Monitoring the Transition from Molecular Surface Passivation to 2D Layer Formation on 3D Perovskite Films
Tim Kodalle a, Raphael Moral b, Lucas Scalon b, Rodrigo Szostak b, Maged Abdelsamie c, Ana Nogueira b, Carolin Sutter-Fella a
a Molecular Foundry, Lawrence Berkeley National Laboratory, California 94720, USA, United States
b Laboratory of Nanotechnology and Solar Energy, Institute of Chemistry, University of Campinas – UNICAMP, P.O. Box 6154, Campinas, 13083-970, Brazil
c Materials Sciences Division, Lawrence Berkeley National Laboratory
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Spring Meeting 2022 (NSM22)
#TSPV22. Towards Stable Perovskite Photovoltaics
Online, Spain, 2022 March 7th - 11th
Organizers: Yana Vaynzof, Feng Gao and Zhuoying Chen
Contributed talk, Tim Kodalle, presentation 342
Publication date: 7th February 2022

While organic-inorganic (hybrid) perovskites achieved competitive power conversion efficiency within an impressive short time [1], they still suffer from insufficient stability [2]. Instability of hybrid perovskite thin films under thermal stress and/or in humid air is one of the major obstacles for the commercialization of perovskite-based solar modules [2,3]. One of the most promising approaches to improve the long-term stability is the incorporation of bulky, hydrophobic molecules into the perovskite layer [4]. While the beneficial effects of these post-deposition treatments are generally accepted, there is an ongoing discussion about the mechanism of this passivation effect. Particularly, it is under debate when optimum surface passivation is reached using different concentrations of bulky molecules where a transition is supposed to happen from the surface attached bulky molecule itself to a 2D layer formation with general structure  R2An−1BnX3n+1 [5], which forms via intermixing of the bulky molecules and PbI2 from the 3D perovskite. Here, A denotes the cation, B the metal ion, and X the halogen anion of the 3D perovskite, R is the bulky organic cation.

Here, we use in situ photoluminescence (PL) measurements during spin-coating and annealing to probe the dynamic deposition of 2-thiophenemethylammonium iodide (2-TMAI) and phenylethylammonium (PEAI) with varied concentration on 3D triple cation perovskites. For both molecules, we find the transition from molecular passivation to the formation of an R2An−1BnX3n+1 layer at concentrations around 4-10 mmol/L. Using higher concentrations, we see the formation of a distinct surface layer. During spin-coating and annealing of these higher concentrated solutions, we furthermore monitor the transition from a single inorganic layer spaced by the bulky cations (n=1) to mixed 2D layers (n=2 and higher) and, in the case of 2-TMAI, to the formation of a mixed disordered phase [6]. The latter may negatively affect the electronic properties of the perovskite layer [7]. Our results illustrate how in situ PL can be used to gain mechanistic understanding on the 2D layer formation, its interaction with the 3D perovskite, and its transformation to the disordered phase. Therefore, it can be utilized to deliberately optimize the annealing sequence targeting an ideal 2D/3D interface satisfying enhanced charge transport and stability.

Combining abovementioned results with in situ GIWAXS measurements, we propose a model for the surface passivation mechanism either via molecular passivation or 2D layer formation, compare the mechanisms for both investigated molecules, and correlate them with device stability proposing an optimized stabilization treatment.

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