AbstractPerovskite has been considered to be a promising optoelectronic material due to its superior properties, yet, it are strongly sensitive to oxygen and water, especially when applied in devices. Here, a highly efficient and stable light-emitting device (LED) is reported by applying solution-processed Mg-doped ZnO (MZO) nanocrystals (NCs) as an interfacial layer. The effect of Mg doping on the optical and electronic properties of NCs is investigated. This study demonstrates that the air stability of perovskite NC film is significantly enhanced because of the decreased oxygen vacancy surface sites of MZO NCs. Incorporation of a MZO layer with favorable electronic energy level to form a suitable band alignment promotes electron injection and enhances the LED performance. Compared to the device without MZO, the LED shows 3059 cd m−2 of luminance, with 1.9 times enhanced current efficiency and 2 times increased external quantum efficiency. In addition, the device with MZO also exhibits better stability. This research provides a potential strategy for realizing stable and efficient perovskite LEDs.

The surface organic ligands of the quantum dots (QDs) play important roles in the performance of QD electronic devices. Here, we fabricated low toxic AgIn5S8/ZnS QDs light-emitting diodes (QD-LEDs) and greatly enhanced the device efficiency through surface ligand exchange treatments. The oleic acid-capped QDs were replaced with a shorter ligand 1,2-ethanedithiol, which was proved by the Fourier transform infrared spectrum measurement. The treated QD films became more compact with higher film mobility and shorter film photoluminescence lifetime. The more conductive QD films fabricated LEDs showed an external quantum efficiency over 1.52%.Keywords: 1,2-ethanedithiol; AgIn5S8/ZnS; ligand exchange; light-emitting diode; quantum dot;

•Improved hole injection modified by MoO3 layer and low pressure plasma.•Higher surface roughness would decrease the injection of holes.•Unexpected leakage current could be depressed by low pressure plasma treatment.•Provide helpful information for the optimization on structure design of OLEDs.In this work, we performed the investigation on surface modification of indium tin oxide (ITO) anode by depositing MoO3 interface layer and treating with low pressure plasma. Experimental results revealed both low pressure oxygen plasma treatment and MoO3 interface layer are efficient in facilitating the injection of holes, while both modifications increase the ITO surface roughness. Interestingly, the electroluminescent (EL) device with 3 nm MoO3 layer displayed the highest EL performances, which were even higher than those of the device with both modifications. By optimizing the selection of gas, treatment time, and applied electric field intensity of low pressure plasma treatment, the combination of 3 nm MoO3 layer and 2 min low pressure nitrogen plasma treatment at 4000 V m−1 was demonstrated to be most efficient in depressing the unexpected leakage current without sacrificing the injection rate of holes.Our experimental results revealed the combination of 3 nm MoO3 layer and 2 min low pressure nitrogen plasma treatment at 4000 V m−1 is most efficient in depressing the unexpected leakage current without sacrificing the injection rate of holes.Download high-res image (156KB)Download full-size image

Over the last two decades, intensive research efforts have been devoted to the suppressions of photoluminescence (PL) blinking and Auger recombination in metal-chalcogenide nanocrystals (NCs), with significant progresses being made only very recently in few specific NC structures. Here we show that nonblinking PL is readily available in the newly synthesized perovskite CsPbI3 NCs and that their Auger recombination of charged excitons is greatly slowed down, as signified by a PL lifetime about twice shorter than that of neutral excitons. Moreover, spectral diffusion is completely absent in single CsPbI3 NCs at the cryogenic temperature, leading to a resolution-limited PL line width of ∼200 μeV.Keywords: Auger recombination; blinking; nanocrystal; Perovskite; spectral diffusion;

Two-photon-pumped lasers have been regarded as a promising strategy to achieve frequency up-conversion for situations where the condition of phase matching required by conventional approaches cannot be fulfilled. However, their practical applications have been hindered by the lack of materials holding both efficient two-photon absorption and ease of achieving population inversion. Here, we show that this challenge can be tackled by employing colloidal nanocrystals of perovskite semiconductors. We observe highly efficient two-photon absorption (with a cross section of 2.7 × 106 GM) in toluene solutions of CsPbBr3 nanocrystals that can excite large optical gain (>500 cm–1) in thin films. We have succeeded in demonstrating stable two-photon-pumped lasing at a remarkable low threshold by coupling CsPbBr3 nanocrystals with microtubule resonators. Our findings suggest perovskite nanocrystals can be used as excellent gain medium for high-performance frequency-up-conversion lasers toward practical applications.

