© 2021 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2021, 2, 2031–2035 | 2031 Cite this: Mater. Adv., 2021, 2, 2031 The performance of conjugated polymers as emitters for triplet–triplet annihilation upconversion† Riley O’Shea, ab Can Gao, c Tze Cin Owyong, ab Jonathan M. White b and Wallace W. H. Wong * ab A series of poly(phenylene–ethynylene) copolymers with various aryl spacer units were synthesized for use as emitters in triplet–triplet annihilation upconversion. The upconversion performance of these conjugated polymers was compared to that of well-known poly(phenylene–vinylene) polymers, MEH-PPV and super yellow, in chloroform solution. The copolymer containing anthracene units outperformed both reference polymers recording a maximum upconversion quantum yield of 0.18%. Introduction Triplet–triplet annihilation upconversion (TTA-UC), also known as triplet fusion upconversion, is a photochemical process by which two lower energy photons can be combined to produce one photon of higher energy. 1 It sees use in raising the efficiency of solar cells above their thermodynamic limit (the Shockley–Queisser limit). 2,3 Two chromophores are required in a typical TTA-UC system – a triplet sensitizer and an annihilator/emitter. 1 The triplet sensitizer absorbs a photon promoting it to its singlet excited state (Fig. 1a). Intersystem crossing (ISC) leads to the formation of the triplet excited state on the sensitizer. This triplet exciton can then be transferred to an emitter molecule via triplet energy transfer (TET). As the triplet exciton population of emitters builds, two triplet excitons can combine leading to triplet–triplet annihilation (TTA) generating a higher energy singlet exciton. This singlet exciton then undergoes radiative decay releasing a photon that is higher in energy than the photon absorbed by the sensitizer. Transition metal complexes are most commonly used as triplet sensitizers but metal chalcogenide quantum dots have also been used. 4,5 As for the emitter component, polycyclic aromatic hydro- carbon molecules have been widely investigated. 6–9 Much less well-established is the use of conjugated polymers as emitters in TTA-UC. 10–14 It was shown in theoretical models that the extended conjugation of these materials leads to improved triplet exciton diffusion which may assist in TTA-UC performance. 15 Measure- ments of TTA-UC systems containing conjugated polymers showed some promise but accurate comparison of TTA-UC efficiency has not been reported. 14,16 In this study, the TTA-UC Fig. 1 Jablonski diagram depicting the process involved for TTA-UC (a). Structures of poly(phenylene–vinylene)s, MEH-PPV and super yellow, and copolymers based on poly(phenylene–ethynylene) investigated as emitters for TTA-UC (b). a ARC Centre of Excellence in Exciton Science, School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia. E-mail: wwhwong@unimelb.edu.au b School of Chemistry, Bio21 Institute, University of Melbourne, Parkville, VIC 3010, Australia c Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China † Electronic supplementary information (ESI) available. CCDC 1992339. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ d1ma00068c Received 27th January 2021, Accepted 19th February 2021 DOI: 10.1039/d1ma00068c rsc.li/materials-advances Materials Advances PAPER Open Access Article. Published on 19 February 2021. Downloaded on 10/16/2021 12:55:27 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue