Lehrstuhl EP2 Uni Bayreuth

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.

Extrinsic traps created by dopants or impurities are ubiquitous in organic semiconductors and can critically influence charge transport. Here we report a comprehensive study, using low-temperature thermally stimulated luminescence measurements complemented by quantum mechanics and molecular dynamics (QM-MD) calculations, as well as kinetic Monte Carlo (KMC) simulations, to investigate trapping and energetic relaxation of charge carriers in amorphous host-guest systems containing the widely investigated thermally activated delayed fluorescence (TADF) emitter 10-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9,9-dimethyl-9,10-dihydro-acridine (DMAC-TRZ) in two different hosts. DMAC-TRZ guest acts as a shallow trap in 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) and a deep trap in 1,3-bis (triphenylsilyl) benzene (UGH3), each forming an additional offset Gaussian density of states (DOS) distribution, as supported by the QM-MD calculations. These calculations accurately reproduce the trap depths 𝜀𝑡
and widths of the DOS distributions 𝜎DOS
in the considered compositions, showing that the nominal trap depth is independent of the host-guest ratio. By experiment and KMC simulations we found that the trapping behavior differs significantly for shallow (𝜀𝑡
 ≤ 3⁢𝜎DOS
) and deep (𝜀𝑡
 > 3⁢𝜎DOS
) cases. Shallow traps do not behave as distinct traps but rather broaden the host DOS and increase the density of low-energy states, causing the average activation energy to depend on guest concentration 𝑐𝑡
, in line with the “effective disorder” concept. By contrast, when a trap is deep, the energy needed to release the trapped charges to the host is independent of the guest concentration. Furthermore, for deep traps, we observe that charge detrapping occurs simultaneously via both guest-to-host and guest-to-guest charge-transfer pathways, with the latter regime becoming dominant for concentrations exceeding 𝑐𝑡
 = 5%. Remarkably, this guest-guest transfer already sets in at only 1% trap concentration. We attribute this to local guest clustering and superexchange-mediated intercluster transfer.