The vapor-phase provides a unique capacity to encode precise composition and morphology in semiconductor materials and interfaces for energy-harvesting functionality. Here, we highlight recent work on the vapor-phase synthetic control of Si nanowires, photoelectrochemical interfaces, and hybrid perovskite materials. Together, these processes provide platforms to design chemically encoded, nanostructured systems for applications ranging from solar water splitting to photovoltaic solar cells. First, we show how abrupt transitions between p-type, intrinsic, and n-type silicon allow nanowire p-i-n superlattices to be synthesized that behave as multijunction photovoltaic devices with extraordinarily large photovoltages. Using spatio-selective photoelectrochemical deposition of hydrogen and oxygen-evolving co-catalysts, water splitting particle suspensions are demonstrated. Second, we show how planar silicon interfaces can be functionalized with nanoscale oxide and graphene layers, facilitating the integration of molecular catalysts for solar-driven CO2 reduction. Finally, we demonstrate the first metal organic chemical vapor deposition (MOCVD) growth of methylammonium lead iodide (MAPbI3). Use of separate vapor precursors for the lead, organic, and halide components allows the tuning of reaction conditions to grow the material directly with high purity. Overall, the projects highlight the precise and tunable control of material composition, morphology, and functionality provided by the vapor phase.