Interface engineering <i>via</i> pre-engineered black phosphorus quantum dots for highly efficient carbon-based hole-transport-layer-free perovskite solar cells

Planar, carbon-electrode-based perovskite solar cells (C-PSCs) without a hole transport layer (HTL) are highly attractive due to their simple fabrication, low cost, and scalability. However, their performance is often limited by inefficient physical and electrical contact at the perovskite/carbon interface, which impedes hole extraction and promotes charge recombination. This study introduces a pre-engineered, multifunctional interlayer for HTL-free C-PSCs utilizing tetrabutylammonium ion (TBA⁺)-intercalated black phosphorus quantum dots (BPQDs). The TBA⁺ intercalation during synthesis pre-engineers the BPQDs with enhanced conductivity, a raised valence band maximum (-5.27 eV), and defect-passivation capabilities. This creates a favorable cascade energy-level alignment between the perovskite absorber (-5.5 eV) and the carbon electrode (-5.0 eV), thereby facilitating efficient hole extraction. The BPQDs interlayer also ensures seamless perovskite/carbon contact, promoting interfacial charge transfer. Additionally, TBA⁺ ions released from BPQDs effectively passivate defects on the perovskite surface, suppressing nonradiative recombination. Consequently, the optimized devices achieve a power conversion efficiency (PCE) of 17.08%, which is 24.1% and 11.9% higher than that of control devices without an interlayer (13.76%) and with a pristine BPQDs interlayer (15.26%), respectively. Furthermore, the encapsulated devices demonstrate improved operational stability, retaining 89.1% of their initial PCE after 360 hours under 1-sun illumination at 85 °C and 85% relative humidity.