While silicon (Si) photovoltaics has almost reached its maximum theoretical efficiency, perovskite solar cells (PSC) have emerged in the last decade as a new generation of photovoltaics, with a record efficiency of 26.1%, almost matching that of Si cells [1]. Taking advantage of both silicium mature technology and perovskite versability (tunable bandgap, ease of fabrication and low cost), PSC/Si tandem based on a silicon sub-cell and a perovskite top-cell have the potential to harvest more than
40% of incoming photons [2]. However, if for the top-cell the inversed architecture (p-i-n) has been identify as substantially more stable than its n-i-p counterpart [3], it still suffers from poor stability compared to the silicon technology, which has so far hindered their commercialization both as single junction and as top-cells in tandems [4]. Recombination at interfaces having been identified as a bottleneck for both long term efficiency and stability, strategies based on the integration of low
dimensional perovskite (displaying higher intrinsic stability) at interfaces have been proposed [5]. Alternatively, the integration of Lithium fluoride (LiF) at the perovskite/ electron transport layer interface (typically C60/SnO2) enables for field effect passivation, however so far at the cost of higher defect densities [6]. We expect both strategies to be highly complementary, as low dimensional perovskite integrated at the perovskite/C60 interface not only passivates the surface but protects 3D perovskite from the subsequent evaporation steps (LiF and C60).

The low dimensional layer (quasi-2D perovskite) is formed at the top surface of 3D perovskite through  he spin coating of bulky ammonium cations on top of 3D perovskite. First, several promising candidates were identified, namely phenethylammonium iodide (PEAI), bromide and chloride (PEABr, PEACl), 4-Fluoro-Phenethylammonium iodide (4F-PEAI) and butylammonium bromide and iodide (BABr, BAI). Solvent engineering strategies as well as the introduction of additives (MASCN, MAI, ethylene diamine) were studied in order to induce the recrystallization of the top surface of 3D
perovskite upon 2D cations addition and control the dimensionality of the quasi-2D perovskite. The quasi-2D layer was systematically characterized by SEM, KPFM, XRD and photoluminescence (PL), revealing major differences depending on the 2D cation used and the deposition strategy. Relying on PL results and I-V characterization, we identified a mixture of BAI and BABr as the most promising candidate to passivate the perovskite surface and boost the efficiency of semi-transparent p-i-n perovskite top cells, with a +0.1V enhancement in VOC.
3D perovskite has been shown to be sensitive to LiF deposition [6]. We hypothesize that the thin layer of quasi-2D perovskite formed at the perovskite top surface could protect the active material from the subsequent evaporation steps and reduce trap density at the perovskite/C60 interface. An important stability study aiming at comparing reference cells (no quasi-2D perovskite, no LiF), cells with LiF and cells with the dual passivation (quasi-2D perovskite and LiF), is planned for the incoming months based on ISOS protocol tests. We intent to demonstrate that the integration of both materials is highly complementary and could enable to take fully advantage of LiF field effect passivation