Silicon Quantum Dots for Light-Emitting Diodes Extending to the NIR-II Window

Biomedical applications rely on semiconductor quantum dots (QDs) exhibiting electroluminescence (EL) properties in the wavelength range between 1.0 and 1.7 μm (called the second near-infrared window, NIR-II). However, developing heavy-metal-free QDs remains a challenge. Herein, we report, for the first time, a colloidal silicon QD light-emitting diode (Si-QLED), which exhibits an EL spectrum with a peak at 1.0 μm along with a high external quantum efficiency (EQE) value of 4.84%, which is close to the record value among reported NIR-II EL devices. SiQDs as optically active layers are synthesized by thermal disproportionation of hydrogen silsesquioxane derived from triethoxysilane, followed by hydrofluoric etching. The photoluminescence quantum yield of SiQDs is improved up to 53% by thermal hydrosilylation of 1-decene, followed by careful extraction of highly emitting QDs using size exclusion chromatography. Si-QLEDs have an inverted device architecture with multilayers, leading to stable emission properties even at a high applied voltage. Furthermore, we demonstrate a spectral shape tuning of the EL spectra by taking advantage of a large Stokes shift between the optical absorption and emission, the property inherent to QDs of indirect band gap Si crystals.