C2-Symmetrical 3,4-Ethylenedioxythiophene Monomers through a Divergent Approach

We present a divergent synthetic approach to C2-symmetrical 3,4-Ethylenedioxythiophene (EDOT) monomers in which functionalities can be introduced as pendant chains from the ethylene bridge. The key synthon, obtained through a high yielding trans-etherification, is the chiral EDOT with bromomethyl pendant groups and is prone to substitution reactions with oxygen-based nucleophiles. Elimination of the key precursor affords a diene that can be elaborated into unprecedented PhEDOT monomers using the Diels-Alder reaction. The strategy is further validated by the synthesis of a dithiane-containing EDOT.

3D Printing of Layered Structures of Metal-Ionic Polymers: Recent Progress, Challenges and Opportunities

Layered Structures of Metal Ionic Polymers, or Ionic Polymer-Metal Composites (IPMCs) are formed by a membrane of an ionic electroactive materials flanked by two metal electrodes on both surfaces; they are devices able to change their shape upon application of an electrical external stimulus. This class of materials is used in various fields such as biomedicine, soft robotics, and sensor technology because of their favorable properties (light weight, biocompatibility, fast response to stimulus and good flexibility). With additive manufacturing, actuators can be customized and tailored to specific applications, allowing for the optimization of performance, size, and weight, thus reducing costs and time of fabrication and enhancing functionality and efficiency in various applications. In this review, we present an overview of the newest trend in using different 3D printing techniques to produce electrically responsive IPMC devices.

Regioselectivity Control in Spirobifluorene Nitration under Mild Conditions: Explaining the Crivello's Reagent Mechanism

The regioselective nitration of 9,9'-spirobifluorene under mild conditions is reported for the first time by operating under Menke's and Crivello's conditions. The optimized protocol allows obtaining 2-nitro and 2,2'-dinitro-9,9'-spirobifluorene in yields of 79 and 95% and, for the first time, 2,2',7-trinitro-9,9'-spirobifluorene with 66% yield. Besides, the role of dinitrate salt in Crivello's protocol has been now clarified, which opens novel scenarios in the preparation of functional materials.

Supramolecular Weaving by Halogen-Bonding in Functionality-Rich Hexasubstituted Aromatic Synthons

Hexasubstituted benzenes are interesting platforms for the generation of functional materials, whose applications span from supramolecular recognition to organic electronics. Their synthesis is difficult to achieve by controlling multiple substitution steps of all hydrogen atoms on the aromatic benzene skeleton, so, often, cycloaddition reactions from disubsituted alkynes are used. In this work, we report a novel, straightforward route to C₃-symmetrical hexasubstituted aromatic synthons with a diverse and rich pattern of functionalities, and we report about their packing mode in the crystals, in which, unprecedentedly, directional, strong halogen bonding interactions are capable of forming bidimensional supramolecular weaving.

Polymer Solar Cells with Active Layer Thickness Compatible with Scalable Fabrication Processes: A Meta-Analysis

Organic photovoltaics (OPV) has been considered for a long time a promising emerging solar technology. Currently, however, market shares of OPV are practically non-existent. A detailed meta-analysis of the literature published until mid-2021 is presented, focusing on one of the remaining issues that need to be addressed to translate the recent remarkable progress, obtained in devices' performance at lab-scale level, into the requirements able to boost the manufacturing-scale production. Namely, the active layer's thickness is referred to, which, together with device efficiency and stability, represents one of the biggest challenges of this technological research field. Papers describing solar cells containing non-fullerene acceptor (NFA) binary and ternary blends, as well as NFA plus fullerene acceptor (FA) ternary blends are reviewed. The common ground of all analyzed devices is their high-thickness active layers, compatible with large-area deposition techniques. By defining a new figure of merit to discuss the OPV thickness (thickness tolerance, TT), it is found that this parameter is not affected by the chemical family's nature of the active blend components. On the other hand, the analysis suggests that there are promising strategies to improve the TT, which are discussed in the conclusion section.

Branched phosphazenium salts as effective, versatile cocatalysts for epoxide/CO2 coupling

Branched phosphazenium salts of general formula [(Me2N)3P=N]4P+X- (X = Cl- or N3-) have been tested as cocatalysts for different catalyst systems, such as salen-type chromium and porphyrin cobalt complexes, in...

