THE PHYSICS GRADUATE PROGRAM invites everyone to the seminar:
Metal halide perovskite materials: beyond solar cells
Ana Flavia Nogueira
Laboratório de Nanotecnologia e Energia Solar (LNES), Chemistry Institute, University of Campinas
Metal halide perovskite materials have been extensively investigated as thin films or colloidal nanoparticles in several technological applications, such as photovoltaic cells (PV), lasers, photodetectors and light emitting diodes (LED).
Perovskite is the name given to materials that have the general formula ABX3 with a structure similar to the mineral CaTiO3. In perovskite materials for PV, the monovalent cation “A” is an organic cation such a methylammonium (CH3NH3+, MA) or formamidinium (CH(NH2)2+, FA). For LED applications, “A” is an inorganic cation as Cs+, Rb+. In both applications, “B” is a divalent cation Pb2+ or Sn2+, and “X” a halogen anion Cl-, Br- or I-. The optoelectronic properties that make perovskites attractive are their direct and tunable band gap (that can be varied due to quantum confinement or composition), high absorption coefficient (similar to GaAs), ambipolar transport of charges, high mobility of electrons and holes compared with organic semiconductors, and electron diffusion lengths that can exceed micrometers in large crystals. Multicomponent perovskite solar cells have reached the recent efficiency breakthrough of 25.2%, higher than silicon polycrystalline photovoltaics
In this presentation we will discuss the development of metal halide perovskites applied to both photovoltaics and light emission in the Chemistry Institute at UNICAMP.
Colloidal perovskite nanocrystals are promising for LED applications. The most used synthetic method to prepare them relies on a mixture of oleylamine (OLA) and oleic acid (OA) as surfactants. The resulting nanocrystals exhibit poor colloidal stability due to facile proton exchange between the oleate and amine surfactants and they readily precipitate from the crude solution. Oleylamine itself can also accelerate the degradation of the nanocrystals. We will discuss an amine-free synthesis that utilizes tetraoctylammonium halides for preparation of OA-capped CsPbX3 PQDs without the need of post-anion exchange methods.
Playing with the halide source (e.g. using SnX4 salts), OLA/OA equilibrium is affected, and the ratio Cs+ /OLA is changed. As consequence, nanoplates can be obtained with a variety of thickness, exhibiting emission that spams the whole visible spectrum.
Metal halide perovskite for photovoltaics are prepared with a myriad of different cations and anions. Such cocktail influences the formation of intermediates, new phases, favours halide homogenization, etc; so that at the end, the efficiency of the device is closely related to not only the optical quality of the film (e.g. crystallinity), but morphology and composition
In the second part of this presentation, we will summarize important results using in situ experiments to probe perovskite formation (2D and 3D), stability and composition. We employed time-resolved grazing incidence wide angle x-ray scattering (GIWAXS), small angle x-ray scattering (SAXS) and high-resolution XRD taken at the Brazilian Synchrotron National Laboratory and SSRL-Stanford.
In situ GIWAXS experiments allowed us to understand the influence of the relative humidity and time to drop the antisolvent during the preparation of perovskite films and get important information about final composition and morphology. It is well known that a 2D layer on the top of a 3D bulk perovskite improves stability and performance. In situ GIWAXS revealed us that during thermal annealing the 2D layer transforms itself into a disorder layer, improving hole transfer and stability. This technique was also employed to identify the first intermediates formed during the degradation of different Cs and Br perovskite compositions under ambient conditions.
 E. Yassitepe, L. G. Bonato, A. F. Nogueira, E. H. Sargent. et.al., “Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes”, Advanced Functional Materials (Wiley), 26 (2016) 8757-8763
 L. G. Bonato, R. F. Moral, G. Nagamine, A. Aló, J. C. Germino, D. S. Silva, D. B. Almeida, L. F. Zagonel, F. Galembeck, L. A. Padilha Jr., A. F. Nogueira, “Revealing the Role of Tin (IV) Halides in the Anisotropic Growth of CsPbX3 Perovskite Nanoplates” Angewandte Chimie, 59, 2-11 (2020)
 R. Szostak, J. C. Silva, S.-H. Turren-Cruz, M. M. Soares, R. O. Freitas, A. Hagfeldt, H. C. N. Tolentino and A. F. Nogueira* “Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films”, Science Advances (Science), 5, eaaw6619, (2019)
 R. Szostak, P. E. Marchezi, A. Marques, J. C. Silva, M. S. Holanda, M. M. Soares, H. C. N. Tolentino and A. F. Nogueira, “Exploring the formation of formamidinium-based hybrid perovskites by antisolvent methods: in situ GIWAXS measurements during spin coating”, Sustainable Energy & Fuels, 3 (2019) 2287-2297.
 A. Sutanto, R. Szostak, N. Drigo, V. Queiroz, P. E. Marchezi, J. C. Germino, H. N. Tolentino, M. Nazeeruddin, A. F. Nogueira, G. Grancini “In Situ Analysis Reveals the Role of 2D Perovskite in Preventing Thermal-Induced Degradation in 2D/3D Perovskite Interfaces”, Nano Letters, 20(5) 3992-3998 (2020)
 P. E. Marchezi, E. M. Therézio, R.Szostak, H. C. Loureiro, K. Bruening, A. Gold-Parker, M. A. Melo Jr., C. J. Tassone, H. C. N. Tolentino, M. F. Toney, A. F. Nogueira, “Degradation mechanisms in mixed-cation and mixed-halide CsxFA1-xPb(BryI1-y)3 perovskite films under ambient conditions” J. Mater. Chem. A, 9, 9302-9312 (2020)
 R. F. Moral, L. G. Bonato, J. C. Germino, W. X. Oliveira, R. Kamat, J. Xu, C. Tassone, D. D. Stranks, M. F. Toney, A. F. Nogueira, “Synthesis of Polycrystalline Ruddlesden-Popper Organic Lead Halides and Their Growth Dynamics”, Chemistry of Materials, 31 (22) (2019), 9472-9479
 R.E Beal, N.Z. Hagström, J. Barrier, A. Gold-Parker, R. Prasanna, K. A. Bush, D. Passarello, L. T. Schelhas, K. Brüning, C.J. Tassone, H.-G., Steinrück, M. D. McGehee, M. F. Toney, A. F. Nogueira “Structural Origins of Light-Induced Phase Segregation in Organic-Inorganic Halide Perovskite Photovoltaic Materials” Matter, v.2, Issue 1, (2020)
Date: September 11, 2020 – (friday) – Place: Sala virtual do ConferênciaWeb da Rede Nacional de Pesquisa – Time: 10:15 a.m.
virtual room access link: https://conferenciaweb.rnp.br/webconf/ppgfsc-ufsc