Visible light-emitting host-guest electrochemical cells using cyanine dyes

Light-emitting electrochemical cells (LECs) can be fabricated as a single emissive organic/salt layer sandwiched between two electrodes, offering cost-effective next generation signage and lighting applications. Cyanine dyes are especially attractive to exploit the low cost potential of LECs. Cyanines denote a large class of fluorescent organic salts with tuneable emission wavelength, inherent conductivity for ionic and electronic charges, and many cyanines are commercially available at low cost. We systematically tested a set of cyanine dyes for visible emitting LECs. To circumvent non-radiative quenching processes in pure cyanine films (monomer fluorescence quantum yields, PLQE < 1.5%) we exploited the efficient resonance energy transfer (RET) from cyanine host to cyanine guest molecules (PLQE maximum = 16.2%). The analysis indicated that specific host-guest interactions or a parallel energy transfer channel to host dimers can reduce the guest PLQE, despite a generally high RET efficiency in cyanine host-guest systems. By comparing single component with host-guest LECs, we found that the PLQE enhancement directly translated into the device efficiency increase, and red-emitting host-guest LECs with an external quantum efficiency (EQE) of 0.36% were achieved, close to the theoretical EQE maximum (0.81%). Chemical approaches that provide sterically demanding (to increase the PLQE) and high bandgap (for emission at smaller wavelengths) cyanines at low cost promise further progress in the field. (C) 2017 Elsevier B.V. All rights reserved.