Eliminating Fermi-level pinning in PbS quantum dots using an alumina interfacial layer

Through a systematic approach we show that the insertion of a thin alumina layer in between a PbS QD layer and an Au substrate can eliminate Fermi level pinning. In this study band edge energies of different sized PbS QD monolayers with different cross-linkers were measured by using ultraviolet photoelectron spectroscopy and electrochemistry. When PbS QDs were immobilized directly on the Au, the measured valence band maximum was found to be insensitive to changes in the QD size or cross-linker indicating Fermi level pinning of the QD valence band to the Au Fermi level. After insertion of a thin film of alumina in between the PbS quantum dot monolayer film and the Au substrate, the measured valence band position revealed a shift that depended on ligand and QD size. These results identify a general method for eliminating Fermi level pinning in QDs and an approach for predictably controlling the energetics at QD-metal interfaces which is beneficial for improving the performance of QD based solar cells.