Semiconductors group

Abstract:

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe₂ possesses two stacking arrangements, the ferroelectric Weyl semimetal T_d phase and the higher-order topological insulator 1T' phase. However, in thin flakes of MoTe₂, it is unclear if the layer stacking follows the T_d, 1T', or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe₂ layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T' and T_d domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe₂ stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe₂ flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

Abstract:

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm² V^-1 s^-1 at 295 K and increasing gradually to ∼13 cm² V^-1 s^-1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Abstract:

Metal halide perovskite semiconductors have shown great performance in solar cells, and including an excess of lead iodide (PbI2) in the thin films, either as mesoscopic particles or embedded domains, often leads to improved solar cell performance. Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbI3) perovskite films reveal the FAPbI3:PbI2 interface to be remarkably coherent. It is demonstrated that such interface coherence is achieved by the PbI2 deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weakly van-der-Waals-bonded layers containing the near-octahedral units. The exact crystallographic interfacial relationship and lattice misfit are revealed. It is further shown that this 3R polytype of PbI2 has similar X-ray diffraction (XRD) peaks to that of the perovskite, making XRD-based quantification of the presence of PbI2 unreliable. Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign. These findings explain why a slight PbI2 excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.

Abstract:

Perovskite solar cells have rapidly advanced due to their exceptional optoelectronic properties, but achieving uniform crystallization and stability remains challenging. This review examines solvent-assisted annealing, including solvent-vapor and anti-solvent treatments as a strategy to modulate perovskite crystallization for enhanced device performance. Solvent vapors (e.g. DMF, DMSO, alcohol mixtures) introduced during thermal annealing sustain a supersaturated environment that extends nucleation and enables Ostwald ripening, yielding markedly larger grain sizes and improved crystallinity. Studies show that solvent annealing can increase MAPbI3 carrier diffusion lengths beyond 1 μm and maintain >14.5% efficiency even for films up to 1 μm thick. Advanced schemes, such as combined DMSO-water vapor annealing, have produced nearly single-crystal grains and devices with 19.5% power conversion efficiency (PCE), by reducing defect-mediated recombination. These microstructural gains translate into higher PCE and stability: solvent-annealed films exhibit fewer trap sites and inhibited moisture degradation. Finally, we address scalability: ambient solvent-antisolvent treatments have yielded >5 μm grains with 100% film coverage in large-area Perovskite solar cells. Overall, solvent annealing emerges as a powerful tool for tailoring perovskite films. This review synthesizes the mechanisms and performance benefits of solvent annealing and evaluates its prospects for scalable, industrialized PSC fabrication. By identifying key challenges and emerging solutions, it aims to guide future research efforts toward more efficient and manufacturable perovskite solar technologies