Origin of the effects of PEG additives in electrolytes on the performance of quantum dot sensitized solar cells

It has been well established that polymer additives in electrolyte can impede the charge recombination processes at the photoanode/electrolyte interface, and improve performance, especially V(oc), of the resulting sensitized solar cells. However, there are few reports about the effect of electrolyte additives on counter electrode (CE) performance. Herein, we systematically investigated the effect of polyethylene glycol (PEG) additives with various molecular weights (M(w) from 300 to 20 000) in polysulfide electrolyte on the performance of two representative CdSe and Zn–Cu–In–Se (ZCISe) quantum dot sensitized solar cells (QDSCs), and explored the mechanism of the observed effects. Electrochemical impedance spectroscopy measurements indicate that all PEG additives can improve the charge recombination resistance at the photoanode/electrolyte interface, therefore suppressing the unwanted charge recombination process, and enhancing the V(oc) of the resulting cell devices accordingly. On the CE side, with the increase of M(w) of PEG additives, the initial effect of reducing the charge transfer resistance at the CE/electrolyte interface evolves into an increasing resistance; accordingly the initial positive effect on FF turns into negative one. Accordingly, low M(w) PEG can improve efficiency for both CdSe (increasing from 6.81% to 7.60%) and ZCISe QDSCs (increasing from 9.26% to 10.20%). High M(w) PEG is still effective for CdSe QDSCs with an efficiency of 7.38%, but falls flat on ZCISe QDSCs (with an efficiency of 9.11%).