Enhancing Performance and Stability of Perovskite Solar Cells through Surface Defect Passivation with Organic Bidentate Lewis Bases.

Organic Lewis bases [2,2'-bipyridine (BPY), 4-hydroxy-1,5-naphthyridine-3-carbonitrile (DQCN), and thenoyltrifluoroacetone (TTFA)] with bi-coordination sites of N and O were employed as perovskite surface defect passivants to address the efficiency and stability issues of perovskite solar cells (PSCs), with typical phenethylammonium iodide (PEAI) and piperazinium iodide (PI) passivants as reference. The surface properties of the perovskite films before and after passivation were characterized by Fourier-transform infrared, ultraviolet-visible, photoluminescence (PL), and time-resolved PL spectroscopy, X-ray diffraction, ultraviolet photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. The characterizations reveal that BPY, DQCN, or TTFA forms coordination bonds with exposed "Pb 2+ ", leading to a slight decrease in the highest occupied molecular orbital or lowest unoccupied molecular orbital energy level and bandgap. These passivants (especially TTFA) can passivate the perovskite surface defects to inhibit non-radiative recombination while having almost no influence on the grain size and surface morphology. Utilizing the passivated perovskite as the light absorption layer, solar cells with an inverted configuration of indium tin oxide/NiO x /passivated MAPbCl x I 3- x /C 60 /BCP/Ag have been fabricated, and power conversion efficiencies of 19.22, 17.85, 16.49, 16.31, and 17.88% have been achieved from PEAI, PI, BPY, DQCN, and TTFA, respectively. All the device performance based on passivated perovskite is superior to that of the control (15.75%) owing to the reduced carrier recombination. The device from TTFA exhibits almost comparable efficiency to that of PEAI and PI controls, indicating that TTFA has an equal excellent passivation effect to state-of-the-art PEAI and PI. Furthermore, the devices based on BPY, DQCN, and TTFA show superior long-term stability with an efficiency loss of only 13.2, 16.7, and 12.9%, respectively, after being stored for 40 days in a ∼12% humidity, low-oxygen level environment, which is 45.4, 38.8, and 44.4% for the control, PEAI, and PI devices, respectively, primarily due to the improved hydrophobicity of the perovskite surface. Our results demonstrate that it is feasible to achieve high-efficiency and long-term-stable perovskite solar cells via selecting the appropriate molecules to passivate perovskite surface defects.