Preparation and properties of an organic light emitting diode with two emission colors dependent on the voltage polarity

We developed an organic light emitting diode (LED) that emits orange light at the forward bias (ITO anode and Al cathode) and green light at the reverse bias (ITO cathode and Al anode). The organic LED has a triple layer structure where an emission layer with different doped guest dye on each side is interposed between two hole transport layers. Its brightness, maximum 20 cd/m2 was not strong because electrons are blocked by the hole transport layer near the cathode. However, the EL efficiency (lm/W) for the reverse bias was 13 times higher than that for the forward bias.