Kuan-Yi Wu

Porphyrin‐Incorporated 2D D–A Polymers with Over 8.5% Polymer Solar Cell Efficiency

A copolymerization strategy is developed to utilize porphyrin as a complementary light-harvesting unit (LHU) in D-A polymers. For polymer solar cells (PSCs), the presence of LHUs increases the short-circuit current density (Jsc ) without sacrificing the open-circuit voltage (Voc ) and fill factor (FF). Up to 8.0% power conversion efficiency (PCE) is delivered by PPor-2:PC71 BM single-junction PSCs. A PCE of 8.6% is achieved when a C-PCBSD cathodic interlayer is introduced.

A Facile PDMS‐Assisted Crystallization for the Crystal‐Engineering of C60 Single‐Crystal Organic Field‐Effect Transistors

Poly(dimethylsiloxane) (PDMS)-assisted crystallization (PAC) is a facile method to produce oriented C60 crystal arrays. Changing the drying mechanism from evaporation to solvent absorption (by PDMS) widens the solvent selection and facilitates the engineering of both the macroscopic shape and the microscopic lattice structure of the crystal arrays. The method also shows the potential to be applied to other organic semiconductors and large-area production.

Synthesis and morphological studies of a poly(5,6-difluorobenzo-2,1,3-thiadiazole-4,7-diyl-alt-quaterchalcogenophene) copolymer with 7.3% polymer solar cell efficiency

To obtain a poly(5,6-difluorobenzo-2,1,3-thiadiazole-4,7-diyl-alt-quaterchalcogenophene) (P(FBT-alt-CP4)) copolymer with a small optical band gap (Eg), and to achieve high short-circuit current (Jsc) in the P(FBT-alt-CP4) : PC71BM polymer solar cells (PSCs), P(FBT-alt-Se2Th2), which contains selenophene-2,5-diyl (–Se–) π-bridges, was synthesized. P(FBT-alt-Se2Th2) shows a Eg of 1.56 eV and is strongly aggregated in solution. Wide angle X-ray diffraction (WAXD) and grazing incidence X-ray diffraction (GI-XRD) results revealed the high solid-state order of P(FBT-alt-Se2Th2) and its edge-on orientation on the substrate. It delivered a high hole mobility (μh) of 0.36 cm2 V−1 s−1 in organic field-effect transistors (OFETs). The strong aggregation tendency of P(FBT-alt-Se2Th2) caused large segregation domains in the P(FBT-alt-Se2Th2) : PC71BM thin film, as is seen in the high-resolution transmission electron microscopy (HR-TEM) images. The addition of 8 vol% of 1-chloronaphthalene (1-CN) effectively suppressed the aggregation and led to more homogeneous active layer morphology. The improved morphology enhanced the Jsc of the PSCs. A superior PCE of 7.34% with a Voc of 0.70 V, a Jsc of 15.8 mA cm−2, and a FF of 66.4% was achieved in the inverted P(FBT-alt-Se2Th2) : PC71BM PSCs. The strong aggregation of P(FBT-alt-Se2Th2) is likely related to its more straight conjugated backbone according to the theoretical calculation results of the FBT-alt-Se2Th2 repeat unit.

The backbone rigidity and its influence on the morphology and charge mobility of FBT based conjugated polymers

A series of three PVTh4FBT polymers containing different alkyl side chain placement was synthesized. The thermochromic behaviors and DFT calculations indicated that the backbone coplanarity and rigidity of the PVTh4FBT polymers can be effectively modulated by adjusting the side chain position and density. Higher ordered and better oriented edge-on lamellar packing was formed by P1, which possesses the most rigid backbone among the three polymers and pre-aggregates in the solution. P1 also delivered the highest hole mobility (0.26 cm2 V−1 s−1) among the three analogues because its thin-film morphology is in favor of charge transport.

