Impacts of Heterogeneous TiO₂ and Al₂O₃ Composite Mesoporous Scaffold on Formamidinium Lead Trihalide Perovskite Solar Cells

Heterogeneous TiO₂ and Al₂O₃ composites were employed as a mesoporous scaffold in formamidinium lead trihalide (FAPbI_3–<i>x</i>Cl_<i>x</i>)-based perovskite solar cells to modify surface properties of a mesoporous layer. It was found that the quality and morphology of the perovskite film were strongly affected by the TiO₂/Al₂O₃ ratio in the mesoporous film. The conversion efficiency of the perovskite solar cell was improved by using a composite of TiO₂ and Al₂O₃ in comparison with TiO₂- and Al₂O₃-based cells, yielding 11.0% for a cell with a 7:3 TiO₂/Al₂O₃ composite. Our investigation shows a change of electron transport path depending on a composition ratio of insulating Al₂O₃ to n-type semiconducting TiO₂ in a mesoporous layer

Amorphous Metal Oxide Blocking Layers for Highly Efficient Low-Temperature Brookite TiO₂‑Based Perovskite Solar Cells

A fully low-temperature-processed perovskite solar cell was fabricated with an ultrathin amorphous TiO_<i>x</i> hole-blocking layer in combination with brookite TiO₂ prepared at temperature <150 °C. Structured with TiO_<i>x</i>/brookite TiO₂ bilayer electron collector, the perovskite solar cells exhibit high efficiency up to 21.6% being supported by high open-circuit voltage and fill factor up to 1.18 V and 0.83, respectively. Compared to SnO_<i>x</i> hole-blocking layer, TiO_<i>x</i> has better electron band alignment with brookite TiO₂ and hence, results in higher efficiency

Controlled Crystal Grain Growth in Mixed Cation–Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation–halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO₂ mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent–voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm² device and 15.2% for a 1.0 × 1.0 cm² device

First Evidence of CH₃NH₃PbI₃ Optical Constants Improvement in a N₂ Environment in the Range 40–80 °C

We study the optical response of CH₃NH₃PbI₃ layers to light solicitation under different environmental gas and temperature conditions. The measurements were performed in nonreactive (Ar or N₂) and reactive (O₂ or humid air) gas in the range 40–80 °C crossing the tetragonal–cubic transition (∼50 °C). With respect to truly inert Ar, the use of N₂ not only assures the reversibility of the optical constants during thermal cycles but also improves the optical response of the material. While in N₂ and Ar atmospheres the optical parameters of the material can be recovered at the end of the cycle, in contrast, the presence of humidity in the air causes the absorption coefficient to monotonically and inexorably decrease in the whole visible range, especially after the lattice has moved to cubic. The use of N₂ thus represents an effective strategy to improve the absorption under thermal operation conditions

Revealing a Discontinuity in the Degradation Behavior of CH₃NH₃PbI₃ during Thermal Operation

The advance of innovative photovoltaics based on hybrid perovskites is currently forced to face their stability and durability through the rationalization of the phenomena occurring into the lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of MAPbI₃ layers by in situ structural and optical analyses upon recursive thermal cycles from 30 to 80 °C in different annealing environments. We reveal an acceleration of the material modification, above what expected, as the threshold of the tetragonal to cubic transition (∼50 °C) is surpassed. This produces discontinuities in the degradation rate, bandgap value, and dielectric behavior of the MAPbI₃ layer. The phenomenon is put in relationship with the order–disorder lattice modifications described by Car–Parrinello molecular dynamics calculations and reveals that the action of species from humid air becomes largely more effective above 50 °C for reasons related to the increased accessibility/reactivity of the lattice which, in turn, impacts on defects generation