Co/Eu co-doped electron transport layer enhances charge extraction and light absorption for efficient carbon-based HTM-free perovskite solar cells

Carbon-based hole-transport materials (HTM)-free perovskite solar cells (C-PSCs) have received increasing attention in photovoltaic fields thanks to the low priced and superior stability. Nevertheless, the C-PSCs still suffer from low power conversion efficiencies (PCEs) in comparison with conventional PSCs. Here, we report a bimetal Co/Eu co-doping strategy to modify the electron transportation layer (ETL), simultaneously improving the charge extraction and light absorption and thus the performance of HTM-free C-PSCs. Systematic experiments suggest that (i) Co doping passivates the electronic trap-states in TiO2 ETL, improving the charge transport properties; (ii) Eu doping enhances the photoresponse of PSCs. Combining them together in ETL, all the photovoltaic parameters of C-PSCs are appreciably enhanced, leading to a PCE leap from 11.12 to 14.06% while keeping a superior device stability. These results may provide possibilities for the exploration of high-efficiency C-PSCs by further optimizing ETL with suitable doping.