Intermittent Electron Transfer Activity From Single CdSe/ZnS Quantum Dots

Intermittent Electron Transfer Activity From Single CdSe/ZnS Quantum Dot

Suppressed Blinking Dynamics of Single QDs on ITO

The exciton quenching dynamics of single CdSe/CdS3MLZnCdS2MLZnS2ML core/multishell QDs adsorbed on glass, In2O3, and ITO have been compared. Single QDs on In2O3 show shorter fluorescence lifetimes and higher blinking frequencies than those on glass because of interfacial electron transfer from QDs to In2O3. Compared to glass and In2O3, single QDs on ITO show suppressed blinking activity as well as reduced fluorescence lifetimes. For QDs in contact with the n-doped ITO, the equilibration of their Fermi levels leads to the formation of negatively charged QDs. In these negatively charged QDs, the off states are suppressed because of the effective removal of the valence band holes, and their fluorescence lifetimes are shortened because of exciton Auger recombination and hole transfer processes involving the additional electrons. This study shows that the blinking of single QDs can be effectively suppressed on the surface of ITO. This phenomenon may also be observable for other QDs and on different n-doped semiconductors

Hole Transfer from Single Quantum Dots

Photoinduced hole transfer dynamics from single CdSe/CdS3ML/CdZnS2ML/ZnS2ML core/multishell quantum dots (QDs) to phenothiazine (PTZ) molecules were studied by single QD fluorescence spectroscopy to investigate the static and dynamic heterogeneities of the hole transfer process as well as its effect on the blinking dynamics of QDs. Ensemble-averaged transient absorption and fluorescence decay measurements show that excitons in QDs dissociate by transferring the valence band hole to PTZ with a time constant of 50 ns for the 1:1 PTZ–QD complex, and the subsequent charge recombination process (i.e., electron transfer from the conduction band of the reduced QD to oxidized PTZ to regenerate the complex in the ground state) occurs mainly on the 100 to 1000 ns time scale. Single QD–PTZ complexes show pronounced correlated fluctuations of fluorescence intensity and lifetime with time. In addition to the dynamic fluctuation, there are considerable heterogeneities of average hole transfer rate among different QD–PTZ complexes. The hole transfer process has little effect on the statistics of the off-states, which is often believed to be positively charged QDs with a valence band hole. Instead, it increases the probability of weakly emissive or “gray” states

Decoupling Interfacial Charge Transfer from Bulk Diffusion Unravels Its Intrinsic Role for Efficient Charge Extraction in Perovskite Solar Cells

In a perovskite solar cell, the overall photoinduced charge-transfer (CT) process comprises both charge diffusion through the bulk to perovskite/electrode interfaces and interfacial electron and hole transfer to electrodes. In this study, we decoupled these two entangled processes by investigating the film thickness-dependent CT dynamics from CH₃NH₃PbI₃ perovskites to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (electron acceptor) and spiro-OMeTAD (hole acceptor). By fitting ultrafast transient absorption kinetics to an explicit “diffusion-coupled charge-transfer” model, we found that the charge diffusion from the film interior to perovskite/electrode interfaces took ∼200 ps to a few nanoseconds, depending on the thickness of perovskite film; the subsequent interfacial charge transfer was ultrafast, ∼6 ps for electron transfer to PCBM and ∼8 ps for hole transfer to spiro-OMeTAD, and led to efficient charge extraction (>90%) to electrodes in a 400 nm thick film. Our results indicate that the picosecond interfacial charge transfer is a key to high-performance perovskite solar cells

Measurement of Electric Double Layer Charging Dynamics on Platinum Electrodes in Aqueous Solutions of Alkali Sulfates and Nitrates

The charging dynamics of electric double layers (EDLs) have been extensively investigated through theoretical and simulation methods, yet the experimental studies remain limited. In this work, we accurately measure the charge dynamics between platinum plate electrodes and the aqueous solutions of seven electrolytes, including sulfuric acid, lithium sulfate, sodium sulfate, potassium sulfate, lithium nitrate, sodium nitrate, and potassium nitrate. This study explores the dependence of EDL charging dynamics on voltage, electrolyte concentration, and electrode distance with an equivalent circuit model employed in data analysis

Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enabled by Built-in Halide Gradient

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH₃NH₃PbBr_<i>x</i>I_3–<i>x</i> single-crystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH₃NH₃PbI₃ NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the low-bandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy

Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO₂ Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy

The presenting surface of TiO₂ is one of the key factors that influence the photoinduced charge injection process from covalently bound chromophores. However, the dependence of electron transfer (ET) on TiO₂ surface properties (structure, defects, and facets) remains poorly understood due to the difficulties of deconvoluting the signal from a multitude of surface binding sites in highly heterogeneous ET systems. In an effort to correlate TiO₂ surface features with ET, we compare the photoinduced ET dynamics from single quantum dots (QDs) to polycrystalline TiO₂ thin films (pc-TiO₂) grown by atomic layer deposition (ALD) with that of porous TiO₂ nanoparticle films (np-TiO₂) by utilizing single-particle fluorescence spectroscopy. Unlike the broad distribution of ET rates (deduced from fluorescence lifetimes) on np-TiO₂, QDs on pc-TiO₂ exhibit two narrowly distributed ET rates that we attribute to reduced site heterogeneity. Variable temperature pc-TiO₂ annealing studies suggest that the double-peaked distribution of ET rates is related to TiO₂ surface defects, where QDs undergo more rapid ET. Further modification of pc-TiO₂ with a submonolayer of Al₂O₃ enables the selective exclusion of the more rapid ET pathway. More generally, this study provides insight into the role of surface defects in photoinduced ET into crystalline semiconductor oxides

Excitation-Power-Dependent Emission Color Tuning in Mn-Doped One-Dimensional Perovskite Single Crystal

One-dimensional (1D) perovskites are ideal broadband-emitting phosphors for white-emitting diode applications. Doping Mn2+ in 1D perovskites provides a new idea to tune the materials’ light-emitting properties. Here, we report an excitation-power-dependent dual-band emission from both self-trapped excitons (STEs) and Mn dopants in individual 1D Mn-doped DMEDAPbBr4 single crystals. By changing the excitation intensity, we realize a broad and continuous color tuning range between blue and orange. The temperature-dependent photoluminescence (PL) and transient spectroscopy measurements demonstrate that the tunable emission color is enabled by the competitive transfers of excitons to the STE state and Mn2+ ions. The Mn-doped 1D perovskites exhibit a high photostability by showing a continuous switch between blue and orange emissions during an uninterrupted operation time of 7.5 h. The high stability and emission-color switchable properties make the Mn-doped 1D perovskites special optically tunable PL materials for potential light-emitting applications

Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)₂(MA)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 (BA = CH₃(CH₂)₃NH₃⁺, MA = CH₃NH₃⁺) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as “<i>n</i> = 4”, are found to be mixture of multiple perovskite phases, with <i>n</i> = 2, 3, 4 and ≈ ∞, that naturally align in the order of <i>n</i> along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-<i>n</i> to large-<i>n</i> phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ∼358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices