Strong Non-Reciprocal Chiroptical Properties in Thin Films of Chiral Alkylthio-Decorated 1,4-Phenylene/Thiophene Dyes

In the context of chiral π-conjugated materials, the use of enantiopure alkylthio appendages represents a valid alternative to conventional alkoxy groups: sulphur atom is bigger and more electron-rich than oxygen, thus allowing for higher polarizability, greater flexibility, larger bulkiness and lower structural anisotropy. In light of these considerations, here we report two new chiral alkylthio-decorated 1,4-phenylene/thiophene dyes, obtained by simple synthetic strategies involving Pd-catalyzed cross-coupling protocols, looking for strong non-reciprocal chiroptical features in thin films. In particular, for the chiral alkylthio-decorated 1,4-phenylene-bis(thiophenylpropynone) (Thio-PTPO) dye, which proved to be the most promising for our purpose, a detailed investigation in thin films was carried out, involving optical and chiroptical spectroscopies in absorption and emission, as well as optical microscopy techniques.

Comparing Adsorption of an Electron-Rich Triphenylene Derivative: Metallic vs Graphitic Surfaces

Crucial to the performance of devices based on organic molecules is an understanding of how the substrate-molecule interface influences both structural and electronic properties of the molecular layers. Within this context we studied the self-assembly of an alkoxy-triphenylene derived electron donor (HAT) in the monolayer regime on graphene/Ni(111). The molecules assembled into a close-packed hexagonal network commensurate with the graphene layer. Despite the commensurate structure, the HAT molecules only had a weak, physisorptive interaction with the substrate as pointed out by the photoelectron spectroscopy data. We discuss these findings in view of our recent reports for HAT adsorbed on Ag(111) and graphene/Ir(111). For all three substrates HAT adopts a similar close-packed hexagonal structure commensurate with the substrate while being physisorbed. The ionization potential is equal for all three substrates, supporting weak molecule-substrate interactions. These findings are remarkable, as commensurate overlayers usually only form at strongly interacting interfaces. We discuss potential reasons for this particular behavior of HAT which clearly sets itself apart from most studied molecule-substrate systems. In particular, these are the relatively weak but flexible intermolecular interactions, the molecular symmetry matching that of the substrate, and the comparatively weak but directional molecule-substrate interactions.

Crystallization of D-A Conjugated Polymers: A Review of Recent Research

D-A conjugated polymers are key materials for organic solar cells and organic thin-film transistors, and their film structure is one of the most important factors in determining device performance. The formation of film structure largely depends on the crystallization process, but the crystallization of D-A conjugated polymers is not well understood. In this review, we attempted to achieve a clearer understanding of the crystallization of D-A conjugated polymers. We first summarized the features of D-A conjugated polymers, which can affect their crystallization process. Then, the crystallization process of D-A conjugated polymers was discussed, including the possible chain conformations in the solution as well as the nucleation and growth processes. After that, the crystal structure of D-A conjugated polymers, including the molecular orientation and polymorphism, was reviewed. We proposed that the nucleation process and the orientation of the nuclei on the substrate are critical for the crystal structure. Finally, we summarized the possible crystal morphologies of D-A conjugated polymers and explained their formation process in terms of nucleation and growth processes. This review provides fundamental knowledge on how to manipulate the crystallization process of D-A conjugated polymers to regulate their film structure.

Enhanced photovoltaic performance of donor polymers effected by asymmetric π-bridges

Asymmetric π-bridge-based donor polymers produced via a simple one-pot chemical synthesis method exhibit enhanced photovoltaic performance.

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.

Synthesis of a soluble adenine-functionalized polythiophene through direct arylation polymerization and its fluorescence responsive behavior

An adenine-functionalized polythiophene is synthesized via direct arylation polymerization using Boc-protection to overcome catalyst deactivation. The resulting copolymer is highly soluble and shows reversible fluorescence quenching.

Random Polymer Donor for High-Performance Polymer Solar Cells with Efficiency over 14

Constructing random copolymers has been regarded as an easy and effective approach to design polymer donors for state-of-the-art polymer solar cells (PSCs). In this work, we develop a naphtho[2,3-c]thiophene-4,9-dione-based copolymer PBN-Cl as a donor material for PSCs, and a moderate power conversion efficiency (PCE) of 11.21% is achieved with a relatively low fill factor (FF) of 0.615. We then incorporate a similar acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) into the polymeric backbone of PBN-Cl to tune its photovoltaic performance, and a significantly higher PCE of 14.05% is achieved from the random polymer PBN-Cl-B80 containing 80% BDD unit. The enhanced PCE of the PBN-Cl-B80-based device mainly relies on the higher FF value, resulting from the improved charge mobility properties, reduced bimolecular and trap-assisted recombination, and more appropriate phase separation. The results demonstrate a feasible strategy to tune the photovoltaic performance of polymer donors by constructing a random polymer with a compatible component.

