Compositional Engineering of Perovskite Light Absorbers for Enhanced Stability and Performance through 2D/3D Interfacing

Abstract

The semiconducting hybrid-organic inorganic halide perovskites have excellent optical and electronic properties that attract the interest of scientists and researchers. Besides these excellent properties, the simplicity of alteration in the structure of the framework leads to wide use in organic electronic devices, for example, light-emitting diodes (LED), transistors, and solar cells. Perovskite solar cells have seen the most rising rate in the chart of solar cell efficiency from 3 to over 25% in more than ten years. Their tunability towards lower bandgap has contributed to the improvements in the efficiency. In the last three years, most researchers have been focusing on the α-FAPbI3-based perovskite solar cells due to having a lower bandgap (1.45 eV) which is closer to the Shockley-Queisser optimum (1.1– 1.4eV) to gain high efficiency. A distinctive feature of FAPbI3 is that it is more thermally stable compared to MAPbI3. However, the α-FAPbI3 perovskite is not stable at room temperature as it converts to the undesirable δphase. Optimizing α-FAPbI3 stability is the main goal of my thesis, using ammonium-iodide-based large molecules. Several reports on the stabilization of FAPbI3 have shown that the 2D- Ruddlesden-Popperphase-doped 3D FAPbI3 enhancing the stability of the structure, albeit the bandgap is increased rendering a less optimal bandgap. The main achievement of this thesis is discovering that the doping of FAPbI3 with large alkyl ammonium moieties enhances efficiency and stability while maintaining the bandgap of pure α-FAPbI3 perovskite phase. I describe the resulting composition by the formula (A)xFAPbI3, where A represents the large alkyl ammonium iodide species and x ranges from 0 to 0.5. In the second chapter, I study the stabilization of α-FAPbI3 via doping with 5-amino valeric acid hydroiodide (AVAI). By using solid-state NMR, we demonstrate the atomic-level interaction between this molecular modulator and the perovskite lattice and propose a structural model of the stabilized three-dimensional structure, further aided by density functional theory (DFT) calculations. We find that onestep deposition of the perovskite in the presence of AVAI produces highly crystalline films with large, micrometer-sized grains and enhanced charge-carrier lifetimes, as probed by transient absorption Abstract ix spectroscopy. Optical measurements confirm that there is no effect on the bandgap of FAPbI3 after doping. The devices based on (AVAI)0.25FAPbI3 exhibit superior operational stability in comparison with neat FAPbI3 while achieving power conversion efficiency of 19%. In the next chapter, a similar approach based on benzylammonium iodide (BzI) has been used. The structural and optical characterization of films based on (BzI)xFAPbI3 composition (x = 0.05, 0.1, 0.25, 0.50) demonstrate that there is no 2D phase forming under these conditions. Moreover, solid-state NMR results show BzI interacting on the atomic level with α-FAPbI3 by binding to the 3D perovskite through hydrogen bonding interaction and stabilizing it against the detrimental α-to-δ phase transition. Perovskite solar cells based on the (BzI)0.25FAPbI3composition achieve power conversion efficiencies exceeding 20%, which is accompanied by enhanced shelf-life and operational stability, maintaining 80% of the performance after one year at ambient conditions. In third chapter, films based on (BzI)0.25FAPbI3 compositions are further investigated upon aging under the ambient conditions, as they show an unexpected transition from black to red color without transition to expected yellow δ phase, unlike the reference FAPbI3, showing a direct transition to δ phase after 2 days. I perform different measurements to investigate the nature of this red phase as a function of annealing temperature of (BzI)0.25FAPbI3 and compare these properties to t