We have long had an interest in engineering exciton behavior and understanding the factors that lead to exciton quenching and efficiency or stability loss in OLEDs. This has included studies directed at manipulating the spatial extent of exciton formation and recombination, the efficiency roll-off under high injection, and the mechanisms for device degradation. Recently, emphasis has been on manipulating molecular orientation in thin film during vapor processing to identify the impact of permanent dipole moment (PDM) alignment on exciton behavior and OLED efficiency. We reported the surprising result that spontaneous orientation polarization (SOP), resulting from PDM alignment in anisotropic films, can lead to significant charge carrier accumulation and quenching in OLEDs. This has impacted views on the factors that limit efficiency and stability, as well as the role of molecular orientation (as dictated by molecular structure and processing) in performance. More recently we have demonstrated how deposition conditions, thin film blending, and device architecture can be used to tune this phenomenon and the associated level of exciton quenching. Ongoing work is focused on studying SOP across a broader range of applications.
We are working to construct nanostructures that could be tailored to emit particular types of linear or circular polarized light. When integrated into OLEDs, these could offer unique functionality to improve display efficiency, as well as new functionality in 3D displays or systems for augmented and virtual reality. In related work funded, we are developing a combined experiment-theory approach to understand the key architectural device design features that impact viewing angle-dependent dispersion in color and intensity. Changes in color and intensity with viewing angle is an undesirable feature in a display, and especially challenging for foldable and flexible displays.
Metal-halide perovskites are among the most exciting emerging materials for solar photoconversion, combining demonstrated high efficiency with low temperature materials processing. One challenge in this area for commercialization is the reliance on solution processing methods that frequently involve the use of toxic solvents to process the metal-halide and organo-halide precursor materials. Our ongoing work in this area is focused on the use of vapor-based processes, and specifically moderate vacuum vapor-transport deposition (VTD) as “dry” analogues to avoid the challenges related to solution processing. Recently we have successfully demonstrated efficient VTD-deposited photovoltaic cells based on the archetype perovskite methylammonium lead iodide (MAPbI3), the impact of VTD on crystallographic texturing in thin film, and are now working to expand the materials chemistries available via VTD to include formamidinium-based perovskites and mixed A-site perovskites to increase efficiency and stability.
Exciton transport plays a central role in the operation of many organic optoelectronic devices, in particular in organic photovoltaic cells (OPVs). We have previously worked to develop effective methods to probe exciton diffusion, especially in device-relevant architectures, as well as a variety of materials- and device-based methods to engineer the exciton diffusion length to increase OPV efficiency.
Recently, our interest in excitonic energy transfer has grown to include the use of photonic structures to engineer light-matter coupling and realize long-range energy transfer. When an exciton in an organic semiconductor interacts strongly with a confined photonic mode, new coupled states termed exciton-polaritons can emerge. Polaritons are fascinating mixed light-matter states that possess properties that are an admixture of those of the exciton and photon. Relevant for energy transfer is the delocalized (photon-like behavior) of the polariton which may enable longer-range transfer than the exciton alone can realize. These states are of potential use for optoelectronic devices as well as optical/photonic circuits. Our interest is in manipulating molecular orientation and thin film crystallinity to tune light-matter coupling and understand the impact of coupling strength on energy transfer. We have recently demonstrated the ability to continually tune molecular orientation with processing conditions, leading to a corresponding tunability in light-matter coupling strength.