The influence of charge carriers in the hole transport layer on stability of quantum dot light-emitting devices

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.