Recently, perovskite solar cells have shown excellent performance under indoor light conditions. In a new approach using directional illumination combined with nanoscale scanning probe microscopy (SPM) characterization, morphology dependent‐charge transport measurements are performed to provide a comprehensive understanding of the optoelectronic behavior of (FAPbI3)0.85(MAPbBr3)0.15 containing 5 vol% cesium (Cs5vol%) with various electron transport layers (ETLs), i.e., SnO2, c‐TiO2, and [6,6]‐phenyl‐C61‐butyric acid methyl ester/SnO2 under indoor light. This approach allows the identification of the charge transport properties of the perovskite film and the perovskite/ETL interface separately. The light is applied from the top of the perovskite film to study the electronic properties of the surface. Lower photocurrent and lower surface photovoltage (SPV) are observed under top‐illumination conditions. The electronic interface behavior is investigated using bottom‐illumination and short excitation wavelengths, such as blue LED light. Higher photocurrent and higher SPV are observed under blue light illumination from the bottom. These results suggest that the charge transport capability is enhanced near the p–n junction. Conductive atomic force microscopy results show that SnO2 enhances the charge collection properties of the perovskite's grain boundaries (GBs). Kelvin probe force microscopy results confirm that SnO2 exhibits homogeneous and high surface potential because of the lowest trap states at GBs.
Mixed-halide perovskites (MHPs) have attracted attention as suitable wide-band-gap candidate materials for tandem applications owing to their facile band-gap tuning. However, when smaller bromide ions are incorporated into iodides to tune the band gap, photoinduced halide segregation occurs, which leads to voltage deficit and photoinstability. Here, we propose an original post-hot pressing (PHP) treatment that suppresses halide segregation in MHPs with a band gap of 2.0 eV. The PHP treatment reconstructs open-structured grain boundaries (GBs) as compact GBs through constrained grain growth in the in-plane direction, resulting in the inhibition of defect-mediated ion migration in GBs. The PHP-treated wide-band-gap (2.0 eV) MHP solar cells showed a high efficiency of over 11%, achieving an opencircuit voltage (V oc ) of 1.35 V and improving the maintenance of the initial efficiency under the working condition at AM 1.5G. The results reveal that the management of GBs is necessary to secure the stability of wide-band-gap MHP devices in terms of halide segregation.
Self-supported electrocatalysts are a new class of materials exhibiting high catalytic performance for various electrochemical processes and can be directly equipped in energy conversion devices. We present here, for the first time, sparse Au NPs self-supported on etched Ti (nanocarved Ti substrate self-supported with TiH) as promising catalysts for the electrochemical generation of hydrogen (H) in KOH solutions. Cleaned, as-polished Ti substrates were etched in highly concentrated sulfuric acid solutions without and with 0.1 M NHF at room temperature for 15 min. These two etching processes yielded a thin layer of TiH (the corrosion product of the etching process) self-supported on nanocarved Ti substrates with different morphologies. While F-free etching process led to formation of parallel channels (average width: 200 nm), where each channel consists of an array of rounded cavities (average width: 150 nm), etching in the presence of F yielded Ti surface carved with nanogrooves (average width: 100 nm) in parallel orientation. Au NPs were then grown in situ (self-supported) on such etched surfaces via immersion in a standard gold solution at room temperature without using stabilizers or reducing agents, producing Au NPs/TiH/nanostructured Ti catalysts. These materials were characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), grazing incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). GIXRD confirmed the formation of AuTi phase, thus referring to strong chemical interaction between the supported Au NPs and the substrate surface (also evidenced from XPS) as well as a titanium hydride phase of chemical composition TiH. Electrochemical measurements in 0.1 M KOH solution revealed outstanding hydrogen evolution reaction (HER) electrocatalytic activity for our synthesized catalysts, with Au NPs/TiH/nanogrooved Ti catalyst being the best one among them. It exhibited fast kinetics for the HER with onset potentials as low as -22 mV vs. RHE, high exchange current density of 0.7 mA cm, and a Tafel slope of 113 mV dec. These HER electrochemical kinetic parameters are very close to those measured here for a commercial Pt/C catalyst (onset potential: -20 mV, Tafel slope: 110 mV dec, and exchange current density: 0.75 mA cm). The high catalytic activity of these materials was attributed to the catalytic impacts of both TiH phase and self-supported Au NPs (active sites for the catalytic reduction of water to H), in addition to their nanostructured features which provide a large-surface area for the HER.
