Highly Stable Interdigitated PbS Quantum Dot and ZnO Nanowire Solar Cells with an Automatically Embedded Electron-Blocking Layer

Abstract

We have constructed heterojunction iodide ligand PbS quantum dot (QD) and ZnO nanowire (NW) solar cells. In these interdigitated structures, PbS QDs are well-embedded within ZnO NWs grown on a dense ZnO layer. A Au back contact is directly formed on the iodide ligand PbS QD surface layer. In the widely studied colloidal QD-based heterojunction solar cells, the PbS QD active layer is sandwiched between the hole-blocking layer (or electron-accepting layer) and the electron-blocking layer (EBL) (or hole-transport layer). In contrast, our solar cells contain no EBL except an additional thin PbS QD capping layer, which was deposited on the interdigitated layer to ensure the complete infiltration of the PbS QDs, and prevent shortening between the ZnO NWs and Au back contact. The interdigitated layer was constructed using a layer-by-layer dip-coating method to achieve sufficient infiltration of PbS QDs in the ZnO NWs. Owing to highly efficient charge separation in the interdigitated structure, our cell achieved a high external quantum efficiency in the visible and infrared regions, despite using only iodide ligand PbS QDs. Using cross-sectional Kelvin probe force microscopy, we confirmed that the PbS QD capping layer formed between the ZnO NW and Au back contact intrinsically functioned as an EBL. Consequently, the solar cells with ZnO NWs and iodide ligand PbS QDs maintained their performance after 500 days of storage in air without encapsulation. This stability performance surpasses most of the previously reported solar cells with PbS QDs. An encapsulated 1.0 cm2 solar cell maintained its power output for over 75 h under continuous one-sun illumination with no significant degradation. The proposed solar cell architecture with an automatically embedded EBL can realize highly efficient and stable colloidal QD-based solar cells. Moreover, the interdigitated architecture concept can be extended to various organic and inorganic hybrid solar cells.