Increasing the efficiency of light-emitting electrochemical cells by limiting the exciton quenching
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Increasing the efficiency of light-emitting electrochemical cells by limiting the exciton quenching Daniel Tordera1, Antonio Pertegás1, Enrique Ortí1 and Henk J. Bolink1 1 Instituto de Ciencia Molecular, Universidad de Valencia C/ Catedrático J. Beltrán 2 ES-46980 Paterna (Valencia), Spain. Email: [email protected] ABSTRACT Light-emitting electrochemical cells (LECs) are one of the simplest electroluminescent devices. The possibility to be processed from solution and to operate with air-stable materials makes them an attractive alternative to organic light emitting diodes (OLEDs). Still their efficiencies are below those obtained in OLEDs. Additionally the best efficiencies were reported at low luminances and sustained for a short period of time. Here we show that for a LEC employing an orange-emitting charged iridium complex that is driven using a pulsed driving scheme high efficiencies of up to 20.5 cd A-1 can be obtained at high luminance and sustained over the device lifetime. It is also shown that the efficiency depends strongly on the current density applied. INTRODUCTION Light-emitting electrochemical cells (LECs) are one of the simplest types of molecular electroluminescent devices. Due to their peculiar mechanism LECs can be prepared from solution and operate with air stable electrodes making them suitable for low cost and large area lighting applications [1-3]. In its simplest form they consist of a single active layer composed of an ionic transition-metal complex (iTMC) which supports all three processes of charge injection, charge transport and emissive recombination [2-5]. By means of improved device architecture and an optimized operation scheme LEC devices can reach very high performances with long lifetimes and high efficiencies at high luminances [6-8]. However their efficiencies are still below those obtained in organic light-emitting diodes (OLEDs) [9]. High efficiencies have been reported at rather low luminances and at very low current densities [10-12]. Moreover, these efficiencies could be sustained only for a short moment during the operation time of the device dropping shortly after to lower values. Here, we show that the key to achieving high efficiencies is the use of low current densities, in particular when applied in a pulsed mode of operation. In this way it is possible to obtain excellent efficiencies throughout the device lifetime in combination with good overall performances. The possible efficiency loss mechanisms are also proposed.
EXPERIMENTAL DETAILS To carry out this study an orange-emitting charged iridium complex [Ir(ppy)2(dtbbpy)][PF6] (1), where ppy is 2-phenylpyridine and dtb-ppy is 4,4’-di-tert-butyl-2,2’-bipyridine, has been used. The molecule was previously synthesized [7] and reported [13]. Poly(3,4ethylenedioxythiophene):poly-styrenesulfonate (PEDOT:PSS) was purchased from Hereaus and solvents used were obtained from Aldrich. Indium tin oxide (ITO)-coated glass plates (15 Ω –1) were patterned using conventional photolithography. The substrates were extensively clean
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