The thin electron injection layers between the cathode and the light emitting polymer layer in polymer light emitting diodes (PLEDs) have been shown to have a big impact on the final device performance. Usually, in PLEDs low work function metals like Ba, Mg or Ca are used to reduce the energy barrier between the cathode and the polymer thus providing a better electron injection from the cathode. Also salts like LiF, NaF, Cs2CO3 and CsF have recently been shown to function as electron injection layers in light emitting devices. From these, especially caesium carbonate (Cs2CO3) results into high efficiency diodes both as a solution processed electron injection layer in PLEDs, as well as an n-dopant in the electron transport layer in vacuum deposited small molecule based OLEDs.
The functional mechanism of Cs2CO3 as a pure interlayer is not yet fully understood. The proposed mechanisms include the n-doping of the organic layer with Cs2CO3, the thermal decomposition of Cs2CO3 and following formation of caesium metal or the formation of an n-doped CsO2 layer.
In this study the phenomena resulting from the combination of a hole-dominant alkoxy-phenyl-substituted poly(phenylene vinylene) (PPV) based light emitting polymer with a highly efficient electron injection layer of Cs2CO3 in light emitting diodes has been investigated. As a result, diodes with about 35 % higher efficiency were achieved with PPV-Cs2CO3 structure in comparison to the traditional PPV-Ba structure. Additionally to the increased efficiency, also the lifetime of the Cs2CO3-diodes is comparable to the Ba-diodes implying that the long-term stability of the diodes is not affected by the optimized Cs2CO3-cathode.
The strong increase in the electron injection of the Cs2CO3 diodes is apparently caused by a highly conductive, n-doped layer resulting from the charge transfer reaction between Cs2CO3 and PPV, where the magnitude of the reaction and resulting effects strongly depend on the amount of the applied Cs2CO3. The conclusion of the n-doped layer can be drawn from the LIV, impedance and photoluminescence measurements of the diodes with Ba and Cs2CO3 cathodes before, during and after electrical stressing.