Halide perovskites, a new class of semiconductors, exhibit remarkable optoelectronic properties, making them particularly promising for solar energy conversion, low-power lighting, photodetection, and quantum technologies. While their performance rivals that of silicon, they are synthesized more easily and at lower cost. However, the resulting polycrystalline thin films contain numerous crystalline defects, which generally degrade the performance of optoelectronic materials. Halide perovskites stand out for their unusual tolerance to these defects, the origin of which remains debated and may involve their soft, polar, and dynamic crystal lattice.
To overcome the difficulty of experimentally identifying these defects in halide perovskites, the GEMaC team studied MAPbI3 (CH3NH3PbI3) single crystals synthesized at the LuMIn laboratory. At a temperature of 4 K (−269 °C), laser excitation of these crystals generated photoluminescence, revealing extremely fine emission lines, less than 500 µeV wide. These optical signatures suggest highly localized emission originating from point defects, where trapped charge carriers recombine, emitting light.
An initial study showed that the energy of these lines fluctuates over timescales of about one second, with amplitudes ranging from a few hundred µeV to a few meV. This spectral diffusion phenomenon, usually associated with a single emitter, actually originates from a set of emitters fluctuating in concert. Furthermore, several lines associated with distinct defects move synchronously, indicating that these defects are subject to the same source of fluctuations in their local crystalline environment, suggesting the existence of slow, correlated lattice deformation domains.
A second study characterized these lines using excitation and polarization spectroscopy. The measurements reveal that the defect emission is strongly linearly polarized (up to 40%) and is enhanced when the excitation energy is slightly below the crystal's light absorption threshold. These observations are consistent with recombination between a defect and a neighboring defect, known as donor-acceptor pair recombination.
These works, carried out as part of Thi Huyen Trang Nguyen's thesis at GEMaC, demonstrate that the ultra-fine lines in single crystals allow defects to be used as sensitive probes of their local environment and crystal lattice dynamics. They pave the way for a better understanding of the electronic properties of defects in halide perovskites, a crucial step in elucidating their defect tolerance.
