Novel Properties of Organic Charge-transfer Complexes under Photoexcitation

Description

Organic charge-transfer complexes (CTCs) formed by mixing donor (D) and acceptor (A) molecules have drawn much attention in recent years. With special charge distributions and electronic interactions at the D/A interfaces, unexpected properties such as ultra-long carrier diffusion distance up to 3.5cm(Nat. Mater.,554,7690 (2018)), abnormally-high electrical conductivities (Nat. Mater.,16,1209 (2017)), and world’s first organic long-persistent luminescence materials (Nat. Mater.,550,384 (2017)) have been reported recently. Forrest et al. commented that D:A mixtures are considered as a game-changer in the future development of optoelectronic devices. Irrespective to all these excitements, current understanding on charge distribution and energy profile in CTCs upon excitation as well as electronic interactions are far from adequate.While UV photoemission spectroscopy (UPS) is a powerful tool for revealing valence electronic structures of the CTCs at their ground states, many recent novel properties of CTCs are observed under photoexcitation. However, current understanding on electronic structures and charge interactions in CTC under photoexcitation is clearly not enough. In view of this limitations, the PI has recently modified a conventional UPS system by equipping an additional Xe-lamp for external photoexcitation. With this modified system, photoinduced electronic interactions such as charge exchange processes, energy level alignments, charge coupling at the D/A interface can be studied. Our preliminary results demonstrate that photoexcitation can cause two different types of changes in the electronic structures of D/A interface. For D/A interface with a “depletion” junction (i.e. electron transfer from A to D) in dark, photoexcitation would flip the charge transfer direction and forms an “accumulation” junction (i.e. electron transfer from D to A). In contrast, such photoinduced flipping of charge transfer direction was not observed in D/A “accumulation” junction.Atomic force microscopy (AFM) is another versatile tool for studying surface properties. In addition to surface morphology, modern AFM actually provides many different modes of electrical characterizations (e.g. Kelvin probe force microscope (KPFM), electrostatic force microscope (EFM) and pico-current mapping image, etc) which can provide high-resolution imaging of charge distribution and behavior. In this project, we plan to combine the UPS results (e.g. local charge distributions, energy levels, charge transfer behavior, etc.) with the AFM results (e.g. contact topographies, surface potential difference, photo-induced electrical signal, etc.) to build a detailed picture of the CTC interfaces under photoexcitation. With such knowledge, we aim to develop strategic approaches for controlling the charge transfer efficiency and custom tuning the electronic properties of CTCs for their novel applications in organic optoelectronic devices.