Direct Calorimetric Observation of the Rigid Amorphous Fraction in a Semiconducting Polymer

The performance of polymeric semiconductors is profoundly affected by the thermodynamic state of its crystalline and amorphous fractions and how they affect the optoelectronic properties. While intense research has been conducted on the crystalline features, fundamental understanding of the amorphous fraction(s) is still lacking. Here, we employ fast scanning calorimetry to provide insights on the glass transition of the archetypal conjugated polymer poly­(3-hexylthiophene) (P3HT). According to the conceptual definition of the glass transition temperature (<i>T</i>_g), that is, the temperature marking the crossover from the melt in metastable equilibrium to the nonequilibrium glass, an enthalpy relaxation should be observed by calorimetry when the glass is aged below <i>T</i>_g. Thus, we are able to identify the enthalpy relaxations of mobile and rigid amorphous fractions (MAF and RAF, respectively) of P3HT and to determine their respective <i>T</i>_g. Our work moreover highlights that the RAF should be included in structural models when establishing structure/property interrelationships of polymer semiconductors

Reversibly Slowing Dewetting of Conjugated Polymers by Light

Dewetting, i.e., the retraction of a fluid from a surface it “dislikes”, is a macroscopic phenomenon controlled through parameters like viscosity and surface tension on length-scales much larger than the size of the molecules. So far, dewetting was known to proceed in the same manner, independent of the dewetting film being illuminated by light or not, e.g., through an optical microscope. Here, we demonstrate that the velocity of dewetting of conjugated polymers can be reversibly tuned through appropriate exposure to light. We relate this observation to the absorption of photons of suitable energy resulting in the generation of excitons which may partially delocalize along and across polymer chains and so induce changes in polymer chain conformation. Such changes, in turn, may cause stiffening or overlap of polymer chains and thus lead to macroscopically detectable differences in behavior of an ensemble of conjugated molecules expressed via material properties like viscosity

Coexisting Glassy Phases with Different Compositions in NFA-Based Bulk Heterojunctions

Organic solar cell (OSC) bulk heterojunctions (BHJ) typically feature a rich phase morphology with the phase composition and distribution significantly affecting processes such as charge generation, recombination and extraction, and in turn, device performance. While fullerene-based BHJs are relatively well understood structurally, especially when blends with a flexible-chain donor are employed, donor: non-fullerene acceptor (NFA) blends are more challenging to elucidate. The reason is that NFAs often display different polymorphs; moreover, their glassy states can be complex. Focusing on blends of the widely investigated donor polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), and the prototype NFA, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC), we reveal here the coexistence of two glassy phases: a molecularly intermixed and an ITIC-rich one. In P3HT-rich blends, both glassy phases are present as nanosized domains, evenly distributed in the BHJ, as visualized via vapor phase infiltration (VPI) “staining”. In contrast, the 1:1 (by weight) and NFA-rich blends show clear, lateral phase separation between large (>500 nm) domains of the glassy phases and thinner polymer-rich domains that are unaffected by annealing. Our observations help to explain earlier P3HT: ITIC device studies; and also highlight the complexity of NFA-based BHJs, emphasizing the need for a deeper understanding of the phase behavior of such systems

Designing Small Molecules as Ternary Energy-Cascade Additives for Polymer:Fullerene Solar Cell Blends

Designing Small Molecules as Ternary Energy-Cascade Additives for Polymer:Fullerene Solar Cell Blend

Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology

We investigate the molecular order and thin film morphology of the conjugated polymer polytellurophene, in order to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer’s backbone planarity and performance in organic transistors. We find that poly­(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the CC vibrational peak to lower wavenumbers (∼1389 cm^–1) and a smaller optical band gap (∼1.4 eV). Resonant Raman spectroscopy revealed that molecular order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temperature, while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temperatures. When blended with the insulator HDPE, all three polymers exhibit little additional disorder and instead form phase-separated networks of high molecular order that are beneficial to percolated charge transport in transistors. We find that molecular order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure–property relationships that determine the performance of these novel organometallic polymers in electronic devices

Bis(triisopropylsilylethynyl)pentacene/Au(111) Interface: Coupling, Molecular Orientation, and Thermal Stability

The assembly and the orientation of functionalized pentacene at the interface with inorganics strongly influence both the electric contact and the charge transport in organic electronic devices. In this study electronic spectroscopies and theoretical modeling are combined to investigate the properties of the bis­(triisopropylsilylethynyl)­pentacene (TIPS-Pc)/Au(111) interface as a function of the molecular coverage to compare the molecular state in the gas phase and in the adsorbed phase and to determine the thermal stability of TIPS-Pc in contact with gold. Our results show that in the free molecule only the acene atoms directly bonded to the ligands are affected by the functionalization. Adsorption on Au(111) leads to a weak coupling which causes only modest binding energy shifts in the TIPS-Pc and substrate core level spectra. In the first monolayer the acene plane form an angle of 33 ± 2° with the Au(111) surface at variance with the vertical geometry reported for thicker solution-processed or evaporated films, whereas the presence of configurational disorder was observed in the multilayer. The thermal annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption at ∼470 K, which leaves the backbone of the decomposed molecule flat-lying on the metal surface as in the case of the unmodified pentacene. The weak interaction with the metal substrate causes the molecular dissociation to occur 60 K below the thermal decomposition taking place in thick drop-cast films

Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide

We report the synthesis of a novel naphthalene diimide (NDI) monomer containing two (tributyl­stannyl)­vinyl groups. The utility of this building block is demonstrated by its copolymerization with five different electron-rich comonomers under Stille conditions. The resulting high molecular weight polymers show red-shifted optical absorptions in comparison to the analogous polymers without the vinylene spacer and a significant increase in the intensity of the low-energy intramolecular charge transfer band. The polymers all exhibit ambipolar behavior in bottom-gate, top-contact organic thin-film transistors. The insertion of a solution-processed barium hydroxide layer between the polymer and the gold electrode led to unipolar behavior with improved electron mobilities

Comparative Optoelectronic Study between Copolymers of Peripherally Alkylated Dithienosilole and Dithienogermole

Here we report a simple methodology for the synthesis of dithienosilole and dithienogermole monomers in which the necessary solubilizing long chain alkyl groups are incorporated into the peripheral 3,5-positions of the fused ring. We report four novel monomers in which methyl or butyl groups are attached to the bridging Si and Ge atom. Copolymers with bithiophene were synthesized by a Stille polymerization in high molecular weight. We report the optical, electrical, electrochemical and morphological properties of the resulting polymers. We find that the nature of the bridging heteroatom (Si or Ge) has only a minor influence on these properties, whereas the nature of the alkyl chain attached to the bridging atom is found to have a much larger effect

Temperature-Dependence of Persistence Length Affects Phenomenological Descriptions of Aligning Interactions in Nematic Semiconducting Polymers

Electronic and optical properties of conjugated polymers are strongly affected by their solid-state microstructure. In nematic polymers, mesoscopic order and structure can be theoretically understood using Maier–Saupe (MS) models, motivating us to apply them to conjugated macromolecular systems and consider the problem of their material-specific parametrization. MS models represent polymers by worm-like chains (WLC) and can describe collective polymer alignment through anisotropic MS interactions. Their strength is controlled by a phenomenological temperature-dependent parameter, υ­(<i>T</i>). We undertake the challenging task of estimating material-specific υ­(<i>T</i>), combining experiments and Self-Consistent Field theory (SCFT). Considering three different materials and a spectrum of molecular weights, we cover the cases of rod-like, semiflexible, and flexible conjugated polymers. The temperature of the isotropic–nematic transition, <i>T</i>_IN, is identified via polarized optical microscopy and spectroscopy. The polymers are mapped on WLC with temperature-dependent persistence length. Fixed persistence lengths are also considered, reproducing situations addressed in earlier studies. We estimate υ­(<i>T</i>) by matching <i>T</i>_IN in experiments and SCFT treatment of the MS model. An important conclusion is that accounting explicitly for changes of persistence length with temperature has significant qualitative effects on υ­(<i>T</i>). We moreover correlate our findings with earlier discussions on the thermodynamic nature of phenomenological MS interactions

Effects of a Heavy Atom on Molecular Order and Morphology in Conjugated Polymer:Fullerene Photovoltaic Blend Thin Films and Devices

We study the molecular order and morphology in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) thin films and their blends with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). We find that substitution of the sulfur atoms in the thiophene rings of P3HT by heavy selenium atoms increases the tendency of the molecules to form better ordered phase; interestingly, their overall fraction of ordered phase is much lower than that of P3HT-based films. The higher tendency of P3HS molecules to order (aggregate) is consistent with more planar chain conformation simulated. The lower fraction of ordered phase (or the higher fraction of disordered phase) in P3HS-based films is clearly identified by in-plane skeleton Raman modes under resonant excitation conditions, such as a smaller ratio of the CC modes associated with the ordered (∼1422 cm^–1) and disordered (∼1446 cm^–1) phases (<i>I</i>_1422cm^–1/<i>I</i>_1446cm^–1 = 1.4 for P3HS and 0.6 for P3HS:PCBM), compared with P3HT-based films (<i>I</i>_1449cm^–1/<i>I</i>_1470cm^–1 = 2.5 for P3HT and 1.0 for P3HT:PCBM) and a larger Raman dispersion of the CC mode: P3HS (17 cm^–1) <i>versus</i> P3HT (6 cm^–1) and P3HS:PCBM (36 cm^–1) <i>versus</i> P3HT:PCBM films (23 cm^–1). The higher fraction of disordered phase in P3HS prevents the formation of micrometer-sized PCBM aggregates in blend films during thermal annealing. Importantly, this lower fraction but better quality of ordered phase in P3HS molecules strongly influences P3HS:PCBM photovoltaic performance, producing smaller short-circuit current (<i>J</i>_sc) in pristine devices, but significantly larger increase in <i>J</i>_sc after annealing compared to P3HT:PCBM devices. Our results clarify the effects of heavy atom substitution in low band gap polymers and their impact on blend morphology and device performance. Furthermore, our study clearly demonstrates resonant Raman spectroscopy as a simple, but powerful, structural probe which provides important information about “fraction/quantity of ordered phase” of molecules, not easily accessible using traditional X-ray-based techniques