The influence of poly(3-hexylthiophene) regioregularity on fullerene-composite solar cell performance

On the Use of Reflection Polarized Optical Microscopy for Rapid Comparison of Crystallinity and Phase Segregation of P3HT:PCBM Thin Films

Rapid, nondestructive characterization techniques for evaluating the degree of crystallinity and phase segregation of organic semiconductor blend thin films are highly desired for in-line, automated optoelectronic device fabrication facilities. Here, it is demonstrated that reflection polarized optical microscopy (POM), a simple technique capable of imaging local anisotropy of materials, is capable of determining the relative degree of crystallinity and phase segregation of thin films of polymer:fullerene blends. While previous works on POM of organic semiconductors have largely employed the transmission geometry, it is demonstrated that reflection POM provides 3× greater contrast. The optimal configuration is described to maximize contrast from POM images of polymer:fullerene films, which requires Köhler illumination and slightly uncrossed polarizers, with an uncrossing angle of ±3°. It is quantitatively demonstrated that contrast in POM images directly correlates with 1) the degree of polymer crystallinity and 2) the degree of phase segregation between polymer and fullerene domains. The origin of the bright and dark domains in POM is identified as arising from symmetry-broken liquid crystalline phases (i.e., dark conglomerates), and it is proven that they have no correlation with surface topography. The use of reflection POM as a rapid diagnostic tool for automated device fabrication facilities is discussed.

Room Temperature Synthesis of a Well-Defined Conjugated Polymer Using Direct Arylation Polymerization (DArP)

While a major improvement to the sustainability of conjugated polymer synthesis, traditional direct arylation polymerization (DArP) still requires high temperatures (typically >100 °C), necessitating a significant energy input requirement. Performing DArP at reduced or ambient temperatures would represent an improvement to the sustainability of the reaction. Here we describe the first report of a well-defined conjugated polymer synthesized by DArP at room temperature. Previous efforts toward room temperature DArP relied on the use of a near-stoichiometric silver reagent, an expensive coinage metal, which makes the reaction less cost-effective and sustainable. Here, room temperature polymerizations of 3,4-ethylenedioxythiophene (EDOT) and 9,9-dioctyl-2,7-diiodofluorene were optimized and provided molar mass (Mn) up to 11 kg/mol PEDOTF, and performing the reaction at the standard ambient temperature of 25 °C provided Mn up to 15 kg/mol. Model studies using other C-H monomers of varying electron density copolymerized with 9,9-dioctyl-2,7-diiodofluorene provided insight into the scope of the room temperature polymerization, suggesting that performing room temperature DArP is highly dependent on the electron richness of the C-H monomer.

Post-polymerisation approaches for the rapid modification of conjugated polymer properties

Post-polymerisation functionalisation provides a facile and efficient way for the introduction of functional groups on the backbone of conjugated polymers. Using post-polymerisation functionalisation approaches, the polymer chain length is usually not affected, meaning that the resulting polymers only differ in their attached functional groups or side chains, which makes them particularly interesting for investigating the influence of the different groups on the polymer properties. For such functionalisations, highly efficient and selective reactions are needed to avoid the formation of complex mixtures or permanent defects in the polymer backbone. A variety of suitable synthetic approaches and reactions that fulfil these criteria have been identified and reported. In this review, a thorough overview is given of the post-polymerisation functionalisations reported to date, with the methods grouped based on the type of reaction used: cycloaddition, oxidation/reduction, nucleophilic aromatic substitution, or halogenation and subsequent cross-coupling reaction. Instead of modifications on the aliphatic side chains of the conjugated polymers, we focus on modifications directly on the conjugated backbones, as these have the most pronounced effect on the optical and electronic properties. Some of the discussed materials have been used in applications, ranging from solar cells to bioelectronics. By providing an overview of this versatile and expanding field for the first time, we showcase post-polymerisation functionalisation as an exciting pathway for the creation of new conjugated materials for a range of applications.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Electrochemical reduction of NO catalyzed by boron-doped C60 fullerene: a first-principles study

