Optoelectronic materials are an important driver of many modern technologies. These materials couple their interaction with light to their electronic properties in a range of—often tunable—ways. Responses such as generation of photocurrent from incident light or generation of light by electro- or photoluminescence are vital to many existing and emerging applications. Other more complex linear and nonlinear properties offer unique avenues for optical control. In this dissertation, we will explore a particular class of optical response, where DC photocurrent is generated in a homogeneous material without an interface. This bulk photovoltaic effect (BPVE) shows promise for sensing applications, characterization of quantum materials, and higher efficiency power conversion, but some aspects remain poorly understood. We first outline a materials design strategy to predict distortions that will have a significant impact on the BPVE and use this method to better understand the photoresponse of monolayer MoS2. Then, we discuss two potential mechanisms that could contribute to the BPVE. While these mechanisms have been discussed in model systems, it is difficult to determine their relevance in real materials. We outline new methods for calculating the current contribution due to electron-phonon scattering that occurs during light excitation, and we estimate another current that arises during carrier recombination. From first-principles calculations, we predict the contribution of these mechanisms to the BPVE in several prototypical materials. Then, we turn our attention to hybrid organic-inorganic perovskites, which are a class of materials well known for their excellent optoelectronic performance. The presence of organic components in these materials lends a high degree of tunability and leads to complex dynamical behavior. We use first principles study to better understand hybrid perovskites. We model the thermodynamic, electronic, and optical properties of a perovskite in a novel structure that incorporates several different organic components that each template different structural motifs. Then, we study how cation rotations and vibrations can affect the optical response. Together, these reports provide a perspective on how ab initio modeling can contribute to the design and understanding of optoelectronic materials.