Since the arrival of organic light-emitting diodes (OLEDs), the possibility of being able to economically print off rolls of electronic newspaper has become an exciting prospect. However, the continuous production of OLEDs under ambient conditions has been frustrated by the requirement for low-work-function cathode materials and a thin active layer. A promising alternative that suffers from neither of these constraints is the light-emitting electrochemical cell (LEC), and an article in the August 14 edition of Nature Communications (DOI: 10.1038/ncomms2002) describes how continuous ambient fabrication of these devices has recently been realized.
A. Sandstrom at Umeå University, Sweden, H.F. Dam at the Technical University of Denmark, and their co-workers used a slot-die roll coater, in which ink is injected onto a continuous roll of material, to print a simple layered device structure onto a flexible, transparent cathode material—poly(terephthalate) coated with ZnO-on-indium-tin-oxide. An active layer of a conjugated polymer (superyellow) and electrolyte (KCF3SO3) was first spread to a (dried) thickness of 1 μm, before coating with a similar thickness of poly(ethylenedioxythiophene): poly(styrenesulfonate) to serve as an anode. Despite the relatively thick and rough nature of the resulting layered LEC, they could achieve a brightness of 150 cd m–2 at 10 V.
When a voltage is applied to the device, the mobile electrolyte ions in a LEC form electric double layers at the anode and cathode that promote hole and electron injection into the active layer, respectively. This leads to both p- and n- doping of the polymer film until the two regions meet, allowing charges to recombine and emit light, even in thick films such as these. The time dependence of this doping process makes turn-on time an important property, measured at 2 s at a current density of 770 A m–2 for these devices.
LECs share with OLEDs a need to be kept free from oxygen and water vapor during operation, so resistance to ambient conditions was effectively conferred to the developed device by drying at high temperature and encapsulating it inside adhesive barrier layers. Given that these steps could also be carried out in a continuous process, the researchers see their research as a first step toward cheap roll-to-roll printing of large areas of light-emitting “paper” and said that optimization of the active material may lead to rapid improvements in its performance.
