Building Blocks of Hybrid Perovskites: A Photoluminescence Study of Lead‐Iodide Solution Species

Abstract

In this work, we present a detailed investigation of the optical properties of hybrid perovskite building blocks, [PbI 2 + n ] nÀ , that form in solutions of CH 3 NH 3 PbI 3 and PbI 2. The absorbance, photoluminescence (PL) and photoluminescence excitation (PLE) spectra of CH 3 NH 3 PbI 3 and PbI 2 solutions were measured in various solvents and a broad concentration range. Both CH 3 NH 3 PbI 3 and PbI 2 solutions exhibit absorption features attributed to [PbI 3 ] 1À and [PbI 4 ] 2À complexes. Therefore, we propose a new mechanism for the formation of polymeric polyiodide plumbates in solutions of pristine PbI 2. For the first time, we show that the [PbI 2 + n ] nÀ species in both solutions of CH 3 NH 3 PbI 3 and PbI 2 exhibit a photoluminescence peak at about 760 nm. Our findings prove that the spectroscopic properties of both CH 3 NH 3 PbI 3 and PbI 2 solutions are dominated by coordination complexes between Pb 2 + and I À. Finally, the impact of these complexes on the properties of solid-state perovskite semiconductors is discussed in terms of defect formation and defect tolerance.

Hybrid perovskite crystallization from binary solvent mixtures: interplay of evaporation rate and binding strength of solvents

“…The absorption bands ascribed to PbI3 -Solv3 and PbI4 2-Solv2 species showed a shift in the peak position, which can possibly indicate further formation of higher-order polyiodide plumbates or iodoplumbate chains. 36 The increased amount of higher-order polyiodide plumbates indicates that the strength of solvent coordinating ability to MAPbI3 increases in order from GBL to DMSO (GBL< NMP < DMF <DMSO). Interestingly, this order of solvents in relation to perovskite coordination ability correlates well with the theoretically predicted trend, calculated through a sum of Gutmann's donor and acceptor numbers.…”

Small-angle scattering to reveal the colloidal nature of halide perovskite precursor solutions

“…One approach to gain further knowledge that has received considerable attention is to study the chemical organization in the precursor solution used to synthesize the perovskite thin film. 5,6 As the crystalline thin film is growing directly from solution, a detailed understanding of the crystallization mechanism is critical to enable control of the perovskite formation. In this regard, the colloidal chemistry of perovskite precursor solutions contributes significantly to the crystallization process and thus to the perovskite morphology, which is known to have a direct impact on the performance of PSCs.…”

“…The role of solvents and coordination chemistry of the central atom Pb is one of the key topics in the field of solution chemistry for perovskite precursors. 6,9,10 Weakly coordinating solvents tend to favour faster crystallization, whereas stronger coordinating solvents like DMSO form an intermediate state, which leads to a slower crystallization process and thus a better film morphology. 11,12 However, though trends are observable and the resulting film properties are characterised in detail, the actual solution chemistry, including the specific structure of the colloidal particles and a mechanism to explain the fast, lowtemperature crystallization, remains unclear.…”

“…UV-vis spectroscopy is frequently used to identify characteristic absorption spectra, which can be used to determine possible species in solution. 6,7 However, it is challenging to apply this method to solution concentrations above 1 mol L -1 typically used for thin-film deposition in solar cell fabrication. 5 Considering that the majority of the PSCs are fabricated by a solution-based deposition process it is of extreme importance to characterize the solution properties in their nature, close to application.…”

“…From previous work, it is known that we can expect iodoplumbates in solution and that the number of iodides surrounding lead should be increasing with concentration. 6,33,34 Thereby experiments, e.g. UV-vis, were mostly carried out for extremely diluted solutions, based on experimental limitations.…”