Polymorphisms and morphological studies of a difluorobenzothiadiazole conjugated copolymer with 7.8% polymer solar cell efficiency

This study presents the structural characterization of a 5,6-difluorobenzo-2,1,3-thiadiazole (FBT)–quaterthiophene (Th4) alternating copolymer (PTh4FBT) and how the crystalline nature of PTh4FBT affects the PTh4FBT–PC71BM morphology and the device performances. The single crystal structure of 5,6-difluoro-4,7-di(thiophen-2-yl)benzothiadiazole (Th2FBT) first confirms the low conformational preference of the FBT containing molecule. Since the crystallization process does not assist in unifying the conformation of Th2FBT, both intrachain conformation and interchain c-shifts in the crystalline state of PTh4FBT have to be scrutinized. By comparing the 2D WAXS pattern of PTh4FBT and the simulated patterns generated from Cerius2 molecular modeling, it was found that the diffraction pattern generated from the lattice containing PTh4FBT with anti-conformation and limited interchain c-shift matches best with the experimental one. The face-to-face interchain stacking of PTh4FBT rendered strong crystalline nature to the polymer and caused large segregation in the PTh4FBT–PC71BM blend system, which was indicated by the TEM and GI WAXS morphological studies. The aggregation size was reduced via the use of the 1-chloronaphthalene additive. The optimized morphology led to improved Jsc and FF. The single-junction PTh4FBT–PC71BM based polymer solar cells deliver a high PCE of 7.75% with a Voc of 0.76 V, a Jsc of 14.36 mA cm−2, and a FF of 71.0%.

Highly-integrated, laser manipulable aqueous metal carbonyl vesicles (MCsomes) with aggregation-induced emission (AIE) and aggregation-enhanced IR absorption (AEIRA)

A highly-integrated, laser manipulable multi-functional metal carbonyl nanovesicle (MCsome) with aggregation-induced emission (AIE) and aggregation-enhanced IR absorption (AEIRA) is created via the self-assembly of a bithiophene tethered-Fp acyl derivative (Fp: CpFe(CO)2) (1). Although 1 is hydrophobic and non-surface-active, the molecule can self-assemble in water into vesicles without detectable critical aggregation concentration (CAC). The water–carbonyl interaction (WCI) is responsible for the colloidal stability. The bilayer membrane structure with the bithiophene moieties associated within the inner wall and the iron-carbonyl units exposed to water is confirmed by transmission electron microscopy (TEM), atomic force microscopy (AFM), and cyclic voltammetry (CV) experiments. The synchrotron small-angle X-ray scattering (SAXS) experiment suggests that the bithiophene groups are interdigitated within the membrane. The spatial segregation of the AIE-active bithiophene domain from the iron-carbonyl units by the butanoyl spacers prevents the quenching effect of the iron and renders the MCsome photoluminescent. The polarizable iron-carbonyl groups on the surface of the MCsome create an enhanced optical field upon infrared (IR) irradiation, resulting in an enhancement (ca. 100-fold) in IR absorption for the carbonyl groups as compared to the same concentration of molecule 1 in THF. When the MCsome interacts with a focused continuous-wave near-IR (NIR) laser beam, a strong gradient (trapping) force is generated allowing the laser trapping of the MCsome without using additives. A sharp contrast in the refractive index (RI) of 1 (RI = 1.71) with water (RI = 1.33) accounts for this laser manipulability that is difficult to be achieved for nanosized liposomes (RI = 1.46). As illustrated, the MCsome of 1 represents a novel group of vesicular colloids, which is amenable to functional materials complementary to extensively studied liposomes and polymersomes.

Roles of 3-Ethylrhodanine in Attaining Highly Ordered Crystal Arrays of Ambipolar Diketopyrrolopyrrole Oligomers

Until now, only limited DPP oligomers delivered ambipolar semiconductor characteristics. To develop a facile strategy of preparing ambipolar mono-DPP oligomers, two dithienyl diketopyrrolopyrrole (DPPT) based-conjugated molecules, DPPT-RD and DPPT-DCV, which contain 3-ethylrhodanine (RD) and dicyano-2-vinyl (DCV) end substituents were synthesized. The influences of the -RD end substituents on the molecular properties, solid-state morphology, and OFET performances of the DPPT oligomer were investigated. The UV-vis absorption and CV results showed that the RD end substituents provide the DPPT oligomer suitable EHOMO and ELUMO for hole and electron injection from the Au source-drain electrodes. Moreover, the RD end substituents also improve the crystalline nature of the DPPT oligomer. That is, DPPT-RD can form crystal arrays with good lattice orientation, larger crystalline size, and without polymorphism. With those properties, DPPT-RD thus display ambipolar characteristic with μh and μe reaching 2.16 × 10-2 and 7.27 × 10-2 cm2 V-1 s-1, respectively.

