Soichi Uchida

Much effort has gone into research on light‐emitting electrochemical cells (LECs) in recent years. LECs have a simple structure and can be fabricated using low‐cost methods and materials and are seen as the next big thing in organic devices after organic light‐emitting diodes (OLEDs). In particular, expectations are high, in that LECs could be used to create a new generation of low‐cost lighting systems, making use of their surface‐emitting property. Getting such systems to the market will require the development of highly efficient white light‐emitting LECs. A variety of methods for obtaining white emission based on the light‐mixing principle have been explored. Among these, the use of exciplexes formed between donor‐type and acceptor‐type molecules is one of the more promising. Exciplex emission is broad in spectrum and can be used to produce LECs with a high color rendering index. In this progress report, the recent developments in research into LECs designed to utilize exciplex emission and present technologies used to obtain white emission are discussed. The potential for using thermally activated delayed fluorescence to improve efficiency is described. Finally, the latest developments in optical engineering techniques for LECs are also discussed.

Application of the concept of three‐color (red (R), green (G), and blue (B)) light‐mixing to obtain white light is the most suitable way to realize white‐light‐emitting devices with very high color‐rendering indices (CRI). White‐light‐emitting devices based on the three‐color‐mixing method could be used to create lighting and display technologies. Here, white‐light‐emitting electrochemical cells (LECs) with very high CRIs are reported, which were fabricated by using blend films composed of a fluorescent π‐conjugated polymer (FCP), poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (PFBT), and a phosphorescent iridium complex, [Ir(ppy)₂(biq)]⁺(PF₆)⁻ (where (ppy)⁻=2‐phenylpyridinate and biq=2,2′‐biquinoline). The LECs fabricated with PFBT, the benzothiadiazole content of which is 0.01 mol %, showed blue electroluminescence (EL) emission originating from the fluorene segments and green EL emission from the benzothiadiazole units simultaneously. White LECs were then realized by adding red‐emitting Ir complexes as guest molecules to the blue‐green‐emitting PFBT. By optimizing the proportions of the PFBT and Ir complexes in the active layers (PFBT/[Ir(ppy)₂(biq)]⁺(PF₆)⁻=1:0.2 (mass ratio)), white‐light emission with Commission Internationale de l′Eclairage (CIE) coordinates of (0.29, 0.34) and a very high CRI value of 91.5 was achieved through RGB color‐mixing. It was noted that the emission mechanism was based on Förster resonance energy transfer and Dexter energy transfer from excited PFBT to [Ir(ppy)₂(biq)]⁺(PF₆)⁻. The utilization of LECs based on blue‐green FCPs and red Ir complexes looks very promising for the prospect of realizing white‐light‐emitting devices with very high CRIs.

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes...