Multi-Band Electronic Structure and Carrier Dynamics in Formammidinium Lead Bromide Perovskite

In recent years, the scientific community has been captivated by the exceptional properties of halide perovskites. These materials have garnered significant interest due to their high absorption coefficients in the visible range (α&gt;10⁵ cm^-1), ultralong carrier diffusion lengths, extended carrier lifetimes, remarkable defect tolerance, and tunable bandgaps [1-2]. Among these perovskites, lead bromide perovskites have emerged due to their relatively large bandgap of approximately 2.3 eV, making them ideal candidates for tandem applications and efficient semitransparent solar cells. In this context, one of the most promising materials is formamidinium lead bromide perovskite (FAPbBr₃) that exhibits remarkable stability under light exposure and possesses a carrier diffusion length that facilitates a straightforward planar heterojunction solar cell configuration [3, 4]. Despite its proven superiority in terms of semitransparency and stability, only a limited number of experimental and theoretical studies have been conducted to explore its properties. In particular, the knowledge of the electronic properties and of the dynamics of photoexcited carriers in FAPbBr₃ remain incomplete. Understanding the charge generation and recombination is of paramount importance as it provides crucial insights for advancing the development and optimization of various technologies such as solar cells, lasers, and LEDs [5].<br/>Here, we present the excited-state properties of a thin film of FAPbBr₃studied combining steady-state light absorption, photoluminescence (PL), femtosecond transient absorption spectroscopy (FTAS), photoelectron spectroscopy (PES) with density functional theory (DFT) calculations. In this way, we give a complete description of the electronic properties of FAPbBr₃, assigning the electronic bands involved in the photoexcitation by UV-Vis radiation and the relative carrier dynamics. By integrating absorption, PL, PES, and DFT, we successfully assign the two photobleaching (PB) signals observed in the transient spectrum to specific electronic transitions. Specifically, the signal at 2.3 eV (PB1) originates from the transition between the first valence band (VB1) and the first conduction band (CB1) [6], while the signal at 3.4 eV (PB2) arises from the transition between the second inner valence band (VB2) and CB1. In the light of this assignation, the rise of the PB1 and PB2 signals were investigated as a function of the carrier density, in order to correlate their dynamics to the thermal relaxation processes involving the carriers excited in the VB1→CB1 and VB2→CB1 transitions. The PB1 signal shows a rise time that slows down with the increase of the carrier density indicating that the dominant processes influencing it are the thermalization and cooling of carriers in VB1 and CB1 [7, 8]. On the other hand, the rise time in PB2 accelerates as the excited carrier density increases, suggesting that this signal is strongly influenced by the thermalization of the hot holes in VB2. In fact, VB2 shows a quasi-flat band structure determining a localization near the band maximum of all the excited holes [9].<br/>In summary, the combination of various spectroscopic techniques and theoretical calculations allows us to unravel the underlying carrier dynamics and establish correlations with thermal relaxation processes, providing valuable insights into the excited-state behavior of FAPbBr₃. These findings enhance our understanding of the material properties and pave the way for further advancements in its utilization for optoelectronic and photovoltaic applications.<br/><br/><b>References</b><br/>[1] C. Wehrenfennig, et al., Adv. Mater., 2014.<br/>[2] K. Jena, et al., Chem. Rev., 2019.<br/>[3] N. Arora et al., Nano Lett., 2016.<br/>[4] A. Amat et al., Nano Lett., 2014.<br/>[5] A. A. Al-Kahtani et al., Coatings, 2021.<br/>[6] B. Anand, et al., Phys. Rev. B, 2016.<br/>[7] M. B. Price, et al., Nat. Comm., 2015.<br/>[8] J. M. Richter, Nat. Comm., 2017.<br/>[9] G. J. Hedley, Sci. Reports, 2018.