Role of Charge- And Energy-Transfer Processes in Ternary Organic Photovoltaic Devices

Description

Ternary organic photovoltaic (OPV) devices with multiple donor (D) and acceptor (A) materials have recently emerged as promising candidates for high efficient and low-cost energy conversion. Despite its rapid development in recent years, fundamental understanding on this emerging technology is far from adequate. So far, there is no conclusive working mechanism and general design rules for this game-changer technology. In conventional binary OPV devices with D-A systems, photogenerated electron-hole pairs are dissociated at the charge-transfer (CT) states formed near the D/A interfaces. The CT states critically affect the dissociation processes and thus the efficiency of the OPV devices. While the CT states are of high importance to the cell performance, their properties and energies in ternary system are far difficult to predict. Additional photoactive component would induce changes in morphologies, molecular orientation, energy disorder and charge coupling which can directly affect the density-of-state (DOS) distribution and the local charge interaction of the CT states. More importantly, some ternary OPV devices are additionally benefited from the energy-transfer interaction between Ds and As. To date, the roles of energy- and charge- interaction to CT states properties in ternary OPV system are under debates, leading to conflicting design rules and mechanisms. Ultraviolet photoemission spectroscopy (UPS) is a powerful tool for probing the electronic properties and charge interaction of organic materials, but the measurement is performedin darkand thus provides little information on the excited CT states properties. In view of the above limitations, this project propose to direct measure the excited CT states properties of the ternary systems by UPS studiesunder external irradiation of selectivewavelength. By selectively exciting molecules of different energy gaps, charge interaction due to energy-transfer process in the ternary system can be examined. By comparing UPS results with and without wavelength-selected irradiation, CT states properties in ternary system due to energy- and/or charge-transfers can be monitored from their energy levels variations. The final goal of this proposed work is to reveal the working mechanisms of the ternary OPV devices, and the energetic factor that describes and controls performance of complicated ternary OPV devices. The success of the project will not only shed light on the design rules of the ternary solar cells, but also launch a new measurement approach for measuring the charge- and energy- transfer interaction of the photoactive materials.