Publications

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.

Using fast scanning calorimetry, we determined the crystallinity of thin films of poly(3-hexylthiophene) crystallized from the melt from measurements of the specific melting enthalpy. A broad range of film thicknesses from 10 µm down to 26 nm was covered. The sample mass was determined from measurements of the specific heat capacity in the molten state allowing a quantitative analysis of the heat flow data. Films with a thickness 400 nm slowly cooled from the melt showed the same crystallinity as bulk samples measured with conventional DSC. Below 350 nm the melting enthalpy decreased strongly. We assign this strongly reduced crystallinity to the restricted crystallization kinetics originating from hindered spherulitic growth under thin film confinement. A higher crystallinity could be partially regained by extended isothermal crystallization at elevated temperatures. Much faster cooling, with rates above about 100 Ks−1 led to a partial suppression of crystallization even for thick films. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1791–1801

The field of conjugated polymers has expanded in the last years considerably and impressive performance, both in field effect transistors and photovoltaic devices has been achieved. After the initial emphasis on improving the performance, more emphasis is recently given to fundamental studies on structure formation. Therefore, this review concentrates on systematic correlation studies of structure formation in solution, in bulk and thin films as well as in photovoltaic blends of donor‐type π‐conjugated polymers. The main focus is on the correlation of structure, morphology and molecular chain orientation as a function of macromolecular properties such as molecular weight, dispersity, non‐covalent intramolecular and intermolecular interactions, solvent interactions and innovative processing techniques. The tools applied for elucidating fundamental information of structure formation and orientation mainly consist of optical spectroscopy and scattering techniques (SAXS/WAXS/GIWAXS). Since the field of conjugated polymers is very vast in terms of chemical structural diversity, only selected examples of donor polymers are covered here and the emerging class of n‐type conjugated polymers are not included. The focus is not on the structural variation or their performance in solar cells or transistors in terms of record efficiencies, but on the systematic studies leading to a structure‐property correlation in donor polymers.

The design, synthesis, characterization, and application of a novel series of copolymers based on the electron deficient thieno[3,4-c]pyrrole-4,6-dione building block, copolymerized with either thieno[3,2-b]thiophene (PTPDTT) or thiophene (PTPDT), are reported. High molecular weights were obtained for PTPDTT via Stille polycondensation. For the PTPDTs, different molecular weights were achieved by varying the polymerization conditions. The increase in molecular weight (PTPDT-2) favors face-on alignment and increases the charge carrier mobility. Grazing-incidence wide-angle X-ray scattering measurements reveal higher crystallinity for PTPDTT with up to 5 orders of lamellar stacking compared to PTPDTs. All polymers show ambipolar charge transport with highly balanced hole and electron mobilities in organic field effect transistors (OFETs), which improve considerably upon thermal annealing. A shift of comonomer from simple thiophene in PTPDT-2 to planar and electron-dense thienothiophene in PTPDTT drastically changes the alignment from face-on to edge-on fashion. Consequently, the charge carrier mobility increases considerably by 1 order of magnitude in PTPDTT, reaching excellent charge carrier mobilities for both holes (0.11 cm² V–¹ s–¹) and electrons (0.17 cm² V–¹ s–¹). PTPDTT was tested as a donor material in combination with PC₇₁BM as well as an acceptor material along with a donor polymer. As a donor material, a power conversion efficiency of 4.3% was reached in combination with PC₇₁BM.

Brush polymers are highly functional polymeric materials combining the properties of different polymer classes and have found numerous applications, for example, in nanomedicine. Here, the synthesis of functional phosphonate-ester-bearing brush polymers based on poly(2-oxazine)s is reported through a combination of cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazine and reversible addition-fragmentation chain transfer (RAFT) polymerization. In this way, a small library of well-defined (Đ ≤ 1.17) poly(oligo(2-ethyl-2-oxazine) methacrylate) P(OEtOzMA)_n brushes with tunable lower critical solution temperature (LCST) behavior and negligible cell toxicity is prepared. Upon deprotection, the phosphonic acid end-group of the P(OEtOzMA)_n brush enables the successful grafting-onto iron oxide nanoparticles (IONPs). Colloidal stability of the particle suspension in combination with suitable magnetic resonance imaging (MRI) relaxivities demonstrates the potential of these particles for future applications as negative MRI contrast agents.

