PbS Quantum Dots: Size, Ligand Dependent Energy Level Structures and Their Effects on the Performance of Heterojunction Solar Cells

Abstract

The conduction band minimum and valence band maximum of colloidal PbS quantum dots with different sizes (2.6-4.5 nm) and different surface ligands (oleic acid or tetrabutylammonium iodide, TBAI) were determined by cyclic voltammetry and absorption measurements. Furthermore, the effect of quantum dot sizes on the performance of PbS quantum dots/TiO2 heterojunction solar cells prepared in air was also studied. The results show that energy level structures of PbS quantum dots strongly depend on the size and ligand on the surface. As the quantum dot size increases from 2.6 nm to 4.5 nm, the conduction band minimum of pristine PbS quantum dots with oleic acid ligands 916 无 机 材 料 学 报 第 31 卷 decreases from –3.67 eV to –4.0 eV, while their valence band maximum increases from –5.19 eV to –4.97 eV. However, for PbS quantum dots passivated by TBAI ligands, their conduction band minimum and valence band maximum change from –4.15eV and –5.61 eV to –4.51eV and –5.46 eV, respectively. Devices made of the PbS quantum dots with size of 3.9 nm show the highest power conversion efficiency of 2.32%, which can be ascribed to their proper bandgap, crystal quality and favorable energy-level alignment at the PbS quantum dot /TiO2 interface.