Multifunctional Carbon Nanoparticle Interlayer Enables Efficient Hole-Transport-Layer-Free Carbon-Based Perovskite Solar Cells

Hole-transport-layer-free carbon-based perovskite solar cells (HTL-free C-PSCs) offer simplified fabrication and enhanced stability but suffer from interfacial contact issues, surface defects, and energy-level misalignment, limiting their power conversion efficiency (PCE). This work develops a “three-in-one” strategy employing a bilayered carbon rear electrode featuring a thin interlayer of tetrabutylammonium iodide (TBAI)-incorporated carbon nanoparticles (∼10 nm) and a conductive carbon layer composed of graphite flakes and carbon black. TBAI acts as a dispersant, enabling deposition of a uniform interlayer that forms a seamless perovskite/carbon contact and facilitates rapid interfacial charge transfer. Furthermore, TBA⁺ in the interlayer reacts with residual PbI₂ to form a 1D perovskite (TBAPbI₃) while I^– fills iodine vacancies, passivating surface defects and suppressing nonradiative recombination. Additionally, the TBAI incorporated interlayer exhibits an increased work function (WF) of 5.21 eV, establishing a cascade band alignment between the valence band of MAPbI₃ (5.5 eV) and the WF of the conductive carbon layer (5.0 eV) to enhance hole extraction. This synergistic approach yields HTL-free C-PSCs with a PCE of 14.84%, representing a 20.2% increase over devices without the interlayer.