View article

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO_x) and vanadium oxide (V₂O_x) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO_x HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO_x and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO_x. The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO_x layer.