POLY(N-ISOPROPYL ACRYLAMIDE) MICROGEL DOPED WITH LUMINESCENT PBS QUANTUM DOTS

Thiol-stabilized PbS quantum dots （QDs） with dimensions 3-5 nm capped with a mixture of 1-thioglycerol/dithioglycerol （TGL/DTG） were coUoidally prepared at room temperature. Room temperature photoluminescence quantum efficiency of freshly prepared PbS QDs （7%-11%） remained higher than 5% upon aging for three weeks when the nanocrystals （NCs） were stored in an ice-bath in the dark, and higher than 5%for at least five weeks when extra DTG ligands were introduced into the nanocrystal solution followed by stirring every two weeks. Poly（N-isopropyl acrylamide） （PNIPAM） microgels were produced via precipitation polymerization with dimensions of ca. 230 nm and polydispersity of 3-5%. Incorporation of PbS QDs into PNIPAM microgels indicated that PbS can be incorporated into the interior of microgel particles and not at the microgel interface. The combination of reasonable room temperature quantum efficiency and strong, efficient luminescence covering the 1.3-1.55 μm telecommunication window makes these nanoparticles promising materials in optical devices and telecommunications.

Crown ether-assisted room-temperature halide passivation for highefficiency PbS quantum dots enabling large-area and long-lifetime near-infrared QD-OLEDs

High-efficiency and low-cost near-infrared(NIR)emitting quantum dots(QDs)are highly desirable for next-generation intrinsically flexible NIR light sources.Halide passivation is commonly employed to passivate surface traps to obtain high-quality NIRemitting PbS QDs,but this procedure requires high temperature and inert atmospheres.Here we develop a facile roomtemperature halide passivation method for highly efficient NIR-emitting PbS QDs by employing crown ethers as a unique auxiliary additive.Experimental and theoretical investigations reveal that the formation of K^(+)-crown ethers complex effectively facilitates the dissociation of KCl in toluene and releases more Cl^(−)ions for extraordinary halide passivation at room temperature and in the air,thus improving the photoluminescence quantum yield from 24%to 35%in solution and further to 44%in blend films.The well-passivated PbS QD films are integrated with red organic light-emitting diodes(OLEDs)and the resulting QDOLEDs exhibit high-performance NIR emission centered at 887 nm,a high external quantum efficiency of 5.2%at a radiance of 10 W·sr^(−1)·m^(−2),superior operational stability with long lifetime T90 of 188 h at the current density of 25 mA·cm^(−2).We also construct a large-area NIR QD-OLED(5 cm×5 cm)with desirable uniform emission.This work opens a new avenue to achieve robust large-area NIR planar light sources for broad applications.

High-performance flexible and broadband photodetectors based on PbS quantum dots/ZnO nanoparticles heterostructure

Flexible and broadband photodetectors have drawn extensive attention due to their potential application in foldable displays, optical communications, environmental monitoring, etc. In this work, a flexible photodetector based on the crystalline PbS quantum dots(QDs)/ZnO nanoparticles(NPs) heterostructure was proposed. The photodetector exhibits a broadband response from ultraviolet-visible(UV-Vis)to near infrared detector(NIR) range with a remarkable current on/off ratio of 7.08×10^3under 375 nm light illumination.Compared with pure ZnO NPs, the heterostructure photodetector shows the three orders of magnitude higher responsivity in Vis and NIR range, and maintains its performance in the UV range simultaneously. The photodetector demonstrates a high responsivity and detectivity of4.54 A W-1and 3.98×10^12Jones. In addition, the flexible photodetectors exhibit excellent durability and stability even after hundreds of times bending. This work paves a promising way for constructing next-generation high-performance flexible and broadband optoelectronic devices.

PbS quantum dots and Ba F_(2):Tm^(3+) nanocrystals co-doped glass for ultra-broadband near-infrared emission [Invited]

With the rapid growth of optical communications traffic,the demand for broadband optical amplifiers continues to increase.It is necessary to develop a gain medium that covers more optical communication bands.We precipitated PbS quantum dots(QDs) and Ba F_(2):Tm^(3+) nanocrystals (NCs) in the same glass to form two independent emission centers.The Ba F_(2)NCs in the glass can provide a crystal field environment with low phonon energy for rare earth (RE) ions and prevent the energy transfer between RE ions and PbS QDs.By adjusting the heat treatment schedule,the emission of the two luminescence centers from PbS QDs and Tm^(3+) ions perfectly splices and covers the ultra-broadband near-infrared emission from 1200 nm to 2000 nm with bandwidth over 430 nm.Therefore,it is expected to be a promising broadband gain medium for fiber amplifiers.

