The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co((II/III))tris(bipyridyl)-based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global 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:

Metal halide perovskite solar cells (PSCs) currently attract enormous research interest because of their high solar-to-electric power conversion efficiency (PCE) and low fabrication costs, but their practical development is hampered by difficulties in achieving high performance with large-size devices. We devised a simple vacuum flash-assisted solution processing method to obtain shiny, smooth, crystalline perovskite films of high electronic quality over large areas. This enabled us to fabricate solar cells with an aperture area exceeding 1 square centimeter, a maximum efficiency of 20.5%, and a certified PCE of 19.6%. By contrast, the best certified PCE to date is 15.6% for PSCs of similar size. We demonstrate that the reproducibility of the method is excellent and that the cells show virtually no hysteresis. Our approach enables the realization of highly efficient large-area PSCs for practical deployment.

A mixture of CsPbI3 and FAPbI3 is thermodynamically stabilized in the perovskite phase with respect to the pure δ phases.