Pulsed Laser Deposition of CsPbBr3 Films: Impact of the Composition of the Target and Mass Distribution in the Plasma Plume

Abstract

All-inorganic cesium lead bromine (CsPbBr3) perovskites have gained a tremendous potential in optoelectronics due to interesting photophysical properties and much better stability than the hybrid counterparts. Although pulsed laser deposition (PLD) is a promising alternative to solvent-based and/or thermal deposition approaches due to its versatility in depositing multi-elemental materials, deep understanding of the implications of both target composition and PLD mechanisms on the properties of CsPbBr3 films is still missing. In this paper, we deal with thermally assisted preparation of mechano-chemically synthesized CsPbBr3 ablation targets to grow CsPbBr3 films by PLD at the fluence 2 J/cm2. We study both Cs rich- and stoichiometric PbBr2-CsBr mixture-based ablation targets and point out compositional deviations of the associated films resulting from the mass distribution of the PLD-generated plasma plume. Contrary to the conventional meaning that PLD guarantees congruent elemental transfer from the target to the substrate, our study demonstrates cation off-stoichiometry of PLD-grown CsPbBr3 films depending on composition and thermal treatment of the ablation target. The implications of the observed enrichment in the heavier element (Pb) and deficiency in the lighter element (Br) of the PLD-grown films are discussed in terms of optical response and with the perspective of providing operative guidelines and future PLD-deposition strategies of inorganic perovskites.

CsPbBr3 deposited by laser ablation: effects of post-growth aging, oxygen adsorption and annealing on film properties

“…Consistently, studies on the reaction kinetics of mechano-chemically synthesized CsPbBr 3 powders handled at room temperature by mixing the precursor salts in stoichiometric quantity report complete reaction occurring in less than 10 min [43]. Since we mixed 1:1 molar ratio PbBr 2 : CsBr, corresponding to a stoichiometric mixture, the reaction 3PbBr 2 + Cs 4 PbBr 6 → 4CsPbBr 3 may not work advantageously to remove the spurious phase in the absence of PbBr 2 in excess [41]. Alternatively, as the transformation of the Cs 4 PbBr 6 phase to CsPbBr 3 can be driven thermally, thermal annealing of the mixed reacted powder was exploited to yield the desired phase.…”

“…The resulting compressed solid pellet was transferred to an oven where it was thermally annealed in two steps: first step, heating at 400 °C for 2 h and, second step, further heating at 500 °C for 2 h. Finally, the oven was switched off and the target was let cool down to room temperature before being kept in a plastic box within a vacuum ball and under darkness until their usage in PLD depositions. A comprehensive discussion of the interplay between composition of the PbBr 2 -CsBr mixture and thermal annealing protocol in preparing the compacted targets was reported elsewhere [41].…”

“…Wet chemistry synthesis is carried out based on two different methods: the single-step approach mixes stoichiometric ratio of both precursor salts in a common solvent that is allowed to evaporate to obtain perovskite powders to be processed depending on the application; the two-step approach dissolves the precursors into different solvents, the obtained suspensions are deposited as thin films and then annealed to form the perovskite material. Although the experimental equipment needed for solution-assisted synthesis of CsPbBr 3 is cheap, criticisms related to the different solubility of the CsBr and PbBr 2 precursor salts in aprotic solvents are responsible of the precipitation of spurious phases alongside the desired CsPbBr 3 phase [19,40,41], hence leading to poor control on reproducibility, uniformity, and composition in particular over large area substrates. More generally, wet (solution-based) methods need hazardous chemicals and capping ligands, suffer from a restricted number of solvents and their inability to work in dissolving effectively all precursors cause solvent residuals, affect morphology and crystallization quality of the final product, and require multistep and time-consuming processing to produce gram-scale amounts of CsPbBr 3 powders to be further processed.…”

“…A technique largely exploited for depositing thin films starting from compacted powder targets is pulsed laser deposition (PLD). It was recently discussed, in terms of stoichiometry transfer and impact of the mass distribution of Cs, Pb, and Br in the plasma plume, to growth good-quality luminescent CsPbBr 3 films using targets prepared by thermally assisted mechano-chemical synthesis [41]. A relevant finding of this preliminary study was that the simultaneous presence of heavy and light species in CsPbBr 3 implies films suffering from Pb-excess and Br-deficiency if a conventional 1:1 molar ratio target is prepared.…”

“…To extend the knowledge of the potential of PLD in growing CsPbBr 3 films, in this paper, we explore the implications of a stoichiometric mixture of PbBr 2 and CsBr powders (molar ratio 1:1) to manufacture CsPbBr 3 compacted targets to be ablated by pulsed excimer laser. As the resulting PLD-deposited films suffer from Br-deficiency, due to the uneven mass of the involved elements and stoichiometric target, it may be interesting to investigate possible strategies to tune and enhance their photoluminescence performances on accounting for the unique ability of PLD to tune composition, morphology, and thickness [41]. This aim is well motivated by several interesting and not fully understood or studied results regarding the controversial influence of the external environment conditions on the emission properties of perovskite materials.…”