The high photoluminescence efficiency, high color purity, and easy tunable bandgap make inorganic perovskite nanocrystals very attractive in luminescent display applications. Here, we report a color-saturated, red light-emitting diode (LED) using an inverted organic/inorganic hybrid structure and perovskite nanocrystals. We demonstrated that through a simple post treatment to the perovskite nanocrystals with polyethylenimine, the surface defects of the perovskite nanocrystals could be well passivated, leading to great enhancements on their absolute photoluminescence quantum yield and photoluminescence lifetime. Through using a well-passivated perovskite nanocrystal film and optimizing the charge balance, we achieved an electroluminescence LED with a current efficiency of 3.4 cd A–1, corresponding to an external quantum efficiency (EQE) of 6.3%, which is the highest value reported among perovskite NC LEDs so far.

Thin active layers in solar cells normally have lower defect density with easier procedure and lower cost than thick ones. PbSe nanocrystal (NC)-based solar cells with thin layers were fabricated using a ligand exchange procedure. An optical spacer was confirmed to be effective in improving the distribution of light field in the devices, which could enhance the photon absorption of the thin active layers. Meanwhile, the transportation of electrons and holes was balanced and optimized by tuning the particle size and the layer thickness, which demonstrated a high short-circuit current of 32.2 mA cm−2 and a 1 sun power conversion efficiency of 4.12%. Besides, the devices with smaller PbSe NCs preserved the high efficiency for tens of hours, which is different from previous studies using large NCs.

With good electrical conductivity, optical transparency, and mechanical compliance, graphene films have shown great potential in application for photovoltaic devices as electrodes. However, photovoltaic devices employing graphene anodes usually suffer from poor hole collection efficiency because of the mismatch of energy levels between the anode and light-harvesting layers. Here, a simple solution treatment and a low-cost solution-processed molybdenum oxide (MoOx) film were used to modify the work function of graphene and the interfacial morphology, respectively, yielding highly efficient hole transfer. As a result, the graphene/MoOx anodes demonstrated low surface roughness and high electrical conductivity. Using the graphene/MoOx anodes in PbSe nanocrystal solar cells, we achieved 1 sun power conversion efficiency of 3.56%. Compared to the control devices with indium tin oxide anodes, the graphene/MoOx-based devices show excellent performance, demonstrating the great potential of the graphene/MoOx anodes for use in optoelectronics.Keywords: graphene; hole transfer; PbSe nanocrystal; solar cell; solution-processed MoOx;

•CdS and CdSe shells were coated on TiO2 nanorods using electrochemical deposition.•Perfect TiO2/CdS/CdSe nanocable arrays were obtained.•The stepwise band-edge electronic structure promoted the quick transfer of the photo-excited electrons.•Photocurrent density of 6.57 mA/cm2 and output potential difference of 1.04 V were achieved.Vertically aligned TiO2/CdS and TiO2/CdS/CdSe nanocable arrays on fluorine-doped tin oxide (FTO) were fabricated by electrochemical deposition of CdS and CdS/CdSe shells over TiO2 nanorod arrays. The morphology, composition, structure and optical absorbance of CdS and CdS/CdSe shells sensitized TiO2 nanorod arrays were characterized by different analytical methods. The CdS shell with a hexagonal structure and the CdSe shell with a face-centered structure were densely and uniformly coated on the tetragonal TiO2 nanorod cores both radially and longitudinally. The photovoltaic measurement showed that the photocurrent density obtained under AM1.5G illumination with a zero bias potential (Ag/AgCl electrode) largely increased from 2.17 to 6.57 mA/cm2, when the TiO2/CdS nanocables were further covered by the CdSe shell.

•The enhanced photoluminescence (PL) for In-rich CuInS2 QDs with [Cu]/[In] molar ratios of 0.31.•The conduction electron-Cu vacancy recombination and DAP were considered to exist and the temperature-independent DAP recombination was enhanced in the In-rich CuInS2 QDs.The enhanced photoluminescence (PL) for In-rich copper indium sulfide quantum dots (CIS QDs) was observed. The conduction electron-Cu vacancy recombination and the donor–acceptor pair (DAP) defect recombination were considered to exist in CIS QDs at the same time. The temperature-dependent PL study showed that the emission of these QDs might be mainly originated from the recombination between electrons in the quantized conduction band and holes in the copper vacancy acceptor when x was 0.500 (CuxIn1−xS). However, the temperature coefficient of PL peak position decreased when x was 0.237. That meant the DAP recombination increased in the In-rich CIS QDs.

