Investigation of excess charge carriers and optimization of InP quantum-dot light-emitting diodes using self-assembled monolayers

InP-based quantum dot (QD) light-emitting diodes (QLEDs) have emerged as promising candidates for next-generation displays, yet their development remains challenging due to charge imbalances caused by the difference in electron and hole injection. Moreover, it is still debated whether the excess carriers are electrons or holes. In this study, we address this challenge by introducing self-assembled monolayers (SAMs) between the electron transport layer (ETL) and QDs. By employing two distinct SAMs with opposite dipole moments—4-methoxybenzoic acid (MBA) and 4-cyanobenzoic acid (CBA)—we provide direct evidence that electrons are in excess in InP QLEDs. The contrasting charge injection modulation effects of these SAMs enabled this clear identification of the excess carriers. Additionally, the SAMs improved surface morphologies and effectively passivated surface defects on the ZnO ETL, mitigating exciton quenching. As a result, MBA-treated QLEDs demonstrated superior device efficiencies compared to pristine or CBA-treated devices, attributed to the suppression of electron injection from the ETL to the QDs. We believe this study offers valuable insights for optimizing QLED performance through precise charge carrier modulation.