PbS QDs Research Articles

Ultimate Charge Extraction of Monolayer PbS Quantum Dot for Observation of Multiple Exciton Generation.

Multiple exciton generation (MEG) has great potential to improve the Shockley-Queisser (S-Q) efficiency limitation for colloidal quantum dot (CQD) solar cells. However, MEG has rarely been observed in CQD solar cells because of the loss of carriers through the transport mechanism between adjacent QDs. Herein, we demonstrate that excess charge carriers produced via MEG can be efficiently extracted using monolayer PbS QDs. The monolayer PbS QDs solar cells exhibit α=1 in the light intensity dependence of the short-circuit current density Jsc (Jsc ∝Iα ) and an internal quantum efficiency (IQE) value of 100 % at 2.95 eV because of their very short charge extraction path. In addition, the measured MEG threshold is 2.23 times the bandgap energy (Eg ), which is the lowest value in PbS QD solar cells. We believe that this approach can provide a simple method to find suitable CQD materials and design interface engineering for MEG.

Simplistic thermal transformation of MIL-125 to TiO2 nano-coins and nano-diamonds for efficient quantum-dot sensitized solar cells

TiO2-based nano-coins and nano-diamonds were successfully incorporated for improved performance of PbS quantum-dot sensitized solar cells (QDSCs) through a simplistic thermal transformation of NH2-MIL-125(Ti). We report a modified heat-treatment process which affirms simple transformation of NH2-MIL-125(Ti) to TiO2 in addition to the removal of organic constituents of screen-printed TiO2 photoanoade, eliminating the discrete heat-treatment cycle for thermal transformation of NH2-MIL-125(Ti) before screen-printing. X-ray diffraction spectroscopy (XRD) revealed the phase transformation and resultant crystal structures of TiO2 while XPS was used to authenticate the fabrication of PbS sensitized TiO2 photoanode. UV–Vis spectroscopy showed that the modified photoanodes with TiO2 nano-coins and nano-diamonds exhibited improved reflectance of light, which resulted in enhanced visible light absorbance in PbS QDs. Relatively, incorporation of simplistically transformed TiO2 nano-diamonds resulted higher photocurrent density (JSC~17 mA/cm2) in QDSCs which was credited to superior absorbance of incident light and inferior charge carrier recombination.

Atom Probe Tomographic Imaging of PbS Quantum Dot Formation on Neodymium Clusters in Silicate Glasses

The first 3-D direct observation of clusters of Nd oxide inside silicate glasses was achieved using atom probe tomography. Three-dimensional elemental maps of major chemical elements in glasses such as Si, Al, Zn and O showed no evidence of regions that had concentrations higher than the average values, whereas the Nd aggregated into regions of high concentration. Elemental maps of Nd and Pb recorded from the glasses containing PbS QDs showed highly-concentrated areas of both elements at the same locations; this result indicates that PbS QDs formation started in association with the Nd clusters.

Superior ZnS deposition for augmenting the photostability and photovoltaic performance of PbS quantum-dot sensitized solar cells

Simply by elevating the successive ionic layer adsorption and reaction temperature for ZnS deposition, we successfully demonstrated improved photostability of PbS-based quantum dot-sensitized solar cells (QDSCs) which was credited to efficient surface coverage of ZnS over PbS QDs and TiO2. Using open-circuit voltage decay and cyclic voltammetry techniques distinctly, it was affirmed that the back electron transfer at TiO2/PbS/electrolyte interface was reduced. Superior deposition of ZnS in terms of surface coverage not only exhibited increase in photocurrent density (from 20.7 mA/cm2 to 22.3 mA/cm2) and power conversion efficiency (from 3.1% to 3.4%), it also stabilized the normalized current significantly.

An Electrochemical Sandwich Immunosensor for the Detection of HER2 using Antibody-Conjugated PbS Quantum Dot as a label

A facile electrochemical sandwich immunosensor for the detection of a breast cancer biomarker, the human epidermal growth factor receptor 2 (HER2), was designed, using lead sulfide quantum dots-conjugated secondary HER2 antibody (Ab2-PbS QDs) as a label. Using Ab2-PbS QDs in the development of electrochemical immunoassays leads to many advantages such as straightforward synthesis and well-defined stripping signal of Pb(II) through acid dissolution, which in turn yields better sensing performance for the sandwiched immunosensor. In the bioconjugation of PbS QDs, the available amine and hydroxyl groups from secondary anti-HER2 and capped PbS QDs were bound covalently together via carbonyldiimidazole (CDI) acting as a linker. In order to quantify the biomarker, SWV signal was obtained, where the Pb2+ ions after acid dissolution in HCl was detected. The plated mercury film SPCE was also detected in situ. Under optimal conditions, HER2 was detected in a linear range from 1–100 ng/mL with a limit of detection of 0.28 ng/mL. The measures of satisfactory recoveries were 91.3% to 104.3% for the spiked samples, displaying high selectivity. Therefore, this method can be applied to determine HER2 in human serum.

Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots

Abstract A simple approach to deposit metal chalcogenide-based quantum-dots on TiO2 (anatase) nanoparticles via pseudo-successive ionic layer adsorption and reaction (p-SILAR) was opted to design and investigate the corresponding photoactivity under visible light. We used TiO2/PbS and TiO2/CdS nanocomposites for photocatalysis after which we prepared solar paints from them to put side by side the resultant photovoltaic performance. We developed solar paint type quantum-dot-sensitized solar cells (QDSCs) from TiO2/PbS and TiO2/CdS nanocomposites to divulge electron-hole generation, electron-hole recombination, and resistance against photo-corrosion. TiO2/PbS exhibited higher electron-hole generation than TiO2/CdS owing to its efficient visible light harvesting capability which resulted in higher photocurrent density (JSC) and thus higher power conversion efficiency (PCE) for respective QDSC; however, it had inferior resistance against photo-corrosion. Additionally, the degradation of an azo-based dye (acid orange-56) reflected an effectual increase in the photocatalytic activity of anatase-TiO2 as a result of deposition of PbS and CdS QDs. TiO2/PbS performed better than TiO2/CdS as a photocatalyst under visible light irradiation.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Synergistic ligand exchange and UV curing of PbS quantum dots for effective surface passivation.

Lead sulfide (PbS) quantum dots (QDs) are promising materials in solution-processed photovoltaic (PV) devices due to their tunable bandgap and low-cost processing. Replacing the long oleic acid ligands of the as-synthesized QDs with shorter ligands is a key step for making functional QD PVs with correctly tuned band energies and reduced non-radiative recombination centers. In this work, we study the effect of ultraviolet (UV) treatment of PbS QD layers on the QD surface states during ligand exchange. We demonstrate that this straightforward approach effectively reduces the surface trap states and passivates the surface of QDs. We find that UV treatment reduces the density of hydroxyl groups attached to the QD surface and improves the bonding of short ligands to the QD surface. Multiple analyses show the reduction of nonradiative recombination centers for the UV-treated sample. The power conversion efficiency (PCE) of our optimized PbS QD device reached 10.7% (vs. 9% for the control device) and was maintained above 10% after 230 h of constant illumination.

Synthesis and Characterization of ZnO/TiO2 Photocatalyst Decorated with PbS QDs for the Degradation of Aniline Blue Solution

Synthesis and Characterization of ZnO/TiO2 Photocatalyst Decorated with PbS QDs for the Degradation of Aniline Blue Solution

Low-Temperature ZnO Thin Film and Its Application in PbS Quantum Dot Solar Cells

Zinc oxide (ZnO) has been widely deployed as electron conducting layer in emerging photovoltaics including quantum dot, perovskite and organic solar cells. Reducing the curing temperature of ZnO layer to below 200 oC is an essential requirement to reduce the cell fabrication cost enabled by large-scale processes such as ink-jet printing, spin coating or roll-roll printing. Herein, we present a novel water-based ZnO precursor stabilized with labile NH3, which allow us to spin coat crystalline ZnO thin films with temperatures below 200 oC. Thin film transistors (TFTs) and diode-type quantum dot solar cells (QD SCs) were fabricated using ZnO as electron conduction layer. In the QD SCs, a p-type 1,2-ethylenedithiol treated PbS QDs with a bandgap of 1.4 eV was spin-coated on top of ZnO layer by a layer-by-layer solid state ligand exchange process. Electron mobility of ZnO was about 0.1 cm2V-1s-1 as determined from TFT measurements. Power conversion efficiency of solar cells: FTO/ZnO/PbS/Au-Ag was 3.0% under AM1.5 irradiation conditions. The possibility of deposition of ZnO at low temperatures demonstrated herein is of important for solution processed electronic and optoelectronic devices. 
 Keywords
 ZnO, low-temperature, quantum dots, solar cells, TFTs
 References
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Enhanced PbS quantum dot loading on TiO2 photoanode using atomic-layer-deposited ZnS interfacial layer for quantum dot-sensitized solar cells

Ultrathin and conformal ZnS film grown by atomic layer deposition was employed in quantum dot-sensitized solar cells (QDSSCs) as an interfacial layer (IL) between mesoporous TiO2 photoanode and successive ionic layer adsorption and reaction (SILAR)-grown PbS QDs. ZnS IL provided more nucleation sites compared to a bare TiO2 photoanode, which enhanced PbS QDs loading remarkably. As a result, the optical absorbance and thus photocurrent density considerably increased. The power conversion efficiency of QDSSCs increased from 3.4% to 4% by introducing the ZnS IL. However, the β-recombination model obtained from electrochemical impedance spectroscopy revealed the evolution of charge carrier recombination inside QDs as a consequence of enhanced QD loading, which partly dilutes this benefit.

