Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >23%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off.

Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on energy and charge transfer dynamics, as well as charge recombination and trapping processes happening over a time scale from fs to ms. As shown in Fig.1, when 2D perovskite is used on top of the 3D, charge transfer happens, while electron hole recombination at the perovskite/hole transporter interface is prevented. This results in improved device efficiency. In concomitance, the stable 2D perovskite is used as a sheath to physically protect the 3D underneath, with the aim to enhance the device stability. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.