Polymer-Assisted Single-Step Slot-Die Coating of Flexible Perovskite Solar Cells at Mild Temperature from Dimethyl Sulfoxide

The industrialization of perovskite solar cells relies on solving intrinsic-to-material issues. To reach record efficiencies perovskite deposition needs to be finely adjusted by multi-step processes, in a humidity free glove-box environment and by means of hardly scalable techniques often associated with toxic solvents and anti-solvent dripping/bath. Herein, the use of polymeric material is proposed to deposit perovskite layers with easy processability. To the scope, a starch-polymer/perovskite composite is developed to suit slot-die coating technique requirement, allowing the deposition of hybrid halide perovskite material in a single straightforward step without the use of toxic solvents, and in uncontrolled humid environment (RH up to 70 %). The starch-polymer increases the viscosity of the perovskite precursor solutions and delays the perovskite crystallization that results in the formation of perovskite films at mild temperature (60 °C) with good morphology. These innovative inks enables the fabrication of flexible solar cells with p-i-n configuration featured by a power conversion efficiency higher than 3 %. . Overall, this approach can be exploited in the future to massively reduce perovskite manufacturing costs related to keeping the entire fabrication line at high-temperature and under nitrogen or dry conditions.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.

One-Pot Regiodirected Annulations for the Rapid Synthesis of π-Extended Oligomers

We demonstrate the broad applicability of the annulation protocol combining, in one pot, a direct arylation and cross aldol condensation for the straightforward synthesis at gram-scale of π-extended thiophene-based scaffolds. The regiospecific direct arylation drives the subsequent cross-aldol condensation proceed under the same basic conditions, and the overall protocol has broad applicability in the synthesis of extended aromatics wherein the thiophene ring is annulated with furans, pyridines, indoles, benzothiophenes, and benzofurans. These scaffolds can be further elaborated into π-extended, highly fluorescent oligomers with a central deficient benzothiadiazole unit with up to nine aromatic rings through coupling reactions.

A relatively wide-bandgap and air-stable donor polymer for fabrication of efficient semitransparent and tandem organic photovoltaics

Organic photovoltaics (OPVs) have attracted tremendous attention in the field of thin-film solar cells due to their wide range of applications, especially for semitransparent devices. Here, we synthesize a dithiaindacenone-thiophene-benzothiadiazole-thiophene alternating donor copolymer named poly{[2,7-(5,5-didecyl-5H-1,8-dithia-as-indacenone)]-alt-[5,5-(5',6'-dioctyloxy-4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]} (PDTIDTBT), which shows a relatively wide bandgap of 1.82 eV, good mobility, and high transmittance and ambient stability. In this work, we fabricate an OPV device using monolayer graphene as top electrode. Due to the stability of PDTIDTBT in air and water, we use a wet transfer technique for graphene to fabricate semitransparent OPVs. We demonstrate OPVs based on the PDTIDTBT:Phenyl-C61/71-butyric acid methyl ester (PCBM) blend with maximum power conversion efficiencies (PCEs) of 6.1 and 4.75% using silver and graphene top electrodes, respectively. Our graphene-based device shows a high average visible transmittance (AVT) of 55%, indicating the potential of PDTIDTBT for window application and tandem devices. Therefore, we also demonstrate tandem devices using the PDTIDTBT:Phenyl-C61-butyric acid methyl ester (PC60BM) blend in both series and parallel connections with average PCEs of 7.3 and 7.95%, respectively. We also achieve a good average PCE of 8.26% with an average open circuit voltage (Voc) of 1.79 V for 2-terminal tandem OPVs using this blend. Based on tandem design, an OPV with PCE of 6.45% and AVT of 38% is demonstrated. Moreover, our devices show improved shelf life and ultraviolet (UV) stability (using CdSe/ZnS core shell quantum dots [QDs]) in ambient with 45% relative humidity.

Light Management in Organic Photovoltaics Processed in Ambient Conditions Using ZnO Nanowire and Antireflection Layer with Nanocone Array

Low carrier mobility and lifetime in semiconductor polymers are some of the main challenges facing the field of organic photovoltaics (OPV) in the quest for efficient devices with high current density. Finding novel strategies such as device structure engineering is a key pathway toward addressing this issue. In this work, the light absorption and carrier collection of OPV devices are improved by employment of ZnO nanowire (NW) arrays with an optimum NW length (50 nm) and antireflection (AR) film with nanocone structure. The optical characterization results show that ZnO NW increases the transmittance of the electron transporting layer as well as the absorption of the polymer blend. Moreover, the as-deposited polymer blend on the ZnO NW array shows better charge transfer as compared to the planar sample. By employing PC70BM:PV2000 as a promising air-stable active-layer, power conversion efficiencies of 9.8% and 10.1% are achieved for NW devices without and with an AR film, indicating 22.5% and 26.2% enhancement in PCE as compared to that of planar device. Moreover, it is shown that the AR film enhances the water-repellent ability of the OPV device.