The electrochemical reduction of nitrogen monoxide (NO) is one of the most promising approaches for converting this harmful gas into useful chemicals. Using density functional theory calculations, the work examines the potential of a single B atom doped C60 fullerene (C59B) for catalytic reduction of NO molecules. The results demonstrate that the NO may be strongly activated over the B atom of C59B, and that the subsequent reduction process can result in the formation of NH3 and N2O molecules at low and high coverages, respectively. Based on the Gibbs free energy diagram, it is inferred that the C59B has excellent catalytic activity for NO reduction at ambient conditions with no potential-limiting. At normal temperature, the efficient interaction between the *NOH and NO species might lead to the spontaneous formation of the N2O molecule. Thus, the findings of this study provide new insights into NO electrochemical reduction on heteroatom doped fullerenes, as well as a unique strategy for fabricating low-cost NO reduction electrocatalysts with high efficiency.

Tuning fullerene miscibility with porphyrin-terminated P3HTs in bulk heterojunction blends

Understanding and manipulating the miscibility of donor and acceptor components in the active layer morphology is important to optimize the longevity of organic photovoltaic devices and control power conversion efficiency. In pursuit of this goal, a "porphyrin-capped" poly(3-hexylthiophene) was synthesized to take advantage of strong porphyrin:fullerene intermolecular interactions that modify fullerene miscibility in the active layer. End-functionalized poly(3-hexylthiophene) was synthesized via catalyst transfer polymerization and subsequently functionalized with a porphyrin moiety via post-polymerization modification. UV-vis spectroscopy and X-ray diffraction measurements show that the porphyrin-functionalized poly(3-hexylthiophene) exhibits increased intermolecular interactions with phenyl-C61-butyric acid methyl ester (PCBM) in the solid state compared to unfunctionalized poly(3-hexylthiophene) without sacrificing microstructure ordering that facilitates optimal charge transport properties. Additionally, differential scanning calorimetry revealed porphyrin-functionalized poly(3-hexylthiophene) crystallization decreased only slightly (1-6%) compared to unfunctionalized poly(3-hexylthiophenes) while increasing fullerene miscibility by 55%. Preliminary organic photovoltaic device results indicate device power conversion efficiency is sensitive to additive loading levels, as evident by a slight increase in power conversion efficiency at low additive loading levels but a continuous decrease with increased loading levels. While the increased fullerene miscibility is not balanced with significant increases in power conversion efficiency, this approach suggests that integrating non-bonded interaction potentials is a useful pathway for manipulating the morphology of the bulk heterojunction thin film, and porphyrin-functionalized poly(3-hexylthiophenes) may be useful additives in that regard.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Donor-acceptor random regioregular π-conjugated copolymers based on poly(3-hexylthiophene) with unsymmetrical monothienoisoindigo units

Donor-acceptor π-conjugated random copolymers based on regioregular poly(3-hexylthiophene), rr-P3HT, with unsymmetrical monothienoisoindigo moieties were obtained by direct arylation polycondensation of 2-bromo-3-hexylthiophene with unsymmetrical monothienoisoindigo motifs under the optimized conditions [palladium-immobilized on thiol-modified silica gel with chloride counter anions, PITS-Cl (2.5 mol%), PivOH (1.0 equiv.), K2CO3 (3.0 equiv.), DMAc, 100 °C, 24 h]. Incorporation of unsymmetrical monothienoisoindigo electron-acceptor units into the polymers tuned their highest occupied and lowest unoccupied molecular orbital levels, which were close to those of the hole transport material (PEDOT) and electron transport material (PCBM), respectively, in thin-film organic solar cells. Alkyl chains of the unsymmetrical monothienoisoindigo units in the polymers tuned their macrostructural order, resulting in the observation of crystalline patterns and specific absorption peaks in thin films. An organic solar cell containing the most crystalline random copolymer showed an efficiency of 1.91%.

