the alternatives, [7] Si is attractive because of its abundance, biocompatibility, [8] compatibility with silicon-based electronics, [9] luminescent properties when formed as QDs and established/tailorable surface chemistry. [10] EL of silicon-based nanomaterials (e.g., porous silicon, [11] Si nanocrystals in solid matrices [12] ) was first reported in the early 1990s and 2000s with low external quantum efficiencies (EQE) from 10 −6 to 1%. Of late, attention has shifted to colloidal silicon quantum dots (SiQDs) as potential active materials in hybrid organic light-emitting diodes (OLED) structures because of their promising EQE of up to 8.6% for near infrared (NIR) EL and 6.2% for red EL. [9a] Even with these improved metrics, the practical potential SiQD-LEDs remains limited by broad EL bandwidths with full-width-at-half-max-

In this study, quantum dot light-emitting diodes based on non-toxic silicon quantum dots functionalized with hexyl and dodecyl organic ligands showed a negative capacitance effect. Current density-voltage (J −V ) measurements revealed the charge transport mechanisms in the QLEDs. The capacitancevoltage (C − V ) characteristics were measured with an LCR meter over a wide range of frequencies (200 Hz to 1 MHz) and at temperatures from −40 • C to 60 • C. The classical heterojunction theory can describe the operation of quantum LEDs, but an effect not predicted by Shockley's theory was observed. Negative capacitance values were recorded in both fabricated LEDs, which were not observed in SiQD-LEDs before. Hence, we investigate the negative capacitance origin and its influence on the device performance. We attribute the negative capacitance to trapmediated recombination, where charges at defect sub-bandgap states contribute to the recombination but cannot be replenished fast enough. As a result, a current flows to re-establish the equilibrium, which lags behind the applied voltage, and the NC appears. By comparing the two functionalizations, we also observed a different temperature dependence of the positive capacitance peak and a stronger negative capacitance effect for dodecyl functionalized SiQDs. This effect is attributed to their improved charge carrier confinement abilities.Index Terms-Silicon quantum dot (SiQD), light-emitting diode (LED), negative capacitance (NC). I. INTRODUCTIONQ ANTUM dot light-emitting diodes (QLEDs) are highly researched as promising electroluminescence (EL) devices. QDs itself combine the advantages of inorganic and organic materials by offering size-tunable emission, [1] high color purity, [2] easy processibility, [3] and high photoluminescence quantum efficiency. [4] Additionally, QLEDs are expected to have improved stability compared to OLEDs because of their inorganic crystalline materials. [5] Many conventional QDs consist of toxic or heavy metal materials, such as Cd or Pb, and therefore, alternative materials are researched. A promising alternative is nanosized silicon,

Luminescent colloidal silicon quantum dots (SiQDs) have been explored as alternatives to metal-based QDs for light-emitting diodes (LEDs) because of the abundance and biocompatibility of silicon. To date, the broad electroluminescence (EL) bandwidth (> 100 nm) and blue-shifting of EL at high applied voltages of SiQD-LEDs have been outstanding challenges that limited competitive spectral purity and device stability. Herein, we report the fabrication and testing of SiQD-LEDs that incorporate a Fabry-Pérot cavity that exhibit a narrow spectral linewidth as low as ca. 23 nm. The presented devices also provide spectral and visual stability from +4 V to +8 V, as well as spectral tunability.