  Sulfide Kesterite Cu2ZnSnS4 (CZTS), a nontoxic and low-cost photovoltaic
material, has always being facing severe charge recombination and poor carrier
transport, resulting in the cell efficiency record stagnating around 11% for
years. Gradient bandgap is a promising approach to relieve these issues,
however, has not been effectively realized in Kesterite solar cells due to the
challenges in controlling the gradient distribution of alloying elements at
high temperatures. Herein, targeting at the Cd alloyed CZTS, we propose a
pre-crystallization strategy to reduce the intense vertical mass transport and
Cd rapid diffusion in the film growth process, thereby realizing front
Cd-gradient CZTS absorber. The Cd-gradient CZTS absorber, exhibiting downward
bending conduction band structure, has significantly enhanced the minority
carrier transport and additionally improved band alignment and interface
property of CZTS/CdS heterojunction. Ultimately, we have achieved a champion
total-area efficiency of 13.5% (active-area efficiency: 14.1%) in the cell and
in particular a high open-circuit voltage of >800 mV. We have also achieved a
certified total-area cell efficiency of 13.16%, realizing a substantial step
forward for the pure sulfide Kesterite solar cell.
