Emerging Three-dimensional and Two-dimensional Hybrid Halide Perovskites: From Synthesis to Thin Film Properties and Solar Cell PerformancePublic Deposited
Three-dimensional (3D) and two-dimensional (2D) hybrid halide perovskites have emerged as front-runners in solar energy conversion applications with the potential to provide low-cost renewable energy. Being at the interface of chemistry, physics, materials science, and electrical engineering, the field of perovskite solar cells has become a top area of interest for researchers from various areas of expertise. The work in this dissertation explores these emerging perovskites from both materials and solar cell device engineering perspectives. The main motivations in this dissertation are: (1) to discover new 2D perovskite materials and crystal structures by manipulating the parent 3D perovskite compounds, (2) to understand the film formation properties and optimize the microstructures to achieve highly performing solar cell devices, and (3) to address the stability issue of the perovskite solar cells. A typical solar cell device is comprised of a perovskite light absorber layer sandwiched between electron and hole injection/collection layers. Delivery of excellent solar cell performance encompasses optimization of light absorption, charge extraction and charge collection via careful manipulation of each layer in the device stack. One important illustration in this work is that the hydrohalic acid “additives” typically employed in the precursor solution prior to spin-casting to improve CH3NH3PbI3 perovskite film morphology are not “innocent” and induce subtle changes in the perovskite crystal structure and optical bandgap. For example, we observe a monotonic decline in the tetragonal → cubic phase transition temperature with increasing HX concentration, resulting in the stabilization of the high-temperature cubic phase at room temperature for the perovskite films treated with high HX concentrations. In addition, this thesis uncovers the fundamental synthetic and thermodynamic limitations present in the 2D homologous series of (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 when going towards higher n-members, illustrating that n = 5 is the limit in this series to crystallize in phase-pure form, while n = 6 and 7 can be formed but only as intergrowths with secondary phases. This phenomenon is rationalized by the substantial increase in the enthalpy formation energies of the 2D compounds when going above n = 5. When the crystals of this series are solution-processed for thin film fabrication, their crystallinity and crystal growth orientation in thin films are found to be sensitive to the perovskite layer thickness n. The work in this dissertation presents controllable tuning of the high n-member compounds for better crystallinity and crystal growth orientation in the thin films via solvent-engineering to attain enhanced solar cell performance.