Scalable Synthesis of Naphthothiophene and Benzodithiophene Scaffolds as π-Conjugated Synthons for Organic Materials

The synthesis on a gram-scale and the full characterization of naphtho[1,2-b]thiophene and benzo[1,2-b:6,5-b']dithiophene 4-carboxylate esters bearing 2-octyldodecyl side chains, and their stannylated and brominated derivatives, suitable for their insertion into π-conjugated polymers is described. The fully soluble and processable synthons are obtained through a cascade sequence of reactions, namely direct arylation and cross aldol condensation, which create an effective pathway for the annulation and π-extension of suitable, commercially available reagents. The newly reported synthesis are compared, whenever possible, using ‘green chemistry metrics’ with literature synthesis, showing dramatic improvements.

Atomistic modelling of entropy driven phase transitions between different crystal modifications in polymers: the case of poly(3-alkylthiophenes)

Polymorphism and related solid-state phase transitions affect the structure and morphology and hence the properties of materials, but they are not-so-well understood. Atomistic computational methods can provide molecular-level insights, but they have rarely proven successful for transitions between polymorphic forms of crystalline polymers. In this work, we report atomistic molecular dynamics (MD) simulations of poly(3-alkylthiophenes) (P3ATs), widely used organic semiconductors to explore the experimentally observed, entropy-driven transition from form II to more common form I type polymorphs, or, more precisely, to form I mesophases. The transition is followed continuously, also considering X-ray diffraction evidence, for poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT), evidencing three main steps: (i) loss of side chain interdigitation, (ii) partial disruption of the original stacking order and (iii) reorganization of polymer chains into new, tighter, main-chain stacks and new layers with characteristic form I periodicities, substantially larger than those in the original form II. The described approach, likely applicable to other important transitions in polymers, provides previously inaccessible insight into the structural organization and disorder features of form I structures of P3ATs, not only in their development from form II structures but also from melts or solutions.

Effect of the Electron Transport Layer on the Interfacial Energy Barriers and Lifetime of R2R Printed Organic Solar Cell Modules

Understanding the phenomena at interfaces is crucial for producing efficient and stable flexible organic solar cell modules. Minimized energy barriers enable efficient charge transfer, and good adhesion allows mechanical and environmental stability and thus increased lifetime. We utilize here the inverted organic solar module stack and standard photoactive materials (a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester) to study the interfaces in a pilot scale large-area roll-to-roll (R2R) process. The results show that the adhesion and work function of the zinc oxide nanoparticle based electron transport layer can be controlled in the R2R process, which allows optimization of performance and lifetime. Plasma treatment of zinc oxide (ZnO) nanoparticles and encapsulation-induced oxygen trapping will increase the absolute value of the ZnO work function, resulting in energy barriers and an S-shaped IV curve. However, light soaking will decrease the zinc oxide work function close to the original value and the S-shape can be recovered, leading to power conversion efficiencies above 3%. We present also an electrical simulation, which supports the results. Finally, we study the effect of plasma treatment in more detail and show that we can effectively remove the organic ligands around the ZnO nanoparticles from the printed layer in a R2R process, resulting in increased adhesion. This postprinting plasma treatment increases the lifetime of the R2R printed modules significantly with modules retaining 80% of their efficiency for ∼3000 h in accelerated conditions. Without plasma treatment, this efficiency level is reached in less than 1000 h.

Synthesis of Dithienocyclohexanones (DTCHs) as a Family of Building Blocks for π-Conjugated Compounds in Organic Electronics

The development and widespread application of organic electronic devices require the availability of simple and cost-effective suitable materials. In this study, the preparation of a new class of conjugated compounds on the basis of a dithienocyclohexanone (DTCH) core is reported. Several synthetic strategies for the preparation of dialkyl DTCH derivatives are explored, with special emphasis on the establishment of a sustainable synthetic access. Two successful synthetic pathways, both consisting of five steps, are identified: the first one featuring readily available 3-thiophenecarboxaldeyde and the second one 3-ethynylthiophene as the starting materials. Both procedures are characterized by reasonably high overall yields (over 30%) and remarkably low E factors (<400). Preliminary evidences of the use of such building blocks in the micellar Suzuki-Miyaura cross-coupling reactions leading to promising molecular semiconductors are also given. Moreover, on a small molecule containing DTCH moiety, solar cell performance was investigated.

Direct Arylation Strategies in the Synthesis of π-Extended Monomers for Organic Polymeric Solar Cells

π-conjugated macromolecules for organic polymeric solar cells can be rationally engineered at the molecular level in order to tune the optical, electrochemical and solid-state morphology characteristics, and thus to address requirements for the efficient solid state device implementation. The synthetic accessibility of monomers and polymers required for the device is getting increasing attention. Direct arylation reactions for the production of the π-extended scaffolds are gaining importance, bearing clear advantages over traditional carbon-carbon forming methodologies. Although their use in the final polymerization step is already established, there is a need for improving synthetic accessibility to implement them also in the monomer synthesis. In this review, we discuss recent examples highlighting this useful strategy.

