Hybrid perovskites are currently one of the most active fields of research due to their huge potential for applications in solar cells (SCs) and light-emitting diodes (LEDs). The performances of perovskite SCs have increased at an incredible rate, reaching efficiencies exceeding 23%. However, the commercialization is hindered by their poor stability. Very recently, low-dimensional 2D perovskites (Ruddlesden-Popper (RP) with formula A2A’n-1BnX3n+1 or Dion-Jacobson (DJ) with formula AA’n-1BnX3n+1) have been identified as promising alternatives to 3D perovskites. In those compounds, A is a large organic spacer cation, A’ is a smaller organic cation capable of forming 3D perovskite slab, B is a divalent metal (commonly Pb2+), X is halide anion, and n is number of inorganic perovskite layers. Lower dimensional perovskites can be considered as self-assembled multiple-quantum-wells, with excitons confined in inorganic “wells”, while the organic spacers serve as potential barriers. Due to their chemical structure, these materials offer better tunability and higher stability compared to their 3D counterparts, but they suffer from reduced efficiency and insufficient understanding of the relationships between their chemical structure and their properties. Consequently, the progress in exploring huge realm of possible compounds in this category is proceeding mainly by trial and error. In contrast, this project proposes a comprehensive study on 2D and quasi-2D perovskites, by tackling the chemical/crystal engineering, detailed structural/physical property measurements and state-ofthe- art theoretical approaches and computing. It will lead to elucidation of relationship between the unique physical properties of 2D perovskites (excitonic properties, electron–phonon coupling, etc.), the choice of perovskite building blocks and resulting crystal structure and orientation, optical properties and optoelectronic performances, thus extending the realm of SCs and LEDs applications.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In particular, we will focus on the tuning of optoelectronic properties through a well-thought compositional engineering. The choice of perovskite building blocks (organic spacer cations A and number of layers in inorganic slab n) will be systematically investigated by exploring: i) the steric effect (branching of alkylammonium cations) ii) the effect of alkylammonium chain length, iii) the effect the alkyl ammonium chain length in the case when cation contains an aromatic ring. We will perform comprehensive experimental and theoretical investigations of the perovskite materials with spacer cations selected to enable investigation of the cation structural effects. Experimental investigations will include optical spectroscopy (UV/Vis and PL including variable temperature and time-resolved techniques), electron microscopy, and ultraviolet photoelectron spectroscopy (UPS) measurements to obtain information on the optical and electronic properties of the samples. Phase purity and crystallinity of thin films will be examined by X-ray diffraction (XRD), and new structures will be solved from the single crystal data or powder data. The explanations behind the expanded chemical complexity will be further pursued by theoretical calculations, including density functional theory (DFT) to determine the formation energies, stability and strain relaxations, electronic structures, defect formation energies and the impact of structural distortions on energy levels, as well as molecular dynamics simulations (MD) to obtain complementary information on the electron-phonon coupling and role of dynamical disorder. Finally, based on the obtained experimental and theoretical results, we will identify the best candidates to prepare LEDs or SCs devices and derive guidelines for rational synthesis of quasi-2D materials for high efficiency and high stability devices. The devices will be comprehensively characterized to evaluate efficiencies and lifetimes.

The proposed study, relying on systematic approach and complementary expertise and synergy of the project team, will result in providing the deeper understanding of interplay between specific structural, compositional and electronic features, tailored by the choice of perovskite building blocks, and their device performance (efficiency and stability). This correlation is still not understood and as a consequence, best results are nowadays unfortunately obtained by chance, rather than by the targeted design. The improved understanding we are aiming for will enable tailored design of spacer cations, leading to perovskite solar cells and/or LEDs with improved efficiency, bringing these devices closer to commercialization. Thus, the technological breakthroughs resulting from improved understanding of tailored design within the class of quasi-2D perovskites are expected to result in significant long-term benefits to society and to industry.