Air exposure oxidation and photooxidation of solution-phase treated PbS quantum dot thin films and solar cells

The air exposure oxidation mechanisms of PbS quantum dot (QD) thin films and solar cells are studied in the current paper. As a novel and advantageous method, QD thin films were prepared by the single-step deposition of colloidal QDs treated with different ligands of butylamine (BA), mercaptopropionic acid (MPA), tetra-butylammonium iodide (TBAI), methylammonium iodide (MAI) and methylammonium lead triiodide (MAPbI(3), perovskite). Photoluminescence (PL) measurements evaluated the stability of different surface treated PbS QDs during the colloidal to thin film transformation, and over the air exposure times. Blue-shift and quenching the PL spectra suggested rapid oxidation of QD thin films at the first times of air exposure. However, the oxidation rates significantly reduced for the QD thin films treated with organic MPA and all the inorganic ligands. According to the X-ray photoelectron spectroscopy (XPS) studies, thin films oxidation accompanied with the formation of PbSO3 and PbSO4 compounds on the (100) facets of PbS QDs. Although MAPbI(3) treatment led to the complete passivation of QDs in the air, perovskite shelling partially oxidized to PbO and PbCO3 compounds. Furthermore, the PL enhancement phenomenon observed at the first times of air exposure for the TBAI and MAPbI(3)-treated QD thin films, as a result of their strong surface passivation as well as the photoenhancement and photoelectrification mechanisms. Eventually, p-n and p-i-n structured solar cells were fabricated by the single-step deposition of solution-phase treated PbS QD inks. In this case, inorganic surface treatments not only increased the power conversion efficiency (PCE) of solar cells, but also led to a high stability of fabricated devices in the air environment (lower than 1% PCE loss after 500 h of storage in the air).