Colloidal Precursor-Induced Growth of Ultra-Even CH₃NH₃PbI₃ for High-Performance Paintable Carbon-Based Perovskite Solar Cells

Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have attracted intense attention due to their relatively high stability. However, their power conversion efficiency (PCE) is still low, especially for the simplest paintable carbon-based PSCs (C-PSCs), whose performance is greatly limited by poor contact at the perovskite/carbon interface. To enhance interface contact, it is important to fabricate an even-surface perovskite layer in a porous scaffold, which is not usually feasible due to roughness of the crystal precursor. Herein, colloidal engineering is applied to replace the traditional crystal precursor with a colloidal precursor, in which a small amount of dimethyl sulfoxide (DMSO) is added into the conventional PbI₂ dimethylformamide (DMF) solution. After deposition, PbI₂(DMSO) adduct colloids (which are approximately tens of nanometers in size) are stabilized and dispersed in DMF to form a colloidal film. Compared with PbI₂ and PbI₂(DMSO) adduct crystal precursors deposited from pure DMF and DMSO solvents, respectively, the PbI₂(DMSO) adduct colloidal precursor is highly mobile and flexible, allowing an ultra-even surface to be obtained in a TiO₂ porous scaffold. Furthermore, this ultra-even surface is well-maintained after chemical conversion to CH₃NH₃PbI₃ in a CH₃NH₃I solution. As a result, the contact at the CH₃NH₃PbI₃/carbon interface is significantly enhanced, which largely boosts the fill factor and PCE of C-PSCs. Impressively, the achieved champion PCE of 14.58% is among the highest reported for C-PSCs.