This work investigates the mechanisms for the enhanced injection of electrons from a solution-processable polyethyleneimine (PEI) interlayer/aluminum (Al), silver (Ag), or gold (Au) electrodes in phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer (SY-PPV)-based polymer light-emitting diodes (PLEDs). The bilayer cathode significantly enhances electron injection and device performance, and is comparable to that of PLEDs using low work function metals, such as calcium (Ca) or lithium fluoride (LiF)/Al as the cathode. The functions of the PEI interlayer are characterized by current density-light intensity-voltage (J-L-V) and photovoltaic measurements, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. The thin PEI film forms a dipole layer to shift the vacuum level of the electroluminescent polymer and decrease the barrier height for electron injection. The existing dipoles are correlated with a large numbers of amino groups in the PEI polymer. As a result, the PLEDs with PEI/Al, PEI/Ag, and PEI/Au bilayer cathodes exhibit markedly enhanced performances. The application of air-stable metals as the cathode allows flexibility in the design of device cathode structures for real applications.
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