Origin of reversible photo-induced phase separation in hybrid perovskites (1606.07366v2)

Abstract: Nonequilibrium processes occurring in functional materials can significantly impact device efficiencies and are often difficult to characterize due to the broad range of length and time scales involved. In particular, mixed halide hybrid perovskites are promising for optoelectronics, yet the halides reversibly phase separate when photo-excited, significantly altering device performance. By combining nanoscale imaging and multiscale modeling, we elucidate the mechanism underlying this phenomenon, demonstrating that local strain induced by photo-generated polarons promotes halide phase separation and leads to nucleation of light-stabilized iodide-rich clusters. This effect relies on the unique electromechanical properties of hybrid materials, characteristic of neither their organic nor inorganic constituents alone. Exploiting photo-induced phase separation and other nonequilibrium phenomena in hybrid materials, generally, could enable new opportunities for expanding the functional applications in sensing, photoswitching, optical memory, and energy storage.

• The paper demonstrates that photo-generated polarons induce strain that nucleates iodide-rich clusters, resulting in reversible phase separation.
• It employs cathodoluminescence imaging and multiscale modeling to capture the nucleation dynamics and saturation of phase-separated domains.
• The findings suggest strategies to mitigate phase separation, paving the way for more stable and efficient perovskite-based optoelectronic devices.

Overview of Photo-Induced Phase Separation in Hybrid Perovskites

Hybrid perovskites, characterized by the chemical formula APbX$_3$, offer substantial promise for diverse optoelectronic applications, such as photovoltaics and light-emitting diodes (LEDs). Their cost-effectiveness, high brightness, and structural defect tolerance make them particularly attractive for these purposes. However, one intriguing barrier to their wider application is the light-induced phase separation of mixed halide compositions, notably MAPb(I$_x$Br$_{1-x}$)$_3$. This paper systematically investigates the mechanism underlying reversible photo-induced phase separation in hybrid perovskites, combining insights from nanoscale imaging and multiscale modeling to elucidate the interaction between photo-generated polarons and halide demixing.

Mechanism of Phase Separation

The research reveals that the phase separation is primarily driven by strain induced by polarons—charge carriers generated upon light exposure, coupled with their resultant lattice distortions. It identifies that these polarons promote the nucleation of iodide-rich clusters within the perovskite lattice. This behavior emerges from the unique electromechanical properties of hybrid perovskites, distinct from either pure inorganic or organic materials alone. A combination of cathodoluminescence imaging and multiscale modeling bridges molecular and mesoscopic length scales to capture this process, highlighting how polarons become trapped in fluctuations of halide composition, exacerbating phase separation under prolonged illumination.

Experimental Observations

A key experimental approach involved fabricating mixed halide perovskite films and subjecting them to controlled illumination. The photo-induced cluster formation was monitored through detailed cathodoluminescence imaging. Notably, upon prolonged exposure to light, these clusters localized predominantly at grain boundaries, indicative of underlying polaronic strain effects. The formation dynamics are characterized by initially latent behavior followed by stochastic cluster nucleation, which then reaches a saturation point dictated by the extent of polaron-induced strain fields.

Theoretical Insights

The paper supplements empirical findings with a theoretical framework based on a Landau-Ginzburg model that accounts for the linear coupling between strain and composition. The role of electron-phonon interaction is highlighted as a critical component, with the simulations showing that substantial lattice stabilization by polarons is necessary for initiating phase separation. The interplay of these factors is further underscored by temperature-dependent behavior noted both experimentally and through phase diagrams generated by the theoretical model.

Implications and Future Outlook

The understanding of nonequilibrium phenomena like photo-induced phase separation in hybrid perovskites could significantly influence the development of new materials and devices. The findings suggest potential pathways for minimizing adverse effects in optoelectronic devices—such as controlling defect concentrations to reduce halide migration or modifying electron-phonon coupling to mitigate phase separation. Moreover, these photo-induced processes can be harnessed to develop new functionalities like optical memory systems and highly sensitive sensors.

The confluence of such photo-induced kinetic effects in hybrid perovskites presents opportunities for novel applications spanning various technology domains. Continuing advancements in imaging and modeling techniques could further explore the unique nonequilibrium properties of trending hybrid materials, paving the way for the next generation of high-performance optoelectronic devices and other functional applications. Given the rapidly evolving field of hybrid materials, it will be crucial for future research to capitalize on these foundational insights to bring innovative device applications to fruition.