Lehrstuhl EP2 Uni Bayreuth

α Phase formamidinium lead iodide (FAPbI3) is a promising perovskite absorber for high-efficiency photovoltaics, yet it is metastable under ambient conditions and converts to the unsuitable hexagonal δ phase. We investigate the combined influence of temperature and pressure on the phase behavior of FAPbI3 using a home-built hot-press system coupled with an in situ optical reflectance spectroscopy setup. δ FAPbI3 powders were subjected to temperatures between 25 and 140 °C and pressures up to 30 MPa. Optical signatures of the δ, 4H, 6H, and α phases were identified and used to construct an approximate temperature–pressure phase diagram. The observed phase evolution is consistent with the Clapeyron equation. Increasing temperature induces the δ → α phase transition, while compression drives a partial δ → 4H conversion. Furthermore, increasing pressure reduces the temperature required for the δ → α phase transition, suggesting lower effective activation barriers via the 4H as intermediate phase.

A reported giant electrostrictive effect in halide perovskites has attracted interest for its physical origin and for potential applications in actuation and sensing. However, despite the reported exceptionally large magnitude of the effect, systematic experimental investigation remains limited. In this work, the electromechanical response of methylammonium lead halide perovskites (MAPbX3; X═I, Br, Cl) in the form of powder-pressed pellets, MAPbBr3 single crystals, and MAPbI3 thin films is investigated using a high-resolution double-modulation interferometer. Reference measurements using a commercial electrostrictive polymer confirm that the setup resolves sub-nanometer electromechanical displacements. In contrast to previous reports of giant electrostrictive compression in halide perovskites, no such intrinsic compressive electrostriction is observed in any of the samples. Instead, an expansion at twice the driving frequency is detected. Analysis of the frequency dependence and step response indicates that the expansion arises primarily from thermal expansion caused by Ohmic currents that lead to Joule heating.

We report a modulation technique applicable to optical two-beam interferometers with unequal arm lengths that allows
for highly precise, low-noise displacement measurements below the nanometer regime with a compact optical layout.
The technique is inspired by frequency modulation (FM) spectroscopy and employs an external electro-optic modulator
with sinusoidally phase modulated laser light in the radio-frequency (RF) regime (MHz frequencies). By combining
the RF modulation with a secondary mechanical modulation at a lower frequency to a super-heterodyne scheme we
achieve continuous sign-resolved tracking of the arm length difference over many wavelengths, and low-frequency
laser noise is effectively suppressed. Our set-up is based on the compact interference lens (iLens) technique and can
measure displacements of samples with a wide variety of surface qualities. We examined the system performance
using two samples, a mirror surface and a commercial aneroid capsule without specular reflectivity. Stable multi-fringe
operation with sub-nanometer resolution is demonstrated; the noise floor reaches 10 pm/√Hz at frequencies above
10 Hz in a regular laboratory environment. This work demonstrates a versatile, simple set-up capable of sign-resolved,
sub-nanometer displacement sensing suitable even for non-ideal surface conditions.