We introduce the concept that free-charge generation in organic photovoltaic (OPV) materials may best be described by competition between long- and short-range electron transfer events, and that the distribution of rates as a function of distance follows the predictions of Marcus theory. Our results reveal the fundamental connection between solution-phase electron transfer research that has been conducted in the chemistry community over many decades, and the younger materials science effort to develop efficient OPV materials. The model that emerges provides insight into how the microstructure of OPV materials influences the electron transfer process via both entropic and quantum-mechanical mechanisms.
With increasing knowledge of the role of the different phases in the bulk heterojunction organic solar cell, the primary site for charge generation is now considered to be the mixed phase, and not the clean interface between neat polymer and neat fullerene. To gain a better understanding of the primary charge generating and recombination steps in this region of the system, we focus our studies on the role of the solid-state microstructure of neat polymers and light-doping of these polymers with a variety of electron-accepting dopants at low concentration.
This presentation will describe some recent work on the doping of polythiophene and polyfluorene derivatives with fullerenes, phthalocyanines and perylenes, which provide a range of reduction potentials that serve to control the driving force for electron transfer processes. Results from flash photolysis, time-resolved microwave conductivity (fp-TRMC), femtosecond transient absorption spectroscopy (fTA) and photoluminescence spectroscopy will be presented.
Matthew Bird, Gina Mauro, Lori Zaikowski, Xiang Li, Obadiah Reid, Brianne Karten, Sadayuki Asaoka, Hung-Cheng Chen, Andrew Cook, Garry Rumbles, John Miller
The diffusion of singlet and triplet excitons along single polyfluorene chains in solution has been studied by monitoring their transport to end traps. Time-resolved transient absorption and steady state fluorescence were used to determine fractions of excitons that reach the end caps. In order to accurately determine the singlet diffusion coefficient, the fraction of polymer ends that have end traps was determined through a combination of NMR and triplet quenching experiments. The distributions of polymer lengths were also taken into account and the resulting analysis points to a surprisingly long singlet diffusion length of 34 nm. Experiments on triplet transport also suggest that the entire 100nm+ chain is accessible to the triplet during its lifetime suggesting a lack of hindrance by defects or traps on this timescale. Time Resolved Microwave Conductivity measurements were also performed on a series of different length oligo- and polyfluorenes in solution allowing a global fit to be performed to extract an accurate intrachain mobility of 1.1 cm2/Vs.
We examined photoinduced charge-generation dynamics of the poly(3-hexylthiophene) (P3HT)/titanyl phthalocyanine (TiOPc) bilayer and the P3HT/TiOPc/C60 trilayer using the combination of flash-photolysis time-resolved microwave conductivity experiments (fp-TRMC) and classic pump-probe transient absorption (TA) spectroscopy following dominant excitation of the P3HT layer. The superlinear increase of φΣμ for the P3HT/TiOPc bilayer, compared to the φΣμ sum of each P3HT and TiOPc layer suggest photoinduced carrier-generation. Furthermore, the superlinear increase of φΣμ of the P3HT/TiOPc/C60 trilayer with respect to the each P3HT/TiOPc and TiOPc/C60 bilayers evinces charge migration from one interface to the other interface. In addition, with selective photoexcitation on the P3HT layer, both amorphous and H-aggregated P3HT domains participate in electron transfer ([P3HT*/TiOPc]→[P3HT•+/TiOPc•-]), contrasting to the previous observation where with selective excitation of the TiOPc layer, only the H-aggregated P3HT domain involves in hole transfer ([P3HT/TiOPc→[P3HT•+/TiOPc•-]) to produce P3HT•+/TiOPc•- in J. Phys. Chem. B 119(24), 7729—7739 (2015). These results under different excitation conditions are consistent with calculated energetic driving force (ΔECS) for charge generation which is -0.58 eV and -0.73 eV for amorphous and H-aggregated P3HT domains under the P3HT layer excitation, while 0.04 eV and -0.11 eV for amorphous and H-aggregated P3HT domains under the TiOPc layer excitation.
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