Monday, 11 December 4:00pm – 5:00pm

This seminar will be delivered in Chemistry Lecture Theatre 4

Speaker: Alison Goldingay

Host: A/Prof. Girish Lakhwani

Title: Dual-direction Energy Harvesting and Strong Light-Matter Coupling in Twisted Perylene Organic Photovoltaics

Abstract: Organic photovoltaic (OPV) solar cells are a promising next-generation solar harvesting technology based on organic polymers and small molecules. They can be flexible and lightweight, and their fabrication can be achieved using environmentally friendly materials with low financial and energy costs of production. They possess tunable absorption spectra, allowing for maximum energy harvesting and for the tailoring of their optical properties for particular applications such as building-integrated solar harvesting. Perylene diimides are a promising class of electron acceptor OPV molecules due to their excellent absorption properties, low cost, ease of synthesis, stability and tunability. These can be paired with the electron donor PTB7-Th to create a suitable material system for solar energy harvesting. We show here that perylene diimide monomers (PDIs) are plagued by strong intermolecular π-π interactions which inhibit their device performance, but we also show that these interactions can be partially overcome by the use of dimers having a twisted plane of conjugation (TPDIs). Intensity dependent device measurements are used to demonstrate that TPDI dimers experience reduced recombination losses due to their more favourable morphology. Using ultrafast spectroscopy, we also demonstrate dual-direction energy harvesting, since charge transfer states are formed from excitation of both the PTB7-Th donor and the (T)PDI acceptors. We demonstrate a method to quantify the contribution of each material to the overall charge production, and show that TPDI blend films demonstrate balanced dual-direction energy harvesting in contrast to PDI blend films.

It is also possible to alter the properties of organic materials by strong light-matter coupling, where a photon in resonance with an optical cavity couples to the excitonic transition of a material, producing a new quasi-particle termed a polariton. The formation of polaritons causes splitting of the excited state energy levels such that new energy levels, termed polariton branches, are created. We fabricate and compare bare and cavity-based OPV devices and demonstrate the presence of strong coupling in cavity-based OPVs through both optical and electrical measurements. We also fabricate cavity-based devices of different thicknesses in order to control the interaction between the exciton in the material and the photon in the cavity. In doing so, we demonstrate that the presence of a resonant cavity within an OPV device enables the selection of certain charge-transfer state energies from within a disordered manifold of states. This has profound implications for the study of device physics in OPVs, since we demonstrate that properties which are normally intrinsic to a material can be changed and manipulated without changing the material itself.

References

¹ Goldingay A, Stuart A, Ghosh P, Patil S, Rao A and Lakhwani G. J. Phys. Chem. C. 2023 (accepted)