Coherent-Twinning-Enhanced Solar Water Splitting in Thin-Film Cu₂ZnSnS₄ Photocathodes

Grain boundary engineering is a proven strategy to improve the efficiency of thin-film solar water splitting. In particular, a photoelectrode with grain-through twins could exhibit efficient charge separation and transport properties. Herein, Cu2ZnSnS4 (CZTS) thin films with coherent twin boundaries (CTBs) were fabricated on Mo-coated soda-lime glass (SLG) substrates via a space-closed sulfurization method. The amount of sulfur addition during annealing determines the density of crystallographic CTBs in CZTS grains. When used as a light absorber in PEC water splitting, the richly twinned Pt/CdS/CZTS photocathode exhibited an exceptionally high photocurrent of −14.9 mA/cm2 at 0 V (vs the reversible hydrogen electrode (RHE), AM 1.5 G), greatly exceeding that of nontwinned (−5.7 mA/cm2) and poorly twinned (−8.7 mA/cm2) counterparts. Systematic studies further revealed that the enhanced PEC performance can be attributed to the myriad minibands of Schottky barriers formed by the high proportion of ordered twins in CZTS films, which effectively facilitated the bulk charge separation and transport efficiency.