No abstract

For a light-emitting diode (LED) to generate light, the minimum voltage required is widely considered to be the emitter’s bandgap divided by the elementary charge. Here we show for many classes of LEDs, including those based on perovskite, organic, quantum-dot and III–V semiconductors, light emission can be observed at record-low voltages of 36–60% of their bandgaps, exhibiting a large apparent energy gain of 0.6–1.4 eV per photon. For 17 types of LEDs with different modes of charge injection and recombination (dark saturation currents of ~10−39–10−15 mA cm−2), their emission intensity-voltage curves under low voltages show similar behaviours. These observations and their consistency with the diode simulations suggest the ultralow-voltage electroluminescence arises from a universal origin—the radiative recombination of non-thermal-equilibrium band-edge carriers whose populations are determined by the Fermi-Dirac function perturbed by a small external bias. These results indicate the potential of low-voltage LEDs for communications, computational and energy applications.

low-cost preparation process, metal halide perovskites are widely considered as a promising class of materials for light emission, [1][2][3][4] among which quasi-2D perovskites are particularly prominent for highly efficient perovskite light-emitting diodes (PeLEDs). [5][6][7][8] Early studies indicate that efficient energy transfer process from wide (low n values) to narrow band gap (high n values) is a main reason for the high PLQY and excellent device performance in quasi-2D perovskites. [9,10] However, it remains a great challenge to control the n values of the quasi-2D perovskites accurately with solution process. Due to the self-aggregation of ligands, the perovskites tend to form smaller n values to bring about inefficient energy transfer, strong electron-phonon coupling, and thus reduced radiative exciton recombination. [11][12][13][14] Meanwhile, the unfavorable 3D perovskites with more defects can also be formed on the upper surface of the vertically non-uniform quasi-2D perovskite film due to the lack of ligand coordination. [15] Furthermore, the aggregated ligands show lower electrical conductivity and stronger charge confinement, resulting in more pronounced Joule heating and Auger recombination which are detrimental to the operational lifetime of PeLEDs. [16,17] Therefore, it is desirable to suppress Quasi-2D perovskites show great promise for light-emitting diodes owing to suppressed non-radiative losses enabled by the energy funneling/cascading nanostructures. However, for red emission quasi-2D perovskites, these ideal energy landscapes for efficient perovskite light-emitting diodes (PeLEDs) can rarely be achieved due to detrimental aggregation of the low-dimensional ligands in perovskite precursors, leading to poor device efficiency and stability. Here, a ligand-modulated dimensionality control strategy is explored to achieve uniform phase distribution and reduce defect density for efficient light emission. In contrast to the model phenethylammonium iodide 2D ligand, the formation of small-n phases can be inhibited by a structurally similar phenoxyethylammonium iodide ligand owing to the weakened aromatic stacking between ligands. Besides, the oxygen atoms can interact with the uncoordinated Pb 2+ ions and promote the NI coordination in the perovskites, which greatly reduces the non-radiative recombination defects in the ionic lattice. With this simple and effective approach, deep-red quasi-2D PeLEDs with record-high external quantum efficiency of 21.6% and decent operational stability are achieved without the need for additional additives. These results highlight the potential of ligand-modulated dimensionality control to achieve highly efficient and stable PeLEDs with a facile fabrication process.