Zeyun Xiao

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

The role of solvent vapor annealing in highly efficient air-processed small molecule solar cells

We demonstrate highly-efficient, solution-processed small molecule solar cells with the best power conversion efficiency (PCE) of more than 5%. The active layer consists of a diketopyrrolopyrrole-based donor molecule (DPP(TBFu)2) and a fullerene derivative (PC71BM) that is spin cast and subsequently treated with solvent vapor annealing (SVA) in air. We find not all solvent vapors lead to the best PCE. Solvents of high vapor pressures and medium donor solubilities, such as tetrahydrofuran or carbon disulfide, are most suitable for SVA in the context of organic solar cell application. On the other hand, acceptor solubility plays an insignificant role in such a treatment. An active layer treated with ideal solvent vapors develops desirable phase separation in both lateral and vertical directions, as revealed by AFM, TEM and TEM tomography. The SVA also leads to enhanced hole mobility. We believe the fast SVA treatment performed in air is a viable way to tune the active layer morphology for printed solar cells.

Effect of molecular weight on the properties and organic solar cell device performance of a donor–acceptor conjugated polymer

Donor–acceptor conjugated polymers with 2-(2-ethylhexyl)-3-hexyl thienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor building block and 5,6-difluorobenzo[c][1,2,5]thiadiazole as acceptor building block have been synthesized by Stille coupling polymerization. The polymerization conditions were optimized to achieve high molecular weight polymers (number-average molecular weight, Mn, up to 139 kg mol−1). The molecular weight dependent polymer properties were studied and compared. Photovoltaic applications of the polymers in bulk heterojunction (BHJ) solar cells revealed that the power conversion efficiency increased significantly (from 0.9% to 4.1%) as the Mn increased from 10 kg mol−1 to 73 kg mol−1 while further increase of the molecular weight had less influence on the solar cell performance.

Construction of Microbelts through the Coassembly of a Disclike Molecule and Primary Alkyl Ammoniums: A Noncovalent Strategy to Mimic Covalently Bonded π-Core Alkyl Chain Structure

In this letter, we report the fabrication of microbelts through the coassembly of hexa-2-pyridyl-hexaazatriphenylen (HPHAT), a disklike pi-conjugated molecule, with primary alkyl ammonium triflate. The strategy is first to construct hydrogen-bonded complexes between HPHAT and primary alkyl ammoniums to mimic covalently bonded pi-core alkyl chain structures, and then the complexes self-assemble into microbelts driven by pi-pi stacking in the pi core and van der Waals interactions between the peripheral alkyl chains. The morphology of as-prepared microbelts has been characterized with scanning electron microscopy (SEM), optical microscopy, polarizing microscopy, and transmission electron microscopy (TEM). Spectroscopic and crystallographic investigations were also carried out to reveal the formation mechanism of the microbelts, through which a sequential self-assembly process has been proposed.

The emergence of oxime click chemistry and its utility in polymer science

The synthesis of new, highly functional and dynamic polymeric materials has risen dramatically since the introduction of click chemistry in 2001. This diverse set of reactions has led to the synthesis of self-healing and dynamic polymers, the creation of hydrogels exhibiting finely tuneable gelation times and mechanical properties and to the temporal and spatial control of chemical reactions enabling the 3D patterning of gels and surfaces with high fidelity. Traditionally, the copper catalysed azide–alkyne cycloaddition (CuAAC), Diels–Alder and thiol–ene click reactions have been utilised but, owing to the demand for more environmentally friendly means of synthesis and the need for more versatile and tolerant chemistry, the imine, hydrazone, and most recently, oxime carbonyl-condensations have seen an astonishing increase in application. This review will focus on the oxime click reaction for the development of functional polymeric materials.

