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(I0.9Br0.1)(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 similar to 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 degrees C. This work provides an effective strategy for developing highly efficient inverted PSCs with dopant-free thin HTLs.