1.5–1.6 eV bandgap Pb‐based perovskite solar cells (PSCs) with 30–31% theoretical efficiency limit by the Shockley–Queisser model achieve 21–24% power conversion efficiencies (PCEs). However, the best PCEs of reported ideal‐bandgap (1.3–1.4 eV) Sn–Pb PSCs with a higher 33% theoretical efficiency limit are <18%, mainly because of their large open‐circuit voltage (Voc) deficits (>0.4 V). Herein, it is found that the addition of guanidinium bromide (GABr) can significantly improve the structural and photoelectric characteristics of ideal‐bandgap (≈1.34 eV) Sn–Pb perovskite films. GABr introduced in the perovskite films can efficiently reduce the high defect density caused by Sn2+ oxidation in the perovskite, which is favorable for facilitating hole transport, decreasing charge‐carrier recombination, and reducing the Voc deficit. Therefore, the best PCE of 20.63% with a certificated efficiency of 19.8% is achieved in 1.35 eV PSCs, along with a record small Voc deficit of 0.33 V, which is the highest PCE among all values reported to date for ideal‐bandgap Sn–Pb PSCs. Moreover, the GABr‐modified PSCs exhibit significantly improved environmental and thermal stability. This work represents a noteworthy step toward the fabrication of efficient and stable ideal‐bandgap PSCs.

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Redox-inactive Zr and Y introduced in BaCo0.4Fe0.4Zr0.1Y0.1O3−δ meliorated redox capability of active ions, so highly improving its OER activity and stability.

Exploring earth-abundant and cost-effective catalysts with high activity and stability for a hydrogen evolution reaction (HER) is of great importance to practical applications of alkaline water electrolysis. Here, we report on A-site Ba2+-deficiency doping as an effective strategy to enhance the electrochemical activity of BaCo0.4Fe0.4Zr0.1Y0.1O3−δ for HER, which is related to the formation of oxygen vacancies around active Co/Fe ions. By comparison with the benchmarking Ba0.5Sr0.5Co0.8Fe0.2O3−δ, one of the most spotlighted perovskite oxides, the Ba0.95Co0.4Fe0.4Zr0.1Y0.1O3−δ oxide has lower overpotential and smaller Tafel slope. Furthermore, the Ba0.95Co0.4Fe0.4Zr0.1Y0.1O3−δ catalyst is ultrastable in an alkaline solution. The enhanced HER performance originated from the increased active atoms adjacent to oxygen vacancies on the surface of the Ba0.95Co0.4Fe0.4Zr0.1Y0.1O3−δ catalyst induced by Ba2+-deficiency doping. The low-coordinated active atoms and adjacent oxygen ions may play the role of heterojunctions that synergistically facilitate the Volmer process and thus render stimulated HER catalytic activity. The preliminary results suggest that Ba2+-deficiency doping is a feasible method to tailor the physical and electrochemical properties of perovskite, and that Ba0.95Co0.4Fe0.4Zr0.1Y0.1O3−δ is a potential catalyst for HER.

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