Benefiting from the strong quantum confinement, PbSe nanocrystals allow their bandgap and absorption edge to be tuned to optimize the absorption of the solar radiation. Here, bandgap engineering-based photovoltaic devices were designed, fabricated, and characterized using two-size PbSe nanocrystals. The fabricated two size particle photovoltaic devices showed 12.8% higher power conversion efficiency compared to that of the single-particle devices, as a result of the enhanced photon absorption and the improved charge transfer.

Near infrared light emitting diodes (NIR LEDs) were fabricated employing blue GaN chips as the excitation source and PbSe quantum dots as the NIR emitting materials. Quantum dots with different emitting wavelengths were selected to fabricate three NIR LEDs corresponding to two typical applications of illumination and optical communication. The variation of emission peak and full width at half-maximum of the devices were investigated under different voltage bias, and the highest external quantum efficiency of 2.52% was achieved, which was comparable to those commercial InGaAsP LEDs and visible quantum dot electroluminescence LEDs.

The time-resolved photoluminescence spectroscopy was employed to analyze the optical properties of Ag2Se quantum dots with different diameters at temperatures of 80–360 K. The photoluminescence lifetime measurement disclosed that in the low-energy electronic structure there were two dominating emissive “in-gap” states associated with surface defect and intrinsic states, which were further confirmed by Gaussian fitting of the photoluminescence spectra. The temperature-dependent emission peak energy was fitted to phenomenological equations to extract the average phonon energy, the Huang–Rhys factor, and the excitonic acoustic phonon coupling coefficient. The relatively large phonon energy and small Huang–Rhys factor were demonstrated, which induced the small variation of emission peak energy in the low-temperature range. Meanwhile, the photoluminescence line width increased with temperature and was analyzed based on the standard equation describing the temperature dependence of the width of the ground state exciton. The variation of both the photoluminescence peak and line broadening was mostly due to the exciton to acoustic phonon coupling.

The power conversion efficiency of photovoltaic devices based on semiconductor perovskites has reached ∼20% after just several years of research efforts. With concomitant discoveries of other promising applications in lasers, light-emitting diodes, and photodetectors, it is natural to anticipate what further excitement these exotic perovskites could bring about. Here we report on the observation of single photon emission from single CsPbBr3 perovskite nanocrystals (NCs) synthesized from a facile colloidal approach. Compared with traditional metal-chalcogenide NCs, these CsPbBr3 NCs exhibit nearly 2 orders of magnitude increase in their absorption cross sections at similar emission colors. Moreover, the radiative lifetime of CsPbBr3 NCs is greatly shortened at both room and cryogenic temperatures to favor an extremely fast output of single photons. The above superior optical properties have paved the way toward quantum-light applications of perovskite NCs in various quantum information processing schemes.Keywords: blinking; nanocrystal; perovskite; single photon emission;

Multigas sensing is highly demanded in the fields of environmental monitoring, industrial production, and coal mine security. Three near-infrared emission wavelengths from PbSe quantum dots (QDs) were used to analyze the concentration of three gases simultaneously through direct absorption spectroscopy, including acetylene (C2H2), methane (CH4), and ammonia (NH3). The corresponding lower detection limits for the three gases were 20, 100, and 20 ppm, respectively, with an accuracy of 2%. This study demonstrates that QDs with tunable emissions have great potential for simultaneous and uninterfered multiplex gas analysis and detection due to the advantages of the easy tunability of multiplex emitting wavelengths from QDs.

•WLEDs were fabricated using blue GaN chips and green- and red-emitting CdSe/CdS/ZnS quantum dots.•Warm and cold white emissions were confirmed with color temperature from 4000 to 9000 K.•Variation of color coordinates and color temperature were investigated and analyzed at deferent bias.•Stability of white emission was analyzed with the increase of working time.Fluorescence-based white-light-emitting diodes (WLEDs) were fabricated using blue GaN chips and green- and red-emitting CdSe/CdS/ZnS quantum dots (QDs). The coordinate and color temperature of the WLEDs could be varied because of the size-tunable emission of CdSe QDs from 510 to 620 nm. Warm and cold white emissions were confirmed with the color temperature ranging from 4000 to 9000 K. Color coordinates were analyzed at different bias. The fast enhancement of blue emission resulted in the shift of color coordinates to the cold side. The stability of white emission during operation was analyzed; stable spectra were achieved within 90 min.

