Towards 26% efficiency in inverted perovskite solar cells via interfacial flipped band bending and suppressed deep-level traps

The strong nonradiative recombination loss of inverted PSCs at the surface and at the perovskite/C60 interface has limited the open-circuit voltage (Voc) and fill factor (FF) of the device and prevented further performance enhancement of PSCs. Here, a new phenomenon was introduced: piperazinium diiodide (PDI) as a surface modifier to suppress deep-level defects at the surface and regulate band alignment at the interface. Instead of penetrating into the lattice and forming a 2D structure, PDI existed mainly in a molecular form thermodynamically prone to deprotonation to prevent the deprotonation reaction between A-site cations and the formation of deep-level defects on the surface. In addition, PDI was shown to partially penetrate into C-60 to modulate interfacial band bending to facilitate electron transport and hinder hole backflow. Accordingly, a more homogeneous surface contact potential difference (CPD) and a higher extraction rate of the hot carriers at the interface were observed in the PDI-treated films. Finally, the optimized inverted devices exhibited a state-of-art power conversion efficiency (PCE) of 26.15%, with a certified PCE of 25.87% (quasi-steady-state: 25.52%). Voc increased from 1.12 V to 1.18 V, benefiting from a 57 mV higher quasi-Fermi level splitting (QFLS). Notably, the devices retained 90.4% and 94.2% of the initial efficiency after being aged at 85(degrees)C for >500 h and tracked at the maximum power point for 1000 h, respectively.