Polymer Electrochromic Cathode Modified Light-emitting Device And Carrier Transport Properties

Polymer light-emitting devices (PLEDs) have a lot of advantages in the manufacturing techniques, such as simple devices' structures, easy fabrication processes and control of precision, as well as low-cost production facilities. Besides that, PLEDs are attracting attention due to their good electroluminescence (EL) properties. They can work under low operating voltage and show vivid colors, wide viewing angle. Especially, PLEDs can be made on flexible substrate making the foldable display devices to be available. Just because of the good qualities mentioned above, PLEDs are becoming more and more attractive as a new technology in multi-color displays field.The EL and I-V properties of PLEDs were studied, whose structure was ITO/PEDOT:PSS/MEH-PPV/buffer/Al. Four pairs of PLEDs are compared. The four pairs are PLEDs with or without PEDOT:PSS as the hole transport layer(HTL), PLEDs used metal barium (Ba) or aluminum (Al) as the single layer cathode, PLEDs with baryta (BaO)/ Al or Ba/Al double layer structure cathode and PLEDs made from MEH-PPV solutions of different concentrations. In order to understand the physical mechanism involved in PLEDs, the transport properties of carriers inside the structure and I-V characteristics of PLEDs are simulated.EL properties of PLEDs made with different cathodes, different MEH-PPV solutions of different concentrations were measured. The results were presented. When the PEDOT:PSS layer was introduced as the HTL, the EL intensity was improved by 120%, and the turn-on voltage decreased by 3V. Besides, when Ba was used as the cathode, the EL intensity of devices becomes three times compared to devices with Al cathode. Then, a ultrathin buffer layer was introduced between the cathode and the EL layer. As a result, no matter Ba or BaO was used, the EL characteristics were enhanced significantly. Also it showed that when Ba was used as the buffer layer the device works even better rather than being used as the single-layer cathode. Finally, based on the results it could be found that PLEDs made fromMEH-PPV solution of higher concentration appeared perform better than those made from lower concentration solution.The charge transport inside PLEDs was study in this article. It was proven that the mobility and density of carrier had few relationships with the depth in the devices. But temperature and electric field have strong influence on them.I-V curves of PLEDs, whose structure was ITO/MEH-PPV/Cathode, were simulated by the Space Charge Limited Current (SCLC) model and the Fowler-Nordheim (F-N) tunneling model respectively. The results show that I-V curves simulated by F-N tunneling model are in better accordance with the curves measured in our experiments. Mean-while, based on F-N tunneling theory relationships between devices' current and thickness of BaO layer were computed, and the results could explain the experimental data well.