Chemistry Colloquium

The complex charging/discharging dynamics of conjugated polymers were investigated and unravelled by a variety of single molecule fluorescence spectroscopy electro-optical techniques see Fig. 1. The injection of positive charge carriers (holes) into conjugated polymer chains is a critical process in solar cells based on organic materials and organic/inorganic hybrid materials. We have observed that hole injection into single conjugated polymer chains is light-assisted and highly cooperative. This unprecedented effect may underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply-trapped-charge layers in polymer light emitting displays and the poor fill factors in organic photovoltaic devices.
(Fig. 1. (A) Hole-injection device structure. (B) HOMO energy levels relative to the work function of the hole-injection electrode for the device shown in A. The black and red lines are Poisson-Boltzmann simulations at 0V (at equilibrium) and 10V (before charging) respectively. (C) single-molecule fluorescence-intensity trajectories (D) Ensemble average of ~100 single-molecule normalized fluorescence-intensity trajectories obtained while applying a triangular bias (top green line) (E,F) Ensemble average of ~100 single-molecule F-V trajectories obtained at: high vacuum (10-7 Torr) (E) and 5 Torr of O2 (F). )
The hole-injection from a layer of carbazole (a strong organic hole-donor), into isolated, single polymer chains of the conjugated polymer (MEH-PPV) imbedded in a multilayer device (Fig 1A) was studied. The experimental amount and rate of hole-injection from the carbazole HTL into individual polymer chains was monitored indirectly by single molecule fluorescence spectroscopy. We assign the injection of holes reported herein to a previously unreported light-induced hole transfer mechanism (denoted by LIHT) involving light assisted injection of holes from the carbazole layer into single MEH-PPV polymer chai