Polyethylenimine (PEI) is sometimes used as a passivation layer at the interface between ZnO electron transport layer and quantum-dots emission layer in quantum-dots light emitting devices (QDLEDs). We recently find that blending ZnO with PEI (ZnO:PEI) is advantageous over using it in a separate layer in terms of device stability. In this work, a comparative study between the ZnO:PEI with a neat ZnO ETL is conducted. The ZnO:PEI ETL results in improvement in both EQE and lifetime of QDLEDs compared to the ZnO ETL. By replacing the ZnO ETL with the ZnO:PEI ETL, delayed EL measurements reveal changes in charge distribution across the QDLED. Applying a reverse bias pulse shows that the reversible delayed EL components in the QDLED with the ZnO:PEI ETL stemmed from the electrons placed in a hole transport layer (HTL). The electrons in the HTL induce an annihilation of accumulated holes at the QD EML/HTL that can be a cause of device degradation. The result provides a new insight into the importance of managing charge distribution across the QDLED via ZnO ETL modification for realizing highly stable QDLEDs.
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.
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