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Lead-based halide perovskites have been widely used as efficient energy materials due to their superior optoelectronic properties and mixed electronic−ionic conductivity. However, lead toxicity has been one of the key challenges for commercialization. Recently Cs 2 AgBiBr 6 , a lead-free double perovskite, has garnered significant interest due to its exceptional stability, nontoxic nature, and promising optoelectronic properties. But because of the low electronic and ionic conductivity of bismuth-based double perovskites, there is a challenge for their use in energy storage applications. To resolve this issue, we have incorporated carbon black and a conducting polymer poly(2,3-dihydrothieno-1,4-dioxin)-poly (styrene sulfonate) (PEDOT:PSS) as electronic and ionic conductivity agents respectively into the Cs 2 AgBiBr 6 porous electrode. This ternary composite exhibits over 40% enhancement in specific capacitance as well as specific energy density compared with a binary composite of carbon black with a perovskite electrode. There is no significant change in the power density. However, only PEDOT:PSS as charge transporting materials in perovskite matrix results in lower energy density and power density despite lower charge transfer resistance (R ct ) at the electrode/electrolyte interface and higher dc ionic conductivity compared to perovskite/carbon composite electrodes. From the electrochemical impedance spectroscopy analysis, it is evident that balanced ionic and electronic conductivities are necessary to achieve optimal performance in lead-free perovskite-based supercapacitors. We also fabricated a solid-state symmetric supercapacitor using a quasi-solid-state gel electrolyte.

Highly fluorescent cesium lead‐based (CsPbX3, X═Br, Cl, I) inorganic metal halide perovskites semiconductors have gained immense popularity in the last decade due to the economic and straightforward fabrication techniques involved in these materials along with their excellent electrical and optoelectronic properties. Cesium lead halide nanocrystals are well known for their fluorescence in the visible region with extremely high internal quantum efficiencies; thus making them highly suitable for the fabrication of efficient light‐emitting diodes, transistors and photodetectors. Although perovskite nanocrystals (NCs) are more fluorescent compared to their bulk counterpart, there have been very few reports on the synthesis and characterization of CsPbX3 perovskite NCs. In this work, we have synthesized and investigated the CsPbBr3 and CsPbBr2I NCs to understand the fundamental optoelectronic properties and structural integrity in mixed halide perovskite NCs. We have estimated ~10 nm average particle size of CsPbBr3 nanocrystals from the high‐resolution transmission electron microscopy (HRTEM) while CsPbBr2I has ~16 nm average particle size with slightly higher polydispersity. Most interestingly, we do not observe any phase segregation of bromide and iodide ions in mixed halide perovskite quantum dots due to finite size effect. This is also confirmed by the energy dispersive X‐ray spectroscopy (EDS) mapping data. However, CsPbBr3 nanocrystals are relatively more stable than the mixed halide perovskite nanocrystals due to fewer defects. Anomalous behavior is observed in the photoluminescence intensity with the variation of precursor concentration indicating a complex nature nanoparticle synthesis.

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