Unexpected large spectral shift from blue to green region in a light-emitting electrochemical cell

Henk J. Bolink; Eugenio Coronado; Rubén D. Costa; Nora Lardiés; Md. K. Nazeeruddin; Enrique Ortí

doi:10.1117/12.781825

25 April 2008 Unexpected large spectral shift from blue to green region in a light-emitting electrochemical cell

Henk J. Bolink, Eugenio Coronado, Rubén D. Costa, Nora Lardiés, Md. K. Nazeeruddin, Enrique Ortí

Proceedings Volume 6999, Organic Optoelectronics and Photonics III; 69992L (2008) https://doi.org/10.1117/12.781825
Event: SPIE Photonics Europe, 2008, Strasbourg, France

Abstract

A new, but archetype compound [Ir(ppy-F₂)₂Me₄phen]PF₆, where ppy-F₂ is 2-(2',4'- fluorophenyl)pyridine and Me₄phen is 3,4,7,8-tetramethyl-1,10-phenanthroline, was synthesized and used to prepare a solid-state light-emitting electrochemical cell (LEC). This complex emits blue light with a maximum at 476 nm when photoexcited in a thin film, with a photoluminescence quantum yield of 52 %. It yields an efficient single-component solid-state electroluminescence device with a current efficiency reaching 5.5 cd/A and a maximum power efficiency of 5.8 Lm/Watt. However, the electroluminiscence spectrum is shifted with respect to the photoluminiscence spectrum by 80 nm resulting in the emission of green light. We demonstrate that this unexpected shift in emission spectrum is not originating from the way of excitation, nor from the presence of large concentrations of ions, but is related to the concentration of the ionic transition metal complex in the thin film. The origin of the concentration dependent emission is extensively commented and argued to be related to the population of either ³LC π-π* or ³MLCT triplet states, in diluted and concentrated films, respectively. Using quantum chemical calculations we demonstrate that three low-energy triplet states are present with only 0.1 eV difference in energy and that their associated emission wavelengths differ by as much as 60 nm from each other.

Citation Download Citation

Henk J. Bolink, Eugenio Coronado, Rubén D. Costa, Nora Lardiés, Md. K. Nazeeruddin, and Enrique Ortí "Unexpected large spectral shift from blue to green region in a light-emitting electrochemical cell", Proc. SPIE 6999, Organic Optoelectronics and Photonics III, 69992L (25 April 2008); https://doi.org/10.1117/12.781825

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KEYWORDS

Thin films

Solids

Absorption

Luminescence

Quantum efficiency

Ions

Solid state electronics

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