Efficient, Stable, and Reproducible Inverted p–i–n Perovskite Solar Cells with a Dopant-Free Spiro-TTB Hole Transport Layer Modified with a PFN-P1 Interfacial Layer

Herein, we report inverted p–i–n perovskite solar cells (PSCs) employing a small molecular dopant-free hole transport layer (HTL), namely, 2,2′,7,7′-tetra( N, N -di- p -tolyl)amino-9,9-spirobifluorene, commonly known as the Spiro-TTB surface modified with an ultrathin layer of poly(9,9-bis(3′-( N, N -dimethyl)-propyl-2,7-fluorene)- alt -2,7-(9,9-dioctylfluorene) known as the PFN-P1 interfacial layer resulting in improved wettability, larger grain size with higher crystallinity, and reduced charge recombination together with better energy level alignment leading to significantly improved device performance. Optimized p–i–n perovskite solar cells using a mixed triple-cation Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.9 Br 0.1 ) 3 (CsMAFA) perovskite absorber layer and an ultrathin PFN-P1 layer over Spiro-TTB HTL show a maximum power conversion efficiency (PCE) of 18.8%, which is significantly higher than that of the Spiro-TTB-only HTL device with a maximum PCE of 15.2%. Moreover, the PSCs having an ultrathin PFN-P1 interfacial layer between the Spiro-TTB HTL and the CsMAFA absorber layer show a reduced hysteresis index (HI) and a higher open circuit voltage ∼1.1 V. Furthermore, the PCE, short circuit current density ( J SC ), and open circuit voltage ( V OC ) have small standard deviations (0.28, 0.18, and 0.01, respectively) out of 20 devices, and the encapsulated devices maintain 80% of their initial efficiencies after 500 h of operation at 65 °C. This work provides an effective strategy for developing highly efficient inverted PSCs with dopant-free thin HTLs.