Benign methylformamidinium byproduct induced by cation heterogeneity inhibits local formation of δ-phase perovskites

Lim, Jihoo, Kim, Jaehui, Davies-Jones, Josh, Danaie, Mohsen, Choi, Eunyoung, Shim, Hongjae, Chen, Liang, Kim, Jincheol, Kim, Judy S., Davies, Philip R. ORCID: https://orcid.org/0000-0003-4394-766X, Seidel, Jan, Green, Martin A., Stranks, Samuel D., Il Seok, Sang and Yun, Jae Sung 2024. Benign methylformamidinium byproduct induced by cation heterogeneity inhibits local formation of δ-phase perovskites. Energy & Environmental Science 10.1039/D4EE03058C Item availability restricted.

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Abstract

Efforts to enhance the efficiency and stability of formamidinium lead triiodide (FAPbI3) perovskite solar cells (PSCs) have primarily focused on employing methylammonium chloride (MACl) as an effective additive. MACl significantly improves the crystallinity and lowers the δ-to-α phase transition temperature of FAPbI3, thereby contributing to the remarkable efficiency of these solar cells. However, upon evaporation with deprotonation of MACl during annealing, the highly reactive methylamine leads to the formation of N-methylformamidinium (MFA+) cations. Despite their potential for significant influence on the properties of FAPbI3 perovskites, the chemical and optoelectronic characteristics of MFA+ in FAPbI3 remain poorly understood. This study investigates the unexplored role of MFA+ in FAPbI3 perovskite with MACl incorporation through advanced nanoscale characterization techniques, including photo-induced force microscopy (PiFM), four-dimensional scanning transmission electron microscopy (4D-STEM), and wavelength-dependent Kelvin probe force microscopy (KPFM). We reveal that MACl induces compositional heterogeneities, particularly formamidinium (FA+) and MFA+ cation inhomogeneities. Surprisingly, MACl selectively promotes the formation of MFAPbI3 at grain boundaries (GBs) and as clusters near GBs. Additionally, we confirm that MFAPbI3 is a wide bandgap, and charge carriers are effectively separated at GBs and clusters enriched with MFAPbI3. This is particularly interesting because MFAPbI3, despite its crystal structural similarity to yellow phase δ-FAPbI3, displays a high surface photovoltage, and does not deteriorate the solar cell performance. This study not only provides insights into the byproduct formation of MFA+ induced by local cation heterogeneity after employing MACl, but also guides a crucial perspective for optimizing formamidinium-based PSC design and performance.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Chemistry Cardiff Catalysis Institute (CCI)
Publisher:	Royal Society of Chemistry
ISSN:	1754-5692
Date of First Compliant Deposit:	23 September 2024
Date of Acceptance:	29 July 2024
Last Modified:	08 Nov 2024 03:00
URI:	https://orca.cardiff.ac.uk/id/eprint/172332

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