Siew Lay Lim

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%.

Improvement of CH3NH3PbI3 Formation for Efficient and Better Reproducible Mesoscopic Perovskite Solar Cells

High-performance perovskite solar cells (PSCs) are obtained through optimization of the formation of CH3NH3PbI3 nanocrystals on mesoporous TiO2 film, using a two-step sequential deposition process by first spin-coating a PbI2 film and then submerging it into CH3NH3I solution for perovskite conversion (PbI2 + CH3NH3I → CH3NH3PbI3). It is found that the PbI2 morphology from different film formation process (thermal drying, solvent extraction, and as-deposited) has a profound effect on the CH3NH3PbI3 active layer formation and its nanocrystalline composition. The residual PbI2 in the active layer contributes to substantial photocurrent losses, thus resulting in low and inconsistent PSC performances. The PbI2 film dried by solvent extraction shows enhanced CH3NH3PbI3 conversion as the loosely packed disk-like PbI2 crystals allow better CH3NH3I penetration and reaction in comparison to the multicrystal aggregates that are commonly obtained in the thermally dried PbI2 film. The as-deposited PbI2 wet film, without any further drying, exhibits complete conversion to CH3NH3PbI3 in MAI solution. The resulting PSCs reveal high power conversion efficiency of 15.60% with a batch-to-batch consistency of 14.60 ± 0.55%, whereas a lower efficiency of 13.80% with a poorer consistency of 11.20 ± 3.10% are obtained from the PSCs using thermally dried PbI2 films.

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.

Pure Blue-Light Emissive Poly(oligofluorenes) with Bifunctional POSS in the Main Chain

Emission of conjugated polymers is known to undergo bathochromic shift from solution to film formation due to π-π stacking in the solid state. In this report, a series of pearl-necklace-like hybrid polymers is designed via the hydrosilylation condensation between bifunctional polyhedral oligomeric silsesquioxanes (B-POSS) and oligofluorene segments. Optoelectronic analyses unequivocally show that the presence of these interconnecting B-POSS can effectively reduce red-shift in photoluminescence and electroluminescence during film formation. These hybrid poly(oligofluorenes) display stable blue emission with high color purity. Thermal analyses also indicate that they are vitrified polymers with high glass transition temperature (up to 125 °C). We believe that this strategy can be extended to other conjugated systems to control color purity in electroactive materials and holds promise as new emissive materials for various applications.