Bimolecular Triplet-Triplet Annihilation Upconversion for Photovoltaics

Abstract

Organic upconversion of photons through triplet energy exchange between two or more molecules (OUC) has been investigated through transient absorption and fluorescence spectroscopy, kinetic rate modelling and morphological analysis of thin films. An OUC system, consisting of one sensitising molecule (PQ4Pd) and an emitting molecule (rubrene), was first studied to explore the possibility of modelling the entire OUC process with a kinetic rate model. Transient absorption spectroscopy allowed for the intermediate steps of OUC to be directly observed and fitted, producing rate constants for each step in the process. This complete model was then optimised against fluorescence measurements from a system containing PtTPBP (sensitiser) and perylene (emitter) to calculate rate constants for that system from a single fluorescence type experiment, as opposed to several orthogonal Stern-Volmer type experiments. The possibility of fabricating a thin film OUC has been investigated through microscopy, fluorescence spectroscopy and a simple Monte-Carlo model. Using a system of PtOEP (sensitiser) and DPA (emitter), it was shown that the maximal efficiency of a thin film containing these molecules suspended in a PMMA matrix is found when the matrix is between 80 and 85 weight% of the total mixture. It was shown that on short timescales (a few seconds to a few minutes), atmospheric oxygen does not adversely affect thin film upconverters of this type as local oxygen is extremely rapidly quenched (less than a ms) and fresh oxygen is not able to diffuse back into the matrix at a rate that is competitive to OUC. It was shown that the degree of intermixing of active materials is of absolute importance in this fabrication, and a novel optical technique for measuring this intermixing in air was developed, some preliminary results are included.