Quantum rod light emitting diodes: Suppressing leakage current and improving external quantum efficiency

Semiconductor quantum rods (QRs) have unique advantages over spherical quantum dots (QDs), such as linearly polarized emission and higher light out-coupling efficiency, which can potentially improve the external quantum efficiency (EQE) of light-emitting diodes (LEDs). However, the EQE of QR LEDs is trailing far behind the QD LEDs, primarily due to the low quantum yield of QRs in the thin films and voids that show up in the film due to the low packing density of the rod-shaped nanoparticles. This study synthesized CdSe/CdS QRs with various aspect ratios and investigated QR LED performance issues. The study found that the main factors impeding the performance of QR LEDs were electron leakage current and exciton quenching, with QR length significantly impacting these factors. To address these issues, without compromising on the QRs shape, we blended an insulator (i.e., paraffin in this case) into the QRs film, which fills the voids/gaps (causing the leakage current) in the QRs film and minimizes infiltration of the charge transporting material in the emitting material, i.e., ZnMgO nanoparticles. As a result, the leakage current in QR LEDs was suppressed over 10 times by blocking direct contact between the electron-transporting material (ETL) and hole-transporting material (HTL). It improved the photoluminescence quantum yield (PLQY) of the QRs film. This approach suppresses exciton quenching, guides both carriers to the QRs and pushes the electron–hole (e–h) recombination zone to the QRs emitting layer. In comparison to the reference QR LEDs, the QR LEDs with paraffin show ~ two-fold improvement in EQE, whereas current efficiency and luminance of the QR LEDs increased from 5.6 to 14 cd·A⁻¹ and 17,500 to 42,300 cd·m⁻², respectively. The efficiency roll-off was also curtailed by 73% at relatively higher current densities. Overall, the use of paraffin in the QR LED fabrication process shows promising improvement in the performance of QR LEDs and overcoming some of the challenges that have impeded their performance compared to QD LEDs.