We fabricated a perovskite solar cell that uses a double layer of mesoporous TiO2 and ZrO2 as a scaffold infiltrated with perovskite and does not require a hole-conducting layer. The perovskite was produced by drop-casting a solution of PbI2, methylammonium (MA) iodide, and 5-ammoniumvaleric acid (5-AVA) iodide through a porous carbon film. The 5-AVA templating created mixed-cation perovskite (5-AVA)x(MA)1- xPbI3 crystals with lower defect concentration and better pore filling as well as more complete contact with the TiO2 scaffold, resulting in a longer exciton lifetime and a higher quantum yield for photoinduced charge separation as compared to MAPbI3. The cell achieved a certified power conversion efficiency of 12.8% and was stable for >1000 hours in ambient air under full sunlight.

TiO 2 /FTO, the cross sectional scanning electron microscopy (SEM) image of which is shown in Fig. 1a. Employing this solar cell configuration, we achieve the best PCE of 21.6% with a PMMA concentration C PMMA =0.6mg/ml. The photovoltaic metrics of the device are as follows: short-circuit current density (J SC ) = 23.7 mA cm -2 , open circuit voltage (V OC ) = 1.14 V, and a fill factor (FF) = 0.78 (Fig. 1b). One of the devices was sent for certification to Newport Corporation PV Lab, an accredited PV testing laboratory, confirming a PCE of 21.02% (supplementary Fig.1 Fig. 3, Tab. 2a and 2b). The photovoltaic metrics of the PSCs fabricated from PTNG method varying C PMMA are summarized in Fig. 1c.Upon increasing the PMMA concentrations (C PMMA , mg/ml) in mixed chlorobenzene and toluene (volume ratio of cholorobenzene and toluene is 9:1) solution, the PCE first augments and subsequently decreases with a peak at C PMMA = 0.6. Upon increasing the C PMMA from 0 to 0.6 mg/ml, the V oc rises from 1.10V to 1.14 V, and the FF from 0.74 to 0.78. To further examine the crystal structure, we conducted thin layer X-ray diffraction (XRD) measurements for perovskite films deposited on m-TiO 2 /blTiO 2 /FTO substrates (Fig.3a). The peak at 12.5 o arises from the (001) lattice planes of hexagonal (2H polytype) PbI 2. The excess PbI 2 is believed to passivate surface defects, increasing the solar cell performance 13 . All the samples show the same trigonal perovskite phase with the dominant (111) lattice reflection. We speculate the (111) plane to exhibit the smallest surface energy because the majority of grains exhibit this orientation to minimize the total Gibbs free energy of the system. With increasing C PMMA , the (111) oriented grains grew faster by consuming neighboring non-oriented crystals, either by regular grain growth or grain attachment, as evidenced by the increased ratio between the (111) and (123) peak at 13.9 ° and 31.5° in the presence of PMMA. By taking the full width at half maximum (FWHM) of the (111) reflection, we calculate the crystallite size using Scherrer's equation. Their dimension increases from 41 to 55, 70, 94, and 112 nm by increasing C PMMA from 0 to 4mg/ml.We attribute the larger crystal sizes to the templating effect of PMMA on the crystal The X-ray photoelectron spectroscopy (XPS) in Fig. 3d shows Pb 4f spectra.There are two main peaks Pb 4f7/2 and Pb 4f5/2 at 138 and 142.8 eV, respectively.We attribute the two small peaks located at 136.4 and 141.3 eV to the presence of In conclusion, we introduce a new method for preparing high-electronic quality perovskite films and implement it for the fabrication of PSC with excellent performance their certified PCE attaining 21 %. Further development of the method will enable further performance gains. Acknowledgement:

Federated learning (FL) is a machine learning setting where many clients (e.g. mobile devices or whole organizations) collaboratively train a model under the orchestration of a central server (e.g. service provider), while keeping the training data decentralized. FL embodies the principles of focused data collection and minimization, and can mitigate many of the systemic privacy risks and costs resulting from traditional, centralized machine learning and data science approaches. Motivated by the explosive growth in FL research, this paper discusses recent advances and presents an extensive collection of open problems and challenges.

In the past few years, organic-inorganic halide perovskites have rapidly emerged as promising materials for photovoltaic applications, but simultaneously achieving high performance and long-term stability has proved challenging. Here, we show a one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient perovskite solar cells with enhanced stability. We modify the surface of methylammonium lead triiodide (CH3NH3PbI3) perovskite by spin-coating its precursor solution in the presence of butylphosphonic acid 4-ammonium chloride. Morphological, structural and elemental analyses show that the phosphonic acid ammonium additive acts as a crosslink between neighbouring grains in the perovskite structure, through strong hydrogen bonding of the -PO(OH)2 and -NH3(+) terminal groups to the perovskite surface. The additives facilitate the incorporation of the perovskite within a mesoporous TiO2 scaffold, as well as the growth of a uniform perovskite layer at the surface, enhancing the material's photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture.