Beyond the crystal lattice of lead halide perovskites: The curious cases of Cs4PbX6, Cs(Pb:Mn)I3, Cs2PbI2Cl2 and CsPb(Cl:Br:I)3 nanocrystals
Quinten A. Akkerman a, Liberato Manna a
a Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT), via Morego 30, I-16163 Genova, Italy
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Quinten A. Akkerman, presentation 101
Publication date: 11th February 2019

Nanocrystals (NCs) of cesium lead halide perovskites (LHP) have recently become an extensive research topic, owing due to their interesting optical properties.1-2 The LHP CsPbX3 phase, with X = Cl-, Br- or I-, is characterized by corner sharing PbX64- octahedra, with the Cs+ cations filling the voids created by four neighboring PbX64- octahedra. Here, we will present recent works focusing on the cesium LHP structure, and how altering the lead-halide framework strongly alters the structural and optical properties of these cesium LHPs.

In this presentation, we will first briefly focuses on Cs4PbX6 NCs, and how these type of NCs exhibit optical absorption with sharp, high energy peak due to transitions between states localized in individual PbX64– octahedra.3 Because of the large band gap of Cs4PbX6 (>3.2 eV), no excitonic emission in the visible range was observed.4 The Cs4PbBr6 nanocrystals can be converted into green fluorescent CsPbBr3 nanocrystals by their reaction with an excess of PbBr2 with preservation of size and size distributions. We will also briefly the case alloyed CsPbxMn1–xI3 nanocrystals.5 These NCs have essentially the same optical features and crystal structure as the parent α-CsPbI3 system, but they are stable in films and in solution for periods over a month. The stabilization stems from a small decrease in the lattice parameters slightly increasing the Goldsmith tolerance factor, combined with an increase in the cohesive energy. Finally, hybrid density functional calculations confirm that the Mn2+ levels fall within the conduction band, thus not strongly altering the optical properties.

Finally, we will focuses on two novel types of LHP related NCs. The vast majority of LHP (both as thin films and NCs) are currently based on either a single halide compositions (CsPbCl3, CsPbBr3, CsPbI3) or an alloyed mixture of bromide with either Cl- or I- (i.e. CsPb(Br:Cl)3 or CsPb(Br:I)3). Here. we present the synthesis, as well as a detailed optical and structural study of two halide alloying cases that have not previously been reported for LHP NCs: Cs2PbI2Cl2 NCs and triple halide CsPb(Cl:Br:I)3 NCs.REF In the case of Cs2PbI2Cl2, we observe for the first time fully inorganic LHP NCs with a Ruddlesden-Popper phase (RPP) crystal structure. Unlike the well-explored organic-inorganic RPP, here, the RPP formation is triggered by the size difference between the halide ions (Cl and I). In the case of the triple halide CsPb(Cl:Br:I)3 composition, the NCs comprise a CsPbBr2Cl perovskite crystal lattice with only a small amount of incorporated iodide, which segregates at RPP planes’ interfaces within the CsPb(Cl:Br:I)3 NCs. Supported by density functional theory calculations and post-synthetic surface treatments to enhance PLQY, we show that the combination of iodide segregation and defective RPP interfaces are most likely linked to the strong PL quenching observed in these nanostructures.

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