Photoactive Donor-Acceptor Composite Nanoparticles Dispersed in Water

Organic Donor-Acceptor-Donor Trimers Nanoparticles Stabilized by Amphiphilic Block Copolymers for Photocatalytic Generation of H2

Photocatalytic generation of H2 via water splitting emerges as a promising avenue for the next generation of green hydrogen due to its low carbon footprint. Herein, a versatile platform is designed to the preparation of functional π-conjugated organic nanoparticles dispersed in aqueous phase via mini-emulsification. Such particles are composed of donor-acceptor-donor (DAD) trimers prepared via Stille coupling, stabilized by amphiphilic block copolymers synthesized by reversible addition-fragmentation chain transfer polymerization. The hydrophilic segment of the block copolymers will not only provide colloidal stability, but also allow for precise control over the surface functionalization. Photocatalytic tests of the resulting particles for H2 production resulted in promising photocatalytic activity (≈0.6 mmol g-1 h-1). This activity is much enhanced compared to that of DAD trimers dispersed in the water phase without stabilization by the block copolymers.

Review of Waterborne Organic Semiconductor Colloids for Photovoltaics

Development of carbon neutral and sustainable energy sources should be considered as a top priority solution for the growing worldwide energy demand. Photovoltaics are a strong candidate, more specifically, organic photovoltaics (OPV), enabling the design of flexible, lightweight, semitransparent, and low-cost solar cells. However, the active layer of OPV is, for now, mainly deposited from chlorinated solvents, harmful for the environment and for human health. Active layers processed from health and environmentally friendly solvents have over recent years formed a key focus topic of research, with the creation of aqueous dispersions of conjugated polymer nanoparticles arising. These nanoparticles are formed from organic semiconductors (molecules and macromolecules) initially designed for organic solvents. The topic of nanoparticle OPV has gradually garnered more attention, up to a point where in 2018 it was identified as a "trendsetting strategy" by leaders in the international OPV research community. Hence, this review has been prepared to provide a timely roadmap of the formation and application of aqueous nanoparticle dispersions of active layer components for OPV. We provide a thorough synopsis of recent developments in both nanoprecipitation and miniemulsion for preparing photovoltaic inks, facilitating readers in acquiring a deep understanding of the crucial synthesis parameters affecting particle size, colloidal concentration, ink stability, and more. This review also showcases the experimental levers for identifying and optimizing the internal donor-acceptor morphology of the nanoparticles, featuring cutting-edge X-ray spectromicroscopy measurements reported over the past decade. The different strategies to improve the incorporation of these inks into OPV devices and to increase their efficiency (to the current record of 7.5%) are reported, in addition to critical design choices of surfactant type and the advantages of single-component vs binary nanoparticle populations. The review naturally culminates by presenting the upscaling strategies in practice for this environmentally friendly and safer production of solar cells.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.

Eco-Friendly Polymer Solar Cells: Advances in Green-Solvent Processing and Material Design

Despite the recent breakthroughs of polymer solar cells (PSCs) exhibiting a power conversion efficiency of over 17%, toxic and hazardous organic solvents such as chloroform and chlorobenzene are still commonly used in their fabrication, which impedes the practical application of PSCs. Thus, the development of eco-friendly processing methods suitable for industrial-scale production is now considered an imperative research focus. This Review provides a roadmap for the design of efficient photoactive materials that are compatible with non-halogenated green solvents (e.g., xylenes, toluene, and tetrahydrofuran). We summarize the recent development of green processing solvents and the processing methods to match with the efficient photoactive materials used in non-fullerene solar cells. We further review progress in the use of more eco-friendly solvents (i.e., water or alcohol) for achieving truly sustainable and eco-friendly PSC fabrication. For example, the concept of water- or alcohol-dispersed nanoparticles made of conjugated materials is introduced. Also, recent important progress and strategies to develop water/alcohol-soluble photoactive materials that completely eliminate the use of conventional toxic solvents are discussed. Finally, we provide our perspectives on the challenges facing the current green processing methods and materials, such as large-area coating techniques and long-term stability. We believe this Review will inform the development of PSCs that are truly clean and renewable energy sources.

