No abstract

Synthesis of a size series of colloidal ZnTe/ZnSe (core/shell) quantum dots (QDs) is reported. Because of the unique Type-II characters, their emission can range over an extended wavelength regime, showing photoluminescence (PL) from blue to amber. The PL lifetime measures as long as 77 ns, which clearly indicates the Type-II characteristics. ZnTe/ZnSe (Core/Shell) QDs can be further passivated by ZnS layers, rendered in water, while preserving the optical and chemical stabilities and thus proved their potentials toward “nontoxic” biological or medical applications that are free from concerns regarding heavy-metal leakage. ZnTe/ZnSe Type-II QD/polymer hybrid organic solar cells are also showcased, promising environmentally friendly photovoltaic devices. ZnTe/ZnSe Type-II QD incorporated photovoltaic devices show 11 times higher power conversion efficiency, when compared to that of the control ZnSe QD devices. This results from the Type-II characteristic broad QD absorption up to extended wavelengths and the spatially separated Type-II excitons, which can enhance the carrier extractions. We believe that ZnTe/ZnSe-based Type-II band engineering can open many new possibilities as exploiting the safe material choice.

A model semiconductor-sensitizer layer of CdSe with under- or overlayers of CdS or ZnS by pre- or postadsorption was prepared on the surface of mesoporous TiO2 films by a series of successive ionic layer adsorption and reaction (SILAR) processes in solutions containing corresponding cations and anions. The growth of each semiconductor layer was monitored by taking UV−visible absorption spectra and high-resolution transmission electron microscopy (TEM) images. The all SILAR-prepared multicomponent sensitizer consisting of CdS/CdSe/ZnS layers was evaluated in a polysulfide electrolyte solution as a redox mediator in regenerative photoelectrochemical cells. The CdS and ZnS layers with the CdSe layer sandwiched in between were found to significantly enhance photocurrents. The best photovoltaic performance was obtained from the CdS/CdSe/ZnS-sensitizer with the ZnS layer on the top, yielding an overall power conversion efficiency of 3.44% with a mask around the active film and 3.90% with no mask. The effect of the mask on short-circuit current (J sc) and overall efficiency (η) measurements was shown to be increasingly critical in semiconductor-sensitized solar cells as they exhibit high photocurrents. The polysulfide electrolyte, which acted as an effective electron transfer mediator for CdS and/or CdSe sensitizers, was not as effective for PbS-based sensitizers prepared by the same SILAR process.

We investigated the temperature-dependent photoluminescence (PL) properties of colloidal CsPbX 3 (X = Br, I, and mixed Br/I) quantum dot (QD) samples in the 30−290 K temperature range. Temperature-dependent PL experiments reveal thermal quenching of PL, blue shifting of optical band gaps, and line width broadening for all CsPbX 3 QD samples with increasing temperature. Interestingly, side-peak emissions that are spectrally separated from the excitonic PL peaks were observed for both CsPbBr 3 and CsPb(Br/I) 3 QD samples at temperatures below ∼250 K. The side-peak emission for the CsPbBr 3 QD sample is located at a lower energy compared to the band-edge peak, whereas that of the Br-rich CsPb(Br/I) 3 alloy QD sample is located at a higher energy than that of the band-edge peak. We found that the CsPbBr 3 QDs have two emissive states, a band-edge state, and one involving shallow defects, which can be spectrally separated by narrowing the emission line widths at low temperature. In the case of the Br-rich CsPb(Br/I) 3 QD sample, the partial halide-segregation-induced heterogeneity of the alloy phase within the ensemble at low temperature leads to blue-shifted radiative recombination channels.

Methoxy‐functionalized triphenylamine‐imidazole derivatives that can simultaneously work as hole transport materials (HTMs) and interface‐modifiers are designed for high‐performance and stable perovskite solar cells (PSCs). Satisfying the fundamental electrical and optical properties as HTMs of p‐i‐n planar PSCs, their energy levels can be further tuned by the number of methoxy units for better alignment with those of perovskite, leading to efficient hole extraction. Moreover, when they are introduced between perovskite photoabsorber and low‐temperature solution‐processed NiO x interlayer, widely featured as an inorganic HTM but known to be vulnerable to interfacial defect generation and poor contact formation with perovskite, nitrogen and oxygen atoms in those organic molecules are found to work as Lewis bases that can passivate undercoordinated ion‐induced defects in the perovskite and NiO x layers inducing carrier recombination, and the improved interfaces are also beneficial to enhance the crystallinity of perovskite. The formation of Lewis adducts is directly observed by IR, Raman, and X‐ray photoelectron spectroscopy, and improved charge extraction and reduced recombination kinetics are confirmed by time‐resolved photoluminescence and transient photovoltage experiments. Moreover, UV‐blocking ability of the organic HTMs, the ameliorated interfacial property, and the improved crystallinity of perovskite significantly enhance the stability of PSCs under constant UV illumination in air without encapsulation.

Temperature dependent photoluminescence (PL) spectroscopy in a range of 5 K to room temperature (RT, 290 K) and single dot blinking behavior were investigated for CdTe/CdSe (core/shell, C/S) quantum dots (QDs). The QDs show type-II characteristics as both of the valence and conduction band levels of the CdTe core are placed higher in energy than those of the CdSe shell. The thickness of the CdSe shell was varied to control the degree of type-II character, and bare CdTe QDs were used as controls. The CdTe/CdSe (C/S) QDs have unique PL properties including (i) high susceptibility to PL thermal quenching with an exciton dissociation energy as small as 18 meV, compared with 46 meV for the CdTe QD, (ii) smaller band gap change showing only half the reduction of the control within the temperature change, and (iii) up to 27% larger PL bandwidth broadening than the control. The unique temperature-dependent properties were enhanced as the type-II character was increased by the thicker CdSe shell. Single dot level PL intermittency characteristics were studied for quasi type-II CdTe/CdSe (C/S) QDs that have alloyed layers at the core-shell interface. The quasi type-II QDs exhibited more frequent PL intensity intermittence blinking on and off at 290 K when compared with the CdTe QDs. However, the blinking kinetics follows similar universal power law on/off probability distributions with the R on and R off exponents evaluated as 1.57 and 1.38, respectively.

Photoexcited electron extraction from semiconductors can be useful for converting solar energy into useful forms of energy. Although InP quantum dots (QDs) are considered alternative materials for solar energy conversion, the inherent instability of bare InP QDs demands the use of passivation layers such as ZnS for practical applications, which impedes carrier extraction from the QDs. Here, we demonstrate that Cu-doped InP/ZnS (InP/Cu:ZnS) QDs improve the electron transfer ability due to hole capture by Cu dopants. Steady-state and time-resolved photoluminescence studies confirmed that electrons were effectively transferred from the InP/Cu:ZnS QDs to a benzoquinone acceptor by retarding the electron–hole recombination within the QD. We evaluated the photocatalytic H2 evolution performance of InP/Cu:ZnS QDs under visible light, which showed outstanding photocatalytic H2 evolution activity and stability under visible light illumination. The photocatalytic activity was preserved even in the absence of a cocatalyst.

True colors: High-quality InP and InP/ZnS quantum dots (QDs) are obtained by means of a simple one-pot method in the presence of polyethylene glycol (PEG). Rapid and size-controlled reactions lead to highly crystalline and nearly monodisperse QDs at relatively low temperatures. The particles emit from cyan blue to far-red, and are successfully used in cellular imaging (see figure).