Metal-Free Inverted Perovskite Solar Cells with N-DMBI-Doped Single-Walled Carbon Nanotubes for Enhanced Charge Transport

4-(1,3-Dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)-N,N-dimethylaniline (N-DMBI) was employed as an n-dopant for single-walled carbon nanotube (SWCNT) thin films, enabling the fabrication of an inverted perovskite solar cell with the structure ITO/hole transport layer (HTL)/CH₃NH₃PbI₃/6,6-phenyl-C61-butyric acid methyl ester (PCBM)/N,N'-bis[3-(dimethylamino)propyl]perylene-3,4,9,10-tetracarboxylic diimide (PDIN)/N-DMBI-doped SWCNT. This structure eliminates the need for conventional metallic back electrodes. When poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) was used as HTL, a power conversion efficiency of 9.9% was achieved. Substituting PEDOT:PSS with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) further improved the PCE to 10.1%. The long-term stability of the device was evaluated under a nitrogen atmosphere for 50 days. The PCE of devices with Ag electrodes decreased by 50%, whereas those with SWCNT electrodes retained 90% of their initial performance. N-DMBI doping of SWCNT thin films reduced the sheet resistance from 73.4 Ω/sq to 32.6 Ω/sq and shifted the ionization potential to a shallower value. Furthermore, the Seebeck coefficient changed from +65.7 μV/K to -22.5 μV/K, confirming the successful formation of n-type SWCNTs. X-ray photoelectron spectroscopy revealed that doping with N-DMBI or PDIN effectively removed oxygen and water molecules from the SWCNT surface. This study introduced PDIN as an interfacial layer between PCBM and the SWCNTs, preventing direct contact between these layers. This structure exhibited J-V characteristics with reduced losses in open-circuit voltage and fill factor by suppressing carrier traps.