White quantum dot light-emtting diodes based on inverted device structure

Abstract

Colloidal quantum dots (QDs) have many advantages given their superior optical and electrical properties when used in optoelectronic devices. High photoluminescence quantum yields, broad absorption and narrow emission spectra, and stability with regard to thermal and optical stimuli make QDs excellent materials for light-emitting diodes (LED). After many years of research and advances in the development of materials and device engineering, quantum-dot light-emitting diodes (QLEDs) have undergone numerous improvements in terms of their performance capabilities and architectures. Among the possible applications, white-light-emitting QLEDs have attracted significant attention due to the simplicity of their device architecture and the good flexibility of their emission spectrum. However, the charge carrier imbalance caused by the relatively large bandgap of blue QDs represents a hurdle preventing the achievement of high-performance white QLEDs. This thesis discusses the engineering of the hole transport layer (HTL) to control the hole injection rate into large-bandgap QDs and subsequent improvements in the device characteristics of white QLEDs. I demonstrate an efficient blue QLED with a larger bandgap through HTL engineering. First, the influence of the highest occupied molecular orbital (HOMO) energy level in the HTL on the performance of QLEDs employing blue QDs with larger bandgaps is evaluated. Specifically, HTLs with various HOMO energy levels, i.e., the CBP, mCP, TCTA, TAPC and NPB, are screened to investigate the device performance capabilities of blue QLEDs. Subsequently, CBP and mCP are chosen for use in the HTL of blue QLEDs. However, mCP when deposited onto the QD layer shows a non-uniform surface caused by a mismatch in the surface energy. Eventually, it was found that the co-deposition of CBP and mCP could successfully enhance the hole injection rate into blue QDs with larger bandgaps and could enable a uniform deposition on top of the QD layer. Blue QLEDs employing a CBP:mCP co-deposited layer exhibit a two-fold enhancement in the hole injection rates and thereby the luminescence efficiency. A highly efficient and color-stable white QLED is then exhibited by implementing the CBP:mCP co-deposited layer. Red, green and blue primary-color QLEDs are fabricated with a CBP or a CBP:mCP co-deposited layer, after which all samples are characterized. Spectral changes and device efficiency rates with respect to the mixing ratio of the red, green and blue QDs are investigated. White QLEDs using a CBP or a CBP:mCP co-deposited layer are fabricated and the relationships between the enhancement of each primary-color QLED and that of white QLEDs are investigated. The co-deposition of CBP and mCP allows a two-fold enhancement of the hole injection rate into the large-bandgap QDs, resulting in a considerable enhancement of the device performance in terms of the luminescence efficiency and the operational stability of corresponding QLEDs. Furthermore, the co-deposited HTL reduces the driving voltage of the large-bandgap QDs and subsequently improves the color stability of white QLEDs for improved temporal and operational dependency. The resulting white QLEDs emit white-emission around an equal energy point in the Commission Internationale de lEclairage 1931 chromaticity diagram and exhibit an external quantum efficiency rate exceeding 5 % at brightness levels ranging from 100 to 10,000 cdm-2. I believe that the suggested device architecture and the technological guideline presented in this thesis will be helpful to those developing efficient and color-stable white QLEDs.