The inorganic lead-free Cs 2 AgBiBr 6 double perovskite structure is the promising development direction in perovskite solar cells (PSCs) to solve the problem of the instability of the APbX 3 structure and lead toxicity. However, the low short-circuit current and power conversion efficiency (PCE) caused by the low crystallization of Cs 2 AgBiBr 6 greatly limit the optoelectronic application. Herein, we adopt a simple strategy to dope singlelayered MXene nanosheets into titania (Ti 3 C 2 T x @TiO 2 ) as a multifunctional electron transport layer for stable and efficient Cs 2 AgBiBr 6 double PSCs. The single-layered MXene nanosheets significantly improve the electrical conductivity and electron extraction rate of TiO 2 ; meanwhile, the single-layered MXene nanosheets change the surface wettability of the electron transport layer and promote the crystallization of the Cs 2 AgBiBr 6 double perovskite in solar cell devices. Therefore, the PCE went up by more than 40% to 2.81% compared to that of a TiO 2 based device, and the hysteresis was greatly suppressed. Furthermore, the device based on Ti 3 C 2 T x @TiO 2 showed the long-term operating stability. After storing the device for 15 days under ambient air conditions, the PCE still remained a retention rate of 93% of the initial one. Our finding demonstrates the potential of Ti 3 C 2 T x @TiO 2 in electron transfer material of high-performance double PSCs.

Photocatalytic reduction of carbon dioxide (CO 2 ) into high-value chemicals is a very effective way to solve the greenhouse effect, improve the utilization ratio of resources, and cope with the energy crisis. However, the low catalytic activity and poor product selectivity of the catalyst have been largely restricting its large-scale application. Herein, we successfully synthesized an ultra-thin two-dimensional trimetallic metal− organic framework (NiZrCu-BDC) nanosheet as a photocatalyst for CO 2 reduction, and the average thickness of NiZrCu-BDC is about 4 nm. The NiZrCu-BDC nanosheet has the ability to reduce CO 2 to methanol (41.05 μmol h −1 g −1 ) and ethanol (36.62 μmol h −1 g −1 ), and the turnover frequency of NiZrCu-BDC is 34 times more than that of NiZr-BDC. Zr and Cu doping enables enrichment of Ni surface charges to promote CO 2 chemisorption, and the ultra-thin structure can shorten the electron transport path. Meanwhile, the electron density of Ni catalytical sites in NiZrCu-BDC is enhanced by doping Cu and Zr to facilitate COOH* and CHO formation, which are deemed as key species for CO 2 reduction reactions to liquid products. This work provides further insights into the photocatalytic reduction of CO 2 based on the multi-metal−organic framework.

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