Origin of the Below-Bandgap Turn-On Voltage in Light-Emitting Diodes and the High VOC in Solar Cells Comprising Colloidal Quantum Dots with an Engineered Density of States.

The turn-on voltage of a light-emitting diode (LED) is an important parameter as it determines the power consumption of the LED and influences the effective power conversion efficiency. LEDs based on nanoscale engineering of the blended PbS [mixture of quantum dots (QDs) with two different bandgaps] colloidal QDs have recently shown record performance in the infrared region. One of the most intriguing findings for these blended devices is the substantially lower-than-bandgap turn-on voltage and the achievement of an open circuit voltage ( VOC), approaching the radiative limit. In this work, we provide new insight into the origin of these phenomena. We posit that the decrease in the effective density of states in the conduction and valence band of the emitter QDs in the blended structures modifies the chemical potential, which controls the turn-on voltage for LEDs through the modification of the photon flux and VOC for photovoltaic devices with quasi-Fermi level modification.