Conjugated Thiophene-Fused Isatin Dyes through Intramolecular Direct Arylation

We report on the design, synthesis, and properties of innovative, planar, π-conjugated compounds in which a thiophene ring is fused with the skeleton of the naturally occurring dye isatin. The synthesis is achieved in high yields making use of an intramolecular direct arylation reaction as the key step, making the overall process potentially scalable. The synthetic sequence has been demonstrated also for an isatin bearing fluorine substituents on the aromatic ring. NMR and X-ray studies demonstrate the crosstalk occurring between the fused, coplanar, and conjugated moieties, making these novel dyes with a donor-acceptor character. Cyclic voltammetry and UV-vis studies confirm very interesting HOMO-LUMO levels and energy gaps for the new compounds.

R2R-printed inverted OPV modules--towards arbitrary patterned designs

We describe the fabrication of roll-to-roll (R2R) printed organic photovoltaic (OPV) modules using gravure printing and rotary screen-printing processes. These two-dimensional printing techniques are differentiating factors from coated OPVs enabling the direct patterning of arbitrarily shaped and sized features into visual shapes and, increasing the freedom to connect the cells in modules. The inverted OPV structures comprise five layers that are either printed or patterned in an R2R printing process. We examined the rheological properties of the inks used and their relationship with the printability, the compatibility between the processed inks, and the morphology of the R2R-printed layers. We also evaluate the dimensional accuracy of the printed pattern, which is an important consideration in designing arbitrarily-shaped OPV structures. The photoactive layer and top electrode exhibited excellent cross-dimensional accuracy corresponding to the designed width. The transparent electron transport layer extended 300 µm beyond the designed values, whereas the hole transport layer shrank 100 µm. We also examined the repeatability of the R2R fabrication process when the active area of the module varied from 32.2 cm(2) to 96.5 cm(2). A thorough layer-by-layer optimization of the R2R printing processes resulted in realization of R2R-printed 96.5 cm(2) sized modules with a maximum power conversion efficiency of 2.1% (mean 1.8%) processed with high functionality.

Reactivity of decafluorobenzophenone and decafluoroazobenzene towards aromatic diamines: a practical entry to donor–acceptor systems

A series of Donor–Accepting–Donor (D–A–D) and Accepting–Donor–Accepting (A–D–A) compounds have been prepared and employed in the synthesis of oligomers potentially useful in optoelectronic applications.

The effect of donor content on the efficiency of P3HT:PCBM bilayers: optical and photocurrent spectral data analyses

A numerical analysis of optical absorption and photocurrent data reveals extensive interdiffusion in P3HT:PCBM bilayer devices.

Neat C₇₀-based bulk-heterojunction polymer solar cells with excellent acceptor dispersion

The replacement of common fullerene derivatives with neat-C70 could be an effective approach to restrain the costs of organic photovoltaics and increase their sustainability. In this study, bulk-heterojunction solar cells made of neat-C70 and low energy-gap conjugated polymers, PTB7 and PCDTBT, are thoroughly investigated and compared. Upon replacing PC70BM with C70, the mobility of positive carriers in the donor phase is roughly reduced by 1 order of magnitude, while that of electrons is only slightly modified. It is shown that the main loss mechanism of the investigated neat-C70 solar cells is a low mobility-lifetime product. Nevertheless, PCDTBT:C70 devices undergo a limited loss of 7.5%, compared to the reference PCDTBT:PC70BM cells, reaching a record efficiency (4.44%) for polymer solar cells with unfunctionalized fullerenes. The moderate efficiency loss of PCDTBT:C70 devices, due to an unexpected excellent miscibility of PCDTBT:C70 blends, demonstrates that efficient solar cells made of neat-fullerene are possible. The efficient dispersion of C70 in the PCDTBT matrix is attributed to an interaction between fullerene and the carbazole unit of the polymer.

Pushing the Envelope of the Intrinsic Limitation of Organic Solar Cells

The photogeneration of Frenkel-type excitons, instead of pairs of free charges, is one of the main drawbacks of organic photovoltaics, when compared with the inorganic counterpart. The strong Coulomb interaction of charge carriers of opposite sign in organic materials is responsible for the complexity of the process of generation of unbound charges, affecting the photogenerated current and still not clearly understood, as well as for the free energy loss of electrons resulting in a diminished open circuit voltage. Despite this practical limitation, record power conversion efficiencies approaching 10% are currently reported for lab-scale single-junction structures made of low-bandgap electron-donating conjugated small molecules or polymers blended with electron-accepting fullerene derivatives. To go beyond, a deep understanding of charge generation dynamics, highly system dependent, is necessary for the definition of the rules for the design of high-performance organic materials for the photovoltaic application and possibly the reduction of exciton binding energy, through the increase of the dielectric constant, which definitively would overcome the practical constraints to high efficiency organic solar cells.