N‐Type Surface Design for p‐Type CZTSSe Thin Film to Attain High Efficiency

Abstract As a low‐cost substitute that uses no expensive rare‐earth elements for the high‐efficiency Cu(In,Ga)(S,Se)₂ solar cell, the Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n‐type Ag₂ZnSnS₄ is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron‐hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved V_OC and J_SC. Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open‐circuit voltage deficit reduced to as low as 0.306 V (63.2% Shockley‐Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.