Green to Blue Light Emitting CsPbBr₃ Perovskite by Ligand Exchange and its Encapsulation by TiO₂ for Tandem Effect in Photovoltaic Applications

Inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite quantum dots (PQDs) have recently attracted tremendous attention for optoelectronic and photonic applications. Here in we demonstrate a facile ligand exchange (LE) method to tune the band structure of CsPbBr3 nanocrystals (NCs) together with encapsulation of these NCs within TiO2 (TiO2/LE-CsPbBr3), for photovoltaic and LED device applications. The binary ligand system of CsPbBr3PQDs is exchanged with a bifunctional ligand. Optical spectroscopy and fluorescence resonance energy transfer (FRET) study substantiate the ligand exchange and shift of band gap toward lower wavelength (blue shift) in LE-PQDs. Time dependent and aging studies indicate the LE to be a gradual process leading to a much stable PQDs. Reduction in particle size and retention of PQD structure on LE is evidenced by XRD and high resolution transmission electron microscopy (HRTEM). The stacking of encapsulated TiO2/LE-CsPbBr3 and TiO2/CsPbBr3 thus show a potency of tandem effect without any compromise in the individual properties of the two NCs. Band gap tuning through ligand exchange and its encapsulation is thus envisaged as an alternative to cause a tandem effect and is promising as a superior method over the established band gap tuning via halide composition, wherein the halide ion exchange and segregation rules out the option of tandem effect.