Band Structure and Exciton Dynamics in Quasi-2D Dodecylammonium Halide Perovskites

Quasi two-dimensional perovskites in Ruddlesden-Popper phase (RPPs) are extensively studied for their flexible tunability of the optoelectronic properties by changing the number of layers, chemical composition and the organic spacer, which makes them particularly suitable for several applications in the field of photovoltaics and light emitters [1-4]. In fact, the reduced dielectric screening and the quantum confinement in a monolayer of RPP materials generate large bandgaps and stable excitons at room temperature with binding energies of the order of hundreds of meV similarly to other 2D semiconductors. Moreover, the physical properties of RPPs are strongly influenced by the number of layers that affects the electronic bandgap and the exciton binding energy which decreases as the number of layers increases. These properties are also correlated with the chemical composition and the structure of the organic spacer used as the interlayer material [5, 6].<br/>In this developing framework, the photoexcited carrier dynamics in these materials is still far to be completely understood although the study of charge generation and recombination path can furnish key information for boosting the development of new materials for new generation optoelectronic. Femtosecond Transient Absorption Spectroscopy (FTAS) has proven to be an important tool to investigate the ultrafast physics of halide perovskites having different dimensionality, morphology and architectures, giving insights into the band structure and the electronic states involved in the photoexcitation processes. Here, we present FTAS investigations on monolayer and multilayer of RPPs DAMAPI (DA₂MA_n-1Pb_lI_3n+1, where DA=CH₃-(CH₂)₁₁-NH₃₊ is the spacer and <i>n</i> indicates the number of the layers). The FTAS measurements, performed at 77 K and room temperature using several different pump energies and excitation densities, allowed the observation of absorption bleaching energies corresponding to different excitonic and electronic transitions in DAMAPI. These results are compared with theoretical simulations based on DFT, the GW approximation and the Bethe-Salpeter equation for the calculation of the excitonic properties. In this way, we have assigned the transient absorption signals to electronic transitions related to the formation of excitons with principal quantum number 1s and 2s. The temporal analysis of these signals indicates the exciton-exciton annihilation process as the main relaxation mechanism in the first picoseconds after the excitation, while exciton radiative recombination is observed at longer time delays (&gt;100 ps). This work demonstrates how transient absorption measurements not only allow the study of the dynamics of the excited carriers but, because of their high sensitivity to the changes of the carrier density induced by the excitation, they also allow the observation of spectral features otherwise not observable with steady-state measurements [7].<br/><br/><br/><b>References</b><br/>[1] L. Mao, et al., J. Am. Chem. Soc. 2019.<br/>[2] C. Smith, et al., Angew. Chem., Int. Ed. 2014.<br/>[3] G. Grancini, et al., Nat. Commun. 2017.<br/>[4] K. Wang, et al., EcoMat 2021.<br/>[5] M. Palummo, et al., ACS Energy Lett., 2020.<br/>[6] G. Giorgi, et al., J. Mater. Chem. C, 2018.<br/>[7] G. Ammirati, et al., Adv. Opt. Mat., 2023.