Although CuIn1-xGaxSe2-yS2 (CIGS) chalcopyrite and Cu2ZnSn(S,Se)4 (CZTSSe) kesterite-related films offer significant potential for low-cost high-efficiency photovoltaic (PV) devices, the complicated multi-element nature of these materials generally leads to the requirement of more complex and costly deposition processes. This talk focuses on employing the unique solvent properties of hydrazine to solution-deposit CIGS and CZTSSe films for high-performance solar cells. CIGS films are deposited by completely dissolving all elements in hydrazine, solution-depositing a molecular precursor film, and heat treating in an inert atmosphere, to yield a single-phase chalcopyrite film (no post-deposition selenization required). Trace additions of Sb improve grain structure in the resulting film and enhance device performance. Devices based on a glass/Mo/spin-coated CIGS/CdS/i-ZnO/ITO structure yield power conversion efficiencies of as high as 13.6% (AM1.5 illumination; NREL certified). Analogous CZTSSe absorber layers have been processed using a hybrid hydrazine-based slurry approach, enabling liquid-based deposition of kesteritetype films and resulting device efficiencies of as high as 9.6% (AM1.5 illumination; NREL certified)-exceeding the previous kesterite performance record by ∼40%. The combination of improved efficiency, In-free absorber and solution-based processing opens opportunities for development of a low-cost and pervasive technology.