The Physics And Application Of Interface Engineering In Polymer Light-emitting Diodes

Nowadays, displays become an integral part of people everyday life. Though LiquidCrystal Display (LCD) is the dominant display technology, Organic Light Emitting Diodes(OLED) has made significant progress both in academics and industry recently. Due to itswide viewing angle, self-emitting property, high efficiency, low power consumption, and thecapability in flexible display and transparent display, OLED is considered as the nextgeneration display technology.OLED can be divided into two categories: small molecular organic light-emitting diodes(SMOLED) and polymer light-emitting diodes (PLED), based on the functional material’smolecule weight. SMOLED is commonly fabricated by thermal evaporation, while PLED isgenerally fabricated through various solution processes, including spin-coating, dip-coating,screen printing, slot-die coating, and ink-jet printing, etc. Since solution process can becarried out in air at low temperature, PLED demonstrates greater development potential overSMOLED. OLED is “dual-injection” device in which electrons and holes are injected fromthe opposite electrodes into an electroluminescent (EL) layer to produce light. It is essentialfor the electrons and holes to be balanced inside the EL layer to achieve high efficiency.Therefore, device performance can be effectively improved by reducing the carrier injectionbarrier and balancing carrier density in active layer. Interface engineering at OLED cathode oranode side has become a popular approach to enhance device performance by improvingcarrier injection and carrier balance.Recently, water/alcohol soluble conjugated polyelectrolytes (CPEs) or their neutralprecursor, and self-assembled monolayers (SAMs) emerged as promising materials fororganic/metal interface engineering. In our study, we discovere that by simply spin-coatingthe solvents commonly used to dissolve CPEs and SAMs, such as ethanol and methanol, ontop of organic active layer, the performance of PLED is significantly enhanced. It has beendemonstrated that various solvents can be used in solvent treatment and this method works forat different organic/metal interfaces. In particular, the quantum efficiency of P-PPV devicewith Ba/Al cathode is increased by as large as58%after ethanol treatment. An interfacedipole between the organic layer and metal layer induced by the solvent, either from theintrinsic dipole or the interaction between the solvent and the cathode metal, is responsible forthe device performance improvement. The interface dipole layer, which is confirmed by theKelvin Probe Force Microscopy and the photovoltaic measurements, lifts the vacuum level onthe metal side, thereby reducing the electron injection barrier at the organic/metal interface, and leading to better device performance.With efficient hole injection layer (HIL), for most PLED devices, the hole possesses asmaller injection barrier at the anode side compared to the electron at the cathode side,inducing extra holes into the emission layer. Furthermore, the hole mobility of somewidely-used conjugated light emitting polymers is much higher than the electron mobility,resulting in a recombination zone close to the cathode which is not a preferable location.Interface engineering at anode side can significantly reduce the hole leakage current to thecathode, and move the recombination zone away from the cathode. By spin-coating severaldrops of the polar solvent on top of the PEDOT:PSS hole injection layer, the maximumluminance efficiency of P-PPV device is increased by as much as83%without sacrificing theoperation voltage. We find out that the combination effects of the reduced work function andthe lowered resistivity of PEDOT:PSS film, decreased the hole leakage current, leading to amore balanced charge density inside the emission layer, thereby enhancing the deviceperformance.Based on the results of solvent treatment utilized in small devices, we applied solventtreatment in the fabrication of polymer light-emitting display. The display brightness,efficiency, and stability, were significantly improved. The application in PLED displaydemonstrates that solvent treatment is a universal interface engineering method. Moreover,the process of solvent treatment to modify the organic/metal and organic/organic interface issimple, efficient, cost-effective and easy to be adopted in industrial manufacturing.