Fermi-Level-Tuned Nitrogen-Doped Carbon Nanotubes for Improved Electron Transport in Formamidinium-Rich Perovskite Solar Cells

Charge-transport losses arising from carrier-specific trapping limit charge collection and the fill factor (FF) of formamidinium-rich perovskite solar cells. Carbon nanotubes (CNTs) are promising candidates for mitigating these losses owing to their outstanding carrier mobility. Unfortunately, the Fermi level of undoped CNTs is located below the conduction band of the TiO2 electron transport layer, which may hinder electron delivery from the nanotubes. Herein, we present nitrogen-doped CNTs (NCNTs) with a tailored Fermi level as electron-selective additives for (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cells. Nitrogen doping lowers the work function of CNTs from 4.6 to 4.2 eV, favorably aligning the Fermi level for electron transfer toward TiO2. Taking advantage of this alignment, NCNTs substantially enhance electron conduction and promote electron delivery to the electron transport layer, preserving hole transport. By contrast, undoped CNTs reduce the electron current, which is possibly attributed to electron trapping at their deep Fermi level. The selective enhancement of electron transport by NCNTs reduces the series resistance and improves the FF, raising the power conversion efficiency from 14.65% to 16.45%. Work function tailoring of conductive nanocarbon additives by heteroatom doping offers a promising materials strategy for efficient electron extraction in perovskite solar cells.