Photo- and electroluminescence from semiconductor colloidal quantum dots in organic matrices: QD-OLED

The results are reported of an experimental study of samples of organic light-emitting diodes (OLEDs) with luminescent layers fabricated on the basis of two types of CdSe/CdS/ZnS semiconductor quantum dots (QDs) with average CdSe core diameters of 3.2 and 4.1 nm and the same overall diameters of 6.5 nm. The dependences of the LED efficiency on the applied voltage are determined. Assumptions are made about ways of optimizing the design of high-efficiency LEDs.     

Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1]. Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2]. A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3]. We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 Å to 58 Å, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED external quantum efficiency, consistent with a Förster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin (∼5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4], emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.     

Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based on Co‐Sensitization of CdS/CdSe

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially assembled onto a nanocrystalline TiO₂ film to prepare a CdS/CdSe co‐sensitized photoelectrode for QD‐sensitized solar cell application. The results show that CdS and CdSe QDs have a complementary effect in the light harvest and the performance of a QDs co‐sensitized solar cell is strongly dependent on the order of CdS and CdSe respected to the TiO₂. In the cascade structure of TiO₂/CdS/CdSe electrode, the re‐organization of energy levels between CdS and CdSe forms a stepwise structure of band‐edge levels which is advantageous to the electron injection and hole‐recovery of CdS and CdSe QDs. An energy conversion efficiency of 4.22% is achieved using a TiO₂/CdS/CdSe/ZnS electrode, under the illumination of one sun (AM1.5,100 mW cm^?2). This efficiency is relatively higher than other QD‐sensitized solar cells previously reported in the literature.     

Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

Solution‐processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra‐QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer‐lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.     

CdTe和CdS量子点的非线性特性对比

应用Top-hat Z-scan技术在波长为532 nm,脉宽为190 fs激光脉冲下研究了CdTe和CdS量子点的光学非线性吸收和非线性折射特性。实验结果表明:在飞秒激光脉冲作用下,CdTe量子点的非线性吸收表现为饱和吸收,CdS量子点表现为反饱和吸收。CdTe量子点的非线性折射表现为自散焦,CdS量子点表现为自聚焦。尺寸分别为2.6、2.4 nm的CdTe量子点和CdS量子点的非线性吸收系数分别为-9.2610-14、0.7810-14 m/W,非线性折射率系数分别为-0.8610-20、1.4610-20 m2/W,三阶非线性极化率分别为2.7210-15、1.3610-15 esu。表明相近尺寸下不同材料的镉类半导体量子点的光学非线性吸收和非线性折射特性不同,并对其机理进行分析。     

Highly Stable Colloidal “Giant” Quantum Dots Sensitized Solar Cells

Colloidal quantum dots (QDs) are widely studied due to their promising optoelectronic properties. This study explores the application of specially designed and synthesized “giant” core/shell CdSe/(CdS)_x QDs with variable CdS shell thickness, while keeping the core size at 1.65 nm, as a highly efficient and stable light harvester for QD sensitized solar cells (QDSCs). The comparative study demonstrates that the photovoltaic performance of QDSCs can be significantly enhanced by optimizing the CdS shell thickness. The highest photoconversion efficiency (PCE) of 3.01% is obtained at optimum CdS shell thickness ≈1.96 nm. To further improve the PCE and fully highlight the effect of core/shell QDs interface engineering, a CdSe_x S_1?x interfacial alloyed layer is introduced between CdSe core and CdS shell. The resulting alloyed CdSe/(CdSe_x S_1?x )₅/(CdS)₁ core/shell QD‐based QDSCs yield a maximum PCE of 6.86%, thanks to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of CdSe_x S_1?x interfacial layer with respect to CdSe/(CdS)₆ core/shell. In addition, QDSCs based on “giant” core/CdS‐shell or alloyed core/shell QDs exhibit excellent long‐term stability with respect to bare CdSe‐based QDSCs. The giant core/shell QDs interface engineering methodology offers a new path to improve PCE and the long‐term stability of liquid junction QDSCs.     

