Unravelling the origin of the photocarrier dynamics of fullerene-derivative passivation of SnO2 electron transporters in perovskite solar cells

Abstract

Fullerene-passivated SnO₂ electron transport layers (ETLs) offer a route for a continuous boost in the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs). However, a detailed understanding on the photocarrier dynamics in PSCs consisting of fullerene-passivated SnO₂ ETLs is still lacking. Here, we use ultrafast pump-probe transient absorption spectroscopy to analyze the corresponding photocarrier dynamics across fullerene-modified SnO₂ ETLs in PSCs. Both the C₆₀ pyrrolidine tris-acid (CPTA)-modified SnO₂ ETL and 6,6-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM)-modified SnO₂ ETL which have a favorable energy band edge alignment to perovskites, result in an enhanced electron injection rate after introducing these two fullerenes into SnO₂-based PSCs compared to the SnO₂ ETL without modification. Moreover, combining chemical measurements and density functional theory simulation results, it is confirmed that the interfacial chemical bond is tailored between the SnO₂ surface and carboxylic acids of CPTA, thus establishing interface dipoles. Due to the existence of interface dipoles, the chemisorption of the CPTA fullerene functions as a recombination suppressor both at the CPTA-passivated SnO₂/perovskite interface and inside perovskites. By contrast, the physisorption of the PC₆₁BM fullerene on the SnO₂ ETL exhibits relatively high recombination rates both at the SnO₂/perovskite interface and inside perovskites. Therefore, the PSC based on the SnO₂/CPTA ETL yields a PCE of more than 19%, which is superior to that of the reference PSC using PC₆₁BM as a passivator (<18%). This study describes that interfacial carrier dynamics in PSCs consisting of fullerene-modified ETLs can be clearly resolved using ultrafast spectroscopy, which further provides guidance for future design for interfacial modification of ETLs.