Packaging methylammonium lead iodide perovskite (MAPbI)-based solar cells with N or dry air is a promising solution for its application in outdoor photovoltaics. However, the effect of N and O on the decomposition chemistry and kinetics of MAPbI is not yet well-understood. With in situ Fourier transform infrared spectroscopy measurements, we show that the effective activation energy for the degradation of MAPbI in N is ∼120 kJ/mol. The decomposition of MAPbI is greatly accelerated by exposure to O in the dark. As a result of the synergistic effect between O and a HeNe laser (633 nm), the degradation rate is further increased with photon flux. This synergistic effect reduces the effective activation energy of degradation of MAPbI to ∼50 kJ/mol. The solid decomposition products after annealing in N and O at 150 °C or below do not have absorbance between 650 and 4000 cm.

Atomic layer deposition (ALD) of oxide film on [6,6]-phenyl-C butyric acid methyl ester (PCBM) shows a great promise to dramatically improve the ambient stability of hybrid halide perovskite. The nucleation of an ALD oxide on PCBM is critical to reliably apply this strategy. In this paper, we present the first study of the nucleation behavior of ALD oxides, including AlO and ZnO, on PCBM. We find that PCBM film acts a gas diffusion barrier blocking the ALD reactants (diethyl zinc) from etching the underlying CHNHPbI. However, ZnO is not able to nucleate on PCBM. We further identify that trimethyl aluminum, a strongly Lewis acid, reacts readily with C═O on PCBM to generate a seeding layer for nucleating ZnO ALD. This new chemical route is highly reliable and can be used to synthesize ALD ZnO coatings over PCBM. The synthesized PCBM/AlO-ZnO dramatically improves the stability of CHNHPbI against the ambience and even against liquid water. The result signifies the importance of understanding of nucleation of ALD in enabling reliable barrier coatings for hybrid halide perovskites.

Al 2 O 3 atomic layer deposition (ALD), which uses trimethylaluminum (TMA) as the metal precursor, shows promise in improving the environmental stability of hybrid halide perovskites. However, it is not yet entirely clear how TMA, a strong Lewis acid, reacts with fresh perovskites and how the reaction affects the nucleation of ALD Al 2 O 3 . Here, the effects of reaction temperature and partial pressure of TMA on the mechanisms of TMA/CH 3 NH 3 PbI 3 reactions are investigated. Our real time mass gain data and in situ mass spectrometry data show that the TMA/CH 3 NH 3 PbI 3 reaction can either remove mass or accumulate mass onto CH 3 NH 3 PbI 3 substrates, depending strongly on the reaction temperature and partial pressure of TMA. The TMA/CH 3 NH 3 PbI 3 reaction probably generates TMA−CH 3 NHx adduct compounds, which protects CH 3 NH 3 PbI 3 from TMA by forming a shell at 25 °C in the vacuum process. However, these adduct compounds decompose at higher temperatures (e.g., 75 °C). This product layer is much thicker than a monolayer, suggesting the interface formed between Al 2 O 3 coating and CH 3 NH 3 PbI 3 is blurring and messy. These results have not yet, but should be, carefully considered to correctly interpret the effect of ALD Al 2 O 3 treatment on optoelectronic properties of CH 3 NH 3 PbI 3 .

Pyridine molecules have been used to passivate surface Pb sites of CHNHPbI, to recrystallize CHNHPbI, and to bleach CHNHPbI. However, these results contradict each other, as recrystallization and optical-bleach require transformation of bulk CHNHPbI, but surface passivation demands the confinement of the reaction at the surface region. The underlying mechanism for these seemly contradicting results is not yet understood. In this paper, we show, at 25 °C, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CHNHPbI: one can modify the surface region of CHNHPbI by using pyridine vapor of pressure 1.15 torr or lower but can transform the whole bulk CHNHPbI film with a pyridine vapor of 1.3 torr or higher. Our result is the first demonstration that the reaction modes, i.e., surface-confined reaction and bulk transformation, are very sensitive to the partial pressure of under-saturated pyridine vapor. Despite the different reaction behaviors, it is interesting that in all pressure ranges, pyridinium ion is a main product from the reaction between pyridine and CHNHPbI. The bulk transformation is due to the formation of a liquid-like film, which increases the mobility of species to catalyze the reaction between pyridine and CHNHPbI. It is important to note 1.3 torr is much smaller than the saturated vapor pressure of pyridine (20 torr at 25 °C). These findings provide a guidance in applying pyridine and other amines to functionalize and transform CHNHPbI and other hybrid halide perovskites. It also highlights the critical role of fundamental studies in controllably modifying CHNHPbI.

Shape-engineered nanocrystals (SENs) promise a better selectivity and a higher activity in catalytic reactions than the corresponding non-shape-engineered ones because of their larger specific surface areas and desirable crystal facets. However, often, it is challenging to apply SENs in practical catalytic applications at high reaction temperatures, where SENs deforms into more stable, less active nanoparticles. In this paper, we show that atomic layer deposition (ALD) of Al2O3 at 200 °C can controllably dope Al cations into the shape-engineered CeO2 nanorods (NRs) to not only increase their shape transition temperature from 400 °C to beyond 700 °C but also greatly increase their specific reversible oxygen storage capacity (srOSC). The substituted Al3+ ions impede the surface diffusion of Ce ions and therefore improve the thermal stability of CeO2 NRs. These Al3+ dopants form −Al–O–Ce–O– clusters, which are new Ce species and can be reversibly reduced and oxidized at 500–700 °C. This low-temperature chemical doping method decouples the synthesis process of SENs from the doping process and maintains the shape of the SENs during the activation of dopants. This concept could be adopted to enable the applications of other SENs in challenging high-temperature environments.