Facile synthesis of histidine functional poly(N-isopropylacrylamide): zwitterionic and temperature responsive materials

N-Isopropylacrylamide is copolymerised with aldehyde functional monomers to facilitate post polymer functionalisation with histidine via reductive amination. This strategy is successfully performed without the use of protecting groups with high yields, demonstrating the facile application of this synthetic strategy with high functional group tolerance. The resulting temperature responsive, histidine functionalised polymers are characterised and their responsive nature is explored. The functionalised polymers exhibit sharp Lower Critical Solution Temperatures (LCST) at histidine incorporations of up to 8%. Upon 12 and 23% incorporation, the LCST transition extends over a wider range of temperature, indicating that the LSCT is being counteracted by an increase in solubility from the histidine moieties which interact strongly with water. Dynamic Light Scattering data indicates responsive self-assembly.

Doubly Dynamic Self-Healing Materials Based on Oxime Click Chemistry and Boronic Acids

The dynamic covalent characteristics of oxime and boronate ester bonds have been explored. A small excess of a competing aldehyde under acidic conditions resulted in oxime polymer degradation from high molecular weights (30 kDa) to low molecular weight oligomers (2.2 kDa). The dynamic nature of oxime bonds imparts oxime cross-linked hydrogels with self-healing properties and the incorporation of phenyl boronic acid groups into the hydrogel network provides a platform for hydrogel functionalization. The addition of a polyphenol (tannic acid) proves a facile means to incorporate a second, dynamic covalent cross-linking network through boronate ester formation which, owing to the increase in the degree of cross-linking, is found to be nearly double the hydrogel strength (storage modulus increased from 4.6 to 8.5 kPa). Finally, the tannic acid cross-linking network is selectively degraded returning the hydrogel storage modulus to its initial value and providing a means for the synthesis of materials with tunable mechanical properties.

Hydrogen bonding in bulk heterojunction solar cells: A case study

Small molecules with dithieno[3,2-b;2′,3′-d]thiophene as central building block and octyl cyanoacetate and octyl cyanoacetamide as different terminal building blocks have been designed and synthesized. The amide containing small molecule can form intermolecular hydrogen bonding between N-H…O = C of the amide group. The photovoltaic properties and active layer morphologies of the two molecules in bulk heterojunction solar cells are compared to study the influence of hydrogen bonding on the active layer morphology. New methanofullerene compound containing amide group has also been synthesized and compared with conventional fullerene electron acceptors.

Thiazolyl substituted benzodithiophene copolymers: synthesis, properties and photovoltaic applications

Three new conjugated polymers based on 5-decylthiazol-2-yl substituted benzodithiophene have been synthesized by Stille cross-coupling polymerization. 1,3-Dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (M1), 2,5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (M2) and 4,6-dibromo-thieno[3,4-b]thiophene-2-dodecyl carboxylate (M3) were used as acceptor building blocks for the synthesis of conjugated donor–acceptor polymers. The thermal, optical, electrochemical, and photovoltaic properties of the synthesized polymers were studied.

Three-Dimensional Printing of pH-Responsive and Functional Polymers on an Affordable Desktop Printer

In this work we describe the synthesis, thermal and rheological characterization, hot-melt extrusion, and three-dimensional printing (3DP) of poly(2-vinylpyridine) (P2VP). We investigate the effect of thermal processing conditions on physical properties of produced filaments in order to achieve high quality, 3D-printable filaments for material extrusion 3DP (ME3DP). Mechanical properties and processing performances of P2VP were enhanced by addition of 12 wt % acrylonitrile-butadiene-styrene (ABS), which reinforced P2VP fibers. We 3D-print P2VP filaments using an affordable 3D printer. The pyridine moieties are cross-linked and quaternized postprinting to form 3D-printed pH-responsive hydrogels. The printed objects exhibited dynamic and reversible pH-dependent swelling. These hydrogels act as flow-regulating valves, controlling the flow rate with pH. Additionally, a macroporous P2VP membrane was 3D-printed and the coordinating ability of the pyridyl groups was employed to immobilize silver precursors on its surface. After the reduction of silver ions, the structure was used to catalyze the reduction of 4-nitrophenol to 4-aminophenol with a high efficiency. This is a facile technique to print recyclable catalytic objects.