Hybrid perovskite solar cells have been creating considerable excitement in the photovoltaic community. However, they still rely on toxic elements, which impose severe limits on their commercialization. Lead-free double hybrid perovskites in the form of Cs2AgBiBr6 have been shown to be a promising nontoxic and highly stable alternative. Nevertheless, device development is still in its infancy, and performance is affected by severe hysteresis. Here we realize for the first time hysteresis-free mesoporous double-perovskite solar cells with no s-shape in the device characteristic and increased device open-circuit voltage. This has been realized by fine-tuning the material deposition parameters, enabling the growth of a highly uniform and compact Cs2AgBiBr6, and by engineering the device interfaces by screening different molecular and polymeric hole-transporting materials. Our work represents a crucial step forward in lead-free double perovskites with significant potential for closing the gap for their market uptake.

We reveal that 2D/3D interfaces are dynamical in nature which is detrimental for long term perovskite solar cells stability.

Engineering 2D/3D perovskite interfaces is a common route to realizing efficient and stable perovskite solar cells. Whereas 2D perovskite’s main function in trap passivation has been identified and is confirmed here, little is known about its 2D/3D interface properties under thermal stress, despite being one of the main factors that induces device instability. In this work, we monitor the response of two typical 2D/3D interfaces under a thermal cycle by in situ X-ray scattering. We reveal that upon heating, the 2D crystalline structure undergoes a dynamical transformation into a mixed 2D/3D phase, keeping the 3D bulk underneath intact. The observed 3D bulk degradation into lead iodide is blocked, revealing the paramount role of 2D perovskite in engineering stable device interfaces.

Engineering two-dimensional (2D) / three-dimensional (3D) perovskites has emerged as an attractive route to efficient and durable perovskite solar cells. Beyond improving the surface stability of the 3D layer and acting as a trap passivation agent, the exact function of 2D/3D device interface remains vague. Here, we provide evidence that 2D/3D perovskite interface that forms a p-n junction is capable to reduce the electron density at the hole-transporting layer interface and ultimately suppress interfacial recombination. By a novel ultraviolet photoelectron spectroscopy (UPS) depth-profiling technique, we show that engineering of the 2D organic cations, in this case by simply varying the halide counter ions in thiophene methylammonium-salts, modifies the 2D/3D perovskite energy alignment. These measurements enable the true identification of the energetic across the 2D/3D interface, so far unclear. When integrated in solar cells, due to the electron blocking nature of the 2D layer, the optimized 2D/3D structures suppress the interfacial recombination losses, leading to opencircuit voltage (VOC) which approaches the potential internal Quasi-Fermi Level Splitting (QFLS) voltage of the perovskite absorber. The devices exhibit an improved fill factor (FF) driven by the enhanced hole extraction efficiency and reduced electron density at the 2D/3D interface. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems in order to surpass the current limitations of hybrid perovskite solar cell performances.Understanding and exploiting interfacial physics is key in perovskite solar cell engineering and optimization. 1,2 That is especially true when interface losses play a dominant role and complex interface functionalization is essential to minimize them. In the field of hybrid perovskite engineering, much attention has been lately focused on multi-dimensional perovskite interfaces consisting of a wider band gap layered (namely, two dimensional-2D) perovskite deposited between the bulk 3D perovskite and the hole transporting layer (HTL) in a standard mesoporous configuration. [3][4][5][6][7][8][9] Such configuration is currently among the most effective strategies to enhance both the efficiency and stability of perovskite solar cells. 3,10,11 It is generally considered that the 2D perovskite acts as both an efficient mean to passivate the surface traps (leading to reduced defect recombination) and an electron blocking layer due to its wider band gap. [12][13][14][15] However, despite these empirical observations, the energetic alignment at the interface and the relative function of the 2D/3D interface is only qualitatively depicted with a only a partial understanding of these