Differentiating the role of charge injection and charge distribution in electroluminescence efficiency of quantum dots

Charge balance in electrically driven light-emitting diodes is crucial for enhancing device efficiency, particularly from the perspective of exciton formation. Specifically, balanced charge injection is closely related to the emission layer. Typically, charge balance is assessed using the apparent current of single-carrier (electron- or hole-only) devices, but this method is unreliable due to various interface barriers within the device. We observed that as temperature decreases, charge injection becomes more imbalanced, yet device efficiency continues to rise, exceeding the contribution from photoluminescence enhancement. Temperature-dependent transient electroluminescence (T-TrEL) measurements were conducted to analyze charge distribution within the quantum dot (QD) emission layer. The results indicate that the spatial overlap between electron and hole distributions decreases with higher driving voltage (current), thereby reducing the likelihood of exciton formation. This is likely a key factor contributing to the low efficiency of QLEDs operating at low current densities. Our findings suggest that, compared to apparent charge balance, charge distribution is a more accurate and reliable parameter for evaluating charge recombination behavior in QLEDs.