Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self‐trapping events and poor selectivity of the contacts, leading to large non‐radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by voltage‐dependent external quantum efficiency, as well as photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole‐extraction and improved electron blocking at the HTM interface.

As global warming caused by the greenhouse effect is becoming one of the major issues of the 21st century, hydrogen as an alternative to fossil-based fuels and other energy carriers has gained importance in current research. One promising approach to produce hydrogen is photoelectrochemical water splitting, which uses solar energy combined with suitable semiconducting photoabsorber electrodes to generate hydrogen and oxygen from water. However, most water splitting applications reported to date suffer from degradation of the photoabsorber, resulting in a loss of activity after just a few seconds or minutes. Here, a new approach using conformal ultra-thin and oxidation-stable protective layers is presented on Mo:BiVO 4 thin films combined with a thin Fe 0.1 Ni 0.9 O water oxidation co-catalyst, applied by electrochemical deposition, to achieve unprecedented photocurrent densities of up to 5.6 mA cm −2 under simulated AM1.5G illumination and a neutral pH while providing more stable electrodes for water oxidation.

A plasma‐enhanced atomic layer deposition (ALD) process is presented, capable of producing thin conformal films of nickel(II) oxide (NiO) on various substrates. Nickelocene (NiCp2) is used as an inexpensive metal precursor with oxygen plasma as the oxidant. The film growth rate saturates with both nickel precursor and plasma exposure. An ALD window is observed between 225 and 275 °C. Linear growth is achieved at 250 °C with a growth rate of 0.042 nm per cycle. The thickness is highly uniform and the surface roughness is below 1 nm rms for 52 nm thick films on Si(100). Substrates with aspect ratios up to 1:10 can be processed. As‐deposited, the films consist of polycrystalline, cubic NiO, and are transparent over the entire visible range with an optical bandgap of 3.7 eV. The films consist of stoichiometric NiO and contain ≈1% of carbon impurities. Two promising applications of these films are showcased in renewable energy conversion and storage devices: The films are pinhole‐free and exhibit excellent electron blocking capabilities, making them potential hole‐selective contact layers in solar cells. Also, high electrocatalytic activity of ultrathin NiO films is demonstrated for the alkaline oxygen evolution reaction, especially in electrolytes containing Fe3+.