Aqueous synthesis of highly monodispersed thiol-capped CdSe quantum dots based on the electrochemical method

A facile green approach is developed to control the growth regime in the aqueous synthesis of CdSe semiconductor quantum dots (QDs) based on the electrochemistry method. The low growth temperature and slow injection of Se precursors are used to prolong the diffusion controlled stage and thus suppress the Ostwald ripening during nanocrystal growth. The experimental results show that a low concentration of Se precursor would definitely influence the growth procedure. The narrow absorption peaks in the UV–visible absorption spectra, as well as transmission electron microscopy images indicate that the as-prepared CdSe QDs have a good monodispersity. The high-resolution transmission electron microscopy (HRTEM) images and powder X-ray diffraction (XRD) pattern suggest that the as-prepared QDs have high crystallinity and cubic structure. The QDs exhibit high fluorescence QYs of about 30% without any postpreparative treatment over a broad spectral range of 560–660 nm, which could be further broadened by long-term refluxing. The current work suggests that the electrochemical method is an attractive approach to the synthesis of high-quality II–VI semiconductor QDs at a large scale.     

Organic light-emitting diodes based on a series of new polythienothiophene complexes and highly luminescent quantum dots

Experimental samples of organic light-emitting diodes with transport layers based on polythienothiophenes and using CdSe/CdS/ZnS semiconductor quantum dots with an internal quantum efficiency up to 85% in the emitting layer are investigated. It is shown that solubility and film-forming properties are key for using polythienothiophenes in light-emitting diodes. The most promising polythienothiophenes are identified on the basis of the results obtained.     

Origins of Low Quantum Efficiencies in Quantum Dot LEDs

The promise for next generation light‐emitting device (LED) technologies is a major driver for research on nanocrystal quantum dots (QDs). The low efficiencies of current QD‐LEDs are often attributed to luminescence quenching of charged QDs through Auger‐processes. Although new QD chemistries successfully suppress Auger recombination, high performance QD‐LEDs with these materials have yet to be demonstrated. Here, QD‐LED performance is shown to be significantly limited by the electric field. Experimental field‐dependent photoluminescence decay studies and tight‐binding simulations are used to show that independent of charging, the electric field can strongly quench the luminescence of QD solids by reducing the electron and hole wavefunction overlap, thereby lowering the radiative recombination rate. Quantifying this effect for a series of CdSe/CdS QD solids reveals a strong dependence on the QD band structure, which enables the outline of clear design strategies for QD materials and device architectures to improve QD‐LED performance.     

Improved performance of CdSe/ZnS quantum dot light-emitting diodes through doping with small molecule CBP

The poor film formation of CdSe/ZnS quantum dots (QDs) during spin-coating makes a substantial impact on the device performance of quantum dot light-emitting diodes (QLEDs). This work proposes a method to improve the morphology of the quantum dot light-emitting layer (EML) by adding small organic molecular 4,4''-Bis(9H-carbazol-9-yl) biphenyl (CBP) into the layer. Its surface roughness reduces from 6.21 nm to 2.71 nm, which guarantees a good contact between hole transport layer (HTL) and EML. Consequently, the CdSe/ZnS QDs:CBP based QLED achieves maximum external quantum efficiency (EQE) of 5.86%, and maximum brightness of 10 363 cd/m2. It is demonstrated that the additive of small organic molecules could be an effective way to improve the brightness and the efficiency of QLEDs.     

Precisely Encoded Barcodes Using Tetrapod CdSe/CdS Quantum Dots with a Large Stokes Shift for Multiplexed Detection

A serious obstacle to the construction of high‐capacity optical barcodes in suspension array technology is energy transfer, which can prompt unpredictable barcode signals, limited barcode numbers, and the need for an unfeasible number of experimental iterations. This work reports an effective and simple way to eliminate energy transfer in multicolor quantum dots (QDs)‐encoded microbeads by incorporating tetrapod CdSe/CdS QDs with a large Stokes shift (about 180 nm). Exploiting this unique feature enables the facile realization of a theoretical 7 × 7‐1 barcoding matrix combining two colors and seven intensity levels. As such, microbeads containing tetrapod CdSe/CdS QDs are demonstrated to possess a powerful encoding capacity which allows for precise barcode design. The ability of the Shirasu porous glass membrane emulsification method to easily control microbead size facilitates the establishment of a 3D barcode library of 144 distinguishable barcodes, indicating the enormous potential to enable large‐scale multiplexed detection. Moreover, when applied for the multiplexed detection of five common allergens, these barcodes exhibit superior detection performance (limit of detection: 0.01–0.02 IU mL^?1) for both spiked and patient serum samples. Therefore, this new coding strategy helps to expand barcoding capacity while simultaneously reducing the technical and economic barriers to the optical encoding of microbeads for high‐throughput multiplexed detection.     

