Revealing the Negative Capacitance Effect in Silicon Quantum Dot Light-Emitting Diodes via Temperature-Dependent Capacitance-Voltage Characterization

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 capacitance-voltage (
$C-V$
) characteristics were measured with an LCR meter over a wide range of frequencies (
$200 \,\mathrm{Hz}$
to
$1 \,\mathrm{MHz}$
) and at temperatures from
$-40 \,{{\mathrm{^{\circ }}{\mathrm{C}}}}\,\mathrm{to}\, 60 \,{{\mathrm{^{\circ }}{\mathrm{C}}}}$
. 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 trap-mediated 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.