Efficient, Stable and Reproducible Inverted Planar Perovskite Solar Cells by Surface Modification of Dopant-Free Spiro-TTB Hole Transport Layer with PFN-P1 Interfacial Layer

Here, we report inverted (p-i-n) perovskite solar cells (PSCs) that employ an ultrathin layer of Poly(9,9-bis(3'-(N,N-dimethyl)-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) known as PFN-P1 interfacial layer with 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 Spiro-TTB. The surface modification of Spiro-TTB HTL with PFN-P1 interlayer results in improved wettability, larger grain size with higher crystallinity, reduced charge recombination, and improved energy level alignment, all of which lead to significantly improved device performance. The maximum power conversion efficiency (PCE) of 18.8% is demonstrated by optimized p-i-n perovskite solar cells using an ultrathin PFN-P1 layer over Spiro-TTB HTL and a mixed triple cation Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.9Br_0.1)₃ (CsMAFA) perovskite absorber layer. This is significantly higher than the maximum PCE of 15.2% for the Spiro-TTB only HTL device. Furthermore, the PSCs with PFN-P1 interfacial layer exhibit a higher open circuit voltage of approximately 1.1 V and a decreased hysteresis index (HI). Moreover, the addition of PFN-P1 layer between the HTL and the absorber layer protects the underneath Spiro-TTB from getting washed off by the DMF:DMSO mixed solvent used in perovskite precursor resulting in higher reproducibility. PFN-P1 modified PSCs exhibit smaller standard deviations (0.28, 0.18, and 0.01 respectively) for PCE, short-circuit current density (J_SC), and open-circuit voltage (V_OC) out of 20 devices, and the encapsulated devices retain 80% of their initial efficiencies after 500 hours of operation at 65 °C. This work offers an effective approach for fabricating reproducible inverted PSCs with high efficiency employing dopant-free thin HTLs.