Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter D (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > D (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than D show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.
Size-controlled soft-template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue-shifted photoluminescence (PL) or so-called an inverse PL shift. Using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND-based optoelectronic devices, including multicolored light-emitting diodes and air-stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.
In
this work, nitrogen-rich carbon nanodots (CNDs) are prepared
by the emulsion-templated carbonization of polyacrylamide. The formation
mechanism and chemical structure are investigated by infrared, nuclear
magnetic resonance, and X-ray photoelectron spectroscopies. Transmission
electron microscopy also reveals that the obtained CNDs have well-developed
graphitic structure and narrow size distribution without any size
selection procedure. We vary the molecular weight of the polymer to
control the size of the CNDs and finally obtain the CNDs rendering
bright visible light under UV illumination with a high quantum yield
of 40%. Given that the CNDs are worth utilizing in phosphor applications,
we fabricate large-scale (20 × 20 cm) freestanding luminescent
films of the CNDs based on a poly(methyl methacrylate) matrix. The
polymer matrix can not only provide mechanical support but also disperse
the CNDs to prevent solid-state quenching. For practical application,
we demonstrate white LEDs consisting of the films as color-converting
phosphors and InGaN blue LEDs as illuminators. Such white LEDs exhibit
no temporal degradation in the emission spectrum under practical operation
conditions. This study would suggest a promising way to exploit the
luminescence from solid-state CNDs and offer strong potential for
future CND-based solid-state lighting systems.
Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames, and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.
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