Improved quantum efficiency for electroluminescence in semiconducting polymers

Abstract

Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence, ,. If the lowest-energy excited states are strongly bound excitons (electron-hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively. But if the electron-hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.

Spin-dependent polaron recombination in conjugated polymers

“…Nonetheless, a large number of observations of much higher quantum efficiencies have been reported in OLEDs made from conjugated polymers, including values as high as 83%. [1][2][3][4][5][6] A considerable amount of theoretical work focused on understanding the mechanisms underlying exciton formation in conjugated polymers has also been reported. [7][8][9][10][11] So far, however, a commonly agreed theoretical picture is not available (for a review, see Ref.…”

Localized triplet excitations and the effect of photo-oxidation in ladder-type poly(p-phenylene) and oligo(p-phenylene)

“…However, it was suggested that the strong difference in the nature of the SE and TE states influences the formation cross-section of SE's and TE's from polarons. 10,11 Therefore it is highly desirable to investigate the nature of TE's and their spin-dependent formation from polaron pairs as well as from SE's. This paper explores the spatial extent of the triplet wave functions through the spin dependent photophysical properties of m-LPPP and the seven-membered ladder-type oligo͑p-phenylene͒ ͑LOPP7͒ as monitored by their X-band photoluminescence ͑PL͒-detected magnetic resonance ͑PLDMR͒ spectra.…”

Predictive modeling of the current density and radiative recombination in blue polymer-based light-emitting diodes

“…[52][53][54][55] However, it has been argued from other studies that it can be strongly enhanced, in particular in polymers. [56][57][58][59][60][61][62][63][64] For poly-phenylene-vinylene (PPV) based polymers, e.g., the singlet fraction as obtained using various methods ranges from approximately 20% (Ref. 52) to approximately 80% (Ref.…”