Surface Passivation of Lead Sulfide Nanocrystalswith Low Electron Affinity Metals:Photoluminescence and Photovoltaicperformance

Abstract

During the last decade, solution-processed colloidal quantum dots (CQDs) have attracted significantattention for low-cost fabrication of optoelectronic devices. In this study, lead sulfide (PbS) CQDs weresynthesized via the hot injection method and the effect of doping elements with low electron affinity,including cadmium, calcium and zinc, on the passivation of trap states was investigated. A red-shiftin the luminescence emission was observed by doping through passivation of lead dangling bonds.Time-resolved photoluminescence measurements showed that the lifetime of charged carriers wassignificantly enhanced by cadmium doping (B80%) which is quite noticeable compared with calciumandzinc-doped nanocrystals. External quantum efficiency measurements on thin solid films (B300 nm)prepared by spin coating supported improved lifetime of carriers through passivation of mid-gap trapstates. In order to show the potential application of the doping process, bulk heterojunction CQD solarcells were fabricated. It was found that the power conversion efficiency (PCE) was improved up toB40%; the highest improvement was observed with the Cd treatment. Finally, density functional theory(DFT) and electrochemical impedance spectroscopy (EIS) were employed to study the effect of dopingon the density of states. The results showed that doping with low electron affinity metals effectivelyreduced the deep trap states of PbS QDs. Figure 1