Titanium (Ti) resists corrosion due to spontaneous passivation and subsequent formation of a stable, substantially inert oxide film. This passive film limits the reducing ability of Ti in wet chemical synthesis. Aggressive anions are applied here to destabilize Ti passivity, with the objective of activating Ti as a reducing agent for wet-chemical synthesis, through the formation of unsupported and supported metallic nanoparticles (MNPs). For instance, within the first minute of Ti immersion in a standard gold solution (SGS), 1000 ± 3 μg Au/mL in 2% HCl, gold nanoparticles (AuNPs) in solution (unsupported) and a considerable population of highly dispersed AuNPs on Ti (Ti-supported AuNPs) are obtained. This occurs at room temperature, without using reducing agents, stabilizers, or any chemical pre-treatments. Electrochemical measurements revealed that the passivity of Ti was destabilized (oxide thinning/dissolution) in the SGS. The addition of F − to the SGS promoted destabilization of the passive film, and hence activated the reducing ability of Ti. This in turn resulted in significant increase in the population of AuNPs on Ti. Reduction of cations in solution by the active nascent (atomic) hydrogen (H) generated by substrate (Ti) dissolution, promoted by the addition of F − , in acidic medium is confirmed here and discussed vs. conductivity of the oxide (passive) film and solution pH. Experimental findings reveal that the relevance of H as the actual reducing agent is limited to instantaneous and non-conductive passive films. Solution pH studies on such passive films show that the Nernst equation (dE H/H + /d(pH) = −0.059 V) controls the wet chemical synthesis of MNPs rather than E o reduction of the base metal (the substrate). Based on polarization measurements, the prepared Ti-supported AuNPs are demonstrated here as highly active electrocatalysts (better than Pt) for the hydrogen evolution reaction (HER) in 0.1 M HCl solution.Nanoparticles constitute a key area of research and development in the burgeoning field of nanotechnology. The attraction of nanoparticles lies in the myriad of attractive characteristics which can be achieved by reducing suitable materials from the bulk to the nanometer size, these characteristics ranging from increased surface/volume ratio to novel quantum confinement effects. 1 Examples of property enhancements include magnetic, optical, biosensing, thermoelectric, semiconducting, catalytic, energy storage and thermal properties. 2-7 An interesting aspect of nanoparticles is the wide range of material classes in which nanoparticles are useful, including semiconductor, dielectric, metallic, ceramic, composite and polymer nanoparticles. For these reasons, metal nanoparticles have attracted a high interest in scientific research and industrial applications. [8][9][10] Metal nanoparticles (MNPs) can be produced by reducing metal salts with reducing agents and stabilizing them with capping ligands, 11 and they can be subsequently attached to another metal. However, it is more desirable to d...
Halide perovskites are currently intensively being investigated for applications in light-emitting diodes for next-generation lighting and display technology. A recent report shows that the control of triplet states is key to efficient light emission in layered quasi-2D perovskite devices. Unlike perovskite solar cells, the effect of spatial variations in the optoelectronic properties of perovskite light-emitting diodes on the overall device performance has scarcely been investigated. Here, we investigate the nanoscale electronic effects of triplet-state management in such materials using scanning probe microscopy under light illumination, in particular, Kelvin probe force microscopy, to study surface potential changes under light illumination. The recently found improvement in the efficiency of light emission can be seen in correlated contact potential differences at grains and grain boundaries under illumination. We also show that surface potential relaxation times after lighting changes depend on the dimensionality of the perovskite material and hole transfer from the perovskite inorganic lattice to the triplet energy level of the 2D spacer layer. Our findings shed new light on the design of halide perovskite-based LEDs and functional materials for improved performance.
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