LetterEfficiency enhancement of organic light emitting diode via surface energy transfer between exciton and surface plasmon
Graphical abstract
Highlights
► OLEDs with surface plasmon enhanced emission have been fabricated. ► Gold nanoclusters has been used for localization of surface plasmons. ► About three times enhancement in intensity has been found. ► Surface energy transfer has been observed between exciton and surface plasmons.
Introduction
The development of organic light emitting diodes (OLEDs) holds great promise for the production of highly efficient light sources [1]. They are fabricated using fluorescent and phosphorescent emissive materials [2], [3]. Phosphorescent emissive materials are preferred over the fluorescent because in phosphorescent materials both triplet and singlet excitons can be exploited for emission while in case of fluorescent, only singlet excitons can emit [4], [5]. The emission rate of a material is inversely proportional to the life time of excited state [6]. For high emission rate, the life time of excited state should be as small as possible. The excited state life time of phosphorescent materials is few μs in comparison to the few ns for fluorescent materials [7], [8]. This is a disadvantage in case of phosphorescent materials.
Recently the coupling between excitons and surface plasmons (SPs) has been actively studied for radiative emission rate enhancement in semiconducting devices [9], [10], [11], [12]. SPs are the collective oscillations of free electrons in a metal at the interfaces between the metal and dielectric [13], [14], [15]. They are propagating waves at metal/dielectric interface and coupling of light to SPs provide a major loss channel. However, the energy confined in the SP modes can be extracted as radiation through naturally formed surface imperfections such as nanostructures. The overlap of local electromagnetic field of the excitons in the emissive layer and SPs results in a coupling effect between them, due to which, effective energy transfer takes place between them creating an alternate channel for emission. Since the scattering of high momentum localized SPs (LSPs) is much faster than the decay of excitons, coupling results in the enhancement of radiation intensity. There are many reports regarding the use of localized SPs for the enhancement of internal quantum efficiency in inorganic LEDs [9], [10]. The use of LSPs in OLEDs is limited to the photoluminescence only due to the difficulty in incorporating them inside the device structure at suitable place [11], [12]. There are few reports available on the use of metallic nanoparticles for enhancing the efficiency of OLED [16], [17], [18], but the results are not significant. Fuziki et al. [16] have reported the use of gold nanoparticles to enhance the efficiency of OLED based on tris-(8-hydroxyquinolinato)aluminum (Alq3) but the luminescence of both the plasmon enhanced and reference OLED was poor (∼10−3 Cd/m2 at 12 V). Yang et al. [17] have used a cathode structure for plasmonic emission but their results on electroluminescence (EL) were not quite significant (e.g. they have got enhancement only for 2–5 V) and in the work of Choulis et al. [18], trapping due to gold nanoparticles is dominant.
In an exciton–SP system, there are two competitive processes; (i) radiation intensity enhancement due to LSPs and (ii) nonradiative losses due to metal. The efficiency of the interaction between LSPs and excitons exponentially decreases with increasing distance from the emissive layer to the metal surface [19]. At the same time, nonradiative quenching of exciton at metal surface occurs when the distance is very much smaller. Therefore for practical application, an appropriate distance between the emissive material and metal surface should be maintained to obtain radiation intensity enhancement. For the maximum coupling between exciton and SPs, the emission wavelength of excitons should match with the absorption wavelength of LSPs which depends upon the size of nanostructures [20], [21]. Therefore, the size of nanostructures should be optimized for emission enhancement.
In this letter GNCs has been deposited by thermal evaporation and their effect on luminescence enhancement has been studied using steady state and time resolved photoluminescence. Size of GNCs and distance from the emissive layer has been optimized for maximum luminescence enhancement. GNCs have been inserted in phosphorescent OLED structure and the effect on the luminescence has been studied.
Section snippets
Experimental
Gold nanoclusters were fabricated by thermal evaporation at a base pressure of 4 × 10−6 Torr. OLEDs were fabricated on indium-tin-oxide (ITO) coated glass substrates having a sheet resistance of 20 Ω/□ and a thickness of 120 nm which were patterned and cleaned using deionised water, acetone, trichloroethylene and isopropyl alcohol sequentially for 20 min using an ultrasonic bath and dried in vacuum oven. Prior to organic film deposition ITO surface was treated with oxygen plasma for 5 min to increase
Results and discussion
Fig. 1(a) shows the AFM image of GNCs thermally evaporated on glass substrates with deposition rate 0.1 nm/s. The image shows a distribution of particle size. The average size of these GNCs has been calculated from the AFM results for different deposition rates and is shown in Fig. 1(b) which shows an increase in particle size with the increase in deposition rate. Since the size of GNCs is very important for the study of LSP resonance (LSPR), the results have been repeated three times and the
Acknowledgement
Authors are thankful to Director, National Physical Laboratory, New Delhi for continuous encouragement. The Authors also gratefully recognize the financial support from the Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR) New Delhi, India for the projects – NWP-25.
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