This dissertation study mainly falls into two parts: simulation study and experimental investigation of the process-structure-property-performance relationship in perovskite solar cells. Herein, a controllable fabrication of annealing-free perovskite films with tunable crystal grain size and morphology via a seeded approach has been developed. Specifically, a solution of lead iodide (PbI_2) was spin-coated on a substrate, and a low concentration solution of Methylammonium iodide (MAI) was dropped onto the PbI_2 film to form perovskite seed before introducing high concentration solution of MAI. The fast, annealing-free seeded nucleation and growth leads to dense and uniform perovskite thin films exhibited controllable crystal grains. In another project, a polymer additive assisted approach to facilitate the growth of uniform, dense, and ultra-smooth perovskite thin films has also been demonstrated. In specific, a polymer, Polyamidoamine (PAMAM) dendrimers, was incorporated into the blend solution of lead iodide (PbI_2) and Methylammonium iodide (MAI) to regulate the nucleation and growth thereby tuning the morphology and crystallinity. The PAMAM addition not only realized compact perovskite thin films without pinholes in it, but also increased the stability. In the simulation study, both the organic bulk heterojunction solar cells and pervoskite solar cells have been systematically investigated to help understand the device operation and guide the experiments.Different electron transport layers (ETL) and hole transport layers (HTL) were used to study the effect of band gap alignment with adjacent layers and improve the transport of charges. The change in band gap not only facilitated in collection of charges but also improved the overall power conversion efficiency (PCE) of the device in study. Recombination of charges in the bulk active region and its effect on overall PCE was also studied.