Molecular-Scale Imaging Enables Direct Visualization of Molecular Defects and Chain Structure of Conjugated Polymers.

Conjugated polymers have become materials of choice for applications ranging from flexible optoelectronics to neuromorphic computing, but their polydispersity and tendency to aggregate pose severe challenges to their precise characterization. Here, the combination of vacuum electrospray deposition (ESD) with scanning tunneling microscopy (STM) is used to acquire, within the same experiment, assembly patterns, full mass distributions, exact sequencing, and quantification of polymerization defects. In a first step, the ESD-STM results are successfully benchmarked against NMR for low molecular mass polymers, where this technique is still applicable. Then, it is shown that ESD-STM is capable of reaching beyond its limits by characterizing, with the same accuracy, samples that are inaccessible to NMR. Finally, a recalibration procedure is proposed for size exclusion chromatography (SEC) mass distributions, using ESD-STM results as a reference. The distinctiveness of the molecular-scale information obtained by ESD-STM highlights its role as a crucial technique for the characterization of conjugated polymers.

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors.

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process.

Block Junction-Functionalized All-Conjugated Donor-Acceptor Block Copolymers.

Junction-functionalized donor-acceptor (D-A) block copolymers (BCPs) enable spatial and electronic control over interfacial charge dynamics in excitonic devices such as solar cells. Here, we present the design, synthesis, morphology, and electronic characterization of block junction-functionalized, all-conjugated, all-crystalline D-A BCPs. Poly(3-hexylthiophene) (P3HT), a single thienylated diketopyrrolopyrrole (Th xDPPTh x, x = 1 or 2) unit, and poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) are used as donor, interfacial unit, and acceptor, respectively. Almost all C-C coupling steps are accomplished by virtue of C-H activation. Synthesis of the macroreagent H-P3HT-Th xDPPTh x, with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-Th xDPPTh x- block-PNDIT2. Morphology is determined from a combination of calorimetry, transmission electron microscopy (TEM), and thin-film scattering. Block copolymer crystallinity of P3HT and PNDIT2 is reduced, indicating frustrated crystallization. A long period lp is invisible from TEM, but shows up in resonant soft X-ray scattering experiments at a length scale of lp ∼ 60 nm. Photoluminescence of H-P3HT-Th xDPPTh x indicates efficient transfer of the excitation energy to the DPP chain end, but is quenched in BCP films. Transient absorption and pump-push photocurrent spectroscopies reveal geminate recombination (GR) as the main loss channel in as-prepared BCP films independent of junction functionalization. Melt annealing increases GR as a result of the low degree of crystallinity and poorly defined interfaces and additionally changes backbone orientation of PNDIT2 from face-on to edge-on. These morphological effects dominate solar cell performance and cause an insensitivity to the presence of the block junction.

In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics.

A detailed investigation of in situ-synthesized all-conjugated block copolymer (BCP) compatibilized ternary blends containing poly(3-hexylthiophene) (P3HT) and poly{[ N, N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dibcarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) as donor and acceptor polymers, respectively, is presented. Both polymers are incompatible and show strong segregation in blends, which renders compatibilization with their corresponding BCPs promising to enable nanometer-phase-separated structures suitable for excitonic devices. Here, we synthesize a ternary block copolymer/homopolymer blend system and investigate the phase behavior as a function of block copolymer molecular weight and different annealing conditions. The device performance decreases on increasing annealing temperatures. To understand this effect, morphological investigations including atomic force microscopy, high-resolution transmission electron microscopy (HR-TEM), and grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are carried out. On comparing domain sizes of pristine and compatibilized blends obtained from GISAXS, a weak compatibilization effect appears to take place for the in situ-synthesized ternary systems. The effect of thermal annealing is most prevalent for all samples, which, for the highest annealing temperature above the melting point of PNDIT2 (310 °C), ultimately leads to a change from the face-on to edge-on orientation of PNDIT2, as seen in GIWAXS. This effect dominates and decreases all photovoltaic parameters, irrespective of whether a pristine or compatibilized blend is used.

C­H arylation of unsubstituted furan and thiophene with acceptor bromides: access to donor­acceptor­donor-type building blocks for organic electronics.

Pd-catalyzed direct arylation (DA) reaction conditions have been established for unsubstituted furan (Fu) and thiophene (Th) with three popular acceptor building blocks to be used in materials for organic electronics, namely 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2), N,N'-dialkylated 2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) (NDIBr2), and 1,4-dibromotetrafluorobenzene (F4Br2). Reactions with BTBr2, F4Br2, and NDIBr2 require different solvents to obtain high yields. The use of dimethylacetamide (DMAc) is essential for the successful coupling of BTBr2 and F4Br2, but detrimental for NDIBr2, as the electron-deficient NDI core is prone to nucleophilic core substitution in DMAc as solvent but not in toluene. NDIFu2 is much more planar compared to NDITh2, resulting in an enhanced charge-transfer character, which makes it an interesting building block for conjugated systems designed for organic electronics. This study highlights direct arylation as a simple and inexpensive method to construct a series of important donor­acceptor­donor building blocks to be further used for the preparation of a variety of conjugated materials.