Post fabrication electric field treatment of polymer light emitting and photovoltaic devices

Conjugated polymers are semiconductors that offer flexibility, simplicity and lower cost in the fabrication of polymer light emitting diodes (PLEDs), polymer transistors and circuits, polymer photodetectors and solar cells. They have a potential to lead to a technology that can monolithically integrate all the semiconductor devices mentioned above as well as polymer sensors and actuators. This paper presents a novel method as a post-fabrication treatment and its improvements over the performance of the light emitting and photovoltaic properties of PLEDs. Investigated PLED is made of indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), poly[2-methoxy-5-(2´-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and aluminum (Al). Following fabrication at room conditions, heat treatment is performed at 130Cdeg for one hour and electric field treatment is realized with voltage levels from 0 to -8 V under 0.2 atm vacuum. Heat treatment after fabrication restores the light emitting function of otherwise not functioning PLEDs, which degrade due to exposure to oxygen and water vapors at normal room conditions. Electric field treatment reduces the turn-on voltage of PLEDs when they are used as light emitting devices. Electric field treatment of -1 volt reduces the turn-on voltage to 3 volts from 10 volts, which is the case for the devices with heat treatment only. It also improves open circuit voltages and short circuit currents of PLEDs by an order of magnitude when they are used as photo-detectors or photocells. Devices treated with heat only show a short circuit current of around 0.5 nA and open circuit voltage of 5 mV under 500 mW/m² light intensity. These values improve to 5 nA and 55 mV respectively after the devices are electrically treated with -1 volt. Electric field treatment after the thermal treatment also improves the stability and uniformity of the devices.