Nanostructured hybrid ZnO@CdS nanowalls grown in situ for inverted polymer solar cells
Nanostructured three-dimensional hybrid ZnO@CdS (ZOCS) nanowalls fabricated on indium tin oxide (ITO) glass by in situ growth were explored as an electron transport layer (ETL) for inverted polymer solar cells (PSCs). Photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) and space charge limited current (SCLC) analysis show that the in situ growth of CdS on the surface of ZnO can not only passivate and repair the surface defects of ZnO to offer close contact and an efficient path for electron transport, but also act as a bridge for interfacial charge transfer to enhance the electron selectivity and reduce the recombination probability of electrons and holes, which is favorable for improving the Jsc and FF of a device. At the same time, compared to the bare ZnO, the hybrid ZOCS nanowalls show reduced work function and can spread the sunlight inside the active layer to improve the capturing efficiency of photons. The photophysics and electronic properties of the hybrid ZOCS nanowalls strongly depend on the thickness of the CdS layer, and a suitable thickness of the CdS layer improves the power conversion efficiency of inverted PSCs based on P3HT:PCBM from 3.18% for bare ZnO to 4.07% for ZOCS with enhanced Jsc and FF. Moreover, the close contact can also prevent oxygen and moisture diffusing into the active layers, which dramatically enhances the environmental stability.