Publication date: 14th September 2021
In this talk we wiil give a bried overview on our research acitivies related to energy materials for optoelectronic device applications. Further the main focus will be on our recent findings of ultrafast triplets excitons in polymeric solids. Due to their potential for high-efficiency optoelectronic devices, the behavior of organic semiconductors has garnered increasing interest in recent years. Understanding the transport of excitons by diffusion of singlets and triplets is essential for developing efficient optoelectronic devices. Measuring the characteristically non-emissive triplets, however, continues to challenge the field. We have introduced a new experimental technique to measure exciton transport in the transverse direction of polymeric semiconductor thin-films. By using pulsed excitation to measured angle resolved delayed electroluminescence (ARDEL), the group’s method traces the position of the triplet exciton in the emissive layer of OLEDs. The triplet excitons are known to be very localized species in the organic polymers due to their parallel spin configuration and the mutual charge exchange between the molecules. The outcome of the work suggests that, in the case of well polymer chain packing, the triplet transport can be as good as the crystalline organic materials. The polyfluorene system exhibits an efficient triplet-triplet fusion process, which provides singlet excitons as delayed fluorescence and becomes a tool to study triplet exciton kinetics.
The results further characterize thin-film kinetics, which can lead to a better design of optoelectronic devices, like solar cells. Organic semiconductors uniquely exhibit singlet and triplet kinetics that share an important role in determining the performance of various optoelectronic devices. Results suggest the diffusion is significantly anisotropic in thinner films. As the thickness of the film increases, anisotropy reduces in triplet transport. Additionally, the group found that in thicker films, diffusivity approaches close to ultrahigh 10-3 centimeters2 per second, nearing that of similarly based crystalline organic thin films. These findings resolve the decade old mystery of unusually thick emissive layer based efficient OLEDs.
We thank Cambridge Display Technology (CDT) Ltd. for supplying the F8BT. We also acknowledge the support of the NCPRE in allowing the use of the device fabrication facility. G.B. acknowledges funding support of DST for the research fellowship. R.H.F. and D.K. acknowledge the funding support from the RCUK- funded SUNRISE project. We also would like to acknowledge Christopher R. McNeil for the discussion on SAXS data. We would like to acknowledge KL Narasimhan for useful discussion. We also acknowledge SUNRISE industry partner Dr. Su Varma from NSG- Pikington for supply of TCO substrates.
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