With growing demands on the stability of perovskite photovoltaics against various degradation factors, understanding and controlling the defect characteristics of devices have become the most essential issues to be resolved. In this work, the organometal halide perovskite is modified with a lithium−fluoride ionic passivator that enables highly stable and efficient solar cells with a power-conversion efficiency of over 21%, retaining up to ∼90% after 1000 h at 85 °C. The thermal degradation regressions of the films and devices have been temporally investigated, and the trap density of states has been scrutinized as a function of time. Surprisingly, the electronic traps of the solar cells exhibit exponential relaxations in both the trap densities and energy levels as thermally stressed, and the incorporation of LiF has greatly enhanced this relaxation with the mitigation of the following degradation. It is suggested that LiF not only passivates the initial formation of the traps but also controls their roles and behaviors under the thermal degradation of devices.

A facile formation of defects in halide perovskite is recently regarded as a main bottleneck for both the efficiency and stability of perovskite solar cells (PSCs). Therefore, understanding and controlling...

In order for the perovskite solar cells to be truly commercialized, long-term stability of the device must be guaranteed; hence, the individual layers with each interface is of prominent importance. [10][11][12][13] In this aspect, electron transport layer (ETL) and its interface with the perovskite is one of the important sites, which ultimately determines the performance and the stability of the device, since defects generated in the bulk of ETL or accumulated at the ETL/perovskite interface can both damage the charge extraction property and stability of the device, by trapping photogenerated carriers and providing degradation sites. [14][15][16][17] Moreover, for the typical normal n-i-p type device, the ETL side of the device is constantly exposed to the incident light, which can initiate photoinduced degradation or light-induced thermal stress on the device. [18][19][20] Therefore, it is important to smartly design an ETL that can maintain long-term stability under the operation condition.Tin oxide is one of the most widely used materials for ETL due to its high conductivity and appropriate Fermi energy relative to the perovskite. SnO 2 layer can also be easily fabricated by a simple spin-coating of commercially available nanoparticle-type SnO 2 colloidal dispersion, which results in thin and compact layer of stacked SnO 2 nanoparticles applicable to various device structures. [21][22][23][24][25] Here, defects or degradation sites may arise from the inherent defects from the SnO 2 nanoparticle, dangling bonds at the nanoparticle surface from imperfect film formation, or mismatch between nanoparticles and perovskite lattice. [26][27][28][29] To address these issues, various additive compounds have been incorporated into the SnO 2 layer, ranging from ionic salts to chained organic molecules with specialized functional groups. [30][31][32][33][34][35][36] Another alternative method is to synthesize compact SnO 2 layer by using tin (II) chloride hydrate (SnCl 2 •xH 2 O) as a precursor. [37][38][39] Some research groups have even fabricated bilayer-type SnO 2 ETL to improve the film quality, consisting of bare and additivemodified SnO 2 nanoparticles, or nanoparticle-type and SnCl 2derived SnO 2 . [40][41][42] Despite the modified ETLs show merits of passivating charge-trapping defects, these methods require sequential deposition steps for the fabrication of ETLs with multiple heat treatments at different temperatures, which adds

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