Resolving the Hydrophobicity of the Me-4PACz Hole Transport Layer for Inverted Perovskite Solar Cells with Efficiency >20%

A [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) self-assembled monolayer (SAM) has been employed in perovskite devices demonstrating high efficiencies. However, a uniform perovskite layer does not form due to the hydrophobicity of Me-4PACz. Here, we tackle this challenge by adding a conjugated polyelectrolyte, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br), to the Me-4PACz in a specific ratio, defined as Pz:PFN. With this mixing engineering strategy using Pz:PFN, the PFN-Br interaction with the A-site cation is confirmed via solution-state nuclear magnetic resonance studies. The narrow full widths at half-maxima of diffraction peaks and photoluminescence spectra of perovskite films reveal improved crystallization at the optimal mixing ratio of Pz:PFN. Interestingly, the mixing of PFN-Br additionally tunes the work function of the Me-4PACz and the built-in voltage in the solar cells. Devices employing the optimized Pz:PFN mixing ratio deliver an open-circuit voltage of 1.16 V and efficiency >20% for perovskites with a bandgap of 1.6 eV with high reproducibility and concomitant stability.