Laboratory of Organic Photonics and Iontronics

Laboratory of Organic Photonics and Iontronics
Laboratory of Organic Photonics and Iontronics

Polymer Light-Emitting Electrochemical Cells


Frozen-Junction LECs

The polymer light-emitting electrochemical cell (LEC) was first demonstrated in 1995 by researchers at Uniax Corp., Santa Barbara.

An LEC is a thin-film light-emitting device based on a mixed ionic/electronic conductor consisting of a solid-state polymer electrolyte and a luminescent polymer. Under a DC voltage bias, the luminescent polymer is electrochemically p-doped on the anode side and n-doped on the cathode side, creating a light-emitting p-n junction when the doped regions meet. In 1997, I first showed that the dynamic LEC junction could be immobilized by cooling the device to below the glass transition temperature of the polymer electrolyte. This created the first frozen-junction LEC (APL 71:1293, 1997), which exhibited fast response time and  a long operating lifetime (JAP 86:4594, 1999; JAP 100:084501, 2006). More important, the frozen-junction LEC operated as a photovoltaic cell with a large open-circuit voltage independent of the electrode work function (APL 91:233509, 2007). The power efficiency of a frozen-junction PV cell was improved with the addition of a more electronegative conjugated polymer to promote charge separation (Advanced Materials 10:692, 1998). Our current effort in this area aims to realize a stable frozen-junction LEC at room temperature. 

Graph showing frozen junction LEC exhibiting fast response time and a long operating time

Extremely large planar LECs

In the 90s and early 2000s, the vast majority of polymer LECs were fabricated in a sandwich configuration that, while highly emissive, did not allow for the direct visualization of the doping process. There were but a few reports of planar LECs, all with a micrometer-sized interelectrode gap making them difficult to image. 

In 2003, our group at Queen's successfully demonstrated planar LECs with an interelectrode gap size of 3 millimeters, 100 times larger than any previous planar cells. The extremely large planar cells were fabricated using shadow masks (vs. photolithography) and imaged directly with a digital camera (APL 83:3027, 2003). The large interelectrode gap slowed down the in situ electrochemical doping process. This allowed for the time-lapse fluorescence imaging of the dynamic LEC turn-on process (APL 84:2778, 2004).

For the first time, we were able to visualize the in situ electrochemical p- and n-doping processes, leading to the formation of a light-emitting p-n junction. By analyzing the images, we showed that the average doping propagation speed increased linearly with the driving voltage and exponentially with the cell temperature (APL 86:153509, 2005). By moderately heating the cell, we were able to turn on planar LECs with an interelectrode spacing of 11 mm (JAP 98:063513, 2005; APL 88:123507, 2006). These extremely large planar LECs had proven to be a very powerful tool in studying the complex LEC processes. Stressing/imaging of millimeter-sized planar LECs allowed us to elucidate how salt cations (APL 89: 253514, 2006; Advanced Materials, 18:2880, 2006), thermal annealing/quenching (Organic Electronics 9:347, 2008), and electrode work function (Advanced Materials 20:3298-3302, 2008) affect the dynamic doping propagation and junction properties.

We recently mapped the potential distribution across a 10.4 mm-planar LEC that was turned on and subsequently frozen in the probe station. The obtained potential profile established that the planar LEC was a graded p-n junction with the p region being much more conductive than the n region (JACS 137:2227, 2011). We are currently exploring new combinations of materials and operational parameters.

Our goal is to develop a quantitative model that can explain the complex doping patterns we have observed.

Images showing large interelectrode gap which slowed down the in situ electrochemical doping process