Ultrasonication‐Assisted Seed Screening Enables Oriented and Efficient Low‐Dimensional Crystal‐Structural Thin‐Film Photovoltaics

Abstract Ultrasonication‐assisted liquid exfoliation of layered materials is widely used to produce a large number of 2D nanosheets due to its simplicity, universality, and mass production. Here, the utility of ultrasonication is extended from liquid exfoliation to seed screening in the case of layered GeSe and GeS. It is found that the ultrasonic stripping force can also be used to selectively remove the lying seeds interacting with substrate through weak van der Waals (vdW) forces while retaining the standing seeds connected with substrate via tightly covalent bonds. The standing seeds then induce the growth of 2D crystal‐structural films with standing orientation. This thereby enables efficient carrier transport within covalently bonded layers, instead of poor transport between layers held together by vdW forces. The resulting standing‐oriented GeSe films exhibit a 21‐times increase in carrier mobility compared to lying‐oriented films. As a result, this study demonstrates the highest power conversion efficiencies of 6.1% and 10.5% under AM1.5G 1‐ and 0.01‐sun illumination reported for GeSe solar cells, respectively.