Abstract:We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt% 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C 61 (PCBM). Three polymers are observed to give amorphous films, attributed to a non-planar geometry of their backbone whilst the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (~ 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt% PCBM resulted in 70 -90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer + and PCBM − polarons, assayed by the appearance of a long-lived, powerlaw decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over two orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find a excellent correlation between the free energy difference for charge separation (ΔG CS rel ) and yield of the long-lived charge generation yield, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL 3 quenching. We suggest this observation is consistent with a model where the excess thermal energy of the initially formed polarons pairs is necessary to overcome their coulomb binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.4

Organic semiconductors are emerging as a viable alternative to amorphous silicon in a range of thin‐film transistor devices. With the possibility to formulate these p‐type materials as inks and subsequently print into patterned devices, organic‐based transistors offer significant commercial advantages for manufacture, with initial applications such as low performance displays and simple logic being envisaged. Previous limitations of both air stability and electrical performance are now being overcome with a range of both small molecule and polymer‐based solution‐processable materials, which achieve charge carrier mobilities in excess of 0.5 cm2 V−1 s−1, a benchmark value for amorphous silicon semiconductors. Polymer semiconductors based on thienothiophene copolymers have achieved amongst the highest charge carrier mobilities in solution‐processed transistor devices. In this Progress Report, we evaluate the advances and limitations of this class of polymer in transistor devices.

The time‐of‐flight method has been used to study the effect of P3HT molecular weight (Mn = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10−4 cm2V−1s−1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.

Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone.