Solution processed naphthalene diimide derivative as electron transport layers for enhanced brightness and efficient polymer light emitting diodes

Increasing the efficiency and lifetime of polymer light emitting diodes (PLEDs) requires a balanced injection and flow of charges through the device, driving demand for cheap and effective electron transport/hole blocking layers. Some materials, such as conjugated polyelectrolytes, have been identified as potential candidates but the production of these materials requires complex, and hence costly, synthesis routes. We have utilized a soluble small molecule naphthalene diimide derivative (DC18) as a novel electron transport/hole blocking layer in common PLED architectures, and compared its electronic properties to those of the electron transport/hole blocking small molecule bathocuproine (BCP). PLEDs incorporating DC18 as the electron transport layer reduce turn on voltage by 25%; increase brightness over three and a half times; and provide a full five-fold enhancement in efficiencies compared to reference devices. While DC18 has similar properties to the effective conjugated polyelectrolytes used as electron transport layers, it is simpler to synthesise, reducing cost while retaining favourable electron transport properties, and producing a greater degree of efficiency enhancement. The impact on device lifetime is hypothesized to be significant as well, due to the air-stability seen in many naphthalene diimide derivatives.