This study reports on substantial improvement of the open-circuit voltage (Voc) of Cu2ZnSnSe4 (CZTSe) thin film solar cells by applying a passivation strategy to both the top and bottom interfaces of the CZTSe absorber, which involves insertion of a thin dielectric layer between the CZTSe and the surrounding layers. The study also presents in-depth material characterizations using transmission electron microscopy, energy dispersive X-ray spectroscopy, low-temperature photoluminescence, and secondary ion mass spectrometry, to reveal the effects of the interface passivation. To passivate the bottom Mo/CZTSe interface, a dielectric layer with patterned local contacts of dimensions down to ~100 nm was prepared using nanosphere lithography. With this, the Voc, short-circuit current, and fill factor (FF) were significantly enhanced due to reduction in carrier recombination at the bottom Mo/CZTSe interface. The top CZTSe/CdS interface was passivated by a thin dielectric layer which prevented inter-diffusion of Cd and Cu at the top interface, thereby improving the junction quality. Application of the top passivation layers resulted in substantial improvement in Voc and FF, thereby achieving the Voc deficit of 0.542 V which is the record value among reported CZTSe solar cells.