Scientists have developed a new method to improve the stability and efficiency of perovskite solar cells using hafnium oxide (HfOx).[1] Hafnium oxide was deposited using atomic layer deposition to stabilize the molecular layers at the interfaces of the solar cell, which are critical to its function.[1] At one interface, n-doped hafnium oxide promoted the binding of phosphonic acids and improved the heat resistance of the molecular layers.[1] At the second interface, p-doped hafnium oxide fixed organic passivators and prevented ion migration.[1] The solar cells achieved a power conversion efficiency of 27.1% (26.6% certified) and retained more than 90% of their original efficiency during approximately 5000 hours of operation at 85 °C in ambient air.[1] This technology opens up new possibilities for extending the lifetime of perovskite solar cells and improving their practical use.[1]