Hybrid Perovskite Photophysics
Understanding Light‑Matter Interaction in Hybrid Perovskites
When a photon is absorbed by a semiconductor, electrons are promoted to excited states that evolve on femtosecond to picosecond time scales. The precise energy, angular momentum and coupling of these states determine how efficiently the material can emit light, transport charge, or convert solar energy. In hybrid perovskites—crystalline compounds that combine an inorganic metal‑halide framework with organic cations—this picture is complicated by structural softness, compositional disorder, and the presence of multiple competing phases. Small changes in composition or lattice arrangement can reshape the landscape of electronic states, leading to markedly different photophysical behavior.
Our group focuses on how the microscopic structure of hybrid perovskites controls the photophysical evolution of these excited states. By probing the earliest moments after photo‑excitation, we aim to map the pathways that lead to radiative emission, non‑radiative loss, or charge transfer across interfaces. The insights gained are directly relevant to improving the performance and stability of perovskite‑based light‑emitting diodes, lasers, and photovoltaic devices.
Scientific Focus
Hybrid perovskites possess a rich space of potential energy minima arising from variations in halide composition, cation orientation, and lattice strain. This creates spatially disordered energy landscapes in which carriers can become transiently trapped, delocalized, or self‑organized into polarons. Our research investigates three interrelated questions:
- How does structural disorder influence the density, distribution and dynamics of electronic states? By correlating crystallographic data with spectroscopic signatures, we identify how local lattice distortions shift band edges and modify carrier relaxation.
- What are the ultrafast structural responses that accompany electronic excitation?
Photo‑excitation can launch coherent phonons, modify bond lengths, and induce transient phase transitions. Understanding these motions clarifies how the lattice either assists or hinders carrier relaxation. - Under what conditions does charge become localized versus remaining delocalized?
Localization affects recombination pathways and, consequently, the quantum efficiency of light emission. We explore how compositional tuning and morphology steers carriers toward desirable recombination channels.
Collectively, these investigations reveal how microscopic control over composition and structure translates into macroscopic optoelectronic performance.
Experimental Approach
To capture processes that unfold on femtosecond time scales, we employ a suite of ultrafast spectroscopic techniques, each providing a complementary view of the excited‑state dynamics.
Transient Absorption Spectroscopy
A pump pulse excites the sample while a broadband probe monitors changes in absorption as a function of delay time. This method yields kinetic traces for excited‑state populations, allowing us to extract lifetimes of free carriers, trapped states, and polaron formation. By varying pump wavelength and fluence, we disentangle contributions from different excitation pathways.
Two‑Dimensional Electronic Spectroscopy (2DES)
Building on transient absorption, 2DES resolves correlations between excitation and emission frequencies. The resulting two‑dimensional spectra separate overlapping transitions, reveal couplings between electronic states, and track coherent vibronic motions. This technique is particularly powerful for identifying the signatures of disorder‑induced localized states that are otherwise hidden in one‑dimensional measurements.
Time‑Resolved X‑ray Diffraction and Electron Diffraction
In collaboration with national synchrotron and ultrafast X-ray facilities, we probe structural dynamics directly. By recording diffraction patterns after photo‑excitation, we quantify lattice expansions, coherent phonon amplitudes, and transient symmetry changes that accompany electronic relaxation.
