This article explores the atomistic origins of high performance in hybrid halide perovskite solar cells. The study uses electronic structure calculations to investigate the materials chemistry and physics of the bulk perovskite. The key factors contributing to the high performance of hybrid perovskite solar cells include their optoelectronic properties, such as a suitable band gap, high optical absorption, and low carrier effective masses. Additionally, the materials are structurally and compositionally flexible, allowing for spontaneous electric polarization. This polarization can be tuned by choosing the organic cation, leading to internal junctions that aid in the separation of photoexcited electron-hole pairs and reduce recombination. The combination of a high dielectric constant and low effective mass promotes Wannier-Mott exciton separation and effective ionization of donor and acceptor defects. The photoferroic effect may enhance the open circuit voltage and contribute to the current-voltage hysteresis observed in perovskite solar cells. Hybrid perovskites, such as MAPbI3, have a unique structure with a polar organic cation at the center of a lead iodide cage. The presence of ferroelectric domains can lead to internal junctions that improve charge separation and reduce recombination. The study also investigates the electronic structure of MAPbI3, finding that the upper valence band is dominated by I 5p orbitals, while the conduction band is formed by Pb 6p. The band gap is predicted to be 1.67 eV, in good agreement with experimental measurements. The effective masses of both carriers are k-dependent, and the materials exhibit relativistic effects due to the heavy ions involved. The study also examines the role of defects and decomposition in hybrid perovskites. The materials are intrinsic semiconductors with relatively high doping densities. However, the solution processing of these materials is more likely to be kinetically controlled, leading to defects in the lattice. The Mott criterion predicts a transition to a degenerate semiconductor at low carrier concentrations. The study also discusses the potential for controlling carrier concentrations through doping and the degradation pathways of hybrid perovskites in the presence of Lewis acids. The spontaneous electric polarization of hybrid perovskites is a key factor in their performance. The magnitude of the bulk polarization has been probed using Berry phase calculations, and the calculated values are comparable to those of ferroelectric oxide perovskites. The strong polarization has two advantages for photovoltaic operation: enhanced charge separation and improved carrier lifetimes, as well as open circuit voltages above the band gap of the material. The internal electric field resulting from lattice polarization is crucial for these effects. The study also discusses the role of ferroelectric domains in hybrid perovskites, proposing that they contribute to exceptionally long carrier diffusion lengths. The molecular dipole can be tuned by modifying the organic cation, and the study suggests that manipulating theThis article explores the atomistic origins of high performance in hybrid halide perovskite solar cells. The study uses electronic structure calculations to investigate the materials chemistry and physics of the bulk perovskite. The key factors contributing to the high performance of hybrid perovskite solar cells include their optoelectronic properties, such as a suitable band gap, high optical absorption, and low carrier effective masses. Additionally, the materials are structurally and compositionally flexible, allowing for spontaneous electric polarization. This polarization can be tuned by choosing the organic cation, leading to internal junctions that aid in the separation of photoexcited electron-hole pairs and reduce recombination. The combination of a high dielectric constant and low effective mass promotes Wannier-Mott exciton separation and effective ionization of donor and acceptor defects. The photoferroic effect may enhance the open circuit voltage and contribute to the current-voltage hysteresis observed in perovskite solar cells. Hybrid perovskites, such as MAPbI3, have a unique structure with a polar organic cation at the center of a lead iodide cage. The presence of ferroelectric domains can lead to internal junctions that improve charge separation and reduce recombination. The study also investigates the electronic structure of MAPbI3, finding that the upper valence band is dominated by I 5p orbitals, while the conduction band is formed by Pb 6p. The band gap is predicted to be 1.67 eV, in good agreement with experimental measurements. The effective masses of both carriers are k-dependent, and the materials exhibit relativistic effects due to the heavy ions involved. The study also examines the role of defects and decomposition in hybrid perovskites. The materials are intrinsic semiconductors with relatively high doping densities. However, the solution processing of these materials is more likely to be kinetically controlled, leading to defects in the lattice. The Mott criterion predicts a transition to a degenerate semiconductor at low carrier concentrations. The study also discusses the potential for controlling carrier concentrations through doping and the degradation pathways of hybrid perovskites in the presence of Lewis acids. The spontaneous electric polarization of hybrid perovskites is a key factor in their performance. The magnitude of the bulk polarization has been probed using Berry phase calculations, and the calculated values are comparable to those of ferroelectric oxide perovskites. The strong polarization has two advantages for photovoltaic operation: enhanced charge separation and improved carrier lifetimes, as well as open circuit voltages above the band gap of the material. The internal electric field resulting from lattice polarization is crucial for these effects. The study also discusses the role of ferroelectric domains in hybrid perovskites, proposing that they contribute to exceptionally long carrier diffusion lengths. The molecular dipole can be tuned by modifying the organic cation, and the study suggests that manipulating the