Electron-beam-induced
damages in methylammonium lead triiodide
(MAPbI3) perovskite thin films were studied by cathodoluminescence
(CL) spectroscopy. We find that high-energy electron beams can significantly
alter perovskite properties through two distinct mechanisms: (1) defect
formation caused by irradiation damage and (2) phase transformation
induced by electron-beam heating. The former mechanism causes quenching
and broadening of the excitonic peaks in CL spectra, whereas the latter
results in new peaks with higher emission photon energy. The electron-beam
damage strongly depends on the electron-beam irradiation conditions.
Although CL is a powerful technique for investigating the electronic
properties of perovskite materials, irradiation conditions should
be carefully controlled to avoid any significant beam damage. In general,
reducing acceleration voltage and probing current, coupled with low-temperature
cooling, is more favorable for CL characterization and potentially
for other scanning electron-beam-based techniques as well. We have
also shown that the stability of perovskite materials under electron-beam
irradiation can be improved by reducing defects in the original thin
films. In addition, we investigated effects of electron-beam irradiation
on formamidinium lead triiodide (FAPbI3) and CsPbI3 thin films. FAPbI3 shows similar behavior as MAPbI3, whereas CsPbI3 displays higher resistance to
electron-beam damage than its organic–inorganic hybrid counterparts.
Using CsPbI3 as a model material, we observed nonuniform
luminescence in different grains of perovskite thin films. We also
discovered that black-to-yellow phase transformation of CsPbI3 tends to start from the junctions at grain boundaries.
We report on the effect of humidity on the structural, optical, and electrical properties of formamidinium lead halide perovskite (FAPbI 3 ; prepared by a solvent engineering method) and the device characteristics of planar FAPbI 3 solar cells. The relative humidity strongly affects the perovskite film morphology, which changes from a uniform, fully covered FAPbI 3 film at low relative humidity (e.g., ~2%) to an inhomogeneous film consisting of many voids (or pinholes) at high humidity (30%-40%). This morphological deterioration with increasing humidity is also accompanied by a reduction of the film crystallinity, decay of optical property, and shorter carrier lifetime. The device based on a planar FAPbI 3 film shows the best conversion efficiency of 16.6% (with the stabilized output efficiency of 16.4%) at a low humidity (~2%). Higher humidity leads to lower device performance, mainly due to the loss of open-circuit voltage and fill factor, which is consistent with the decrease of recombination resistance.
Organic–inorganic perovskites have emerged as an important class of next generation solar cells due to their remarkable low cost, band gap, and sub-900 nm absorption onset.
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