Two-dimensional (2D) and quasi-2D lead-free halide double perovskites (LFHDPs) have gained significant attention as promising alternatives to conventional lead-based perovskites due to their superior environmental stability, reduced toxicity and versatile structural chemistry. This review provides a comprehensive overview of recent advances in the structural design, scalable synthesis strategies, and optoelectronic functionalities, with particular emphasis on bandgap tunability, strong excitonic effects, and photoluminescence quantum yields of 2D LFHDPs. We examine emerging trends in dimensional engineering, particularly the development of Ruddlesden–Popper and Dion–Jacobson phases incorporating monovalent and trivalent cations, which enable enhanced moisture resistance and tailored quantum confinement. Key challenges remain, including indirect bandgaps, limited carrier mobilities, defect-related trap states, and difficulties in large-area processing. To address these issues, innovative approaches such as compositional alloying, halide mixing, defect passivation, and hybrid device architectures are discussed. Furthermore, we discuss challenges such as limited light absorption, poor carrier mobility, and defect tolerance, and explore innovative strategies to overcome these restrictions. Finally, we outline future research directions that couple computational screening with experimental synthesis, integrate LFHDPs into flexible and neuromorphic devices, and pursue sustainable, low-cost compositions, emphasizing their potential in sustainable optoelectronic technologies.

Compact, electrically driven terahertz (THz) sources remain a major bottleneck for chip-scale spectroscopy, imaging and high-speed wireless links. AlGaN/GaN two-dimensional electron gas (2DEG) systems have been investigated for THz emission, but experimental demonstrations have almost exclusively used Ga-polar platforms. Here we report a room-temperature THz signal from N-polar GaN grating-gate high-electron-mobility transistors (HEMTs) under pulsed electrical excitation. In the reversed-polarization heterostructure, the GaN channel lies above the AlGaN barrier, a geometry that places the channel close to the surface grating. The detected THz power increases with lateral electric field and square-wave duty cycle, whereas the apparent conversion efficiency falls at high duty cycle, consistent with increased Joule heating. Spectral measurements reveal a broad low-frequency feature and a gated/ungated spectral difference. The long-pulse room-temperature response is consistent with predominantly thermal emission, and the gated/ungated difference may reflect geometry-dependent thermal emissivity. These measurements do not establish a separate collective 2DEG contribution or a plasmonic resonance. These results establish an experimental N-polar GaN HEMT platform for studying electrically driven THz radiation and motivate calibrated emissivity, polarization-resolved and gate-tunable measurements.