We report a comparison of charge carrier kinetics and recombination dynamics correlated with the device performances of PBDTTT-C and PBDTTT-CT in non-fullerene P(NDI2OD-T2) solar cells. The nanoscale morphological characteristics are found to be remarkably different for these two polymers in all-polymer bulk heterojunction (BHJ) blends. Important insights into the carrier dynamics and crystalline packing features of these different donor materials are provided in detail. The higher device performance of the PBDTTT-CT:P(NDI2OD-T2) device (2.78% compared to 1.56% for the PBDTTT-C:P(NDI2OD-T2) cell) is shown to result from efficient charge generation and faster carrier separation, reducing non-geminate recombination during charge collection, which is attributed to the perfect intermixing between the donor and acceptor polymer networks.

One-dimensional nanowires and microfibers from small molecules or π-conjugated polymers are important building blocks for the fabrication of nanoelectronic devices and provide model systems for charge transport in organic semiconductors under nanoscopic confinement. We investigated the relationship between molecular structure, supramolecular arrangement of the chromophores, and charge transport within self-assembled nanowires from perylene bisimides with oligopeptide-polymer side chains. The preparation of hierarchically structured microfibers of aligned nanofibrils allowed for a comprehensive structural characterization on all length scales with molecular level precision.⁽¹⁾ We prove that conformational flexibility between the hydrogen-bonded oligopeptides and the π-π stacked chromophores is a key requirement for synergistically enhanced π-π interactions and hydrogen bonding. The combination of beneficial π-π stacking, long-range order of the π-conjugated segments, and the directionality of the nanowires within thin films result in remarkably high conductivity after n-type doping. Moreover, we show that the capability to efficiently transport charge carriers through such nanowires ultimately depends on a delicate balance between π-π and other supramolecular interactions.⁽²⁾ Our results show that there is a complex interplay between the universal translation of molecular chirality into supramolecular helicity and the molecules&’ inherent propensity to form β-sheet-like aggregates.⁽³⁾ The result is a twofold odd-even effect in electronic circular dichroism spectra, depending on both the number of L-alanine units in the oligopeptides and the length of the alkylene spacer between the chromophores and oligopeptide substituents. We demonstrate that even derivatives with similar molecular structures and self-assembly behavior can have significantly different electron mobilities that appear to correlate with the nature of their electronic CD spectra. It is, hence, not generally sufficient to prepare one-dimensional aggregates of π-conjugated molecules in order to obtain self-assembled nanowires. Instead, the exact molecular arrangement within the aggregates and, consequently, even supposedly minor details of the molecular structure may play an important role.
(1) Marty, R.; Szilluweit, R.; Sanchez-Ferrer, A.; Bolisetti, S.; Adamcik, J.; Mezzenga, R.; Spitzner, E.-C.; Feifer, M.; Steinmann, S. N.; Corminboeuf, C.; Frauenrath, H. ASC Nano 2013, 7, 8498-8508.
(2) Marty, R.; Nigon, R.; Leite, D.; Frauenrath, H. J. Am. Chem. Soc. 2014, 136, 3919-3927.
(3) Marty, R.; Helbing, J.; Frauenrath, H. J. Phys. Chem. B. 2014, 118, 11152-11160.

Convective self-assembly (CSA) is a method generally used for depositing in a controlled manner nano- to micro-sized colloids onto a solid substrate under the action of solvent evaporation and capillary forces. By controlling some parameters such as substrate temperature, solution concentration, ambient humidity and temperature while translating the triple-contact line (solvent-air-substrate) a large variety of ordered structures have been obtained from different kinds of colloids (e.g. polymer, metallic)^1.2.3. Here, we adapt the CSA method to polymer solutions and show that by controlling the deposition speed, substrate temperature and oligomer/polymer concentration, CSA can lead to thin films comprised of molecules possessing highly oriented chain conformation, including crystals. For example, performing CSA on short chains of conjugated oligothiophenes (TH13) leads to highly ordered superstructures. Their optoelectronic properties are visibly changed in comparison to spin casted films of the same material that adopts rather disordered microstructure. Significantly different morphologies (porous films or random nanoparticles) with different optoelectronic properties can also be obtained when performing CSA on conjugated polyfluorene (PFO). The advantage of CSA method relies on its reproducibility and applicability over large surface area (i.e. squared centimetres) on any type of substrate that is compatible with the solvent used in solution preparation process.
References:
¹C. Leordean et al., J. Raman Spectrosc.45, 627 (2014).
²V. Saracut et al., ACS Applied Materials & Interfaces5, 1362 (2013).
³C. Farcau et al., Nanoscale4, 7870 (2012).

In spite of impressive improvement in device performance of the polymer-fullerene bulk-heterojunction photovoltaic cells over the past decade, the fact that their light absorption relies mostly on the polymer suggests inefficient use of the solar spectrum on single device level. Here, as a proof of concept, we demonstrate that blends of solution-processible titanyl phthalocyanines (TiOPc) nanoparticles [17-19] and polymer semiconductors [20, 21] can be fabricated into functioning bulk-heterojunction photovoltaic cells. Being a strong absorber in the red and near-IR region, TiOPc nanoparticles can contribute to significantly enhanced light absorption. The higher contrast in electron density between these TiOPc nanoparticles and polymer semiconductors also facilitates characterization of the nanoscale morphology by electron microscopy, making the establishment of a definitive structure-property relationship possible.

