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Durham University

Department of Engineering

Staff Profile

Publication details for Dr Mujeeb Ullah Chaudhry

Ullah, Mujeeb, Yambem, Soniya D., Moore, Evan G., Namdas, Ebinazar B. & Pandey, Ajay K. (2015). Singlet Fission and Triplet Exciton Dynamics in Rubrene/Fullerene Heterojunctions: Implications for Electroluminescence. Advanced Electronic Materials 1(12): 1500229.

Author(s) from Durham

Abstract

The role of triplet excitons in rubrene/C60 heterojunctions is investigated through detailed spectroscopic studies of triplet generation routes in the neat and heterojunction films of rubrene and C60. Time-correlated single-photon counting experiments on rubrene and rubrene/C60 give a long-lived component with lifetime >200 ns, and triplets are found to live longer in rubrene/C60. A distinct reduction at short time scales in fluorescence lifetime of rubrene/C60 gives the indication of singlet exciton dissociation via formation of charge-transfer (CT) states. Using ultrafast transient absorption spectroscopy, it is found that triplets are generated via singlet-fission in neat rubrene films at t ≈ 1.8 ps, whereas a delayed population buildup of triplets in rubrene/C60 occurs at t ≈ 8 ps. The slow rise of triplet population confirms the role of CT-state-mediated triplet energy transfer in rubrene/C60. The recombination of triplets via triplet–triplet annihilation in organic light-emitting diode (OLED) operation of rubrene/C60 is shown to generate extra singlets, which lift the spin branching ratio to values >25%. It is concluded that triplet excitons in rubrene/C60 are instrumental in bringing lower turn-on voltages, brighter emission, and higher external quantum efficiency of electroluminescence in OLED and light-emitting field effect transistors.