Organic light-emitting devices with multi-shell quantum dots connected with polythiophene derivatives

The results of an experimental study of organic light-emitting diodes with poly-2,5-(3,4-diamino thieno[2,3-b]thiophene)-4,4’-amidoarylene transport layers and CdSe/CdS/ZnS quantum dots, the CdSe core of which has an average diameter of 4.1 nm, are presented. A suggestion is made regarding the possible conjugation of quantum dots and polythiophenes, which would substantially improve the characteristics of the light-emitting diodes in the case of optimization of their structure.     

Improving the Power Conversion Efficiency of Organic Solar Cell by Blending with CdSe/ZnS Core–Shell Quantum Dots

We have blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with CdSe/ZnS core–shell quantum dots (QDs) as the active layer to produce organic solar cells (OSC). The size of the CdSe/ZnS core–shell QDs was determined to be about 4 nm using transmission electron microscopy. The OSC were characterized by measuring the absorption spectra, current–voltage characteristics, and external quantum efficiency (EQE) spectra. The samples doped with 0.5 wt.% CdSe/ZnS core–shell QDs exhibited higher power conversion efficiency (PCE) than samples doped with other concentrations of QDs. The PCE of the OSC increases from 2.10% to 2.38% due to an increase of the short circuit current density (J _sc) from 6.00 mA/cm² to 6.25 mA/cm². The open circuit voltage (V _oc) was kept constant when comparing OSC that were undoped and doped with 0.5 wt.% CdSe/ZnS core–shell QDs. These CdSe/ZnS core–shell QDs can increase optical absorption as well as provide extra exciton dissociation and additional electric pathways in hybrid OSC.     

Energy transfer from TPD to CdSe/CdS/ZnS colloidal nanocrystals

The efficiency of electronic-excitation energy transfer from organic semiconductor TPD to CdSe/CdS/ZnS nanocrystals passivated with different organic ligands is investigated. It is shown that the rate of energy transfer from TPD to nanocrystals decreases with increasing thickness of the passivation coating. It is suggested that the Förster mechanism is responsible for the excitation transfer.     

Saturated and Multi‐Colored Electroluminescence from Quantum Dots Based Light Emitting Electrochemical Cells

Novel light emitting electrochemical cells (LECs) are fabricated using CdSe‐CdS (core‐shell) quantum dots (QDs) of tuned size and emission blended with polyvinylcarbazole (PVK) and the ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIM‐PF₆). The performances of cells constructed using sequential device layers of indium tin oxide (ITO), poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS), the QD/PVK/IL active layer, and Al are evaluated. Only color saturated electroluminescence from the QDs is observed, without any other emissions from the polymer host or the electrolyte. Blue, green, and red QD‐LECs are prepared. The maximum brightness (≈1000 cd m^‐2) and current efficiency (1.9 cd A^‐1) are comparable to polymer LECs and multilayer QD‐LEDs. White‐light QD‐LECs with Commission Internationale d'Eclairage (CIE) coordinates (0.33, 0.33) are prepared by tuning the mass ratio of R:G:B QDs in the active layer and voltage applied. Transparent QD‐LECs fabricated using transparent silver nanowire (AgNW) composites as the cathode yield an average transmittance greater than 88% over the visible range. Flexible devices are demonstrated by replacing the glass substrates with polyethylene terephthalate (PET).     

Enhanced Photocurrent Generation in Proton‐Irradiated “Giant” CdSe/CdS Core/Shell Quantum Dots

Group II–VI quantum dots (QDs) possess tunable electrical and optical properties that make them very attractive for high‐tech applications and power generation. The effects of proton irradiation on both the structural and physical properties of “giant” CdSe/CdS core–shell QDs (g‐CS QDs) are investigated. These experiments shed light on photoelectron delocalization in g‐CS QDs, where current linkages and strong variations in optical emission result from the spatial extension of the photoelectron wavefunctions over the conduction bands of CdSe and CdS. Monte Carlo simulations of ion–matter interactions show that the damaging rates can be set from the energy of impinging protons to promote the formation of structural defects in the core or shell. The formation of nanocavities is demonstrated after irradiation doses higher than ≈10¹⁷ H⁺ cm^?2, while a continuous decrease in luminescence intensity is observed for increasing proton fluencies. This feature is accompanied by a concomitant lifetime decrease marking the rise of nonradiative phenomena and the occurrence of greater photocarrier transfers between CdS and CdSe. Current‐to‐voltage characterizations evidence that proton implantation can be implemented to enhance the photocurrent generation in g‐CS QDs. This increase is attributed to the delocalization of photoelectrons in the CdS shell, whose improvement is found to promote electron–hole pair separation.     