Dehalogenative or Deprotonative? The Preparation Pathway to the Organometallic Monomer for Transition-Metal-Catalyzed Catalyst-Transfer-Type Polymerization of Thiophene Derivatives

Due to a wide range of applications in electronic materials, polythiophenes attract considerable attention in organic and polymer syntheses as well as in materials science. For the purpose of developing the practical synthetic protocol, this review focuses on the deprotonative pathway in the preparation of thiophene organometallic monomer, which was shown to be effective employing 2-halo-3-substituted thiophene as a monomer precursor. The thus metallated thiophene monomer was shown to undergo polymerization by nickel(II) complex catalysis, with which highly regioregular head-to-tail (HT)-type polythiophenes were obtained with controlled molecular weight and molecular weight distribution. Several polythiophene derivatives with modified thiophene-ring and side-chain structures were shown to be designed in order to achieve the designed functionality as materials.

Role of Molecular and Interchain Ordering in the Formation of a δ-Hole-Transporting Layer in Organic Solar Cells

Interface engineering, especially the realization of Ohmic contacts at the interface between organic semiconductors and metal contacts, is one of the essential preconditions to achieve high-efficiency organic electronic devices. Here, the interface structures of polymer/fullerene blends are correlated with the charge extraction/injection properties of working organic solar cells. The model system-poly(3-hexylthiophene) (P3HT):phenyl-C₆₁-butyric acid methyl ester (PCBM)-is fabricated using two different degrees of P3HT regioregularity to alter the blend interchain order and molecular packing, resulting in different device performances. Investigations by electroabsorption spectroscopy on these devices indicate a significant reduction (≈1 V) in the built-in potential with an increase in the P3HT regioregularity. This observation is also supported by a change in the work function (WF) of high regioregular polymer blends from photoelectron spectroscopy measurements. These results confirm the presence of a strong dipole layer acting as a δ-hole-transporting layer at the polymer/MoO₃/Ag electrode interface. Unipolar hole-only devices show an increase in the magnitude of the hole current in high regioregular P3HT devices, suggesting an increase in the hole injection/extraction efficiency inside the device with a δ-hole-transporting layer. Microscopically, near-edge X-ray absorption fine structure spectroscopy was conducted to probe the surface microstructure in these blends, finding a highly edge-on orientation of P3HT chains in blends made with high regioregular P3HT. This edge-on orientation of P3HT chains at the interface results in a layer of oriented alkyl side chains capping the surface, which favors the formation of a dipole layer at the polymer/MoO₃ interface. The increase in the charge extraction efficiency due to the formation of a δ-hole-transporting layer thus results in higher short circuit currents and fill factor values, eventually increasing the device efficiency in high regioregular P3HT devices despite a slight decrease in cell open circuit voltage. These findings emphasize the significance of WF control as a tool for improved device performance and pave the way toward interfacial optimization based on the modulation of fundamental polymer properties, such as polymer regioregularity.

Manipulation of PBDT-DTNT:PCBM photoactive layers for a stability increment by core–shell and core–mantle–shell supramolecules

Two types of core–shell and core–mantle–shell supramolecules were designed based on grafted-CNTs and PBDT-DTNT chains and employed in the active layers of PBDT-DTNT:PC₇₁BM solar cells to stabilize morphology and performance.

Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers are important conjugated polymers due to their exceptional optical and conductive properties, over the past few decades many researchers have designed novel strategies to reach more efficient materials for electronic applications.

Self-ordering promoted by the nanoconfinement of poly(3-hexylthiophene) and its nanocomposite with single-walled carbon nanotubes

Nanostructuration and self-ordering of semiconducting organic materials are required to fabricate highly efficient photovoltaic and photoemissive devices. In this work, we investigated the combined effect of melt-assisted template processing and self-ordering of high purity regio-regular poly (3-hexylthiophene) (P3HT) to obtain nanofibers of P3HT and of P3HT-single-walled carbon nanotubes (SWNT) nanocomposites. An original ordering of the polymer and the carbon nanotubes within the nanofibers, as well as their surprising anisotropic photoluminescent properties were determined by vibrational and optical spectroscopy. It was attributed to the combined effect of the melt-assisted wetting confined within alumina nanopores, altogether with the self-organization of both P3HT chains on the one hand, and of the P3HT charged with SWNT on the other hand. It is proposed that the well-ordered regio-regular P3HT matrix orientation is promoted by the interaction with the alumina pore surface and the 1D confinement. For the composite case, the P3HT matrix imposes additionally a preferential orientation of the SWNT transversal to the nanofiber axis. This original organization is responsible for the unexpected polarization of the composite nanofibers photoluminescence. This work opens the way to alternative methods for tackling challenges of nanofabrication to obtain more efficient optoelectronic nanodevices.

