Reversible Growth of 2D/3D Single Crystal Hybrid-Perovskite Heterostructure

In solar cell technology, 2D Ruddlesden-Popper perovskites have been used with polycrystalline (PC) 3D hybrid perovskites (HPs) as ultrathin passivation layers to improve stability and charge extraction. Most 2D/3D heterostructures involve PC thin films on PC 3D HPs, however this approach offers limited control over orientation and crystalline phase, leading to defects at grain boundaries and interfaces. These defects promote the presence of traps for charge carriers, ion migration, and water permeation, significantly impacting device performance.
While 2D HPs were considered unsuitable for photovoltaics due to high exciton binding energies, recent studies show that large polarons prevent exciton formation, allowing free carriers instead [1]. Early theories attributed exciton dissociation on polycrystalline grain boundaries and defects [2], yet more fundamental research on single crystal revealed that is an intrinsic property, occurring even in low-defect-density single crystals [3], suggesting new opportunities for 2D single crystals in optoelectronics [4].
Despite these promising insights, the application of single crystal HPs for both 2D/3D heterostructures and pure 2D solar cells remains challenging. Polycrystalline films with an additional 2D passivation layer have shown superior performance compared to single crystal solar cells which show high surface charge traps, caused by residual crystal growth solution, which worsens surface quality and charge recombination.
In this study, we investigate the growth of single crystal 2D/3D heterostructures, focusing on improving crystal growth and minimizing surface defects. We demonstrate the reversible growth of 2D/3D single crystal thin film heterostructures through a one-step synthesis process, proposing interface engineering strategies to mitigate defect formation and enhance charge transport. By controlling the process parameters, we can obtain 2D/3D SC heterostructures with a well-defined composition, allowing us to selectively promote the growth of a desired phase. This precise control over the growth conditions enables us to fine-tune the crystalline structure, ensuring that the preferred phase dominates while minimizing the presence of unwanted phases.
Moreover, this work includes computational simulations of crystal growth processes, focusing on the kinetics of SC film formation. These simulations provide insights into the interplay between growth conditions and defect formation, offering a predictive tool to fine-tune the growth process for improved heterostructure quality.

REFERENCES:
[1] A. Simbula, R. Pau, Q. Wang, F. Liu, V. Sarritzu, S. Lai, M. Lodde, F. Mattana, G. Mula, A. Geddo Lehmann, I.D. Spanopoulos, M. G. Kanatzidis, D. Marongiu, F. Quochi, M. Saba, A. Mura, G. Bongiovanni. Adv. Optical Mater. 2021, 2100295.
[2] Srimath Kandada et al., J. Phys. Chem. Lett. 11, 3173–3184 (2020).
[3] A. Simbula, L. Wu, F. Pitzalis, R. Pau, S. Lai, F. Liu, S. Matta, D. Marongiu, F. Quochi,M. Saba, A. Mura, G. Bongiovanni. Nat Commun 14, 4125 (2023).
[4] A. Simbula, V. Demontis, F. Quochi, G. Bongiovanni, and D. Marongiu. ACS Omega 2024, 9, 36865−36873.