Cocatalyst‐Free Photocatalysts for Efficient Visible‐Light‐Induced H2 Production: Porous Assemblies of CdS Quantum Dots and Layered Titanate Nanosheets

Highly efficient, visible‐light‐induced H₂ generation can be achieved without the help of a Pt cocatalyst by new hybrid photocatalysts, in which CdS quantum dots (QDs) (particle size ≈2.5 nm) are incorporated in the porous assembly of sub‐nanometer‐thick layered titanate nanosheets. Due to the very‐limited crystal dimension of component semiconductors, the electronic structure of CdS QDs is strongly coupled with that of the layered titanate nanosheets, leading to an efficient electron transfer between them and the enhancement of the CdS photostability. As a consequence of the promoted electron transfer, the photoluminescence of CdS QDs is nearly quenched after hybridization, indicating the almost‐suppression of electron‐hole recombination. These Pt‐cocatalyst‐free, CdS‐layered titanate nanohybrids show much‐higher photocatalytic activity for H₂ production than the precursor CdS QDs and layered titanate, which is due to the increased lifetime of the electrons and holes, the decrease of the bandgap energy, and the expansion of the surface area upon hybridization. The observed photocatalytic efficiency of these Pt‐free hybrids (≈1.0 mmol g^?1 h^?1) is much greater than reported values of other Pt‐free CdS‐TiO₂ systems. This finding highlights the validity of 2D semiconductor nanosheets as effective building blocks for exploring efficient visible‐light‐active photocatalysts for H₂ production.     

Spatial Extent of Fluorescence Quenching in Mixed Semiconductor–Metal Nanoparticle Gel Networks

In this work, mixing and co-gelation of Au nanoparticles (NPs) and highly luminescent CdSe/CdS core/shell nanorods (NRs) are used as tools to obtain noble metal particle-decorated macroscopic semiconductor gel networks. The hybrid nature of the macrostructures facilitates the control over the optical properties: while the holes are trapped in the CdSe cores, the connected CdSe/CdS NRs support the mobility of excited electrons throughout the porous, hyperbranched gel networks. Due to the presence of Au NPs in the mixed gels, electron trapping in the gold NPs leads to a suppressed radiative recombination, namely, quenches the fluorescence in certain fragments of the multicomponent gel. The extent of fluorescence quenching can be influenced by the quantity of the noble metal domains. The optical properties are monitored as a function of the NR:NP ratio of a model system CdSe/CdS:Au. By this correlation, it demonstrates that the spatial extent of quenching initiated by a single Au NP exceeds the dimensions of one NR, which the Au is connected to (with a length of 45.8 nm ± 4.1 nm) and can reach the number of nine NRs per Au NP, which roughly corresponds to 400 nm of total electron travel distance within the network structure.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.     

Tuning the spectrometric properties of white light by surface plasmon effect using Ag nanoparticles in a colour converting light-emitting diode

We report on the spectral tunability of white light by localized surface plasmon (LSP) effect in a colour converting hybrid device made of CdSe/ZnS quantum dots (QDs) integrated on InGaN/GaN blue light-emitting diodes (LEDs). Silver (Ag) nanoparticles (NPs) are mixed with QDs for generating LSP effect. When the plasmon absorption of Ag NPs is synchronized to the QW emission at 448 nm, the NPs selectively absorb the blue light and subsequently enhance the QD emission. Using this energy transfer scheme, the (x, y) chromaticity coordinates of the hybrid white LED was tuned from (0.32, 0.17) to (0.43, 0.26), and thereby generated warm white light emission with correlated colour temperature (CCT) around 1800 K. Moreover, a 47% enhancement in the external quantum efficiency (EQE) was realized.