Triplet Exciton Diffusion and Quenching in Matrix-Free Solid Photon Upconversion Films

Karolis Kazlauskas^a, Steponas Raišys^a, Ona Adomėnienė^a, Povilas Adomėnas^a, Alexander Rudnick^b, Anna Köhler^b

^aInstitute of Photonics and Nanotechnology, Vilnius University, Lithuania, Saulėtekio av. 3, LT-10257 Vilnius,, Lithuania

^bSoft Matter Optoelectronics, Department of Physics, University of Bayreuth, Bayreuth 95440, Germany

Proceedings of International Conference on Advances in Organic and Hybrid Electronic Materials (AOHM19)

Dubrovnik, Croatia, 2019 March 17th - 20th

Organizers: Alejandro Briseno, Thuc-Quyen Nguyen and Natalie Stingelin

Oral, Karolis Kazlauskas, presentation 016
DOI: https://doi.org/10.29363/nanoge.aohm.2019.016
Publication date: 8th January 2019

Efficient triplet exciton diffusion in amorphous solid films is essential for triplet–triplet annihilation (TTA) and TTA-mediated photon upconversion (UC). The detailed TTA-UC energy scheme is provided in Fig. 1.

Fig. 1. TTA-UC energy scheme. ISC - intersystem crossing, TET - triplet energy transfer, TTA - triplet-triplet annihilation, UC - upconversion.

Generally, poor triplet diffusion is believed to cause low UC quantum yield (< 3%) of rigid solid UC systems typically consisting of sensitizer and emitter molecules dispersed in a polymer matrix.[1] Since the diffusion relies on short-range Dexter-type energy transfer, UC systems containing high concentrations of chromophores or ultimately matrix-free systems are preferred. However, our study on high-emitter-content UC films shows that enhanced triplet diffusion can severely reduce TTA-UC performance in the presence of triplet quenching sites, which can be related to unintentionally introduced impurities or some other defects.[2] Evaluation of UC temperature dynamics and triplet diffusivity of matrix-free solid UC systems based on rationally-designed bisfluorene-anthracene (BFA) emitters and standard octaethylporphyrin (PtOEP) sensitizer revealed that the diffusion-facilitated triplet quenching constitutes major energy losses. Even above glass transition temperature of BFA/PtOEP films, whereby translational molecular motions are sufficient to promote TTA, these losses outcompete TTA resulting in degraded overall UC performance. Elimination of the triplet quenching channel is estimated to boost UC quantum yield in the amorphous BFA/PtOEP films well above 7% at room temperature. The current study implies that the UC performance of a particular matrix-free UC system with the triplet quenchers present can be optimized by slightly increasing the intermolecular separation. This is expected to restrict the access of the triplets to the quenching sites while enabling the domination of TTA.

References:

[1] Simon, Y. C.; Weder, C. Low-power photon upconversion through triplet–triplet annihilation in polymers. J. Mater. Chem. 2012, 22, 20817–20830.

[2] Raišys, S.; Kazlauskas, K.; Juršėnas, S.; Simon, Y. C. The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses. ACS Appl. Mater. Interfaces 2016, 8, 15732–15740.

Acknowledgements:

The research at Vilnius University was funded by a grant (No. SMIP- 17-77) from the Research Council of Lithuania.

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