Three solution processable n-type semiconducting perylene bisimides (PBI) with an unsymmetrical substitution pattern are evaluated in terms of their charge transport properties, morphology, and crystal structure. The nature of the substituents is varied from hydrophobic alkyl chains to hydrophilic oligoethyleneglycol (OEG) chains to control intermolecular interactions and to tune self-assembly properties of the compounds. A correlation of structure and morphology with charge transport properties is attempted. Bulk X-ray diffraction (XRD) data are indicative for a lamello-columnar packing motif in the case of PBI 1 and PBI 3 and a columnar hexagonal packing for PBI 2. Further, OEG chains induce liquid crystalline phases while the alkyl substituted compound is crystalline. In the amorphous state after film formation all three materials have a low electron mobility in the range of 10–5 cm2 V–1 s–1. After annealing in the ordered state the mobility of the liquid crystalline compounds increases by 2 orders of magnitude up to 7 × 10–3 cm2 V–1 s–1, while the mobility of the crystalline material decreases by a factor of 4.

Abstract Aggregate formation in poly(3‐hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 ± 5%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field‐effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well‐defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm 2 /Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

An organic electrochemical transistor operates in accumulation mode with high transconductance. The channel comprises a thiophene-based conjugated polyelectrolyte, which is p-type doped by anions injected from a liquid electrolyte upon the application of a gate voltage. The use of ethylene glycol as a co-solvent dramatically improves the transconductance and the temporal response of the transistors.

Abstract The design principles of acceptor 1 –acceptor 2 copolymers featuring alternating diketopyrrolopyrrole (DPP) and thienopyrrolodione (TPD) moieties are investigated. The investigated series of polymers is obtained by varying the aromatic linker between the two acceptor motifs between thiophene, thiazole, pyridine, and benzene. High electron affinities between 3.96 and 4.42 eV, facilitated by the synergy of the acceptor motifs are determined with optical gaps between 1.37 and 2.02 eV. Grazing incidence wide‐angle X‐ray scattering studies reveal a range of film morphologies after thermal annealing, including face‐on, end‐on and superstructure edge‐on‐like crystallites. Conversely, all materials form thin edge‐on layers on the polymer–air interface, as demonstrated by multi‐elemental near‐edge X‐ray absorption fine‐structure spectroscopy. The benefit of the electron‐deficient linkers thiazole and pyridine is evident: In organic field effect transistors, electron mobilities of up to 4.6 × 10 −2 cm 2 V −1 s −1 are obtained with outstanding on/off current ratios of 5 × 10 5 , facilitated by the absence of detectable hole transport in these materials. Viability for all‐polymer solar cells is assessed in active layer blends with the donor polymer PM6, yielding a maximum average power conversion efficiency of 4.8% and an open circuit voltage above 1 V.

The solar cell performance and microstructure of DPP-based polymers with different degrees of fluorination are reported. DPP-based polymers with thiophene–phenyl–thiophene comonomer and thiophene flanking units are studied, with the degree of fluorination of the phenyl unit varied. With bifluorination of the phenyl ring, a higher open circuit voltage is achieved whilst maintaining or even improving the overall solar cell efficiency. While tetrafluorination leads to a further 0.1 V increase in VOC, reaching a high photo voltage of 0.81 V, overall solar cell performance significantly drops. Microstructural studies using AFM, TEM, Grazing incidence wide-angle X-ray scattering (GIWAXS), and Resonant soft X-ray scattering (R-SoXS) reveal that bifluorination largely preserves the microstructure of the nonfluorinated system, whereas tetrafluorination results in coarse phase separation between the polymer donor and the fullerene acceptor. Our results demonstrate that the use of an extended comonomer is a promising strategy for optimizing the beneficial effects of fluorination for DPP-based polymer solar cells, especially in improving the open circuit voltage. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 49–59