PbSe quantum dots (QDs) were employed as real-time and on-chip temperature sensors to monitor the surface temperature of GaN LED chips. The temperature-dependent photoluminescence spectra were achieved and confirmed to be a good method for surface temperature sensing in a micro- to nano-region. The nanosized QD sensors did not influence the LED emission spectrum due to their infrared emission and little absorption. The surface temperature of GaN LED chips was analyzed at different working times and voltages. The temperature sensitivity characterized by the photoluminescence peak position of PbSe QDs was found to be 0.15 nm °C−1 in a range of 30–120 °C and the precision was determined to be ±3 °C. The QD surface temperature sensors were confirmed to have good reversibility and repeatability.

Colloidal ZnCuInS/ZnSe/ZnS core/shell/shell quantum dots (QDs) with average particle sizes of 2.3, 2.7, and 3.3 nm were prepared in a noncoordinating solvent. The size-dependent optical band gap and photoluminescence (PL) band shift due to the quantum confinement effect were observed. Because the PL band showed a large Stokes shifts over 400 meV, the origin of the PL band was related to the electronic transition via defect levels. A time-resolved PL measurement indicated that the PL lifetime of the QDs was a characteristic feature of three dominating transitions from the conduction band to surface defect level, from the conduction band to an acceptor level, and from the donor level to an acceptor level. It was investigated as a function of temperature in the range from 50 to 373 K to understand the radiative and nonradiative relaxation processes and fitted with two empirical expressions, from which the Huang–Rhys factor and the phonon energy were calculated. According to the fitting data, the size-dependent parameters were analyzed including the Huang–Rhys factor, the average phonon energy, and the excitonic-acoustic phonon coupling coefficient. The temperature coefficient was about −2.32 × 10–4 eV/K. The results showed that, in the temperature range from 50 to 373 K, the variations of the energy band gap and the photoluminescence line broadening were predominantly due to an optical transition from the band edge to the defect-related level and the coupling of the carriers to acoustic phonon, respectively.

⁡Carbon-dot based light-emitting diodes (LEDs) with driving current controlled color change are reported. These devices consist of a carbon-dot emissive layer sandwiched between an organic hole transport layer and an organic or inorganic electron transport layer fabricated by a solution-based process. By tuning the device structure and the injecting current density (by changing the applied voltage), we can obtain multicolor emission of blue, cyan, magenta, and white from the same carbon dots. Such a switchable EL behavior with white emission has not been observed thus far in single emitting layer structured nanomaterial LEDs. This interesting current density-dependent emission is useful for the development of colorful LEDs. The pure blue and white emissions are obtained by tuning the electron transport layer materials and the thickness of electrode.Keywords: carbon dots; color switchable; current density; light emitting diode; voltage-dependent

A theoretical model was established in this paper to analyze the properties of 3.50 and 4.39 nm PbSe quantum dot-doped liquid-core multi-mode fiber. This model was applicable to both single- and multi-mode fiber. The three-level system-based light-propagation equations and rate equations were used to calculate the guided spontaneous emission spectra. Considering the multi-mode in the fiber, the normalized intensity distribution of transversal model was improved and simplified. The detailed calculating results were thus obtained and explained using the above-mentioned model. The redshift of the peak position and the evolution of the emission power were observed and analyzed considering the influence of the fiber length, fiber diameter, doping concentration, and the pump power. The redshift increased with the increases of fiber length, fiber diameter, and doping concentration. The optimal fiber length, fiber diameter, and doping concentration were analyzed and confirmed, and the related spontaneous emission power was obtained. Besides, the normalized emission intensity increased with the increase of pump power in a nearly linear way. The calculating results fitted well to the experimental data.

Thin active layers in solar cells normally have lower defect density with easier procedure and lower cost than thick ones. PbSe nanocrystal (NC)-based solar cells with thin layers were fabricated using a ligand exchange procedure. An optical spacer was confirmed to be effective in improving the distribution of light field in the devices, which could enhance the photon absorption of the thin active layers. Meanwhile, the transportation of electrons and holes was balanced and optimized by tuning the particle size and the layer thickness, which demonstrated a high short-circuit current of 32.2 mA cm−2 and a 1 sun power conversion efficiency of 4.12%. Besides, the devices with smaller PbSe NCs preserved the high efficiency for tens of hours, which is different from previous studies using large NCs.