Organic–inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention from researchers since their first demonstration in 20091–3 due to the low-cost solution processing,4 tunable bandgap,5 high absorption coefficient,6 low recombination rate,7 and high mobility of charge carriers.8 The power conversion efficiency (PCE) of single-junction PSCs has rapidly increased from 3.8% to a certified value of 25.7%.9–11 The long-term operational stability of unencapsulated PSCs has also exceeded 1,000 hours in full sunlight after the interface engineering of device structures and molecular passivation of the perovskite layer.2,12−14 Therefore, PSCs represent a promising next-generation photovoltaic technology.
Over the past few years, it has been demonstrated that the elimination of deep-level defects, which act as detrimental nonradiative recombination centers, is critical for realizing high-performance solar cells.15–18 To date, I− vacancy defects constitute the majority of non-radiative recombination centers in the FAPbI3 perovskite layer that are very difficult to mitigate.19,20 These defects are mainly caused by iodine losses during film fabrication or the I− ion migration during device operation, which produces deep-level defects and directly leads to the degradation of photoelectric properties.15–18 Furthermore, the I− ions desorbed from the inorganic framework can be easily oxidized to I0 species, which initiate chemical chain reactions accelerating the formation of deep-level defects in perovskite layers.21 This phenomenon is detrimental for devices operating under complex conditions (including high temperature, continuous light illumination, electrical bias, or their combination) and negatively affects the long-term stability of operational PSCs.22,23 Therefore, inhibiting the formation and migration of I− vacancy defects is critical for stabilizing photoactive perovskite layers and preserving their good photoelectric properties.
Several attempts have been made to passivate anion vacancy defects through the introduction of additives such as quaternary ammonium halide anions and cations,24 caffeine and theobromine,19 CsI–DB21C7 complex,25 naphthalene-1,8-dicarboximide and perylene-3,4-dicarboximide.26 However, these additives are typically used to passivate undercoordinated Pb2+ ions after anion species have already escaped from the crystal lattice. To inhibit the formation of anion vacancy defects radically, anions should be pinned to the crystal lattice without the possibility of escaping, which can be theoretically realized through a judicious chemical design by molecular engineering. There are five potential benefits of this approach. First, the introduced materials should be strongly bonded to the Pb–I framework to pin anions to the crystal lattice and suppress the formation of anion vacancies in the source. Second, strong chemical bonding should localize anions that have already escaped from the crystal lattice, which would inhibit the migration of delocalized anions. Third, the space occupied by the introduced materials in the Pb–I framework should not be too large to ensure efficient charge transport between different Pb–I frameworks. Fourth, the introduced materials should be stable under thermal stress and illumination. Fifth, the introduced materials should act as growth-controlling agents to promote the crystallization of perovskites. In previous studies, various additives such as phosphonopropionic acid13 and amino-based organic ligands27–29 were used as sacrificial agents to inhibit the formation of anion vacancy defects. However, phosphonopropionic molecules tend to degrade upon heating, while amino-based organic ligands cannot form strong chemical bonds to pin anions to the crystal lattice. Furthermore, the large barrier layer of these organic compounds in the Pb–I framework negatively affected charge transport and led to the formation of a charge extraction barrier, making them unsuitable for high-efficiency stable solar cells.
In this work, we propose a rational design strategy for anion fixation and suppressing the formation of anions vacancy defects synergistically by a sustainable pinning effect while retaining the efficient charge transport using amidino-based organic molecules to construct highly efficient and stable PSCs. 3-amidinopyridine (3AP) molecules form strong chemical bonds with the Pb–I framework to effectively pin anions and considerably increase the energy barrier of the formation and migration of anion vacancies. Therefore, this one-stone-for-two-birds strategy enables us to achieve a PCE as high as 25.3% (certified at 24.8%) and significantly improved operation stability following the ISOS-L-1 stability protocol. The present work paves the way for the anion-vacancy defect engineering via molecule–perovskite coordination, providing an effective and simple solution towards efficient and stable perovskite optoelectronics.