ZnO/Ti3C2Tx monolayer electron transport layers with enhanced conductivity for highly efficient inverted polymer solar cells

MXenes, a novel intriguing family of two-dimensional (2D) transition metal carbides and nitrides, have a wide spectrum of applications owning to their unique optical and electronic properties. Herein, we use Ti3C2Tx, a representative of MXenes, as an additive in zinc oxide (ZnO) to fabricate novel ZnO/Ti3C2Tx nanohybrid composite film. The addition of Ti3C2Tx nanosheets constructs new electron transport pathways between the ZnO nanocrystals, and passivates the surface of ZnO by forming the Zn-O-Ti bonding on the ZnO surface. The novel ZnO/Ti3C2Tx nanohybrid film exhibits excellent photoelectric characteristics, and is used as electron transport layers (ETLs) in fullerene and non-fullerene polymer solar cells for the first time. As a result, the power conversion efficiency (PCE) of the photovoltaic devices based on PBDB-T:ITIC with the ZnO/Ti3C2Tx ETLs is 12.20%, up from 10.56% for the corresponding device utilizing pristine ZnO as ETL, a relative increase of 15.53%. Moreover, PM6:Y6 based IPSCs achieve a champion PCE of 16.51% from 14.99% for the reference device, suggesting the good applicability of the ZnO/Ti3C2Tx ETL. The enhancement of PCE is mainly due to the increased transfer and collection of charges in IPSCs. More interestingly, devices based on ZnO/Ti3C2Tx composite ETL display relatively good stability compared with the control device. The layered Ti3C2Tx should be responsible for such enhancement.