Electroluminescence of regioregular poly(alkylthiophenes)

Abstract

Regioregular poly(4,4′-didecyl-2,2′-bithiophene) with exclusively head-to-head and tail-to-tail couplings between adjacent alkylated thiophene rings was studied as a potential material for the active layer in light-emitting diodes (LEDs). Readily visible green light was derived from a single layer LED structure. The electroluminescence (EL) spectrum peaks at 2.2 eV. Excitation of photoluminescence (PL) resulted in green light with the spectrum peaked at 2.3 eV. Additionally to the electroluminescence and photoluminescence measurements, the absorption spectrum is reported. Determination of the optical energy gap gave Eg equal to 2.9 eV. The same experiments performed for another regioregular polyalkylthiophene system (with only head-to-tail couplings) showed extremely poor EL and PL intensity, below the sensitivity of our spectrometer. The difference in yield of photo- and electroluminescence is likely interpreted in terms of trapping of excitons in conjugated blocks separated by head-to-head dyads, where they decay. Non-planarity of the polymer backbone also influences the relative positions of singlet and triplet states.