Highly efficient, ultrathin (~400 nm) pure sulfide kesterite Cu2ZnSnS4 (CZTS) solar cells have been realized by interface reaction route controlling and self-organized nano-pattern at the back contact. The Al2O3 intermediate layer introduced at the Mo/CZTS interface can effectively inhibit the detrimental interfacial reaction between CZTS and Mo in the initial stage of sulfurization, and then turns into a self-organized nanopattern yielding a nanoscale opening for electrical contact. With this interface modification, the traditional issues of phase segregation (secondary phases) and voids at the back contact region can be well addressed, which greatly improves uniformity and reduces back contact recombination. As a result, this interface modification not only leads to beyond 8% ultrathin CZTS solar cells but also yields two certificated world record efficiencies: 9.26% for 0.237 cm2 small area and 7.61% for 1 cm2 standard kesterite CZTS solar cells (normal thickness).