title: The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses.
abstract: Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.
authors.affiliation: Institute of Applied Research, Vilnius University , Saulėtekio 3, LT-10222 Vilnius, Lithuania.
authors.forename: Steponas
authors.initials: S
authors.lastname: Raišys
authors.affiliation: Institute of Applied Research, Vilnius University , Saulėtekio 3, LT-10222 Vilnius, Lithuania.
authors.forename: Karolis
authors.initials: K
authors.lastname: Kazlauskas
authors.affiliation: Institute of Applied Research, Vilnius University , Saulėtekio 3, LT-10222 Vilnius, Lithuania.
authors.forename: Saulius
authors.initials: S
authors.lastname: Juršėnas
authors.affiliation: Adolphe Merkle Institute, University of Fribourg , Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland.|School of Polymers and High Performance Materials, The University of Southern Mississippi , 118 College Drive, Hattiesburg, Mississippi 39406, United States.
authors.forename: Yoan C
authors.initials: YC
authors.lastname: Simon
keywords: diphenylanthracene; exciton diffusion length; platinum octaethylporphyrin; sensitized light upconversion; triplet exciton diffusion; triplet−triplet annihilation