Low-Work-Function Surface Formed by Solution-Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate

The nanometer-sized interfacial layer between the metal cathode and the organic semiconductor plays a critical role in controlling the performance of organic light-emitting devices (OLEDs). Cs2CO3 has been shown to be a very efficient electron-injection material in OLEDs, including in OLEDs based on small molecules and polymers (polymer lightemitting diodes (PLEDs)). It has been shown that there is a three-fold increase in the efficiency of a white-light-emitting PLED upon the insertion of a thin Cs2CO3 layer between the light-emitting polymer (LEP) and the Al cathode. It is also known that Cs2CO3 is more effective than LiF in terms of facilitating electron injection, because devices using a Cs2CO3 electron-injection layer have a lower driving voltage, and hence exhibit a higher power efficiency. In addition, more materials can be used as the cathode metal when Cs2CO3 is used as the electron-injection layer. Furthermore, the electron-injection ability of the interfacial layer can be precisely controlled by adjusting the thickness of Cs2CO3, and thus balanced electron and hole currents can be achieved to realize optimal device performance. Owing to the above reasons, Cs2CO3 has been increasingly used in organic electronic devices. For example, it has been used as the connecting unit in tandem OLED cells, as an electrode material in inverted photovoltaic devices, and as a dopant for other electron-transport materials. Notably, Cs2CO3 can be processed either by thermal evaporation or spin-coating. Cs2CO3 has a high solubility in polar solvents such as water and alcohol, and is almost completely insoluble in most other organic solvents such as toluene, p-xylene, and chlorobenzene. Thus, the deposition of Cs2CO3 layers is compatible with the solution processing of multilayer PLED structures. Furthermore, once all the device parameters have been optimized, there is no obvious difference in device efficiency using these two processes. This point is discussed in more detail below. However, in contrast to well-known LiF, it is still not clear how Cs2CO3 works in improving electron injection. Here, we report a systematic study of the origin of the good electron-injection capabilities of Cs2CO3 layers fabricated by solution processing or thermal deposition. The electron-injection capabilities of Cs2CO3 layers have been studied using PLEDs with cathodes fabricated from several different metals. The devices have been first characterized by current–voltage (I–V) measurements to demonstrate increased electron injection by Cs2CO3. Secondly, the increased electron injection is explored