| Colloidal quantum dots(CQDs)are attracting increasing attention because their excellent properties including tunable bandgaps,broad-band light absorption,solution processability and potential multiple exciton generation effect,making them suitable for low-cost next-generation electronic and optoelectronic device applications,especially photovoltaic devices.Recently,many techniques including ligand engineering of CQDs and device structural engineering have been used to improve the photovoltaic performance of Pb S CQD solar cells.Tetrabutylammonium iodide(TBAI)and 1,2-ethanedithiol(EDT)are the most widely used inorganic and organic ligands,respectively,for the passivation of PbS CQDs.Although heterojunction solar cells using TBAI-passivated PbS CQDs(PbS-TBAI)as the main light-absorbing layer and EDT-passivated PbS CQDs(PbS-EDT)as the hole-transport layer have achieved the power conversion efficiency(PCE)of more than 10% and exhibited better stability,these devices usually need short-term air exposure to reach their optimal performance.Unfortunately,the underlying mechanism still remains elusive.This work aims to understand the internal mechanisms and further guide the preparation and optimization of devices.We compared the photovoltaic performance and air stability of PbS CQDs/ZnO nanoparticle heterojunction solar cells with Pb S-TBAI,PbS-EDT and PbS-TBAI/PbS-EDT films as light absorption layers,respectively.Combing various surface chemistry,optical and electronic measurements,we have demonstrated the possible mechanisms at play that underpin the improved performance of PbS devices with bi-ligands after short-term air exposure,as well as their physicochemical origins.The main results are as follows:(1)ZnO nanoparticles and PbS CQDs with an excitonic absorption peak at 950 nm were synthesized.The optical properties,surface chemistry and band energy of PbS CQDs passivated by different ligands were determined by absorption spectroscopy,Fourier transform infrared spectroscopy and cyclic voltammetry measurements.The results showed that both TBAI and EDT ligands could achieve good ligand replacement for original oleic acid ligands on PbS CQDs with a small amount of residual oleic acid in the Pb S CQDs films.The conduction band minimum and valence band maximum of the Pb S CQDs treated with TBAI is-3.86 eV and-5.12 eV,while they increases to-3.74 eV and-4.99 eV,respectively,for the PbS CQDs treated with EDT.Among the three devices,the Pb S-TBAI/PbS-EDT device achieved optimal performance with an average PCE of 4.25%.In air exposure experiments,the PbS-TBAI/PbS-EDT device and Pb S-TBAI single ligand device exhibited similar performance evolution and achieved optimal performance after three days of air exposure.The PbS-EDT single ligand device exhibited poor air stability and its photovoltaic performance reduced to a quarter of the optimal value after three days of air exposure.(2)we have demonstrated the possible mechanisms at play that underpin the improved performance of the PbS-TBAI/PbS-EDT device after short-term air exposure,as well as their physicochemical origins using a combination of X-ray photoelectron spectroscopy,optical and electronic measurements.The results shows that the PbS-TBAI film plays a dominant role in the initial device performance improvement.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 their energy bands 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.The PbS-EDT film is p-type doped upon short-term air exposure(dozens of minutes)owing to the increase of Pb-O and/or Pb-OH species.However,after longer term(one to three days)air exposure,insulting layers can be formed on the surface of the PbS-EDT QDs and defect density increases due to EDT ligand desorption. |
PDF Full Text Request