Performance enhancement of ZnO nanowires/PbS quantum dot depleted bulk heterojunction solar cells with an ultrathin Al_2O_3 interlayer

Depleted bulk heterojunction （DBH） PbS quantum dot solar cells （QDSCs）, appearing with boosted short-circuit current density （Jsc）, represent the great potential of solar radiation utilization, but suffer from the problem of increased interfacial charge recombination and reduced open-circuit voltage （Voc）. Herein, we report that an insertion of ultrathin A1203 layer （ca. 1.2 A thickness） at the interface of ZnO nanowires （NWs） and PbS quantum dots （QDs） could remarkably improve the performance of DBH-QDSCs fabricated from them, i.e., an increase of Voc from 449 mV to 572 mV, J^c from 21.90 mA/cm2 to 23.98 mA/cm2, and power conversion efficiency （PCE） from 4.29% to 6.11%. Such an improvement of device performance is ascribed to the significant reduction of the interfacial charge recombination rate, as evidenced by the light intensity dependence on Jsc and Voc, the prolonged electron lifetime, the lowered trap density, and the enlarged recombination activation energy. The present research therefore provides an effective interfacial engineering means to improving the overall performance of DBH-QDSCs, which might also be effective to other types of optoelectronic devices with large interface area.

Improving the Performance of PbS Quantum Dot Solar Cells by Optimizing ZnO Window Layer

Comparing with hot researches in absorber layer,window layer has attracted less attention in PbS quantum dot solar cells(QD SCs). Actually, the window layer plays a key role in exciton separation, charge drifting, and so on.Herein, ZnO window layer was systematically investigated for its roles in QD SCs performance. The physical mechanism of improved performance was also explored. It was found that the optimized ZnO films with appropriate thickness and doping concentration can balance the optical and electrical properties, and its energy band align well with the absorber layer for efficient charge extraction. Further characterizations demonstrated that the window layer optimization can help to reduce the surface defects, improve the heterojunction quality, as well as extend the depletion width. Compared with the control devices, the optimized devices have obtained an efficiency of 6.7% with an enhanced V_(oc) of 18%, J_(sc) of 21%, FF of 10%, and power conversion efficiency of 58%. The present work suggests a useful strategy to improve the device performance by optimizing the window layer besides the absorber layer.

Characterization of hot carrier cooling and multiple exciton generation dynamics in PbS QDs using an improved transient grating technique

Multiple exciton generation （MEG） dynamics in colloidal PbS quantum dots （QDs） characterized with an im- proved transient grating （TG） technique will be reported. Only one peak soon after optical absorption and a fast decay within 1 ps can be observed in the TG kinetics when the photon energy of the pump light hv is smaller than 2.7Eg （Eg： band gap between LUMO and HOMO in the QDs）, which corresponds to hot carrier cooling. When hv is greater than 2.7Eg, however, after the initial peak, the TG signal decreases first and soon increases, and then a new peak appears at about 2 to 3 ps. The initial peak and the new peak correspond to hot carriers at the higher excited state and MEG at the lowest excited state, respectively. By proposing a theoretical model, we can calculate the hot carrier cooling time constant and MEG occurrence time constant quantitatively. When MEG does not happen for hv smaller than 2.7Eg, hot carrier cools with a time con- stant of 400 fs. When MEG occurs for hv larger than 2.7Eg, hot carrier cools with a time constant as small as 200 fs, while MEG occurs with a time constant of 600 fs. The detailed hot carrier cooling and MEG occurrence dynamics characterized in this work would shed light on the further understanding of MEG mechanism of various type of semiconductor QDs.

Surface-Modified Graphene Oxide/Lead Sulfide Hybrid Film-Forming Ink for High-Efficiency Bulk Nano-Heterojunction Colloidal Quantum Dot Solar Cells

Solution-processed colloidal quantum dot solar cells(CQDSCs) is a promising candidate for new generation solar cells.To obtain stable and high performance lead sulfide(PbS)-based CQDSCs,high carrier mobility and low non-radiative recombination center density in the PbS CQDs active layer are required.In order to effectively improve the carrier mobility in PbS CQDs layer of CQDSCs,butylamine(BTA)-modified graphene oxide(BTA@GO) is first utilized in PbS-PbX2(X=I-,Br-) CQDs ink to deposit the active layer of CQDSCs through one-step spin-coating method.Such surface treatment of GO dramatically upholds the intrinsic superior hole transfer peculiarity of GO and attenuates the hydrophilicity of GO in order to allow for its good dispersibility in ink solvent.The introduction of B TA@GO in CQDs layer can build up a bulk nano-heterojunction architecture,which provides a smooth charge carrier transport channel in turn improves the carrier mobility and conductivity,extends the carriers lifetime and reduces the trap density of PbS-PbX2 CQDs film.Finally,the BTA@GO/PbS-PbX2 hybrid CQDs film-based relatively large-area(0.35 cm2) CQDSCs shows a champion power conversion efficiency of 11.7% which is increased by 23.1% compared with the control device.

