Wallace W. H. Wong

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.

Organic Solar Cells Using a High‐Molecular‐Weight Benzodithiophene–Benzothiadiazole Copolymer with an Efficiency of 9.4%

A high molecular weight donor-acceptor conjugated polymer is synthesized using the Suzuki polycondensation method. Using this polymer, a single-junction bulk-heterojunction solar cell is fabricated giving a power conversion efficiency of 9.4% using a fullerene-modified ZnO interlayer at the cathode contact.

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.

Tetrakis(imidazolium) macrocyclic receptors for anion binding

New (tetrakis)imidazolium macrocyclic receptor systems of variable cavity size have been synthesised by stepwise alkylation reactions of bis(imidazolium) precursor compounds. Proton NMR titration studies reveal the macrocycles to strongly bind halide and benzoate anions, with two receptor systems displaying notable selectivity for fluoride in competitive acetonitrile-water (9:1) solvent media.

Concentrating Aggregation-Induced Fluorescence in Planar Waveguides: A Proof-of-Principle

The photophysical properties of fluorescent dyes are key determinants in the performance of luminescent solar concentrators (LSCs). First-generation dyes – coumarin, perylenes, and rhodamines - used in LSCs suffer from both concentration quenching in the solid-state and small Stokes shifts which limit the current LSC efficiencies to below theoretical limits. Here we show that fluorophores that exhibit aggregation-induced emission (AIE) are promising materials for LSC applications. Experiments and Monte Carlo simulations show that the optical quantum efficiencies of LSCs with AIE fluorophores are at least comparable to those of LSCs with first-generation dyes as the active materials even without the use of any optical accessories to enhance the trapping efficiency of the LSCs. Our results demonstrate that AIE fluorophores can potentially solve some key limiting properties of first-generation LSC dyes.

High-performance polymer solar cells with a conjugated zwitterion by solution processing or thermal deposition as the electron-collection interlayer

We report highly efficient polymer solar cells (PSCs) with rhodamine 101, a conjugated zwitterion with positive and negative charges on the same molecule, as the electron-collection interlayer. Rhodamine 101 can be processed by solution processing techniques or thermal deposition. The rhodamine 101 interlayer simultaneously improves the short-circuit current, open-circuit voltage, fill factor so as to improve the photovoltaic efficiency of the PSCs with poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active materials in comparison with control PSCs with Al or Ca/Al as the cathode. The photovoltaic efficiency reaches 6.15% for the PSCs with rhodamine 101/Al as the cathode under AM1.5G illumination, whereas the efficiencies are only 3.81% and 4.57% for the control PSCs with Al and Ca/Al as the cathodes, respectively. On the other hand, the improvement on the photovoltaic performance by the rhodamine 101 interlayer is less remarkable for the PSCs with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or PC71BM as the active materials. The photovoltaic efficiency is 4.25% for the PSCs with rhodamine 101/Al as the cathode, almost the same as that (4.20%) of the control PSCs with Ca/Al as the cathode. The high efficiency of the PSCs with the rhodamine 101 interlayer is ascribed to the lowering of the work function of metals by rhodamine 101 that has a strong dipole moment. The different effects of the rhodamine 101 interlayer on the two PSCs with PCDTBT and P3HT as the donor materials are attributed to different reactivities of the two polymers with active metals like Ca and Al. PCDTBT consisting of both electron-donating and electron-withdrawing units is more reactive with the active metals. The reaction between PCDTBT and Ca leads to low photovoltaic efficiency of the PSCs with Ca/Al as the cathode.

Hexa-peri-hexabenzocoronene in organic electronics

Polycyclic aromatic hydrocarbons (PAHs) are in a class of functional organic compounds with increasing importance in organic electronics. Their tunable photophysical properties and typically strong intermolecular associations make them ideal materials in applications where control of charge mobility is essential. Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped PAH that self-associates into columnar stacks through strong π–π interactions. By decorating the periphery of the HBC molecule with various substituents, a range of properties and functions can be obtained including solution processability, liquid crystallinity, and semiconductivity. In this review article, the synthesis, properties, and functions of HBC derivatives are presented with focus on work published in the last five years.

Silicon Analogues of Polyfluorene as Materials for Organic Electronics

Poly(dibenzosilole)s have been increasingly reported as an alternative to polyfluorene in organic electronic materials. Poly(dibenzosilole)s show similar optical properties to polyfluorene, but with improved resistance to oxidation and thermal stability. Several poly(dibenzosilole)s and their co-polymers have been incorporated into organic electronic devices, such as light emitting diodes and solar cells. These materials have shown improved performance over their polyfluorene-based counterparts.

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.

Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor-acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.