Abstract Organic light emitting diode contain couples of thin film layer, and efficient balanced holes and electrons injection is needed so as to have efficient device. Here, the injection is critical at around organic thin-film layers between the metal/transparent conductive oxide (TCO) contact of anode and cathode, that is, injection as the charge transfer across the interface between a metal/TCO contact and an adjacent organic layer in the device. In OLEDs charge injection is the process of transporting electrons or holes from an electrode to the HOMO or LUMO in the organic layers of OLEDs. The charge injected into the OLED device is affected bythe density of mobile carriers, heights of the injection barrier,temperature and the overall charge injection efficiency at the interface. Studies on temperature dependence of the injection current for barrier heights indicated a possibility of the crossover from injection-limited current (ILC) and space-charge-limited current (SCLC)with decreasing temperature irrespective of the injection barrier height. The charge injection efficiency of either holes or electrons determines the charge balance, which intern determine the efficiency OLEDs. Thus, considering one of the dominant holeinjection layer(HIL),poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), the charge injection needs to be optimized, where in this case the PEDOT:PSS work function can be tuned by time dependent UV-ozone exposure of the polymer. Similarly, the work function of other HIL materials can be controlled by different menses - indicated by different studies, and the device efficiency can be improved significantly. Thus, balanced charge injection is important in OLEDs to enhance the performance of OLEDs. Key words: Organic light emitting diode, organic thin-film layers, transparent conductive oxide and injection-limited current .