200-nm long TiO_2 nanorod arrays for efficient solid-state Pb S quantum dot-sensitized solar cellsR

To ensure the infiltration of spiro-OMeTAD into the quantum dot-sensitized photoanode and to consider the limit of the hole diffusion length in the spiro-OMeTAD layer, a rutile TiO2 nanorod array with a length of 200 nm, a diameter of 20 nm and an areal density of 720 ram 2 was successfully prepared using a hydrothermal method with an aqueous-grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 ℃ for 75 min. PbS quantum dots were deposited by a spin coating-assisted successive ionic layer adsorption and reaction （spin-SILAR）, and all solid-state PbS quantum dot-sensitized TiO2 nanorod array solar cells were fabricated using spiro-OMeTAD as electrolytes. The results revealed that the average crystal size of PbS quantum dots was -78 nm using Pb（NO3）2 as the lead source and remain unchanged with the increase of the number of spin-SILAR cycles. The all solid-state PbS quantum dot-sensitized TiO2 nanorod array solar cells with spin-SILAR cycle numbers of 20, 30 and 40 achieved the photoelectric conversion efficiencies of 3.74%, 4.12% and 3.11%, respectively, under AM 1.5 G illumination （100 mW/cm2）.

Stable PbS colloidal quantum dot inks enable blade‑coating infrared solar cells

Infrared solar cells are more efective than normal bandgap solar cells at reducing the spectral loss in the near-infrared region,thus also at broadening the absorption spectra and improving power conversion efciency.PbS colloidal quantum dots(QDs)with tunable bandgap are ideal infrared photovoltaic materials.However,QD solar cell production sufers from small-areabased spin-coating fabrication methods and unstable QD ink.Herein,the QD ink stability mechanism was fully investigated according to Lewis acid–base theory and colloid stability theory.We further studied a mixed solvent system using dimethylformamide and butylamine,compatible with the scalable manufacture of method-blade coating.Based on the ink system,100 cm2 of uniform and dense near-infrared PbS QDs(~0.96 eV)flm was successfully prepared by blade coating.The average efciencies of above absorber-based devices reached 11.14%under AM1.5G illumination,and the 800 nm-fltered efciency achieved 4.28%.Both were the top values among blade coating method based devices.The newly developed ink showed excellent stability,and the device performance based on the ink stored for 7 h was similar to that of fresh ink.The matched solvent system for stable PbS QD ink represents a crucial step toward large area blade coating photoelectric devices.

Efficient quantum dot infrared solar cells with enhanced lowenergy photon conversion via optical engineering

Infrared(IR)solar cells are promising devices for improving the power conversion efficiency(PCE)of conventional solar cells by expanding the utilization region of the sunlight spectrum to near-infrared range.IR solar cells based on colloidal quantum dots(QDs)have attracted extensive attention due to the widely tunable absorption spectrum controlled by dot size and the unique solution processibility.However,the trade-off in QD solar cells between light absorption and photo-generated carrier collection has limited the further improvement of PCE.Here,we present high-performance PbS QD IR solar cells resulting from the combination of boosted light absorption and optimized carrier extraction.By constructing an optical resonance cavity,the light absorption is significantly enhanced in the range of 1,150–1,300 nm at a relatively thin photoactive layer.Meanwhile,the thin photoactive layer facilitates efficient carrier extraction.Consequently,the PbS QD IR solar cells exhibit a highly efficient photoelectric conversion in the IR region,resulting in a high IR PCE of 1.3%which is comparable to the highest value of solution-processed IR solar cells based on PbSe QDs.These results demonstrate that constructing an optical resonance cavity is a reasonable strategy for effective conversion of photons in the devices aiming at light in a relatively narrow wavelength range,such as IR solar cells and narrow band photodetectors.

Broadband, sensitive and spectrally distinctive SnS_(2) nanosheet/PbS colloidal quantum dot hybrid photodetector

Photodetectors convert photons into current or voltage outputs and are thus widely used for spectroscopy,imaging and sensing.Traditional photodetectors generally show a consistent-polarity response to incident photons within their broadband responsive spectrum.Here we introduced a new type of photodetector employing SnS_(2) nanosheets sensitized with PbS colloidal quantum dots(CQDs)that are not only sensitive(~105AW−1)and broadband(300–1000 nm)but also spectrally distinctive,that is,show distinctive(positive or negative)photoresponse toward incident photons of different wavelengths.A careful mechanism study revealed illumination-modulated Schottky contacts between SnS_(2) nanosheets and Au electrodes,altering the photoresponse polarity toward incident photons of different wavelengths.Finally,we applied our SnS_(2) nanosheet/PbS CQDs hybrid photodetector to differentiate the color temperature of emission from a series of white light-emitting diodes(LEDs),showcasing the unique application of our novel photodetectors.

Integrated colloidal quantum dot photodetectors with color-tunable plasmonic nanofocusing lenses

High-sensitivity photodetection is at the heart of many optoelectronic applications,including spectroscopy,imaging,surveillance,remote sensing and medical diagnostics.Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors,as the noise sources scale with the area of the detector.This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity.Currently,such designs are based on diffraction-limited approaches using optical lenses.Here,we employ a plasmonic flat-lens bull’s eye structure(BES)to concentrate and focus light into a nanoscale colloidal quantum dot(CQD)photodetector.The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity.Herein,we demonstrate the first CQD photodetector with a nanoscale footprint,the optically active area of which is determined by the BES;this detector represents an exciting opportunity for high-sensitivity sensing.