Guest emission processes in doped organic light-emitting diodes: Use of phthalocyanine and naphthalocyanine near-IR dopants

A series of near-IR emitting organic light-emitting diodes (OLEDs) have been prepared using poly(vinylcarbazole) (PVK) as the hole transport polymer, aluminum quinolate (Alq(3)) or a sulforamide derivative (Al(qs)(3)) as the host dye (H), and phthalocyanine (Pc) or naphthalocyanine (NPc) dopants as guest dyes (G). The separation of the emission spectrum of the guest dye from that of the host dye allows for independent assessment of their relative efficiencies and correlation of these efficiencies with their tendencies toward charge capture and recombination reminiscent of solution electrogenerated chemiluminescent reactions. Although the absorbance spectra of the guest dyes are significantly separated from the emission spectra of the host dyes, the high molar absorptivities of the Pc and NPc dopants lead to Forster radii and relative energy-transfer (ET) efficiencies comparable to values previously obtained for quinacridone or rubrene dopants, which have been used to create intense green or yellow emission in Alq(3)-based OLEDs. We show that hole capture by the guest dye from oxidized PVK, PVK+./G --> PVK/G(+.), followed by cross-reactions, Alq(3)(-.)(Al(qs)(3)(-.))/G(+.) --> Alq(3)(Al(qs)(3))/G*, can be an important charge-trapping (CT), light-producing pathway at sufficiently high concentrations of the guest dye. Relative efficiencies of Pc* or NPc* emission in these single-layer devices increased exponentially with the electrochemically predicted excess free energy for the oxidation of G to form G(+.) by PVK+..