Bulk heterojunction organic photovoltaics from water-processable nanomaterials and their facile fabrication approaches

Facile synthesis of water-dispersible poly(3-hexylthiophene) nanoparticles with high yield and excellent colloidal stability

There has been growing interest in water-processable conjugated polymers for biocompatible devices. However, some broadly used conjugated polymers like poly(3-hexylthiophene) (P3HT) are hydrophobic and they cannot be processed in water. We herein report a facile yet highly efficient assembly method to prepare water-dispersible pyridine-containing P3HT (Py-P3HT) nanoparticles (NPs) with a high yield (>80%) and a fine size below 100 nm. It is based on the fast nanoprecipitation of Py-P3HT stabilized by hydrophilic poly(acrylic acid) (PAA). Py-P3HT can form spherical NPs at a concentration up to 0.2 mg/mL with a diameter of ∼75 nm at a very low concentration of PAA, e.g., 0.01-0.1 mg/mL, as surface ligands. Those negatively charged Py-P3HT NPs can bind with metal cations and further support the growth of noble metal NPs like Ag and Au. Our self-assembly methodology potentially opens new doors to process and directly use hydrophobic conjugated polymers in a much broader context.

Amphiphilic PTB7-Based Rod-Coil Block Copolymer for Water-Processable Nanoparticles as an Active Layer for Sustainable Organic Photovoltaic: A Case Study

We synthetized a new rod-coil block copolymer (BCP) based on the semiconducting polymerpoly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and poly-4-vinylpyridine (P4VP), tailored to produce water-processable nanoparticles (WPNPs) in blend with phenyl-C71-butyric acid methyl ester (PC₇₁BM). The copolymer PTB7-b-P4VP was completely characterized by means of two-dimensional nuclear magnetic resonance (2D-NMR), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS), size-exclusion chromatography (SEC), and differential scanning calorimetry (DSC) to confirm the molecular structure. The WPNPs were prepared through an adapted miniemulsion approach without any surfactants. Transmission electron microscopy (TEM) images reveal the nano-segregation of two active materials inside the WPNPs. The nanostructures appear spherical with a Janus-like inner morphology. PTB7 segregated to one side of the nanoparticle, while PC₇₁BM segregated to the other side. This morphology was consistent with the value of the surface energy obtained for the two active materials PTB7-b-P4VP and PC₇₁BM. The WPNPs obtained were deposited as an active layer of organic solar cells (OSCs). The films obtained were characterized by UV-Visible Spectroscopy (UV-vis), atomic force microscopy (AFM), and grazing incidence X-ray diffraction (GIXRD). J-V characteristics of the WPNP-based devices were measured by obtaining a power conversion efficiency of 0.85%. Noticeably, the efficiency of the WPNP-based devices was higher than that achieved for the devices fabricated with the PTB7-based BCP dissolved in chlorinated organic solvent.

Surfactant Engineering and Its Role in Determining the Performance of Nanoparticulate Organic Photovoltaic Devices

The fabrication of organic photovoltaics (OPVs) from non-hazardous nanoparticulate (NP) inks offers considerable promise for the development of eco-friendly large-scale printed solar modules. However, the typical NP core-shell morphology (driven by the different donor/acceptor affinities for the surfactant used in NP synthesis) currently hinders the photovoltaic performance. As such, surfactant engineering offers an elegant approach to synthesizing a more optimal intermixed NP morphology and hence an improved photovoltaic performance. In this work, the morphology of conventional sodium dodecyl sulfate (SDS) and 2-(3-thienyl) ethyloxybutylsulfonate (TEBS)-stabilized poly(3-hexylthiophene) (P3HT) donor:phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) acceptor NPs is probed using scanning transmission X-ray microscopy, UV-vis spectroscopy, grazing-incidence X-ray diffraction, and scanning electron microscopy. While the SDS-stabilized NPs exhibit a size-independent core-shell morphology, this work reveals that TEBS-stabilized NPs deliver an intermixed morphology, the extent of which depends on the particle size. Consequently, by optimizing the TEBS-stabilized NP size and distribution, NP-OPV devices with a power conversion efficiency that is ∼50% higher on average than that of the corresponding SDS-based NP-OPV devices are produced.

Recent advances in non-fullerene organic photovoltaics enabled by green solvent processing

Solution-processed organic photovoltaic (OPV) as a new energy device has attracted much attention due to its huge potential in future commercial manufacturing. However, so far, most of the studies on high-performance OPV have been treated with halogenated solvents. Halogenated solvents not only pollute the environment, but are also harmful to human health, which will negatively affect the large-scale production of OPV in the future. Therefore, it is urgent to develop low-toxic or non-toxic non-halogen solvent-processable OPV. Compared with conventional fullerene OPVs, non-fullerene OPVs exist with stronger absorption, better-matched energy levels and lower energy loss. Processing photoactive layers with non-fullerenes as the acceptor material has broad potential advantages in non-halogenated solvents. This review introduces the research progress of non-fullerene OPV treated by three different kinds of green solvents as the non-halogenated and aromatic solvent, the non-halogenated and non-aromatic solvent, alcohol and water. Furthermore, the effects of different optimization strategies on the photoelectric performance and stability of non-fullerene OPV are analyzed in detail. The current optimization strategy can increase the power conversion efficiency of non-fullerene OPV processed with non-halogen solvents up to 17.33%, which is close to the performance of processing with halogen-containing solvents. Finally, the commercial potential of non-halogen solvent processing OPVs is discussed. The green solvent processing of non-fullerene-based OPVs will become a key development direction for the future of the OPV industry.

