| dc.description.abstract | Quantum dot light-emitting devices (QLEDs) are promising candidates for use in the next-generation flat panel displays. QLEDs operate based on the electroluminescence (EL) from quantum dots (QDs) as the emissive layer. QLEDs have gained attraction due to the QDs' intriguing properties such as high photoluminescence quantum yield (PLQY) of nearly 100%, narrow emission full-width at half maximum (FWHM < 30 nm) which provides a wide color gamut, the tunability of their peak luminescence wavelengths across the entire visible spectrum, and their solution-processability which makes them compatible to low-cost and flexible fabrication techniques. These features make QLEDs superior to their currently commercialized organic light-emitting devices (OLEDs) rivals, as they provide more natural images. However, QLEDs EL stability for the three red, green, and blue primary colors are not similar. Particularly, blue QLEDs (B-QLEDs) are the least stable which makes it the bottleneck for QLEDs commercialization.
Despite the massive efforts on the QLEDs' development and progress in achieving efficient devices, the B-QLEDs still suffer from poor EL stability. Developing effective strategies to improve the EL stability of B-QLEDs requires that the underlying degradation mechanisms must first be identified. Although high EL stability refers to maintaining the EL level when the device is under electrical bias, i.e. electrical stability, it is imperative that the high EL stability first requires that the materials do not change with time in the absence of bias, i.e. high shelf stability. Unfortunately, there is no clear consensus in the field on the fundamental factors limiting the B-QLEDs EL stability, and the roles of electrical bias versus just the temporal stability of the devices remain entangled.
The main focus of this thesis is to pinpoint the underlying reasons governing the B-QLEDs EL stability and explain the possible mechanisms for the B-QLEDs EL loss. This work utilizes the upright QLED structure, in which the light output is through the anode, which consists of an organic hole transport layer (HTL), CdSe-based QDs, and an inorganic ZnMgO electron transport layer (ETL). The focus of this work is B-QLEDs although red and green QDs are also used as a reference for comparison at some point.
To systematically study the EL stability of B-QLEDs as the objective of this work, the B-QDs PLQY stability is monitored in storage (shelf life) and under electrical stress with different scenarios. Firstly, the PLQY stability of blue QDs (B-QDs) as a thin film is studied over time. The B-QDs are placed in contact with each of the ETL or HTL as in the B-QLEDs structure to determine if interactions between the different materials in the device stack influence the shelf life of the B-QDs PLQY. It is found that B-QDs PLQY is stable intrinsically or in contact with the HTL. However, the ETL is found to negatively affect the B-QDs PLQY over time, an effect that arises from the diffusion of species from the ETL into the QD layer as well as morphological changes in the ETL that both result in the QDs PLQY drop.
Next, the effect of bias is investigated, first focusing on its effect on the B-QDs PLQY. Single carrier devices are fabricated to be able to exclusively study the potential role of positive (holes) and negative (electrons) carriers upon electrical stress. The results show that HTL undergoes degradation upon hole current flow leading to the B-QDs PLQY decay, whereas the electron current flow has minimal effect on the B-QDs PLQY. The organic HTL is replaced with a more robust thermally crosslinked HTL. Although the new HTL sustained the QDs PLQY, it is not advantageous for improving the B-QLEDs EL stability whereas it can improve the green QLEDs EL stability with the same structure by a factor of two. The results indicate that neither HTL degradation nor B-QDs PLQY drop is the predominant reason for the B-QLEDs' fast EL loss.
The study is further continued by probing the electrical characteristics of the single-carrier devices. The results show that charge injection efficiency in B-QLEDs changes over time due to electrical aging. The results show that the changes worsen the initial charge balance condition during the device operation and thereby a significant EL quenching leads to the EL loss in B-QLEDs. However, the changes are observed to be partially reversible so that driving the B-QLEDs under pulsed current instead of constant current doubles the electrical stability of B-QLEDs.
Ultimately, the investigations continued, and it is found that B-QLEDs suffer from holes leaking into the ZnMgO layer and it causes a significant degradation to the ZnMgO layer. The findings indicate that the damage to ZnMgO layer induced by holes results in more defect density of states in the ETL and worsens the ETL electron injection capacity. This also serves as another contributing factor to the B-QLEDs poor EL stability. | en |
