Letter pubs.acs.org/NanoLett
Hybrid III-Nitride/Organic Semiconductor Nanostructure with High Efficiency Nonradiative Energy Transfer for White Light Emitters R. Smith, B. Liu, J. Bai, and T. Wang* Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, S1 3JD, Sheffield, United Kingdom ABSTRACT: A novel hybrid inorganic/organic semiconductor nanostructure has been developed, leading to very efficient nonradiative resonant-energy-transfer (RET) between blue emitting InGaN/GaN multiple quantum wells (MQWs) and a yellow light emitting polymer. The utilization of InGaN/GaN nanorod arrays allows for both higher optical performance of InGaN blue emission and a minimized separation between the InGaN/GaN MQWs and the emitting polymer as a color conversion medium. A significant reduction in decay lifetime of the excitons in the InGaN/GaN MQWs of the hybrid structure has been observed as a result of the nonradiative RET from the nitride emitter to the yellow polymer. A detailed calculation has demonstrated that the efficiency of the nonradiative RET is as high as 73%. The hybrid structure exhibits an extremely fast nonradiative RET with a rate of 0.76 ns−1, approximately three times higher than the InGaN/GaN MQW nonradiative decay rate of 0.26 ns−1. It means that the RET dominates the nonradiative processes in the nitride quantum well structure, which can further enhance the overall device performance. KEYWORDS: InGaN/GaN nanorod, nonradiative resonant energy transfer, lighting polymer, radiative recombination lifetime, nonradiative recombination lifetime
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stability of the phosphors also require consideration, along with the color quality. Compared with the existing phosphors, a light emitting polymer as a down-conversion material has a number of advantages. It can cover the whole visible spectrum and can have significantly higher photoluminescence efficiencies. Because of an intrinsically large Stokes-shift, which can be around 100 nm or even larger for a lighting polymer, the selfabsorption issue can be eliminated. Furthermore, unlike the existing phosphors, which are normally prepared in a form of grains with a typical size of tens of micrometers, lighting polymers can be dissolved in a solvent, which can be used to obtain homogeneous microstructures and also perfectly match standard spin-coating techniques widely used for the fabrication of inorganic or organic semiconductor optoelectronics. Therefore, they can simplify the process for fabrication of white LEDs, which is particularly attractive to industry. It is also worth highlighting another advantage: because of the large number of high efficiency light emitting polymers (red and green) available, it is very easy (compared to traditional phosphors) to control the color rendering index (CRI) by blending different polymers. The most important advantage for such a hybrid white LED using a lighting polymer as a down conversion material is that
t is vitally important to develop high efficiency lighting in order to meet the energy challenges of the future due to concerns about climate-change and energy crisis. Developments in solid-state lighting are occurring at pace and will lead to ultimate lighting sources, which are mainly based on III-nitride semiconductors. The current commercial state-of-the-art lighting so far depends on blue emission from InGaN/GaN-based light emitting diodes (LEDs) radiatively pumping downconversion phosphor materials to providing the longer wavelength emission component(s), leading to the generation of white light. The past decade has seen major achievements in the development of InGaN-based solid-state lighting using the “blue LED + yellow phosphor”. In particular, the performance has been significantly improved through ultilisation of “blue LED + green and red phosphors” approach recently. However, the performance still lags well behind the potential luminous efficacy levels of up to 350 lm/W for white light,1 meaning that white LEDs still have huge potential for further improvement. In the meantime, scientists have started to devote their efforts to developing phosphor free white LEDs due to a number of drawbacks by using phosphors.2 An important, but often neglected, issue for the fabrication of phosphor-conversion white LEDs is the self-absorption of the phosphor involved in producing the white light. This puts additional limits on the overall efficiency, implying that we still have room for further enhancement in terms of color rendering. Another factor is how to further improve the efficiency of the energy transfer from the blue-LED to the down-conversion phosphor. Quenching and © XXXX American Chemical Society
Received: February 15, 2013 Revised: June 12, 2013
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dx.doi.org/10.1021/nl400597d | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
InGaN/GaN MQWs of the hybrid structure as a result of a fast nonradiative RET process. The extremely fast RET process between the blue MQWs and the light emitting polymer exhibits a very high efficiency with a rate of as high as 73%. These demonstrate the superior performance of our structure to the current state-of-the-art in three separate ways, enhanced and highly efficient nonradiative energy transfer to the downconversion material, improved optical performance of the blue emitting MQW, and, because of the extremely fast nonradiative RET rate, the potential to further increase the overall efficiency due to the competition with the nonradiative recombination rate of the MQW structure. Figure 1a presents a schematic of our hybrid structure, showing that arrays of nanorods, on a scale of 100s of nm, are
the efficiency of the color conversion can be significantly enhanced through so-called nonradiative Fö rster energy transfer, namely, nonradiative resonant energy transfer (RET),3−13 which cannot be achieved using the existing phosphors. The hybrid structure takes advantage of direct excitonic coupling between the donor (InGaN) and acceptor (lighting polymer) in the near field.3−13 The major challenge of employing nonradiative RET is related to it being a shortrange, near-field coupling, with the nonradiative RET rate being highly sensitive to donor−acceptor exciton separation. The energy transfer relies on Coulomb interactions, and thus, the distance between the InGaN emitting layer and the proximal polymer is critical.3−5 The energy transfer rate Γ can be simply described as: Γ ∝ R−4, where R is the distance between InGaN and the polymer.3−5 Therefore, reducing the separation of donor and acceptor excitons to sufficiently small distances (typically
