The effort to develop earth-abundant kesterite solar cells has led to an approximate doubling of the power conversion efficiency over the past five years to 12.6%, primarily due to increases in short-circuit current and fill factor; open-circuit voltage has resisted similar change, limiting further efficiency improvement. In the present investigation, Auger nanoprobe spectroscopy, X-ray/ultraviolet photoelectron spectroscopy, and device characterization are used to provide a comprehensive understanding of the role of grain boundaries and interfaces in limiting performance in kesterite-based devices. High photovoltaic performance is found to correlate with grain boundaries that are Cu-depleted and enriched with SnOx. The formation of this bulk-like oxide at grain boundaries with type I band offset provides a unique effective passivation that limits electron-hole recombination. Building on these new insights, photovoltaic device simulations are performed that show optimized electrostatic designs can compensate for bulk defects, allowing efficiencies closer to the theoretical limit. The effect of top surface and grain boundary composition, measured by Auger nanoprobe microscopy, on the performance of polycrystalline copper-zinc-tin-sulfide/selenide (CZTSSe) solar cells is demonstrated. Using photovoltaic device simulation, an approach to overcome the bulk defects and further improve the efficiency of the solar cells is proposed.