorphous films are currently used and studied for applications, because these disordered films can be reproducibly formed into pinhole free films. Crystalline heterojunctions promise combined high exciton diffusion lengths and carrier mobilities and, when used in vertical heterojunction devices, crystalline films can dramatically expand the application domain of organic electronics. This project will 1) enable important technological and industrial breakthroughs in organic electronic devices, and 2) strengthen technological leadership of the U.S. and prepare the next generation of STEM graduates, including women and underrepresented groups, to follow a STEM career. The PI will engage with the public and students, and is particularly involved in active learning activities to establish role models across generational gaps.In this project, the PI will study organic semiconductor films and heterojunctions with crystal domain sizes of up to 1 mm, very ordered when compared to amorphous or nanocrystalline films employed today. Notably, this will be accomplished while still maintaining a low surface roughness and pinhole free coverage. The fact that organic single crystals reveal highly attractive properties with respect to disordered films means that organic semiconductors hold significantly more promise than what can be realized with the disordered films of today. This is in terms of charge and exciton transport, central to the operation of organic electronic devices. Charge carrier mobility is up to two orders of magnitude more in crystals compared to disordered films (up to 20 vs. 0.1-1 cm2/Vs) along with evidence of band transport, whereas exciton diffusion lengths are up to 2 microns vs. 5-30 nm. These large differences between disordered films and single crystals are thought to source from grain boundaries and structural disorder. Two scientific goals that will allow for future applications are: 1) demonstrate high-performance crystalline heterojunction solar cells with greatly reduced energy losses; 2) unveil the role of grain boundaries on exciton and charge transport.