Micro-spectroscopic in-situ analysis of FAPI films exposed to humidity
Lars Krakowski a, Lars Bergenholtz a, Lorenzo Angiolini a, Jinzhao Li b, Eva Unger b, Christof Schultz a, Andreas Bartelt a
a University of Applied Sciences – HTW Berlin, Wilhelminenhofstr. 75a, D-12459 Berlin, Germany
b Helmholtz-Center Berlin for Materials and Energy, Kekuléstraße 5, D-12489 Berlin, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Poster, Andreas Bartelt, 279
Publication date: 20th April 2022

The stability of metal-halide perovskites (MHP) is the key requirement for further development of MHP solar cells. Among others, the degradation induced by humidity is a major stress factor. Formamidinium-lead iodid (FAPI) solar cells are attractive due to higher chemical stability and better suited band gap energies compared to methylaminium lead iodide (MAPI). However, the transformation from well-performing alpha-phase to photovoltaically irrelevant delta-phase poses a serious problem. In this poster we present results of a spectro-microscopic in-situ analysis on spin-coated formamidinium lead iodid films with an additive of 10% methylammonium chlorid (FAPbI3:10%MACl). We investigated transformation and degradation processes of the microcrystalline material on microscopic length scales when subjected to 50% humidity, which mimics the humidity usually present in ambient air. Oxygen was excluded.

Pristine FAPI films were analyzed inside an atmospheric chamber, which was connected to a humidifier using nitrogen as carrier gas. Multidimensional scanning photoluminescence (PL) microscopy with submicron spatial as well as time and spectral resolution was performed.  The spectrally resolved fluorescence liftetime microscope images of identical spots on the samples were recorded during a period of four weeks as the samples underwent transformations due to the H2O interaction with the FAPI films. For correlation, we will compare the PL images with scanning electron microscope images of the same spots.  

Due to the interaction with the H2O molecules, we observed the formation of 3 - 8 µm sized domains on the films. Initially the domains showed increased PL intensity of the alpha phase (around 800 nm). Within one week dark holes developed in the centers of the domains, while the entire domains enlarged and became frayed. From inside out, the domains subsequently darkened, and reversed their material quality compared to the rest of the film.  Small delta phase (500 nm – 700 nm) and lead iodid (~510 nm) island formed (~ 1 µm), which were scattered across the samples. We found the lead-iodide islands to be formed mostly at the degraded domain edges, while the delta phase islands were formed inside the bright functioning alpha phase domains.

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