Dr. Daniela Marongiu

Abstract:

In the context of solar cell technology, 2D Ruddlesden-Popper perovskite phases have been utilized alongside polycrystalline (PC) 3D hybrid perovskites (HPs) as ultrathin passivation layers to enhance stability and charge extraction. The majority of the reported 3D/2D heterostructures consist of PC thin films deposited on top of PC 3D HPs. This method offers limited control over the orientation and crystalline phase, leading to a high concentration of defects at grain boundaries and interfaces. These defects promote the presence of traps for charge carriers, ion migration, and water permeation.

On the other hand, pure 2D HPs have been considered less suitable for photovoltaic applications due to their large exciton binding energies, which theoretically hinder charge separation and result in significant energy losses. Surprisingly, the presence of large polarons - charge carriers coupled to lattice deformations - prevents the formation of excitons [1]. One of the first explanations was based on exciton dissociation caused by polycrystalline grains boundaries, suggesting that the formation of free carriers actually requires a defective material [2]. However, fundamental studies performed on singles crystals showed that exciton dissociation into unbound carriers is an intrinsic phenomenon, taking place also in single crystals with low defect densities [3]. This can enable more possibilities for 2D single crystals for optoelectronic applications, including photovoltaic ones.

Despite the potential benefits, the use of single crystal (SC) HPs for both 2D/3D heterostructures and pure 2D film devices remains challenging and, at the moment, the best performances are attributed to polycrystalline films possibly with a 2D passivating layer on top. Indeed, the best performing single crystal solar cells show an efficiency gap with respect to the polycrystalline counterpart which is attributed to the high surface charge trap density that results from the contamination of residual crystal growth solution, strongly affecting the surface quality and charge recombination. 

In this study, we investigate single crystal 2D perovskites and 2D/3D heterostructures. We demonstrate the growth of 2D HP single crystal thin films using various additives and analyze their optical and structural properties together with the electrical characterization. We also present single crystal 2D/3D thin film heterostructures and propose several strategies for interface engineering. Additionally, we provide a critical comparison of the photophysics and transport properties between single crystal and polycrystalline samples.