Pressure-Engineered Structural and Optical Properties of Two-Dimensional (C ₄ H ₉ NH ₃ ) ₂ PbI ₄ Perovskite Exfoliated nm-Thin Flakes

Resolving the structure-property relationships of two-dimensional (2D) organic-inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0-10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C ₄ H ₉ NH ₃ ) ₂ PbI ₄ , from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI ₄ ^2- layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in Pb-I-Pb bond angle and increase in Pb-I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P2 ₁ /a phase above 1.4 GPa increases the Pb-I-Pb bond angle and decreases the Pb-I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C ₄ H ₉ NH ₃ ) ₂ PbI ₄ as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C ₄ H ₉ NH ₃ ) ₂ PbI ₄ , where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P2 ₁ /a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C ₄ H ₉ NH ₃ ) ₂ PbI ₄ nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.