Reducing interface recombination boosts the V _oc for perovskite solar cells.

Opening charge transport pathways: In perovskite solar cells, the insulating nature of passivation layers needed to boost open-circuit voltage also increases the series resistance of the cell and limits the fill factor. Most improvements in power conversion efficiency have come from higher open-circuit voltage, with most fill factor improvements reported for very small-area cells. Peng et al. used a nanostructured titanium oxide electron transport layer to boost the fill factor of larger-area cells (1 square centimeter) to 0.84 by creating local regions with high conductivity. Science , this issue p. 390

The trade-off between activity and stability of oxygen evolution reaction (OER) catalysts in proton exchange membrane water electrolyzer (PEMWE) is challenging. Crystalline IrO ₂ displays good stability but exhibits poor activity; amorphous IrO _x exhibits outstanding activity while sacrificing stability. Here, we combine the advantages of these two materials via a lattice water–incorporated iridium oxide (IrO _x · n H ₂ O) that has short-range ordered structure of hollandite-like framework. We confirm that IrO _x · n H ₂ O exhibits boosted activity and ultrahigh stability of >5700 hours (~8 months) with a record-high stability number of 1.9 × 10 ⁷ n _oxygen n _Ir ⁻¹ . We evidence that lattice water is active oxygen species in sustainable and rapid oxygen exchange. The lattice water–assisted modified OER mechanism contributes to improved activity and concurrent stability with no apparent structural degradation, which is different to the conventional adsorbate evolution mechanism and lattice oxygen mechanism. We demonstrate that a high-performance PEMWE with IrO _x · n H ₂ O as anode electrocatalyst delivers a cell voltage of 1.77 V at 1 A cm ⁻² for 600 hours (60°C).