Synthesis and crystallization behavior of regioregular-block-regiorandom poly(3-hexylthiophene) copolymers

Regioregular–regiorandom poly(3-hexylthiophene) copolymers, synthesized by chain-transfer polycondensation, show strong crystallinity due to their one-sided distribution of regiodefects.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

ipso-Arylative polymerization as a route to π-conjugated polymers: synthesis of poly(3-hexylthiophene)

Palladium-catalyzed ipso-arylative polymerization of diphenylcarbinol-substituted thiophene derivatives proceeds to reasonably high molecular weights, but is limited by side reactions.

Stereoregular Brush Polymers and Graft Copolymers by Chiral Zirconocene-Mediated Coordination Polymerization of P3HT Macromers

Two poly(3-hexylthiophene) (P3HT) macromers containing a donor polymer with a polymerizable methacrylate (MA) end group, P3HT-CH₂-MA and P3HT-(CH₂)₂-MA, have been synthesized, and P3HT-(CH₂)₂-MA has been successfully homopolymerized and copolymerized with methyl methacrylate (MMA) into stereoregular brush polymers and graft copolymers, respectively, using chiral ansa-zirconocene catalysts. Macromer P3HT-CH₂-MA is too sterically hindered to polymerize by the current Zr catalysts, but macromer P3HT-(CH₂)₂-MA is readily polymerizable via either homopolymerization or copolymerization with MMA in a stereospecific fashion with both C₂-ligated zirconocenium catalyst 1 and Cs-ligated zirconocenium catalyst 2. Thus, highly isotactic (with mm% ≥ 92%) and syndiotactic (with rr% ≥ 93%) brush polymers, it-PMA-g-P3HT and st-PMA-g-P3HT, as well as well-defined stereoregular graft copolymers with different grafted P3HT densities, it-P(M)MA-g-P3HT and st-P(M)MA-g-P3HT, have been synthesized using this controlled coordination-addition polymerization system under ambient conditions. These stereoregular brush polymers and graft copolymers exhibit both thermal (glass and melting) transitions with Tg and Tm values corresponding to transitions within the stereoregular P(M)MA and crystalline P3HT domains. Acceptor molecules such as C60 can be effectively encapsulated inside the helical cavity of st-P(M)MA-g-P3HT to form a unique supramolecular helical crystalline complex, thus offering a novel strategy to control the donor/acceptor solar cell domain morphology.

Regioregular narrow-bandgap-conjugated polymers for plastic electronics

Progress in the molecular design and processing protocols of semiconducting polymers has opened significant opportunities for the fabrication of low-cost plastic electronic devices. Recent studies indicate that field-effect transistors and organic solar cells fabricated using narrow-bandgap regioregular polymers with translational symmetries in the direction of the backbone vector often outperform those containing analogous regiorandom polymers. This review addresses the cutting edge of regioregularity chemistry, in particular how to control the spatial distribution in the molecular structures and how this order translates to more ordered bulk morphologies. The effect of regioregularity on charge transport and photovoltaic properties is also outlined.

The continued search of high-performance semiconductors for organic electronics calls for smart molecular designs. Here, Ying et al. review the recent development of synthesizing regioregular narrow bandgap conjugated polymers and the effect of regioregularity on transistor and photovoltaic performance.

Synthesis of regioblock copolythiophene by Negishi catalyst-transfer polycondensation using tBu2Zn·2LiCl

Regioblock copolythiohenes consisting of head-to-tail poly(3-hexylthiophene) (HT-P3HT) and head-to-head/tail-to-tail P3HT (HHTT-P3HT) segments could be synthesized by Negishi catalyst-transfer polycondensation (NCTP) using ^tBu₂Zn·2LiCl.

Facile one-pot access to π-conjugated polymers via sequential bromination/direct arylation polycondensation

The synthesis of π-conjugated polymers starting from unfunctionalized aromatic monomers via sequential bromination/direct arylation polycondensation was investigated.