Black-phase formamidinium lead iodide perovskite (FAPbI3), whilst the most promising species for efficient perovskite photovoltaics, is energetically unfavored at room temperature, and is thus always accompanied by undesirable yellow phases during crystallization 1,2,3,4 . The challenge to formulate the fast crystallization process of perovskite has limited the community in deriving unified guidelines for governing the formation of black-phase FAPbI3 5,6 . Here, through in-situ monitoring of the perovskite crystallization process, we report an oriented nucleation mechanism that acted as the key to avoid undesirable phases. This concept was applicable to improving the photovoltaic device performance under different film-processing scenarios. The small-area device demonstrated a power conversion efficiency of 25.4% (certified 25.0%), and the module (27.83 cm 2 ) achieved a champion aperture efficiency of 21.4% (certified).
MainFormamidinium lead iodide perovskite (FAPbI3) features desirable bandgap and thermal resistance, and has thus emerged as the most promising candidate among the perovskite family for highly efficient photovoltaic devices 1,2,3,7,8 . However, the photoactive black-phase FAPbI3 is not energetically favorable at room temperature 4,9,10 .Polytype formation and other intermediate non-photoactive phases can readily occur, which undermines the photovoltaic performance. A few approaches have been developed to promote the formation of black-phase FAPbI3 at room temperature, such as adduct formation with PbI2 and solvent engineering using ionic liquids 11,12 .
In this paper, a rapid, simple and highly sensitive method with dual-readout (colorimetric and fluorometric) based on the nanometal surface energy transfer (NSET) between nitrogen-doped carbon quantum dots (NCQDs) and gold nanoparticles (AuNPs) for detection of biothiols is described. Highly luminescent NCQDs were prepared via a simple one-step hydrothermal method by applying sucrose and glycine as carbon and nitrogen sources. The results showed the obtained NCQDs had an average particle diameter of 5 nm and highly luminescent. The maximum emission wavelength was 438 nm with an excitation wavelength of 360 nm. In this system, NCQDs and AuNPs were respectively treated as energy donors and energy acceptors, which enable the nanometal surface energy transfer (NSET) from the NCQDs to the AuNPs, quenching the fluorescence. However, biothiols was used as a competitor in the NSET by the strongly Au-S bonding to release NCQDs from the Au surface, which subsequently produces fluorescent signal recovery and the red-to-purple color change quickly. This probe showed rapid response, high selectivity and sensitivity for biothiols with dual colorimetric and fluorescent turn-on signal changes. The low detection limit was calculated as 20 nM by using L-cysteine acted as target melocules. The method was also successfully applied to the determination of biothiols in human serum samples, and the results were satisfying.
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