Charge Transfer Dynamics in Layered Hybrid Quasi 2D-Perovskites

Halide perovskites are revolutionizing the field of optoelectronic devices, encompassing applications from solar cells to LEDs, thanks to their outstanding optical and electrical properties. These materials are characterized by high absorption coefficients, long carrier diffusion lengths, and efficient charge transport, positioning them as frontrunners in advanced technology development. Perovskite solar cells (PSCs) have now exceeded 26% efficiency^[1], making them the leaders among thin-film technologies. However, despite their high efficiency, the widespread commercialization of three-dimensional (3D) perovskites is hindered by their instability, particularly in humid environments, as their soft ionic lattice degrades upon exposure to moisture and oxygen, presenting a significant challenge to their long-term viability. To address this, quasi-two-dimensional (2D) perovskites have emerged as promising alternatives. Their layered structure, incorporating larger organic cations, enhances environmental stability by shielding the inorganic framework from degradation. In addition, in these materials the electronic properties, such as bandgaps and exciton binding energies, can be precisely adjusted, essential for optimizing device performance.
One of the most important features of quasi-2D perovskites is the natural dimensionality gradient that arises within their films. Lower-dimensional layers (e.g., n=1) typically form near the substrate, while higher-dimensional phases (n>1) accumulate near the surface^[2,3]. This gradient facilitates more efficient charge separation and extraction. Through careful structural engineering of quasi-2D perovskites, it is possible to finely control carrier and exciton dynamics, leading to significant improvements in device performance.
In this study, we examine the electronic and morphological properties of thin films made of a mixture of 2D perovskites, focusing on the role of different dimensionalities in charge transport/separation. We employ Femtosecond Transient Absorption Spectroscopy (FTAS) and photoluminescence (PL) to investigate charge transfer mechanisms in the active material for photovoltaics^[4]. In particular, we have followed the hole transfer process from the n=4 of PEA₂MA_n−1Pb_nI_3n+1 to the lower dimensionality phases with FTAS, exciting the sample at 1.91 eV, resonant with the bandgap of n=4. This approach, allowed us to evaluate the dynamics of the hole transfer following the rising of the photobleaching (PB) signals related to the n=3 and n=2, demonstrating the efficiency of the charge separation process from higher-n to lower-n phases. The absence of the PB signal for the n=1 phase suggests weak coupling with the other phases, thereby limiting hole transport. This uncoupling between the n=1 and n=2-4 phases is further confirmed by PL measurements. Our findings emphasize the critical importance of controlling the dimensionality gradient in active materials composed of mixed 2D perovskites, optimizing charge transport and separation by promoting efficient transitions between low-n and high-n phases, while minimizing the formation of the n=1 phase.


References

[1] Best research-cell efficiency chart, https://www.nrel.gov/pv/cell-efficiency.html. (Accessed October 2024).
[2] Q. Shang, et al., J. Phys. Chem. Lett. 2017, 8, 18, 4431–4438 [10.1021/jacs.6b12581].
[3] J. Liu, et al., J. Am. Chem. Soc. 2017, 139, 4, 1432–1435 [10.1021/jacs.6b12581].
[4] G. Ammirati, et al., Adv. Optical Mater. 2024, 12, 2302013 [10.1002/adom.202302013].