Side chain engineering in DTBDT-based small molecules for efficient organic photovoltaics

A new small-molecule donor with a dithieno[2,3-d:2',3'-d']-benzo[1,2-b:4,5-b']-dithiophene (DTBDT) core and both alkyl and alkylthio substituents is designed and synthesized to improve the miscibility between DTBDT-based small molecules and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). The alkyl substituent on the 4-position and the alkylthio substituent on the 5-position of the substituted thiophene are expected to improve intermolecular interactions and prevent severe aggregation of the small molecules. The new small molecule, DTBDT-S-C8-TTR, exhibits a homogenous blend morphology with small domains and edge-on-oriented crystalline structures in blends with PC₇₁BM, and give a maximum power conversion efficiency (PCE) of 8.43%. To recover the crystallinity of the DTBDT-S-C8-TTR small molecules weakened after being blended with PC₇₁BM, a solvent vapor annealing (SVA) treatment is performed. The SVA-treated blend films reveal well-developed crystalline domains with interconnected fibrillar structures. This blend morphology allows efficient charge carrier transport in blends and leads to increased PCEs. The maximum PCE of 9.18% achieved using DTBDT-S-C8-TTR suggests that substituting both alkylthio and alkyl groups into DTBDT can yield small-molecule-based organic photovoltaics (OPVs) displaying improved photovoltaic performances.

Thioalkyl- and sulfone-substituted poly(p-phenylene vinylene)s

Poly(p-phenylene vinylene)s (PPVs) have been studied for decades, but new applications like in bioimaging keep emerging and even simple structural variations are still waiting to be explored, as we highlight by this work.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by Environmentally Friendly Solvent Processing

Nowadays, most of the solution-processed high-efficiency polymer solar cell (PSC) devices are fabricated by halogenated solvents (such as chlorobenzene, 1,2-dichlorobenzene, chloroform, etc.) which are harmful to people and the environment. Therefore, it is essential to develop high-efficiency PSC devices processed by environmentally friendly solvent processing for their industrialization. In this regard, we report a new alkylthio chain-based conjugated polymer PBDB-TS as donor material for environmentally friendly solvent-processed PSCs. PBDB-TS possesses a low-lying HOMO energy level at -5.42 eV and a good solubility in toluene and o-xylene. By using o-xylene and 1% N-methylpyrrolidone as processing solvent, following by the thermal annealing treatment for PBDB-TS:ITIC blend films, well-developed morphological features, and balanced charge transport properties are observed, leading to a high power conversion efficiency (PCE) of 9.85%, higher than that of the device cast from halogenated solvent (PCE = 9.65%). The results suggest that PBDB-TS is an attractive donor material for nonhalogen solvents-processing PSCs.

Thiophene-Based Organic Semiconductors

Thiophene-based π-conjugated organic small molecules and polymers are the research subject of significant current interest owing to their potential use as organic semiconductors in material chemistry. Despite simple and similar molecular structures, the hitherto reported properties of thiophene-based organic semiconductors are rather diverse. Design of high performance organic semiconducting materials requires a thorough understanding of inter- and intra-molecular interactions, solid-state packing, and the influence of both factors on the charge carrier transport. In this chapter, thiophene-based organic semiconductors, which are classified in terms of their chemical structures and their structure-property relationships, are addressed for the potential applications as organic photovoltaics (OPVs), organic field-effect transistors (OFETs) and organic light emitting diodes (OLEDs).

Boosting Up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3-d:2',3'-d']benzo[1,2-b:4',5'-b']di thiophene-Based Copolymers by Advantageous Vertical Phase Separation

The photovoltaic cells (PVCs) from conjugated copolymers of PDTBDT-BT and PDTBDT-FBT with 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene as electron donor moieties and benzothiadiazole and/or 5,6-difluorobenzothiadiazole as electron acceptor moieties are optimized by employing alcohol-soluble PFN (poly(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) as cathode modification interlayer. The power conversion efficiencies (PCEs) of inverted PVCs (i-PVCs) from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO₃/Ag are increased from 4.97% to 8.54% and 5.92% to 8.74%, in contrast to those for the regular PVCs (r-PVCs) with devices configuration as ITO/PEDOT:PSS/active layer/Ca/Al under 100 mW/cm² AM 1.5 illumination. The optical modeling calculations and X-ray photoelectron spectroscopy (XPS) investigations reveal that the r-PVCs and i-PVCs from the copolymers exhibit similar light harvesting characteristics, and the enhancements of the PCEs of the i-PVCs from the copolymers are mainly contributed to the favorable vertical phase separation as the strongly polymer-enriched top surface layers and slightly PC₇₁BM (phenyl-C₇₁-butyric acid methyl ester)-enriched bottom surface layers are correspondingly connected to the anodes and cathodes of the i-PVCs, while they are opposite in the r-PVCs. As we known, it is the first time to experimentally verify that the i-PVCs with alcohol-soluble conjugated polymers cathode modification layers enjoy favorable vertical phase separation.

A D–A copolymer donor containing an alkylthio-substituted thieno[3,2-b]thiophene unit

A D–A copolymer (PSTTF2T) based on alkylthio-substituted thieno[3,2-b]thiophene was prepared. PSTTF2T is compatible with fullerene and non-fullerene acceptors in solar cells.

Manipulating the photovoltaic properties of small-molecule donor materials by tailoring end-capped alkylthio substitution

The capability of alkylthio chain as end groups in tuning the photovoltaic properties of small-molecule donor materials is investigated.