Impact of charge carrier injection on single-chain photophysics of conjugated polymers

Abstract

Charges in conjugated polymer materials have a strong impact on the photophysics and their interaction with the primary excited state species has to be taken into account in understanding device properties. Here, we employ single-molecule spectroscopy to unravel the influence of charges on several photoluminescence (PL) observables. The charges are injected either stochastically by a photochemical process, or deterministically in a hole-injection sandwich device configuration. We find that upon charge injection, besides a blue-shift of the PL emission and a shortening of the PL lifetime due to quenching and blocking of the lowest-energy chromophores, the non-classical photon arrival time distribution of the multichromophoric chain is modified towards a more classical distribution. Surprisingly, the fidelity of photon antibunching deteriorates upon charging, whereas one would actually expect the number of chromophores to be reduced. A qualitative model is presented to explain the observed PL changes. The results are of interest to developing a microscopic understanding of the intrinsic charge-exciton quenching interaction in devices.

MEH-PPV photophysics: insights from the influence of a nearby 2D quencher

“…After taking into consideration the impulse response function (IRF) of our system, the fluorescence lifetime is ∼900 ns. This is typical of values obtained in single molecule experiments for PPV derivative polymers [27,28]. As can be seen in the figure, the PL lifetime shortens in the presence of the quencher indicating that graphene has opened up an additional non-radiative decay path for the excitons.…”

“…An oligomer of length N can be excited directly by absorbing light or indirectly by energy transfer from a nearby shorter oligomer. Once excited, besides returning to the ground state via non-radiative decay (with between 90% [3] and 50% [27] probability, not shown explicitly in the diagram), the oligomer may either emit light (with rate k r of the order of 1 (ns −1 ) [27,28]) or the exciton may diffuse to a nearby longer oligomer. The energy transfer from one oligomer to another may occur either by interchain energy transfer (k inter )-the through-space Forster transfer (FRET) between two polymer segments that are in close proximity-or intrachain energy transfer (k intra )-energy transfer between two neighboring segments along the backbone of a single polymer chain.…”

“…In this case, the system acts like a traditional donor-accepter system in which the donor starts to emit light once the accepter is quenched. [10,28]. In the presence off photobleaching events, equation (9) for the extended chain is then modified as:…”

Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

“…The photon statistics are obtained under ambient conditions and with pulsed excitation at a repetition rate of 10 MHz. Only the first second of the acquisition period is taken into account to minimize photo bleaching and blinking effects, which can reduce the quality of photon antibunching 56 , 57 . We measure multiple single aggregates of each material to obtain sufficient photon statistics for the correlation plot.…”

Efficient Charge Transport in Disordered Conjugated Polymer Microstructures

“…Following single charges in extended films at room temperature is a challenging feat, but an exciting recent report was able to detect the motion of individual charges by tracking the perturbation of the emission spectra (Stark shift) and photophysics (blinking) of reporter chromophores embedded in an insulating polymer . Further advances could lead to more sensitive probes of charge motion in conjugated materials, including a better understanding of the effect of charges in the photophysics of individual polymer molecules . The ability to compare macroscopic charge transport measurements and single charge transport observations can aid in the identification of those states and charge transfer steps which are most critical for the operation of optoelectronic devices based in conjugated polymers that display considerable disorder.…”