Conjugated Polymer/Graphene Oxide Nanocomposites—State-of-the-Art

Graphene oxide is an imperative modified form of graphene. Similar to graphene, graphene oxide has gained vast interest for the myriad of industrial applications. Conjugated polymers or conducting polymers are well known organic materials having conducting backbone. These polymers have semiconducting nature due to π-conjugation along the main chain. Doping and modification have been used to enhance the electrical conductivity of the conjugated polymers. The nanocomposites of the conjugated polymers have been reported with the nanocarbon nanofillers including graphene oxide. This review essentially presents the structure, properties, and advancements in the field of conducting polymer/graphene oxide nanocomposites. The facile synthesis, processability, and physical properties of the polymer/graphene oxide nanocomposites have been discussed. The conjugated polymer/graphene oxide nanocomposites have essential significance for the supercapacitors, solar cells, and anti-corrosion materials. Nevertheless, the further advanced properties and technical applications of the conjugated polymer/graphene oxide nanocomposites need to be explored to overcome the challenges related to the high performance.

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.

Effects of non-halogenated solvent on the main properties of a solution-processed polymeric thin film for photovoltaic applications: a computational study

Organic photovoltaic (OPV) devices have reached high power conversion efficiencies, but they are usually processed using halogenated toxic solvents. Hence, before OPV devices can be mass-produced by industrial processing, it would be desirable to replace those solvents with eco-friendly ones. Theoretical tools may be then a powerful ally in the search for those new solvents. In order to better understand the mechanisms behind the interaction between solvent and polymer, classical molecular dynamics (MD) calculations were used to produce a thin film of poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7-Th), processed using two different solvents. PTB7-Th is widely applied as a donor material in OPVs. The first solvent is ortho-dichlorobenzene (o-DCB), which is a highly toxic solvent widely used in lab-scale studies. The second solvent is ortho-methylanisole (o-MA), which is an eco-friendly solvent for organic photovoltaic (OPV) manufacturing. Here we use a solvent evaporation protocol to simulate the formation of the PTB7-Th film. We demonstrate that our theoretical MD calculations were able to capture some differences in the macroscopic properties of thin films formed by o-DCB or o-MA evaporation. We found that the interaction of the halogenated solvent with the polymer tends to break the bonds between the lateral thiophenediyl groups and the main chain. We show that those defects may create traps that can affect the charge transport and also can be responsible for a blue shift in the absorption spectrum. Using the Monte Carlo method, we also verified the influence of the resulting MD morphology on the mobility of holes. Our theoretical results showed good agreement with the experimental measurements and both demonstrate that o-MA can be used to make polymer thin films without any loss of key properties for the device performance. The findings here highlight the importance of theoretical results as a guide to the morphological optimization of green processed polymeric films.

Sulfonated Thiophene Derivative Stabilized Aqueous Poly(3-hexylthiophene):Phenyl-C61-butyric Acid Methyl Ester Nanoparticle Dispersion for Organic Solar Cell Applications

Aqueous dispersions of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) nanoparticles (NPs) have been fabricated using a thiophene-based surfactant 2-(3-thienyl)ethyloxybutylsulfonate sodium salt (TEBS) for the first time via the mini-emulsion process. The use of TEBS resulted in a stable colloidal dispersion of P3HT:PCBM NPs, of which the effect of various fabrication parameters is investigated. The fabricated NPs were characterized by dynamic light scattering, scanning electron microscopy, UV-visible spectroscopy, contrast-variation small and ultra-small angle neutron scattering, and cyclic voltammetry. The internal structure and electrochemical performance of TEBS-stabilized P3HT:PCBM NPs were compared to those of sodium dodecyl sulfate-stabilized core-shell (PCBM-P3HT) NPs at the same surfactant concentration. Neutron scattering and cyclic voltammetry results reveal a homogeneous distribution of small de-mixed P3HT and PCBM domains in the internal structure of TEBS-stabilized P3HT:PCBM NPs, reminiscent of cast film. Moreover, electron microscopy images show evidence of diffused NP surface/interface upon drying (without annealing), which indicates that the thiophene-containing TEBS may improve compatibility and film-forming properties of fabricated P3HT:PCBM NPs, and consequently be more suited for conventional film-processing methods for organic solar cell applications.

Fast interfacial charge transfer in α-Fe2O3-δCδ/FeVO4-x+δCx-δ@C bulk heterojunctions with controllable phase content

The novelties in this paper are embodied in the fast interfacial charge transfer in α-Fe₂O_3−δC_δ/FeVO_4−x+δC_x−δ@C bulk heterojunctions with controllable phase compositions. The carbon source-glucose plays an important role as the connecting bridge between the micelles in the solution, forming interfacial C-O, C-O-Fe and O-Fe-C bonds through dehydration and polymerization reactions. Then the extra VO₃⁻ around the FeVO₄ colloidal particles can react with unstable Fe(OH)₃, resulting the phase transformation from α-Fe₂O₃ (47.99–7.16%) into FeVO₄ (52.01–92.84%), promoting photocarriers’ generation capacities. After final carbonization, a part of C atoms enter into lattices of α-Fe₂O₃ and FeVO₄, forming impurity levels and oxygen vacancies to increase effective light absorptions. Another part of C sources turn into interfacial carbon layers to bring fast charge transfer by decreasing the charge transition resistance (from 53.15 kΩ into 8.29 kΩ) and the surface recombination rate (from 64.07% into 7.59%). The results show that the bulk heterojunction with 90.29% FeVO₄ and 9.71% α-Fe₂O₃ shows ideal light absorption, carriers’ transfer efficiency and available photocatalytic property. In general, the synergistic effect of optimized heterojunction structure, carbon replacing and the interface carbon layers are critical to develop great potential in stable and recoverable use.