Size-dependent fluorescence of conjugated polymer dots and correlation with the fluorescence in solution and in the solid phase of the polymer

Three conjugated polymers (CPs) were synthesized to obtain CPs with the same backbone but with different compositions of repeat units (phenylene and benzoselenadiazole (BSD)). The dominant composition of phenylene units and a smaller amount of BSD in the CP backbone enabled the CPs to emit different fluorescence colors according to their condition (solution or solid), which was caused by the difference in intermolecular electron transfer between CP backbones. Inspired by this, we fabricated polymer dots (Pdots) with various sizes using the CPs to control the number of CP chains within a spherical Pdot. This implied that smaller Pdots, where the chance of intermolecular electron transfer would be at a minimum, would accommodate fewer polymer chains than larger ones. The minimum chance for intermolecular electron transfer resulted in a short-wavelength emission, which was the identical emission color encountered in liquid CP solution. A more frequent intermolecular electron transfer was expected in larger Pdots, exhibiting long-wavelength emission, which was the same as observed in solid CPs. White-light-emitting Pdots that showed Commission Internationale de 1'Eclairage (CIE) coordinates of (0.34, 0.31) were fabricated simply by controlling the Pdot size.

Size-Dependent Photophysical Behavior of Low Bandgap Semiconducting Polymer Particles

The photophysics of water and propan-1-ol suspensions of poly [N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'- benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles has been studied by ultrafast spectroscopy. High molar mass polymer (HMM > 20 kg/mol) forms nanoparticles with around 50 nm diameter via mini-emulsion post-polymerization, while low molar mass (LMM < 5 kg/mol) polymer prepared by dispersion polymerization results in particles with a diameter of almost one order of magnitude larger (450 ± 50 nm). In this study, the presence of excited-states and charge separated species was identified through UV pump and visible/near-infrared probe femtosecond transient absorption spectroscopy. A different behavior for the HMM nanoparticles has been identified compared to the LMM mesoparticles. The nanoparticles exhibit typical features of an energetically disordered conjugated polymer with a broad density of states, allowing for delayed spectral relaxation of excited states, while the mesoparticles show a J-aggregate-like behavior where interchain interactions are less efficient. Stimulated emission in the red-near infrared region has been found in the mesoparticles which indicates that they present a more energetically ordered system.

Control of Shell Morphology in p-n Heterostructured Water-Processable Semiconductor Colloids: Toward Extremely Efficient Charge Separation

This article describes p-n heterostructured water-borne semiconductor naonoparticles (NPs) with unique surface structures via control of shell morphology. The shell particles, comprising PC60-[6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁ BM) composite, having n-type semiconductor characteristics, notably influence the charge carrier behavior in the core-shell NPs. A one- or two-phase methodology based on a PC60 surfactant-water phase and PC₆₁ BM n-type semiconductor-organic phase provides highly specific control over the shell structure of the NPs, which promote their superior charge separation ability when combined with poly-3-hexyl-thiophene (P3HT). Moreover, the resulting water-borne NP exhibits shell morphology-dependent carrier quenching and stability, which is characterized via luminescence studies paired with structural analysis. Corresponding to the results, outstanding performances of photovoltaic cells with over 5% efficiency are achieved. The results suggest that the surrounding shell environments, such as the shell structure, and its electronic charge density, are crucial in determining the overall activity of the core-shell p-n heterostructured NPs. Thus, this work provides a new protocol in the current fields of water-based organic semiconductor colloids.

Optimisation of purification techniques for the preparation of large-volume aqueous solar nanoparticle inks for organic photovoltaics

In this study we have optimised the preparation conditions for large-volume nanoparticle inks, based on poly(3-hexylthiophene) (P3HT):indene-C₆₀ multiadducts (ICxA), through two purification processes: centrifugal and crossflow ultrafiltration. The impact of purification is twofold: firstly, removal of excess sodium dodecyl sulfate (SDS) surfactant from the ink and, secondly, concentration of the photoactive components in the ink. The removal of SDS was studied in detail both by a UV-vis spectroscopy-based method and by surface tension measurements of the nanoparticle ink filtrate; revealing that centrifugal ultrafiltration removed SDS at a higher rate than crossflow ultrafiltration even though a similar filter was applied in both cases (10,000 Da M_w cut-off). The influence of SDS concentration on the aqueous solar nanoparticle (ASNP) inks was investigated by monitoring the surface morphology/topography of the ASNP films using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and photovoltaic device performance as a function of ultrafiltration (decreasing SDS content). The surface morphology/topography showed, as expected, a decreased number of SDS crystallites on the surface of the ASNP film with increased ultrafiltration steps. The device performance revealed distinct peaks in efficiency with ultrafiltration: centrifuge purified inks reached a maximum efficiency at a dilution factor of 7.8 × 10⁴, while crossflow purified inks did not reach a maximum efficiency until a dilution factor of 6.1 × 10⁹. This difference was ascribed to the different wetting properties of the prepared inks and was further corroborated by surface tension measurements of the ASNP inks which revealed that the peak efficiencies for both methods occurred for similar surface tension values of 48.1 and 48.8 mN m^-1. This work demonstrates that addressing the surface tension of large-volume ASNP inks is key to the reproducible fabrication of nanoparticle photovoltaic devices.