There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Cesium titanium(IV) halide perovskites (HPs) are promising all-inorganic, Pb-free materials for perovskite solar cells (PSCs). Here we show that high-quality, uniform thin films of Cs 2 TiBr 6 HP can be prepared through a facile low-temperature vaporbased method. These thin films exhibit a favorable bandgap of 1.8 eV, long and balanced carrier-diffusion lengths >100 nm, suitable energy levels, and superior intrinsic and environmental stability. The first planar-heterojunction PSCs based on Cs 2 TiBr 6 thin films show a stable efficiency of up to 3.3%.

Framework nitrogen atoms of carbon nitride (C3N4) can coordinate with and activate metal sites for catalysis. In this study, C3N4 was employed to harvest visible light and activate Co2+ sites, without the use of additional ligands, in photochemical CO2 reduction. Photocatalysts containing single Co2+ sites on C3N4 were prepared by a simple deposition method and demonstrated excellent activity and product selectivity toward CO formation. A turnover number of more than 200 was obtained for CO production using the synthesized photocatalyst under visible-light irradiation. Inactive cobalt oxides formed at relatively high cobalt loadings but did not alter product selectivity. Further studies with X-ray absorption spectroscopy confirmed the presence of single Co2+ sites on C3N4 and their important role in achieving selective CO2 reduction.

Tin-based halide perovskite solar cells (PSCs) hold the most promise among lead-free PSCs, but they are plagued with inadequate environmental stability and power-conversion efficiency (PCE). Here we demonstrate that the optimum incorporation of a bulky divalent organic cation, 4-(aminomethyl)­piperidinium (4AMP), in FASnI3 thin films improves stability, optoelectronic properties, and PSC performance. The optimized PSC yields a maximum PCE of 10.9% and good 500-h operational stability under continuous illumination. This is attributed to the unique thin-film structure, where the strong ionic bonding afforded by divalent 4AMP may provide near-full-coverage functionalization (encapsulation) of FASnI3 grain surfaces and grain boundaries, retarding O2/H2O ingression and mitigating Sn-defects for reduced photocarrier nonradiative recombination.

Dispersed atomic catalysts can achieve high catalytic efficiency and have the potential to enable chemical transformation of inert molecules like CO 2 . The effect of surface defects on photocatalytic reduction of CO 2 using supported single atom catalysts however requires clarification. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (O v ) and photoexcited electrons on supported single atom Cu catalysts and CO 2 reduction. Adsorption of Cu was strong to the TiO 2 surface, and charges of the Cu atoms were highly dependent on whether surface defects were present. Cu atoms with O v aided in the adsorption of activated bent CO 2 , which is key to CO 2 reduction. Our results also show that CO 2 dissociation (CO 2 * → CO* + O*), which is a proposed initial step of CO 2 reduction to hydrocarbon products, occurs very readily for a single Cu atom in an O v , with barriers of ∼0.19 eV. Such low barriers do not occur with Cu over a stoichiometric surface. Furthermore, the presence of a photoexcited electron leads to a substantial increase in reaction rate for Cu over a stoichiometric surface; the Cu/TiO 2 surface is largely inert in the absence of photoexcited electrons. Experimental results corroborate these theoretical calculations and show that activation of CO 2 occurs most readily for TiO 2 catalysts with dispersed Cu and O v . CO 2 photoreduction also occurs most readily for TiO 2 catalysts with dispersed Cu and O v , compared to TiO 2 or Cu over stoichiometric TiO 2 catalysts. We also modeled atomic Pt to understand how metals besides Cu may behave. We found that Pt over TiO 2 also activates CO 2 but that dissociation of CO 2 over Pt with O v does not occur as readily as for Cu with O v . Our results show that tailoring TiO 2 surfaces with defects in conjunction with specific atomic catalysts like Cu may lead to fast desirable photoreduction of CO 2 .