Colloidal quantum dot (QD)-based light emitting devices are poised to become the leading technology in next generation flat panel displays but their electroluminescent (EL) stability is still insufficient for commercial applications. We recently found that using a cascaded hole transport layer (HTL) structure can lead to significant EL stability enhancements, prolonging device EL lifetime by 25 times. Introducing modifications to the ZnO electron transport layer can lead to similar benefits. Investigations show that the stability enhancement in both cases is associated with a better management of charge and exciton distributions in the HTL. Results from these investigations will be discussed.
While the narrow emission spectrum and high quantum yield of quantum dots (QDs) is desirable for light emitting devices (LEDs), the mechanisms that limit electroluminescent QDLED stability must be understood before they can be used in high brightness applications. The deep energy levels of Cd-based QDs allow for relatively easy electron injection but comparably difficult hole injection, resulting in an imbalance of charge carriers in the emission layer (EML) that can reduce efficiency via non-radiative recombination. The incorporation of a multi-component hole transport layer (HTL) consisting of materials with sequentially deeper highest occupied molecular orbital (HOMO) energy levels in a cascading HTL (CHTL) architecture has been shown to improve QDLED lifetime by 20x while also enhancing luminous efficiency. Prompt and delayed electrical and spectroscopic measurements indicate that the CHTL structure shifts excessive hole accumulation away from the QD/HTL interface, resulting in less degradation of the HTL in contact with the QD EML, and reduces leakage current by blocking electron transport to the anode. The trade-off between exciton density in the HTL vs. QDLED efficiency and stability highlights the importance of the HTL in long-term device performance.
The exceptional luminescence properties of colloidal quantum dots (QDs) make them advantageous for use as an electroluminescent material in light emitting devices (QDLEDs). Drastic improvements in the performance of QDLEDs have been achieved through the use of inorganic electron transport layers and organic hole transport layers (HTLs), yet the electroluminescence stability of QDLEDs remains insufficient for commercial applications. To address the issue of QDLED stability, significant work has been done to reduce charge imbalance and Auger recombination in the QDs which arises from the large energy level mismatch between the valence band of the QD and the highest occupied molecular orbital (HOMO) of the HTL. This work identifies morphological stability within QDLEDs as an additional degradation mechanism limiting device stability. Interaction between the HTL and surface roughness of the underlying layers appears to be a critical parameter to address in QDLED design. Studies of QDLEDs using electrical measurements and electroluminescence imaging elucidate upon the role that morphological stability plays in the degradation of electroluminescent QDLEDs.
It is well-known that hole transport layers (HTLs) in organic light emitting devices (OLEDs) are more sensitive to morphological changes than other organic layers due to the lower glass transition temperatures. A high operational temperature can alter the HTL morphology, severely impacting OLED performance and stability. Although joule heating is a known factor affecting OLED stability during operation, its effect in experimental studies is typically simulated through thermal annealing of the devices rather than applying current. In this work, a comparison of the effects of joule heating vs thermal annealing on the morphological stability of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB) and N,N′-Dicarbazolyl-4,4′-biphenyl (CBP) HTLs and the impact this has on OLED performance is investigated. While thermal annealing of an OLED can be used as an approximation of joule heating, the temperature distribution profile of the OLED is different under the two stress conditions and thus can impact the morphology of the HTL differently. However, joule heating introduces a confounding factor whereby the OLEDs experience intrinsic degradation by the flow of current, aside from thermal stress. Therefore, in this work, joule heating is studied in unipolar devices that comprise solely of the HTL. Device JVL and morphology as a function of temperature for both joule heating and thermal annealing are presented as a means to evaluate stability and performance.
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