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

Abstract

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.

High Optical Performance of Cyan‐Emissive CsPbBr3 Perovskite Quantum Dots Embedded in Molecular Organogels

“…Simultaneously, the emission peak position of Hx‐PQDs shows a slight blueshift along the time, starting at 490 nm and moving to 480 nm. Unlike the ligand exchange process, where it was described the substitution of OA and OLEA with another capping ligands, [ 20 ] we associate this phenomenon to the gradual ligand passivation provided by Hx matrix, since some OA and OLEA ligands are removed from PQDs surface after purification step. Thus, more active sites are available for Hx to interact with the PQDs surface.…”

“…This fact induces a better uniformity and monodispersity of PQDs, which could slightly contribute to the blueshift in the PL. [ 20,28 ] Conversely, the bigger PQDs size and wider distribution obtained in presence of Chx are associated to the formation of agglomerates, generating lower uniformity in size and causing a redshift in the PL. These effects are also observed in the absorption measurements of the different samples, as shown in Figure S4 (Supporting Information).…”

“…As mentioned before, it has been reported the use of amino acids such as α‐amino butyric acid after the synthesis of CsPbBr 3 PQDs, and the impact on their photophysical properties. [ 20 ] Because of the bifunctional structure of the α‐amino butyric acid, it is proposed a ligand exchange process with conventional OA and OLEA, causing better surface passivation, and thereby the enhancement in the optical performances. This fact includes a blueshift in the PL peak, indication of the surface modification with a short‐capping ligand, also mediating the decrease of PQDs size.…”

“…[ 18,19 ] A recent work has reported a blueshift in the emission of CsPbBr 3 PQDs after a ligand exchange and this blueshift was explained by the reduction of particle size. [ 20 ] In addition, PQDs have been also embedded in some interesting matrices such as SiO 2 , [ 21 ] poly(methylmethacrylate), [ 22 ] CaF 2 hierarchical nanospheres, [ 23 ] and so on, showing an efficient material encapsulation to improve long‐term stability even in a polar ambient or moisture. However, the embedding process requires the mixture of PQDs with polar solvents in some cases, promoting the formation of agglomerates and deteriorating their photophysical properties due to the emergence of carrier traps.…”

Varying the Concentration of Organic Acid and Amine Ligands Allows Tuning between Quantum Dots and Magic-Sized Clusters of CH3NH3PbBr3 Perovskite: Implications for Photonics and Energy Conversion

“…Perovskite quantum dots (PQDs) have received increasing interest due to their novel properties, such as high photoluminescence (PL) quantum efficiency and narrow but tunable emission bandwidth. The emission can be easily tuned over the entire visible spectrum by controlling the crystal size and halide ion composition, which are attractive for light-emitting diodes (LED), − solar cells, − and other optoelectronic applications. − ,,− Compared with perovskite bulk materials, PQDs exhibit tunable properties due to the quantum confinement effect and an extremely large surface-to-volume (S/V) ratio that allows for easy surface functionalization for different applications. − Compared with conventional PQDs, perovskite magic-sized clusters (PMSCs) with single size or narrow size distributions have narrower and bluer optical absorption bands and better-defined structures that are desired for fundamental studies. − For example, they are good model systems for understanding the growth of larger nanostructures including QDs or QD solids. − …”