Low-Temperature Stabilization of MA-Free FAPbI₃ via Binary Solvents and Cesium Doping

Riccardo Pallotta^a, Simone Filippi^a, Weidong Xu^b^,^c, Samuele Mattioni^d^,^e, Sergio Marras^f, Alice Scardina^a, Ruggero Sala^a, Matteo Degani^a, Beatríz Martín-Garcia^d^,^g, Samuel Stranks^b^,^c, Giulia Grancini^a

^aDepartment of Chemistry and INSTM, University of Pavia, Via T. Taramelli 1,4, 27100 Pavia, Italy

^bDepartment of Chemical Engineering & Biotechnology, University of Cambridge, Philippa Fawcett drive, Cambridge cB3 0AS, UK

^cDepartment of Physics, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge cB3 0he, UK

^dCIC nanoGUNE BRTA, Tolosa Hiribidea, 76, Donostia-San Sebastián 20018, Spain

^eDepartamento de Polímeros y Materiales Avanzados: Física, Química y Tecnología, University of the Basque Country (UPV/EHU), 20018 Donostia-San Sebastián, Spain

^fIstituto Italiano di Tecnologia Via Morego 30, 16163 Genova, Italy

^gIKERBASQUE-Basque Foundation for Science, Bilbao, 48009, Spain

NIPHO26
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO26)

Pavia, Italy, 2026 June 8th - 9th

Organizers: Giulia Grancini and Robert Hoye

Oral, Simone Filippi, presentation 017
Publication date: 22nd April 2026

Research on Hybrid Halide Perovskite (HHP) has reshaped photovoltaic research in the last decade and has recently approached the commercial scale in tandem configuration. While a wide range of material’s band gaps have been explored to adapt to the different applications, single-junction devices require a band gap of approximately 1.5 eV, indicating formamidinium lead triiodide (FAPbI₃) as the ideal solution. However, the main challenge when working with FA-based perovskites is the high temperature (~160°C) required for the phase transition to occur from the optically inactive non-perovskite δ-phase to the optically active cubic α-phase. Preventing the photoactive phase from degradation - especially at low processing temperatures (<150°C) - remains therefore a key challenge. Conventional strategies to stabilize the cubic FAPbI₃ phase rely on MA-assisted formulation; on one hand, this strategy successfully reduces the annealing temperature of the perovskite active layer to around 100 °C, but on the other hand the inclusion of smaller cations inevitably widens the band gap and introduces phase segregation.

In this context, we present a new strategy to stabilize the cubic α-phase of FAPbI₃ without the addition of MA-based additives and at low processing temperatures. We employ 2% of Cs inside the precursor solution and a binary solvent system with balanced polarity by introducing a polar alcoholic component in the anti-solvent system. This combination enables the phase conversion from δ- to α-FAPbI₃ at just 100 °C temperature, thus stabilizing the photoactive phase. Interestingly, when comparing FAPbI₃ processed using this method with FAPbI₃ prepared using conventional MACl additives, we observe a reduced band gap (1.53 eV) and larger lattice parameters. To demonstrate the effectiveness of our strategy, we fabricated solar cell devices in an inverted (p–i–n) configuration using FAPbI₃ films processed with both a binary solvent system and single solvents. While the devices prepared using single solvents exhibited low PCE, mainly due to the incomplete conversion of the active layer, the ones fabricated with the binary solvent system achieved PCE approaching 22%, thus opening a new path for low temperature processed FA-based solar cells.

Acknowledgements:

The authors acknowledge the European Research Council (ERC) Starting Grant 2018 under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 802862) that founded the “HY-NANO” project and the ERC Proof of Concept “SPIKE” (grant agreement no. 101068936) granted to G.G. The authors are grateful to the project for infrastructures funded by Regione Lombardia RL3776, for supplying the AFM instrument. The authors are also thankful to The Ministero dell’Università e della Ricerca (MUR), University of Pavia through the program “Dipartimenti di Eccellenza 2023–2027” for fundings and FARE (Framework per l’Attrazione e Il Rafforzamento delle Eccellenze per la ricerca in Italia) Project EXPRESS (exploring photoferroelectricity in halide perovskites for optoelectronics) “Development and characterization of halide perovskites for photoferroelectrics applications”. The authors acknowledge the Fondazione Cariplo Economia Circolare 2021 Project “Green flexible hybrid perovskite solar module for the market: from smart lead manipulation to recycling” (FLHYPER, no 20201067), funded under the “Circular Economy-2020” call. G.G. acknowledges the GOPV project (CSEAA_00011), which received funds from Bando Ricerca di Sistema—CSEA—TIPO A Piano triennale 2019–2021 Decreto direttoriale 27 Ottobre 2021 del Ministero della Transizione Ecologica.

nanoGe is a prestigious brand of successful science conferences that are developed along the year in different areas of the world since 2009. Our worldwide conferences cover cutting-edge materials topics like perovskite solar cells, photovoltaics, optoelectronics, solar fuel conversion, surface science, catalysis and two-dimensional materials, among many others.

MATSUS

Previously nanoGe Spring Meeting (NSM) and nanoGe Fall Meeting (NFM), MATSUS is a multiple symposia conference focused on a broad set of topics of advanced materials preparation, their fundamental properties, and their applications, in fields such as renewable energy, photovoltaics, lighting, semiconductor quantum dots, 2-D materials synthesis, charge carriers dynamics, microscopy and spectroscopy semiconductors fundamentals, etc.

International Conference on Hybrid and Organic Photovoltaics

International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.

Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics

The main topics of the Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP) are discussed every year in Asia-Pacific for gathering the recent advances in the fields of material preparation, modeling and fabrication of perovskite and hybrid and organic materials. Photovoltaic devices are analyzed from fundamental physics and materials properties to a broad set of applications. The conference also covers the developments of perovskite optoelectronics, including light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.

International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics

The International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics (NIPHO) is the best place to hear the latest developments in perovskite solar cells as well as on recent advances in the fields of perovskite light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.