Photosynthetic organisms efficiently capture light and transport excitons to the reaction center by tuning the nature and magnitude of energetic disorder in their constituent antenna proteins. Due to their long-range order, large domain sizes and high oscillator strengths, one-dimensional tubular cyanine dye based J-aggregates have become a model system for understanding excitonic antennae and represent a route toward creating organic excitonic wires. We probe the temperature dependent disorder of matrix stabilized tubular J-aggregates, and observe signatures of exciton-exciton annihilation at extremely low photon fluxes, suggesting micron scale exciton diffusion lengths at room temperature, and band-like transport as we tune dynamic disorder in the system. In the past, photochemical instability has hindered the study of these materials and their implementation in practical devices. We stabilize these J-Aggregates against photobleaching and cryogenic damage in a sugar-based matrix enabling quantification of disorder, and robust measurements of exciton diffusion. The long diffusion lengths are among the largest ever measured for singlet excitons in a non covlalent organic supramolecular complex, and argue that quantum coherence can play a role in enhancing energy transfer in analogous highly ordered biological aggregates such as the chlorosome.

Perylenediimides (PDIs) are a promising alternative to fullerenes in organic photovoltaics (OPVs) but have historically performed poorly due to their tendency to form large crystalline domains, leading to excimer formation and trapping of charge carriers. These problems can be avoided by controlling the packing morphology of the PDI molecules so that excimer and trap formation is avoided, or by developing amorphous materials. Here we explore the properties of planar PDI monomers which are substituted at the 2,5,8,11-positions ("Headland Positions") and find that structures with substituents in these positions pack in slip-stacked columns which, despite a high degree of crystallinity, have greatly reduced rates of excimer formation when compared to unsubstituted PDI. Materials with substituents in the headland position undergo nearly quantitative charge transfer when mixed with a polymer donor and in the case of 2,5,8,11-tetraphenyl-PDI (Phenyl-PDI) geminate recombination is also minimized leading to efficient charge carrier generation and high OPV device efficiencies. The structure of the PDI can be further tuned by substituting the solubilizing alkyl chain at the imide position. When the solubilizing chain is sterically small (n-octyl) the molecules stack in a slip-stacked orientation and when it is large (1-ethylpropyl) they pack in a herringbone structure, but when the solubilizing chain is of intermediate size (3,7-dimethyloctyl) the packing structure can be controlled with solvent additives allowing for the fine tuning of active layer morphology. This degree of control allows for the fabrication of efficient OPV devices while maintaining the crystallinity of PDI.

In this talk the influence of backbone fluorination on the optical, electronic and morphological properties of various thiophene containing polymers will be discussed. We report that fluorination leads to an increase in polymer ionisation potential without a change in optical band gap. Increasing levels of backbone fluorination also result in an increase in the dielectric constant of the polymers. Fluorination is found to have a pronounced effect of the tendency of the polymers to aggregate, both in solution and the solid state, which leads to improved performance in field effect transistors in comparison to the non-fluorinated analogues.

Polymer solar cells attract considerable attention as a potential renewable energy technology. Typically, the light-harvesting active layer is composed of a fine blend of a polymeric electron-donor and an electron-acceptor such as a fullerene derivative. The precise nanostructure of these so-called bulk-heterojunctions is critical for achieving an optimal photovoltaic performance. However, polymer/fullerene blends are metastable materials and hence their nanostructure and photovoltaic performance tend to evolve with time. Thus, in particular elevated processing and operating temperatures pose a formidable challenge to the long-term stability of polymer solar cells. In the first part of my talk I will discuss the thermal stability of bulk-heterojunctions with respect to the glass transition temperature of the blend as well as the nucleation and growth kinetics of fullerene crystals. Then I will introduce two tools based on 1) nucleating agents and 2) fullerene mixtures, which permit to considerably enhance the thermal stability of this promising class of materials.

Many applications of conjugated polymers depend critically on excitation energy transport. Although bulk measurements regarding these processes have the inherent disadvantage of averaging over interesting areas, single-molecule spectroscopy is limited to single conjugated polymer chains, which may not reveal bulk-related mechanisms. Both experimental techniques have their unique advantages and have served well in obtaining a detailed understanding of organic materials. We use in situ solvent vapor annealing (SVA) to generate highly ordered aggregates composed of individual conjugated polymer chains to advance into the regime between a single chain and bulk film, that is, the mesoscopic size domain. The model system used shows a drastic change in photoluminescence characteristics in going from well-dissolved single chains to the bulk film, making it especially useful to follow the evolution from a single chain towards a mesoscopic object. By employing single-molecule and aggregate spectroscopy, we unravel the following non-intuitive behavior in the mesoscopic size regime: aggregation of multiple chains leads to coherent coupling between chromophores, resulting in a ten-fold increase in the excited state lifetime. This coupling between chromophores in aggregates is so efficient that strong photon antibunching evolves. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, while the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.

Organic conducting polymers are promising electrode materi