Polymer‐Based White‐Light‐Emitting Electrochemical Cells with Very High Color‐Rendering Index Based on Blue‐Green Fluorescent Polyfluorenes and Red‐Phosphorescent Iridium Complexes

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Abstract
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.

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