Surface Chemistry Control of Colloidal Quantum Dot Band Gap

Surface chemistry modification of as-synthesized colloidal inorganic semiconductor nanocrystals (QDs), commonly referred to as ligand exchange, is mandatory toward effective QD-based optoelectronic and photocatalytic applications. The widespread recourse to ligand exchange procedures on metal chalcogenide QDs often narrows the optical band gap, although little consensus exists on explanation of this experimental evidence. This work attempts at providing a comprehensive description of such a phenomenon by exploiting rationally designed thiol ligands at the surface of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime: the thiol(ate)-induced QD optical band gap reduction almost linearly scales with the inorganic core surface-to-volume ratio and mainly depends on the sulfur binding atom, which is here suggested to contribute occupied 3p orbitals to the valence band edge of the QDs. As opposed to QD models based on the analogy with core/shell heterostructures, the indecomposa...

Hybrid PbS Quantum-Dot-in-Perovskite for High-Efficiency Perovskite Solar Cell.

In this study, a facile and effective approach to synthesize high-quality perovskite-quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH3 NH3 PbI3 (MAPbI3 ) precursor to form a QD-in-perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X-ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI3 -PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over ≈0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.

Electrochemical immunoassay for the detection of IgM antibodies using polydopamine particles loaded with PbS quantum dots as labels

Here, we report for the first time, an electrochemical immunoassay to detect IgM antibodies using lead sulfide quantum dots (PbS QDs) as electrochemical labels. In this sense, dendritic-like polydopamine particles loaded with PbS QDs were synthesized by the self-polymerization of dopamine in basic media in the presence of QDs (PbS@PDA) and further tagged with anti-IgM antibodies, dengue specific antigens, and streptavidin moieties. The analytical features of the sandwich immunoassay on ELISA microplate were carried out with the PbS@PDA-labeled anti-IgM as secondary antibody. The system was interrogated by acid dissolution of PbS@PDA, followed by differential pulse anodic stripping voltammetry in the presence of Bi(III) ions using carbon screen-printed electrodes. The results indicate that the voltammetric current increased with the increasing of the concentration of target IgM within a range of 0–0.5 mg mL−1. The limit of detection of this electrochemical immunoassay was evaluated to 130 ng. The measures of satisfactory recoveries from 88.5% to 114% of spiked samples indicate that such a method has good specificity and is applicable to the quantification of IgM antibodies in complex biological samples. No significant differences at the 0.05 significance level were encountered in the analysis of IgM samples between the electrochemical immunoassay and a Bradford assay.

A simple UV-assisted, room temperature approach for synthesis of water soluble PbS and PbS/CdS core-shell QDs

In this work PbS and PbS/CdS core-shell QDs, were synthesized via a simple UV-assisted approach. Prepared QDs were characterized by means of XRD, FTIR, SEM, TEM, UV–vis and PL analysis. Synthesized QDs indicated cubic phase with a spherical morphology and the mean particle size was obtained about 4 nm. PbS QDs showed an emission with a peak at about 850 nm which it was tunable with the UV-illumination time. CdS shell growth resulted in the shift of the XRD pattern peaks to the higher angles, blue shift of the PL peak and considerably enhancement of the PL intensity. PbS QDs indicated strong photocatalyst activity for Methylene blue (MB) photo-degradation which it was increased significantly after CdS shell growth.

Enhancing silicon solar cells with singlet fission: the case for Förster resonant energy transfer using a quantum dot intermediate

One way for solar cell efficiencies to overcome the Shockley–Queisser limit is downconversion of high-energy photons using singlet fission (SF) in polyacenes like tetracene (Tc). SF enables generation of multiple excitons from the high-energy photons, which can be harvested in combination with Si. In this work, we investigate the use of lead sulfide quantum dots (PbS QDs) with a band gap close to Si as an interlayer that allows Förster resonant energy transfer (FRET) from Tc to Si, a process that would be spin-forbidden without the intermediate QD step. We investigate how the conventional FRET model, most commonly applied to the description of molecular interactions, can be modified to describe the geometry of QDs between Tc and Si and how the distance between QD and Si, and the QD bandgap affects the FRET efficiency. By extending the acceptor dipole in the FRET model to a 2-D plane, and to the bulk, we see a relaxation of the distance dependence of transfer. Our results indicate that FRET efficiencies from PbS QDs to Si well above 50% are possible at very short but possibly realistic distances of around 1 nm, even for QDs with relatively low photoluminescence quantum yield.

Influences of Morphology of Window Layer on the Characteristics of PbS Quantum Dot Solar Cells

Background: In QDs-based solar cell devices, the PbS QDs layer was mainly focused to optimize. The ZnO electron acceptor layer attracts less attention whereas it shows the key roles in extracting and transporting charge carriers in heterojunction. The utilization of 1-D ZnO structures has been demonstrated to be large interface areas and good carrier pathways for efficient carrier collection. However, the influences of the morphology of metal oxide nanostructures on the photovoltaic performance of QD-based solar cells have been few in-depth reports. Objective: In this work, ZnO NRs/PbS QD based solar cells were fabricated. The influences of the ZnO NRs array structures on characteristics of ZnO NRs/PbS QD based solar cells were investigated. Method: ZnO NRs/PbS QD based solar cells were fabricated via spin coating method. XRD, SEM, UV-VIS-NIR spectrophotometer, I-V and EQE measurement systems were utilized to investigate the fabricated samples. Results: We have found optimum combinations of the linked parameters of ZnO NRs, their length of (230 ± 5) nm and density of (1.50 ± 5)x1010 # of rods.cm-2, that exhibit maximum efficiency of ∼2.5% for the ZnO NR/PbS QDs based solar cell. Conclusion: The influences the ZnO NRs structures on the solar cell characteristics, including the absorption, external quantum efficiency, and current density-voltage curves, were investigated. There seems to be an optimum between NR length and their density for resulting in maximum efficiency. This could be due the interplay of solar flux absorption and junction area controlled by these two parameters of ZnO NR morphology.

(Invited) Controlling the Properties of Colloidal Quantum Dots for Energy Conversion Applications

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells, and light emitting diodes.We studied the how the optical absorption enhancement and bandage positions of PbS QDs are modified upon ligand exchange from oleate to a series of cinnamate ligands. We find that band edge positions of PbS QDs, can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust, and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. We find that within the cinnamate/PbS QD system the optical absorption is enhanced and the enhancement scales linearly with the HOMO/LUMO gap of the ligand and not their electron donating/withdrawing character, indicating that the ligand/QD coupling occurs equally efficient between QD VB/ligand HOMO levels and the QD CB/ligand LUMO levels. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the resulting emergent optical and electrical properties. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We demonstrate that the optical enhancement can be used to extract the ligand binding isotherms for a given QD/ligand complex by following the absorption enhancement during ligand exchange reactions. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.

In Situ Passivation for Efficient PbS Quantum Dot Solar Cells by Precursor Engineering.

Current efforts on lead sulfide quantum dot (PbS QD) solar cells are mostly paid to the device architecture engineering and postsynthetic surface modification, while very rare work regarding the optimization of PbS synthesis is reported. Here, PbS QDs are successfully synthesized using PbO and PbAc2 · 3H2 O as the lead sources. QD solar cells based on PbAc-PbS have demonstrated a high power conversion efficiency (PCE) of 10.82% (and independently certificated values of 10.62%), which is significantly higher than the PCE of 9.39% for PbO-PbS QD based ones. For the first time, systematic investigations are carried out on the effect of lead precursor engineering on the device performance. It is revealed that acetate can act as an efficient capping ligands together with oleic acid, providing better surface coverage and replace some of the harmful hydroxyl (OH) ligands during the synthesis. Then the acetate on the surface can be exchanged by iodide and lead to desired passivation. This work demonstrates that the precursor engineering has great potential in performance improvement. It is also pointed out that the initial synthesis is an often neglected but critical stage and has abundant room for optimization to further improve the quality of the resultant QDs, leading to breakthrough efficiency.

Self-assembled synthesis of PbS quantum dots supported on polydopamine encapsulated BiVO4 for enhanced visible-light-driven photocatalysis

Abstract A simple self-assembled method for the deposition of PbS quantum dots (PbS QDs) on the surface of polydopamine (PDA)-encapsulated BiVO4 nanoparticles is proposed to synthesize PbS@PDA/BiVO4 nanocomposite photocatalysts with the assistance of ultrasonication. In the PbS@PDA/BiVO4 nanocomposites that was designed to improve the charge separation efficiency, PDA served as a natural adhesive and a stabilizer that can help to prevent PbS quantum dots from agglomeration, and PbS quantum dots served as the multiple exciton donor. The photocatalytic degradation of glyphosate experiments indicated that PbS@PDA/BiVO4 exhibited the enhanced photocatalytic activity under visible light irradiation. As compared with that of BiVO4, PbS@BiVO4 and PDA/BiVO4, the photocatalytic activity of PbS@PDA/BiVO4 was improved by 3.03, 1.48 and 1.58 times, respectively. The enhanced activity of PbS@PDA/BiVO4 can be attributed to the rapid transport of a mass of photoelectrons generated by multiexciton generation from PbS quantum dots to the surface of BiVO4 and the enhanced charge separation efficiency of photogenerated charge carrier.