Laboratory of Organic Photonics and Iontronics

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

Polymer Bulk Homojunction Cells


Throughout our studies of large planar LECs, we observed that electroluminescence (EL) always originates from narrow p-n junctions, which account for less than 1% of the total cell area. To achieve a higher EL output, we devised a simple and elegant approach to forming multiple light-emitting p-n junctions through the addition of conductive indium tin oxide (ITO) particles into the LEC film. As a result, when a large voltage bias was applied, thousands of tiny light-emitting p-n junctions were turned on within the bulk of the planar LEC. 

The unique “bulk homojunction” LECs exhibited a much faster EL turn-on and EL efficiency that was approx. 40 times higher than a control LEC with a single junction. This device also exhibited a giant open-circuit voltage (VOC) of 25.5 V when operated as a PV cell under illumination (APL87:143502, 2005). The bulk homojunction formation was not limited to specific luminescent polymers or ITO particles.

We successfully demonstrated bulk homojunction LECs in all three primary colors by using various commercially available (gold, silver, zinc) or custom-made (bismuth) metallic particles (JAP 100:104503, 2006).  Recently, we discovered that the underlying operating mechanism of the bulk homojunction LEC was that of bipolar electrochemistry. And bulk homojunction LEC was the first demonstration of bipolar electrochemistry in its solid state. The new field we helped to create, solid-state bipolar electrochemistry, will be a main area of our research.

The following image shows how a single metallic BPE can lead to the formation of four light-emitting p-n junctions. 

Image shows how a single metallic BPE can lead to the formation of four light-emitting p-n junctions