Studies On Two-Color Light-emitting Electrochemical Cells

ABSTRACT: Polymer light-emitting diode (PLED) is widely concerned as a new flat panel display. Currently, full-color display becomes a main pursuit for monocolor display devices have nearly matured. Voltage controlled two-color light-emitting electrochemical cells provide a new method which make full-color display device more simplistic and more practical. Polymer light-emitting electrochemical cell (LEC) is a new concept based on the traditional PLED and lithium cell. In LEC, the active layer is a conjugated polymer doped with solid polymer electrolyte.First, we researched the concentration of polymer electrolyte doped in conjugated polymer. Three types of LEC with different doping density were fabricated. The structure of LEC is ITO/MEH-PPV+PEO+LiCF3SO3/Al. MEH-PPV, PEO and LiCF3SO3 with the mass ratio 10:5:1,5:5:1 and 5:10:2 respectively. The comparisons of electricity features, light intensity and spectral characteristics show that the LEC with mass ratio of 5:5:1 has the best properties. On these bases, the green-lighting LEC is fabricated with PPV as luminescent layer.Second, we fabricated the two-color LEC. The structure of two-color LEC is ITO/PPV+PEO+LiCF3SO3/MEH-PPV+PEO+LiCF3SO3/Al. When forward biased, the LEC emits red-orange light from the MEH-PPV layer. When reverse biased, the LEC emits green light from the PPV layer. We change the structure of two-color LEC. The new structure of two-color LEC is ITO/MEH-PPV+PEO+LiCF3SO3/PPV+PEO+LiCF3SO3/Al. By comparing the situation of luminescence, we found that the p-n junction's position has nothing to do with the nature of the polymer, but is related to the direction of electric field. In other words, the light-emitting p-n junction is closer to the cathode.Third, the influence of the thickness of luminescent layer has been researched. The consequences illustrate that the LEC can emit different colors in the forward and backward voltages only if the thickness of two films is nearly the same. In the mean while, this shows that the position of p-n junction is closer to one of the electrodes.