Solution phase surface functionalization of PbS nanoparticles with organic ligands for single-step deposition of p-type layer of quantum dot solar cells

Semiconducting p-type layers of heterojunction quantum dot solar cells (QDSCs) are often prepared by the layer-by-layer (LBL) method. Due to the intermittent nature of quantum dot (QD) deposition, solid-state ligand exchange and washing steps, the LBL process typically involves a high waste of materials and a relatively low surface quality of the applied layers. To introduce a simplified method for deposition of QD layer, this paper investigates the preparation of pre-exchanged QD inks for the single-step deposition of QD layers of QDSCs. Various pre-exchanged QD inks were prepared through the solution phase exchange of oleate ligands with organic ligands of different functional groups, chain lengths and chain structures. Oxidation and colloidal stability of the prepared inks during long-time air exposure storage were investigated by the means of UV-Vis-NIR spectroscopy, photoluminescence (PL) and zeta potential measurements. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) and combustion CHNS analyses were used to study the surface chemistry of the exchanged QDs and the mechanism of ligand exchange. Various inks of pre-exchanged QDs were deposited on different substrates in a single-step method. The best surface topography and the lowest oxidation rate were achieved for layers prepared by the single-step deposition of 3-mercaptopropionic acid (MPA)-capped QD ink. The QDSCs fabricated by such layers had higher efficiency than those fabricated by the common LBL method. Furthermore, annealing the MPA-capped QD layer increases the power conversion efficiency of the ink based solar cells up to 3.22% under simulated AM1.5 solar illuminations.