Solid-state light-emitting electrochemical cells employing phosphor- sensitized fluorescence Hai-Ching Su, * a You-Heng Lin, b Chih-Hao Chang, b Hao-Wu Lin, b Chung-Chih Wu, b Fu-Chuan Fang, c Hsiao-Fan Chen c and Ken-Tsung Wong c Received 16th February 2010, Accepted 14th April 2010 First published as an Advance Article on the web 2nd June 2010 DOI: 10.1039/c0jm00429d We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy) 2 (SB)] + (PF 6  ) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A 1 ) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A 1 and 21.3 lm W 1 , respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, 50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes. Introduction Solid-state light-emitting electrochemical cells (LECs) possess several advantages over conventional organic light-emitting diodes (OLEDs). In LECs, electrochemically doped regions induced by spatially separated ions under a bias form ohmic contacts with electrodes, giving balanced carrier injection, low operating voltages and consequently high power efficiencies. 1,2 As such, LECs generally require only a single emissive layer, which can be easily processed from solutions, and can conve- niently use air-stable electrodes, while OLEDs typically require more sophisticated multilayer structures and low-work-function cathodes. 3 Compared with conventional polymer LECs, which are usually composed of an emissive conjugated polymer, a salt and an ion-conducting polymer, 1,2 LECs based on cationic transition metal complexes show several further advantages and have attracted much attention in recent years. 4–15 In such devices, no ion-conducting material is needed since these metal complexes are intrinsically ionic. Furthermore, higher electroluminescent (EL) efficiencies are expected, due to the phosphorescent nature of the metal complexes. As in OLEDs, the host–guest approach, i.e. spatially dispersing the emitting guest into a matrix complex (host), had been reported to be an effective way to tune the emission color or to improve the emission efficiency of solid-state LECs based on cationic transition metal complexes. 11,16 Thus far, all host–guest LECs based on previously reported cationic complexes were implemented by doping the phosphorescent cationic complexes (guest) into the host cationic complexes, which were also phos- phorescent. Nevertheless, doping fluorescent guests into phos- phorescent hosts may be an alterative way to achieve efficient/ color-tunable host–guest LECs, since, in conventional OLED technologies, phosphor-sensitization had been proved to be useful in raising the efficiency of fluorescent OLEDs to similar levels to phosphorescent OLEDs. In phosphor-sensitized fluo- rescence, the heavy-metal center of the phosphorescent host mediates rapid inter-system crossing for efficient intramolecular singlet-to-triplet energy transfer, and thus subsequent effective F€ orster energy transfer 17 from triplet excitons of the phosphor host to singlet excitons of the fluorophore guest, harvesting both singlet and triplet excitons in hosts. 18,19 The shorter excited-state lifetimes of fluorescent guests could also possibly reduce the triplet–triplet annihilation issues in purely phosphorescent devices, which is associated with high triplet populations induced by long excited-state lifetimes of triplet excitons. As a result, device efficiencies of phosphor-sensitized fluorescent OLEDs could approach those of phosphorescent OLEDs. 18,19 Despite its successful use in OLEDs, however, up to date, there is no demonstration or report of employing phosphor-sensitized fluorescence in host–guest solid-state LECs based on cationic transition metal complexes. In this work, we report investiga- tions on Ir-complex-based solid-state LECs utilizing phosphor- sensitized fluorescence. 18,19 Results and discussion PL and photophysical studies of phosphor sensitization The chemical structures of the host and guest materials used in this study are shown in Fig. 1. The previously reported green- emitting cationic Ir complex [Ir(dFppy) 2 (SB)] + (PF 6  ) (where a Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan, Taiwan, 71150, Republic of China. E-mail: haichingsu@mail.nctu.edu.tw; Fax: +886-6-3032535; Tel: +886-6- 3032121-57792 b Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan, 10617, Republic of China c Department of Chemistry, National Taiwan University, Taipei, Taiwan, 10617, Republic of China This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20, 5521–5526 | 5521 PAPER www.rsc.org/materials | Journal of Materials Chemistry Downloaded by National Taiwan University on 25 November 2010 Published on 02 June 2010 on http://pubs.rsc.org | doi:10.1039/C0JM00429D View Online