Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes.

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm-1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

Morphology, Crystal Structure and Charge Transport in Donor-Acceptor Block Copolymer Thin Films.

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

The Impact of Polydispersity and Molecular Weight on the Order-Disorder Transition in Poly(3-hexylthiophene).

Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct conformations: a coiled phase and a better ordered aggregated phase. Employing steady state absorption and fluorescence spectroscopy, we measure the course of aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K to 170 K. We show that aggregation is a temperature controlled process, driven by a thermodynamic order-disorder transition. The transition temperature increases with the molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies a smearing out of the phase transition in samples with increasing polydispersity and erodes the signature of a first order phase transition. The detection of a hysteresis when undergoing cooling/heating cycles further substantiates this reasoning.

Polymer crystallization as a tool to pattern hybrid nanostructures: growth of 12 nm ZnO arrays in poly(3-hexylthiophene).

Well-ordered hybrid materials with a 10 nm length scale are highly desired. We make use of the natural length scale (typically 10-15 nm) of the alternating crystalline and amorphous layers that are generally found in semicrystalline polymers to direct the growth of a semiconducting metal oxide. This approach is exemplified with the growth of ZnO within a carboxylic acid end-functionalized poly(3-hexylthiophene) (P3HT-COOH). The metal-oxide precursor vapors diffuse into the amorphous parts of the semicrystalline polymer so that sheets of ZnO up to 0.5 µm in size can be grown. This P3HT-ZnO nanostructure further functions as a donor-acceptor photovoltaic system, with length scales appropriate for charge photogeneration.