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Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in a perovskite film by the use of time-resolved terahertz spectroscopy. Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant.

Stomatopods are shallow-water crustaceans that employ powerful dactyl appendages to hunt their prey. Deployed at high velocities, these hammer-like clubs or spear-like devices are able to inflict substantial impact forces. Here we demonstrate that dactyl impact surfaces consist of a finely-tuned mineral gradient, with fluorapatite substituting amorphous apatite towards the outer surface. Raman spectroscopy measurements show that calcium sulphate, previously not reported in mechanically active biotools, is co-localized with fluorapatite. Ab initio computations suggest that fluorapatite/calcium sulphate interfaces provide binding stability and promote the disordered-to-ordered transition of fluorapatite. Nanomechanical measurements show that fluorapatite crystalline orientation correlates with an anisotropic stiffness response and indicate significant differences in the fracture tolerance between the two types of appendages. Our findings shed new light on the crystallochemical and microstructural strategies allowing these intriguing biotools to optimize impact forces, providing physicochemical information that could be translated towards the synthesis of impact-resistant functional materials and coatings.

This paper reports on highly active and stable amorphous Pd nanoparticle electrocatalysts for the oxygen reduction reaction. The amorphous catalysts were synthesized by a remarkably facile and quick electroless deposition process newly developed in this study (process time <5 min). An electrode substrate (glassy carbon, carbon cloth) was sequentially dipped for 10 s into two separate solutions of a reducing agent (sodium hypophosphite (NaH2PO2)) and Pd ions to deposit amorphous Pd nanoparticles containing phosphorus (Pd-P). By repeating the deposition cycles, the specific and mass activities of the Pd nanoparticles can be actively tuned. Owing to the nanoscale amorphous nature, the obtained Pd-P nanoparticle electrocatalysts exhibited superior specific and mass activities compared with crystalline Pd nanoparticles synthesized by another reducing agent (N2H4) and commercial Pt-loaded carbon (Pt/C) and Pd-loaded carbon (Pd/C). The specific and mass activities of the amorphous Pd-P nanoparticles were over 4.5 times and 2.6 times higher than previously reported values of Pd and Pt catalysts.