Investigating and Optimizing Charge Balance and Its Effect on Efficiency and Lifetime of QLEDs

Abstract

Colloidal quantum dot light-emitting devices (QLEDs) have gained significant attention as next-generation display and solid-state lighting technologies, offering exceptional color purity, tunable emission wavelengths, high photoluminescence quantum yield (PLQY), and solution-processable fabrication techniques. The ability of quantum dots (QDs) to exhibit size-dependent optical properties through quantum confinement effects enables highly saturated red, green, and blue (RGB) emissions. Since their first demonstration in 1994, QLEDs have shown tremendous improvements in performance, achieving external quantum efficiencies (EQE) close to theoretical limits (~20%) and operational lifetimes (T95@100 cd/m²) exceeding 300,000 hours. Despite significant advancements, several unresolved challenges continue to limit the full commercial potential of QLEDs, particularly blue QLEDs (B-QLEDs), which exhibit notably lower electroluminescence (EL) stability compared to their red and green counterparts. A key factor impacting both device efficiency and lifetime is charge imbalance, which can lead to significant reduction in both performance metrics. This imbalance not only reduces the immediate efficiency of the device but also accelerates its long-term operational instability. Additionally, while many developments have focused on red or single-color QLEDs, less attention has been given to conducting comparative studies that evaluate the performance differences among red, green, and blue QLEDs. This leaves gaps in understanding how charge dynamics and performance vary across these colors, particularly in B-QLEDs where the charge dynamics may differ due to changes in the QDs' bandgap. As a result, the exact mechanisms by which charge imbalance affects both efficiency and lifetime, especially in B-QLEDs, remain unclear. Therefore, further research is necessary to better understand these mechanisms and address the remaining challenges. The primary goal of this thesis is to investigate and optimize charge balance in inverted QLEDs, with a focus on studying charge dynamics to better understand the electron and hole supply properties in red, green, and blue QDs. Additionally, this work explores the interactions between excitons and excess charges, examining how excess carriers, either electrons or holes, impact the EL performance of QLEDs. Based on the findings, new charge transport layers (CTLs) are explored to improve the device's charge balance. This research also investigates the EL loss mechanisms in upright B-QLEDs, which remain a major challenge for the commercialization of QLED technology. Finally, new material systems are tested to enhance the EL stability of upright B-QLEDs. To investigate whether commonly used HTLs in highly efficient upright structures can improve charge balance in inverted QLEDs, polymeric HTLs are employed in these devices. To address the challenge of HTL solvent erosion, a suitable recipe is developed for depositing polymeric HTLs on the QDs-EML. The fabricated inverted R-QLEDs exhibit relatively low efficiency, attributed to a poor interface between the QDs-EML and HTL, though not due to damage to the QDs-EML. However, the introduction of a wide bandgap interlayer between the QDs-EML and the HTL results in significant improvements in both efficiency and lifetime, primarily due to enhanced hole supply. Then, electron/hole supply properties of R, G, and B-QDs are investigated in single carrier devices. Results show that widening the bandgap of QDs increases hole supply efficiency but decreases electron efficiency. To investigate their respective electron and hole supply efficiencies, a comparative study is conducted on inverted R, G, and B-QLEDs to explore charge injection characteristics. The findings reveal that in R-QLEDs, the e/h ratio in the QDs-EML is greater than 1, whereas in G- and B-QLEDs, the e/h ratio is less than 1, with charge balance conditions being significantly worse in the case of B-QLEDs. Additionally, photophysical measurements show that, compared to electrons, holes lead to a stronger Auger quenching effect, which is proposed as one possible reason for the poor efficiency and lifetime of B-QLEDs. Finally, by studying spontaneous electron transfer effect at the ETL/QDs-EML interface, it is observed that injecting electrons into the QDs-EML first, rather than holes, is more favorable for efficiency enhancement, suggesting that efficient electron supply is a prerequisite for achieving efficient QLEDs. Based on the findings regarding the charge supply properties of each color, new CTLs are implemented to develop an inverted QLED structure with improved charge balance. The results from this structure show that employing a double ZnMgO ETL approach, where each ETL is annealed at different temperatures to enhance electron supply, along with an HTL featuring a relatively shallow lowest unoccupied molecular orbital (LUMO), leads to EQEs exceeding 23% for inverted R- and G-QLEDs. Additionally, doping the ZnMgO ETL with graphene QDs is proposed as another method to further enhance electron supply in inverted G-QLEDs, leading to improved device efficiency and extended lifetime due to enhanced charge balance. Next, the EL loss mechanism in upright B-QLEDs is systematically studied using non-invasive marking layers at both HTL and ETL to detect charge leakage. The results indicate that electron injection is stronger than hole injection in upright B-QLEDs. Additionally, findings from EL and PL measurements confirm that, aside from some partially reversible deterioration in the PLQY of the QDs-EML, the rapid EL loss in upright B-QLEDs is primarily due to increased electron leakage across the HTL. This leakage causes damage to the hole injection layer (HIL) and diverts holes away from the QDs-EML, leading to a deterioration in hole supply to the QDs-EML. Ultimately, the functional layers in upright B-QLEDs are modified to reduce electron supply and enhance hole supply. The results show that replacing the Al cathode with an Ag/Al electrode, featuring a deeper work function to limit electron injection, results in a 30X improvement in the device's EL stability. Additionally, introducing an insulating polymer layer between two ZnMgO layers to limit electron supply significantly extends the device lifetime by 12X. Furthermore, utilizing a new HTL to block electron leakage and improve hole supply leads to an 8X improvement in EL stability.