Kok-Haw Ong

A Versatile Low Bandgap Polymer for Air‐Stable, High‐Mobility Field‐Effect Transistors and Efficient Polymer Solar Cells

Polymer-based organic thin-fi lm transistors (OTFTs) and organic photovoltaics (OPVs) have attracted much interest in recent years due to their solution processability and mechanical fl exibility, which potentially allow them to be manufactured using low-cost, high-throughput processes such as roll-to-roll printing and inkjet printing. [ 1 ] In recent years, the development of novel materials for these applications has contributed to signifi cant improvements in device performance. In the fi eld of OTFTs, the development of polythiophene derivatives such as poly(3,3 ′ ′ ′ -dialkyl-quaterthiophene) (PQT) and poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) has resulted in OTFTs with hole mobilities of between 0.2 and 0.6 cm 2 V − 1 s − 1 . [ 2 , 3 ] The development of low bandgap polymer donors has meanwhile helped to lift solar cell power conversion effi ciencies (PCEs) to above 6%. [ 4–6 ] However, there have been few reports of polymers that perform well in both OTFTs and OPVs. The development of such versatile polymers could be benefi cial for applications which incorporate both types of devices. Although the high hole mobility of polythiophenes is an attractive property for OPV applications, their relatively large bandgap of around 1.9 to 2 eV [ 7 ] means that they are unable to absorb and harvest photons above 650 nm. The incorporation of acceptor moieties into the polythiophene backbone allows the bandgap to be reduced and can thereby improve solar cell performance. A commonly used acceptor moiety in D–A polymers is 2,1,3-benzothiadiazole, which has been copolymerized with donor moieties such as fl uorene, [ 8 ] carbazole, [ 4 , 9 ] and cyclopentadithiophene [ 10 , 11 ] to produce low bandgap polymers and associated solar cells with PCEs up to 6.1%. Copolymers of thiophene and benzothiadiazole have previously been studied in OPVs, with the number of thiophene units in the repeat unit ranging from three to eight. Despite the lowered bandgap and improved absorption properties of the polymers, the PCEs of the resultant devices have been relatively low, ranging from 0.13% to 2.23%. [ 12–15 ]

Design and modification of three-component randomly incorporated copolymers for high performance organic photovoltaic applications

In this study we report the molecular design, synthesis, characterization, and photovoltaic properties of a series of diketopyrrolopyrrole (DPP) and dithienothiophene (DTT) based donor-acceptor random copolymers. The six random copolymers are obtained via Stille coupling polymerization using various concentration ratios of donor to acceptor in the conjugated backbone. Bis(trimethylstannyl)thiophene was used as the bridge block to link randomly with the two comonomers 5-(bromothien-2-yl)-2,5-dialkylpyrrolo[3,4-c]pyrrole-1, 4-dione and 2,6-dibromo-3,5-dipentadecyl-dithieno[3,2-b;2′,3′-d] thiophene. The optical properties of these copolymers clearly reveal a change in the absorption band through optimization of the donor-acceptor ratio in the backbone. Additionally, the solution processability of the copolymers is modified through the attachment of different bulky alkyl chains to the lactam N-atoms of the DPP moiety. Applications of the polymers as light-harvesting and electron-donating materials in solar cells, in conjunction with PCBM as acceptor, show power conversion efficiencies (PCEs) of up to 5.02%.

Synthesis of thieno[3,2-b]thiophene derived conjugated oligomers for field-effect transistors applications

A novel series of soluble thieno[3,2-b]thiophene (TT) oligomers with alternating TT and bithiophene or fluorene triad architectures have been synthesized for field-effect transistor (FET) applications. Their optical, thermal and electronic properties were investigated using UV-Vis and photoluminescence spectroscopy, thermal gravimetric analysis, and cyclic voltammetry. Compared with α,α′-dihexylsexithiophene (D6HT), these oligomers exhibit blue-shifted absorption spectra, 0.1 eV to 0.3 eV lower than the highest occupied molecular orbital (HOMO) energy levels and accordingly higher ambient stability. Their crystallinity and morphology features of these oligomers were further investigated with X-ray diffraction and atomic force microscopy using vacuum-deposited thin film on Si/SiO2 substrate. Symmetrically structured TT-oligomers including HT2TT, HTTT2, HTTTT and DDFTT exhibit ordered film morphology and promising FET performance with devices fabricated by either vacuum deposition or solution processing techniques. Interestingly, HTTT2 shows interconnected terrace island morphology, which is often observed for pentacene. All p-type transistors show promising performance, with HT2TT demonstrating a hole mobility up to 0.025 cm2V−1s−1 and an on/off current ration ∼1.2 × 103 with vacuum sublimated film deposited on 70 °C substrate.

Effects of fluorination on the electrochromic performance of benzothiadiazole-based donor–acceptor copolymers

A series of thiophene and benzothiadiazole-based copolymers (PDAT-DTBT) is synthesized through Stille coupling polymerization with two mono- and di-fluorinated benzothiadiazole analogues: PDAT-DTBT-F (1F) and PDAT-DTBT-2F (2F). The introduction of fluorine atoms onto the conjugated polymer backbone is found to have a pronounced effect on the optical, electrochemical and morphological properties, which in turn, influences the electrochromic performance of the fabricated absorption/transmission type devices greatly. All the polymers switch reversibly between the colored neutral states (green/blue) to transmissive oxidized states. Systematic enhancement in the reduction process to sub-second speeds (<0.5 s), high coloration efficiency (748 cm2 C−1) and substantially improved ambient stability are observed upon fluorination of electron acceptors. Long-term stability testing of the PDAT-DTBT-F electrochromic device is carried out for up to 10 000 repeated redox cycles between the applied potentials of +1.6 and −1.6 V, without the observation of significant degradation.

An alternating copolymer based on dithienothiophene and diketopyrrolopyrrole units for thin-film transistors and organic solar cells

An alternating donor–acceptor copolymer using dithienothiophene as the donor unit and diketopyrrolopyrrole as the acceptor unit has been synthesized and characterized. Preliminary studies of OTFT devices based on this copolymer showed a high annealing-free saturation hole mobility of 0.13 cm2 V−1 s−1. Initial bulk heterojunction solar cells based on the blends of the copolymer with PC71BM had a power conversion efficiency of 3.4% under 100 mW cm−2 AM1.5 solar illumination.

Design and synthesis of benzothiadiazole–oligothiophene polymers for organic solar cell applications

A series of p-type copolymers based on oligothiophene and benzothiadiazole units was synthesised via Stille coupling and their physical, electrochemical and photovoltaic properties were evaluated. The polymers varied in the number and type of oligothiophene units as well as in the position of the solubilizing alkyl chains relative to the benzothiadiazole unit. These differences in the polymer structure had a significant impact on the thin-film transistor mobility and the solar cell power conversion efficiency, thus illustrating the importance of these design factors for polymers in optoelectronic applications.

High-Mobility Ambipolar Organic Thin-Film Transistor Processed From a Nonchlorinated Solvent.

Polymer semiconductor PDPPF-DFT, which combines furan-substituted diketopyrrolopyrrole (DPP) and a 3,4-difluorothiophene base, has been designed and synthesized. PDPPF-DFT polymer semiconductor thin film processed from nonchlorinated hexane is used as an active layer in thin-film transistors. As a result, balanced hole and electron mobilities of 0.26 and 0.12 cm(2)/(V s) are achieved for PDPPF-DFT. This is the first report of using nonchlorinated hexane solvent for fabricating high-performance ambipolar thin-film transistor devices.