All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb) can be embedded into a "cation cascade" to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.

Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu 2 O/Ga 2 O 3-buried p-n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over + 1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm −2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO 2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu 2 O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO 4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency.

Three-dimensional (3D) perovskite materials display remarkable potential in photovoltaics owing to their superior solar-to-electric power conversion efficiency, with current efforts focused on improving stability. Two-dimensional (2D) perovskite analogues feature greater stability toward environmental factors, such as moisture, owing to a hydrophobic organic cation that acts as a spacer between the inorganic layers, which offers a significant advantage over their comparatively less stable 3D analogues. Here, we demonstrate the first example of a formamidinium (FA) containing Dion–Jacobson 2D perovskite material characterized by the BFA n–1Pb n I3n+1 formulation through employing a novel bifunctional organic spacer (B), namely 1,4-phenylenedimethanammonium (PDMA). We achieve remarkable efficiencies exceeding 7% for (PDMA)­FA2Pb3I10 based 2D perovskite solar cells resisting degradation when exposed to humid ambient air, which opens up new avenues in the design of stable perovskite materials.

Optimizing the morphology of metal halide perovskite films is an important way to improve the performance of solar cells when these materials are used as light harvesters, because film homogeneity is correlated with photovoltaic performance. Many device architectures and processing techniques have been explored with the aim of achieving high-performance devices, including single-step deposition, sequential deposition and anti-solvent methods. Earlier studies have looked at the influence of reaction conditions on film quality, such as the concentration of the reactants and the reaction temperature. However, the precise mechanism of the reaction and the main factors that govern it are poorly understood. The consequent lack of control is the main reason for the large variability observed in perovskite morphology and the related solar-cell performance. Here we show that light has a strong influence on the rate of perovskite formation and on film morphology in both of the main deposition methods currently used: sequential deposition and the anti-solvent method. We study the reaction of a metal halide (lead iodide) with an organic compound (methylammonium iodide) using confocal laser scanning fluorescence microscopy and scanning electron microscopy. The lead iodide crystallizes before the intercalation of methylammonium iodide commences, producing the methylammonium lead iodide perovskite. We find that the formation of perovskite via such a sequential deposition is much accelerated by light. The influence of light on morphology is reflected in a doubling of solar-cell efficiency. Conversely, using the anti-solvent method to form methyl ammonium lead iodide perovskite in a single step from the same starting materials, we find that the best photovoltaic performance is obtained when films are produced in the dark. The discovery of light-activated crystallization not only identifies a previously unknown source of variability in opto-electronic properties, but also opens up new ways of tuning morphology and structuring perovskites for various applications.

Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO2 scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)2](bis(trifluoromethylsulfonyl)imide)2 and [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)2](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 μm-thick mesoscopic TiO2 scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO2 and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 μs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.