White Polymer Light-Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent π-Conjugated Polymers Based on Concepts of Band-Structure Engineering

The authors report on white polymer light-emitting electrochemical cells (PLECs) fabricated with a polymer blend film composed of a blue fluorescent π-conjugated polymer (blue FCP), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD), and a red-orange FCP, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), based on concepts of band-structure engineering. Polymer blending is one of the simplest and most promising methods for fabrication of van der Waals interfaces, which convert electricity to light in PLECs. By optimizing the composition of PFD, MEH-PPV, poly(ethylene oxide) (PEO), and salt (KCF₃SO₃) in the active layer, white-light emission with Commission Internationale de l'Eclairage (CIE) coordinates of (x = 0.33, y = 0.31) can be achieved through light mixing of blue exciton emission from PFD and red-orange exciton emission from MEH-PPV at an applied voltage higher than the threshold voltage, V_th ^blue-FCP, which corresponds to E_g ^blue-FCP/e, where E_g ^blue-FCP is the band gap of PFD and e is the elemental charge. The white light produced by light mixing of PFD and MEH-PPV emissions can be obtained at a low MEH-PPV concentration, while only red-orange emissions from MEH-PPV are obtained at high MEH-PPV concentrations. The emission color of FCP-blend PLECs can be explained by Förster resonance energy transfer (FRET) from the excited PFD to the MEH-PPV because the photoluminescence (PL) spectrum of PFD overlaps with the UV-vis absorption spectrum of MEH-PPV. However, FRET was limited by the presence of PEO in the active layers of the FCP-blend PLECs. This meant it was much easier to tune the emission colors compared to FCP-blend polymer light-emitting diodes (PLEDs), in which FRET occurs predominantly. Utilization of a polymer blend film of blue and red-orange FCPs in PLECs is a very effective and promising method for fabrication of white light-emitting devices.