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Using ultrafast transient absorption spectroscopy, we investigated the surface carrier trapping dynamics in various sized PbS quantum dots (QDs) when either a hot or cold exciton is photogenerated by different pump-energy. We observed that hot carriers exhibit distinctly different surface trapping dynamics from the cold exciton, in which their corresponding transient absorption (TA) spectral evolutions show clear differences in the long wavelength region (less than a band gap energy, Eg). We observed a rapid growth in the degree of surface trapping with an increase in the pump-energy. On the other hand, the degree of surface trapping in terms of the number of created excitons (⟨Nx⟩) shows negligible variation upon photoexcitation at any given wavelength. The photoinduced electron–hole separation followed by carrier trapping was characterized by ultrafast trapping rate. The surface trapping rate was solely dependent on the PbS QD size; the surface trapping rate becomes faster as the QD size increases. Furt...

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2-ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, which give the trace of band bending and its saturation. Consequently, the energy-level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.

ZnO thin films fabricated by a low-temperature sol–gel conversion method (L-ZnO) were employed as n-type electron-transporting layers (ETLs) in depleted-heterojunction quantum dot solar cells (DH-QDSCs). Thin films of PbS (∼200 nm) fabricated by spin-coating a colloidal PbS QD solution layer by layer were used as the p-type photoactive layers. The L-ZnO films functioned as efficient n-type ETLs by fully completely depleting the PbS active layers, and they displayed performances much higher than those of conventional ZnO nanoparticle-based ETLs. The morphologies and chemical compositions of the L-ZnO ETLs varied with the annealing conditions. These factors, in turn, had a marked effect on the charge-transfer characteristics at the L-ZnO/PbS interfaces of the DH-QDSCs. The power conversion efficiency (PCE) of the DH-QDSCs using the optimized L-ZnO films as ETLs was 3.93%, with the fill factor (FF) being 0.60, whereas the PCE of the cells using the ZnO nanoparticle-based films was 1.62%, with the FF being 0....

This paper reports on the enhanced photoluminescence (PL) of nanocomposites through the layered structuring of phosphor and quantum dot (QD). Green phosphor of Sr2SiO4:Eu, red QDs of CdSe/CdS/CdZnS/ZnS core-multishell, and thermo-curable resin were used for this study. Two kinds of composite (layered and mixed) were prepared, and the schemes for optical energy transfer between QD and phosphor were suggested and investigated based on PL decay characteristics. It was found that the layered structure is more effective than the mixed one with respect to PL intensity, PL decay, and thermal loss. When this layered nanocomposite (QDs on phosphor) is used to make a white light emitting diode (LED), the brightness is increased by 37%, and the color rendering index (CRI) value is raised to 88.4 compared to the mixed case of 80.4.

Electrical conductivity in quantum dot solids is crucial for application in devices. In addition to the well-known ligand exchange strategies for enhanced conductivity, the current study examined the optical, structural, and electrical properties of ethanedithiol-treated layer-by-layer (LbL) assembled quantum dot solid (QDS) films following low-temperature annealing (room temperature to 170 °C). As the annealing temperature increased, it was induced that the average separation between nanocrystal quantum dots is decreased, and accordingly, the overall conductivity of the QDS increased exponentially. From a simplified percolation model, the activation energy of temperature-dependent quantum dot attachment was estimated to be around 0.26−0.27 eV both for PbS and PbSe quantum dot solids. Furthermore, the results of this study indicated that device applications requiring higher conductivity, attainable through high-temperature annealing, may also require repassivation after annealing.