An insight into non-emissive excited states in conjugated polymers

Abstract

Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2–5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20–30%. A series of mixed quantum-classical atomistic simulations illustrate that dielectric induced stabilization of nonradiative charge-transfer (CT) type states can lead to similar drastic reduction in fluorescence QY as seen experimentally. Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors

“…Hu et al observed that long-range exciton transport depends strongly on aggregate order [26], further highlighting the importance of interchain interactions on exciton dynamics. In a further study, Hu et al demonstrated that dielectric-induced stabilization of nonradiative charge-transfer states very sensitively controls PL quantum efficiencies in suspended P3HT aggregates [27], consistent with the role proposed by Yamagata et al of charge-transfer components of excitonic coupling [25], indicating once again the key role the environment plays in dictating photophysics in aggregates.…”

Time-Resolved EPR Study on Photoinduced Charge-Transfer Trap State in Thiophene-Thiazolothiazole Copolymer Film

“…The photoexcitation will also occur at the polymer crystalline phase and conduct the efficient exciton-diffusion. Such exciton may not contribute to the highly stable interchain CT generation because of the lack of the electronic stabilizations [5] in the CT character at the crystalline phase in Fig. 4, resulting in dominant radiative deactivations by the fluorescence.…”

“…Such crystalline phases are known to play a key role for the mobilities of the charges and the excitons in the organic devices. Recent studies suggested that non-fluorescent CT characters exist as a competitive channel of the formation of emissive exciton-type states and determine the light emission efficiencies of the LED films [5].…”

“…In the organic photoactive layers, it is well known that the polymer molecules are selforganized to generate the bulk heterojunction (BHJ) [4] interfaces. Apart from the OPV applications, pristine films of the conjugated polymers are also utilized for the FET and LED applications [5]. The polymers usually form two dimensional lamellae structures as a result of close interchain packing interaction [4].…”

Medindo a constante dielétrica em líquidos: um estudo de caso para elaboração de uma proposta para formação de físicos experimentais

“…3(b). O líquido escolhido para realizar este experimento foi o hexano (marca ISOFAR), cuja constante dielétrica estática é bem determinada na literatura [7] e o grau de pureza é industrialmente garantido (98,5%). Aqui é importante ressaltar que a água não foi utilizada como meio dielétrico, pois os experimentos realizados com esta indicam que mesmo água filtrada possui alto grau de impurezas químicas uma vez que a mesma é previamente tratada pelo sistema de abastecimento.…”