Effect of Co-Solvents on the Crystallization and Phase Distribution of Quasi-2D Perovskites

Alessandro Caiazzo

Onderzoeksoutput: Bijdrage aan congresAbstractAcademic


Quasi-2D perovskites (q2D PVKs) are promising candidates for stable and highly efficient solar cells. On top of the superior environmental stability compared to their 3D counterparts, q2D PVKs possess tunable optoelectronic properties, as it is possible to change the number of inorganic layers (n) sandwiched between the organic spacers to tune bandgap and exciton binding energy. However, centering the phase distribution on a specific n-value is challenging. Together with a variety of processing techniques, solvent engineering has previously been used in the literature to tune both phase purity and device efficiency. Nevertheless, no reports have investigated the effect of co-solvents on the crystallization mechanism and the reasons behind the changes in phase distribution.

To fill this gap, we first investigated the kinetics of film formation of BA2MA3Pb4I13 processed from DMF/co-solvent mixtures (DMSO and NMP) via in-situ absorption measurements conducted during room-temperature spin coating or thermal annealing. By combining these results with photoluminescence (PL) spectroscopy and angle-dependent grazing-incidence wide-angle X-ray scattering (GIWAXS), we concluded that the crystallization starts as a 3D-like perovskite from the liquid-air interface, both when the q2D PVK forms during spin coating (low amounts of DMSO or NMP in the solvent mixture) and during annealing (high amounts). The structure of the 3D-like perovskite on the top of the film, however, is impacted by the crystallization at the bottom, as suggested by the redshift of the absorption onset.

In agreement with previous reports, we observed a 2D-3D gradient in the film, with q2D PVKs formed mostly at the bottom of the film and 3D-like PVKs at the top. Even though the crystallization mechanism is the same when using DMSO and NMP as co-solvents, the phase distribution at the bottom of the film is different. More specifically, for the 4:1 DMF/NMP ratio, the distribution shifts toward small n-values, mainly n = 2. On the contrary, when using DMSO as a co-solvent with the same volumetric ratio, a broader phase distribution appears. During the crystallization, as described before, a 3D-like layer is initially formed on the top of the film, while underneath a wet film consisting of perovskite precursors has yet to crystallize. The interactions of the co-solvents with the precursors in the wet film are important in shifting the resulting phase distribution, as they influence the availability of MAI or BAI molecules to participate in the crystallization process. We investigated the ternary interactions between BAI or MAI, PbI2, and the solvents with FT-IR and DFT. By analyzing the S=O and C=O stretching peaks, we observed that NMP interacts more strongly with MAI-PbI2 than with BAI-PbI2, indicating that BAI is less bound to the ternary complex, compared to MAI, and thus more available to participate in the formation of quasi-2D perovskite phases. This explains the phase distribution shifted to smaller n-values when using NMP. The same analysis showed that DMSO, instead, interacts similarly in ternary complexes with both MAI and BAI, agreeing with a broader phase distribution.

Overall, we propose a crystallization mechanism for BA2MA3Pb4I13 in a DMF/co-solvent mixture where 3D-like perovskites crystallize first at the liquid-air interface. While the crystallization proceeds, the phase distribution in the underlying perovskite layer is influenced by the availability of MAI and BAI molecules, which is in turn influenced by the co-solvent’s choice. This conclusion confirms the importance of the co-solvents (or additives) in the challenging formation of phase-pure quasi-2D perovskites. By carefully tuning the DMF/co-solvent ratio, we finally fabricated a more phase-pure q2D PVK with device efficiencies up to 11%, a low degree of 2D-3D vertical gradient, and a narrow phase distribution.
Originele taal-2Engels
StatusGepubliceerd - 2021
Evenement2021 MRS Fall Meeting & Exibit - Boston, Verenigde Staten van Amerika
Duur: 29 nov. 20219 dec. 2021


Congres2021 MRS Fall Meeting & Exibit
Land/RegioVerenigde Staten van Amerika
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