Modulation in charge distribution via polyethylenimine-incorporation into ZnO electron transport layer and its impact on quantum-dots light emitting device stability

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.