Selenium‐Graded Sb 2 (S 1−x Se x ) 3 for Planar Heterojunction Solar Cell Delivering a Certified Power Conversion Efficiency of 5.71%

To efficiently convert solar energy into electricity at low cost with long‐term stability is one of the major tasks in solar cell research and applications. Antimony sulfide‐selenide [Sb 2 (S 1−x Se x ) 3 ] with a tunable bandgap in the range of 1.1–1.8 eV are considered promising photovoltaic materials due to their low‐toxicity, long‐term durability, and abundant element availability. Herein, selenium‐graded Sb 2 (S 1−x Se x ) 3 is synthesized through diffusion controlled solid‐state reaction between selenium and pre‐formed Sb 2 S 3 film. In the device, sulfur‐rich Sb 2 (S 1−x Se x ) 3 with large bandgap leads to high voltage output, while narrow‐bandgap selenium‐rich Sb 2 (S 1−x Se x ) 3 expands spectral response toward longer wavelength. As a consequence, the device yields an open‐circuit voltage comparable to Sb 2 S 3 solar cell, along with a significantly enhanced photocurrent density of 19.43 mA cm −2 , finally delivering a certified power conversion efficiency of 5.71%, which is the highest certified value in planar heterojunction solar cells based on Sb 2 (S 1−x Se x ) 3 . Initial stability examination shows that the device can maintain 88% efficiency after storing for 90 days in moderate humidity and ambient light irradiation. This investigation offers an effective strategy to the fabrication of composition‐graded Sb 2 (S 1−x Se x ) 3 for long‐term stable devices. The methodology may be extended for the fabrication of a broad class of composition‐graded metal sulfide/selenide for solar cell performance enhancement.