Strong Acceptors Based on Derivatives of Benzothiadiazoloimidazole

Despite the rapid progression of organic semiconductors, developing high-air-stability n-type organic semiconductors are still challenging. Herein, novel strong acceptors based on benzothiadiazoloimidazole units are reported. The results reveal that the strong acceptor BTI-NDI-BTI-a has good solubility and high electron affinity (3.94 eV), accompanied by 1D slipped-stacking crystals. Notably, the material presents promising potential for developing into air-stable n-type organic semiconductor materials.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Photoprecursor Approach Enables Preparation of Well-Performing Bulk-Heterojunction Layers Comprising a Highly Aggregating Molecular Semiconductor

Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the "photoprecursor approach" can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.

Rational tuning of high-energy visible light absorption for panchromatic small molecules by a two-dimensional conjugation approach

We have demonstrated a rational two-dimensional (2D) conjugation approach towards achieving panchromatic absorption of small molecules. By extending the conjugation on two orthogonal axes of an electron acceptor, namely, bay-annulated indigo (BAI), the optical absorptions could be tuned independently in both high- and low-energy regions. The unconventional modulation of the high-energy absorption is rationalized by density functional theory (DFT) calculations. Such a 2D tuning strategy provides novel guidelines for the design of molecular materials with tailored optoelectronic properties.

Photovoltaic properties of 3,3′-(ethane-1,2-diylidene)-bis(indolin-2-one) based conjugated polymers

3,3′-(Ethane-1,2-diylidene)-bis(indolin-2-one) (EBI) is a π-conjugated structure in which two oxindoles are connected by a 1,3-butadiene. Two new polymers based on EBI have been synthesized and their photovoltaic properties have been studied.

Random terpolymer with a cost-effective monomer and comparable efficiency to PTB7-Th for bulk-heterojunction polymer solar cells

A new random terpolymer PTB7-Th-T2 was designed and synthesized for high performance PSCs by incorporating a significantly lower-cost 2,2′-bithiophene monomer to the famous PTB7-Th skeleton.

Highly transmissive blue electrochromic polymers based on thieno[3,2-b]thiophene

Polymers based on a thieno[3,2-b]thiophene core modulated by alkylphenyl groups show high optical contrast and deep blue to highly transmissive electrochromic performances.

New PDI-based small-molecule cathode interlayer material with strong electron extracting ability for polymer solar cells

A solution-processed and thickness-insensitive small-molecule perylene diimide derivative, namely PDI-N3I, was synthesized and successfully applied to conventional PSCs.

Synthesis and field-effect transistor properties of a diseleno[3,2-b:2′,3′-d]silole-based donor–acceptor copolymer: investigation of chalcogen effect

We have designed and synthesized a tricyclic diseleno[3,2-b:2′,3′-d]silole (DSS) wherein the 3,3′-position of a biselenophene is bridged by a dioctylsilyl moiety.

Organic memory effect from donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole

A novel D–A polymer is designed for resistance memory devices with a large off ratio, good endurance, and long retention time.