2,2′-Bis(trifluoromethyl)biphenyl as a building block for highly ambient-stable, amorphous organic field-effect transistors with balanced ambipolarity

Novel conjugated polymers, PBPV–TPA, PBPV–CBZ, PBPV–MEH and PBPV–FLO, consisting of trifluoromethyl-substituted biphenyl (2,2′-bis(trifluoromethyl)biphenyl, CF3-BP) as the acceptor with various aromatic donors (triphenylamine TPA, carbazole CBZ, 2-(2-ethylhexyloxy)-5-methoxybenzene MEH and fluorene FLO, respectively) have been successfully synthesized. Their amorphous features, confirmed by wide-angle X-ray scattering, small-angle X-ray scattering and atomic force microscopy, are attributed to lateral side chains on the CBZ, MEH and FLO units, and the propeller geometry of the TPA unit. By integration of CF3-BPs into the polymer backbone, the ambipolarities and well-balanced charge mobilities of OFETs based on these polymers were successfully demonstrated. OFETs based on PBPV–FLO exhibit the highest hole mobility of 0.0152 cm2 V−1 s−1 and electron mobility of 0.0120 cm2 V−1 s−1. In addition, these OFETs also exhibit annealing-free characteristics and ambient stability. The OFET performance without encapsulation remained nearly unchanged in ambient conditions for up to 90 days. This could be attributed to the enhanced oxidative stability from their relatively deep HOMO energy levels and better moisture resistance from the trifluoromethyl substitution. Considering the ambipolar, annealing-free and ambient-stable properties of these CF3-BP-based amorphous conjugated polymers, the electron-accepting CF3-BP unit can be considered as a promising building block in preparing the easily processable conjugated polymers used in high-stability optoelectronic applications.

Conformational Preferences and the Phase Stability of Fullerene Hexa-adducts

Molecular conformation and the assembly structure determine the spatial arrangements of the constituent units and the functions of a molecule. Although, fullerene hexa-adducts (FHAs) have been known as functional materials with great versatility, their conformational preferences and phase stability remain a complicate issue. By choosing bithiophene (T2 ) and dodecyl bithiophene (C12 T2 ) as the peripheral units of FHA, and using microscopic, scattering and diffraction characterizations, our study reveals how the intramolecular interaction and environmental stimulus affects the conformational preferences and phase stability of FHAs.

Influences of Conjugation Length on Organic Field-Effect Transistor Performances and Thin Film Structures of Diketopyrrolopyrrole-Oligomers

Here, two diketopyrrolopyrrole (DPP)-based oligomers, DPP-4T and DPP-6T, are studied to reveal the influences of conjugation length on thin-film morphology and organic field-effect transistor (OFET) performances. PDMS-assisted crystallization in a solvent-annealing chamber is applied to prepare crystal arrays of DPP-4T and DPP-6T to optimize the quality of charge channels for OFET characterizations. To deliver insights into microstructure and morphology of thin films, a characterization procedure for determining molecular packing in thin film and crystallinity of the crystal arrays is presented via grazing incidence wide-angle X-ray scattering, electron diffraction, and lattice simulation software package (Cerius2). With the lattice parameters derived from analyses of grazing incidence wide-angle X-ray scattering (GIWAXS) and electron diffraction (ED), the lattice modeling results indicate that the inferior organic field-effect transistor (OFET) performances of DPP-6T are attributed to longer π-stacking distance. Also, less-ordered molecular arrangement and lower continuity of crystalline domains, both of which are revealed from crystallinity results, lead to lower mobility of DPP-6T. In this case, longer conjugated backbones with more conformational degrees of freedom thus cause inherent crystal defects during the crystal growth process, despite the potential to enhance intermolecular π-orbital overlap. Therefore, to achieve better OFET performance, suitable backbone length makes conjugated oligomers give high intermolecular π-orbital overlap and low density of structural disorder, which are the priorities for constructing good charge channel.