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Tandem organic light-emitting diodes (OLEDs) with an organic–organic intermediate connector, consisting of an n-type doped electron-transporting layer (e.g. Li-doped Alq as shown in the figure) and nondoped hexaazatriphenylene hexacarbonitrile (HAT-CN), exhibit reduced drive voltage, improved voltage stability, and improved power efficiency. Moreover, this intermediate connector can be formed by thermal evaporation at temperatures lower than 300 °C, which simplifies the fabrication process of the OLEDs.
Liao et al. (2007) studied this question.