Towards understanding the initial performance improvement of PbS quantum dot solar cells upon short-term air exposure

An initial improvement in performance of PbS quantum dot solar cells composed of one thick layer of PbS quantum dots (QDs) treated with tetrabutylammonium iodide (PbS-TBAI) and one thin layer of PbS QDs capped with 1,2-ethanedithiol (PbS-EDT) over short-term air exposure is widely observed. However, the underlying mechanisms still remain elusive. In the work, we sought to understand the mechanisms as well as their physicochemical origins using a combination of X-ray photoelectron spectroscopy (XPS) and various electronic measurements. It is found that the PbS-TBAI film plays a dominant role in the initial device performance improvement compared with the PbS-EDT film. The PbS-TBAI film is compensation doped upon short-term air exposure (one to three days) owing to the increase of Pb-O and/or Pb-OH species, enabling its energy band to align better with the electron transport layer for more efficient charge extraction. Moreover, it is demonstrated that the short-term air exposure is capable of reducing defects in the devices and improving the diode quality, resulting in an initial increase in device performance. This work contributes to the fundamental understanding of the surface chemistry changes of PbS quantum dots treated by different ligands over air-exposure and the role of surface chemistry of quantum dots in optimizing their photovoltaic performance.