Negative capacitance as a diagnostic tool for recombination in purple quantum dot LEDs

Abstract

Impedance spectroscopy is a powerful and nondestructive tool for studying charge carrier dynamics in quantum dot light-emitting diodes (QLEDs). We report here that QLEDs exhibit unique capacitance behavior that strongly depends on the ligand chemistry of the quantum dots (QDs). At low frequencies and under bipolar injection, the capacitance of the QLEDs becomes negative before it returns to positive values at even lower frequencies. This behavior is fundamentally different from that observed in organic light-emitting diodes and is attributed to the accumulation of charge carriers within the ligand shells during operation. The capacitive response depends on both the conductivity and the length of the QD ligands and can be used as a diagnostic tool for understanding the luminescent recombination efficiency of a QLED. We find that short and conductive ligands result in positive device capacitances only and this correlates with enhanced device efficiency.