Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing

Thiophene-Based Trimers and Their Bioapplications: An Overview

Certainly, the success of polythiophenes is due in the first place to their outstanding electronic properties and superior processability. Nevertheless, there are additional reasons that contribute to arouse the scientific interest around these materials. Among these, the large variety of chemical modifications that is possible to perform on the thiophene ring is a precious aspect. In particular, a turning point was marked by the diffusion of synthetic strategies for the preparation of terthiophenes: the vast richness of approaches today available for the easy customization of these structures allows the finetuning of their chemical, physical, and optical properties. Therefore, terthiophene derivatives have become an extremely versatile class of compounds both for direct application or for the preparation of electronic functional polymers. Moreover, their biocompatibility and ease of functionalization make them appealing for biology and medical research, as it testifies to the blossoming of studies in these fields in which they are involved. It is thus with the willingness to guide the reader through all the possibilities offered by these structures that this review elucidates the synthetic methods and describes the full chemical variety of terthiophenes and their derivatives. In the final part, an in-depth presentation of their numerous bioapplications intends to provide a complete picture of the state of the art.

All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies

The high efficiency all-small-molecule organic solar cells (OSCs) normally require optimized morphology in their bulk heterojunction active layers. Herein, a small-molecule donor is designed and synthesized, and single-crystal structural analyses reveal its explicit molecular planarity and compact intermolecular packing. A promising narrow bandgap small-molecule with absorption edge of more than 930 nm along with our home-designed small molecule is selected as electron acceptors. To the best of our knowledge, the binary all-small-molecule OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-molecule photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.

High-yielding Pd2(dba)3·C6H6-based four-fold Sonogashira coupling with selenophene-conjugated magnesium tetraethynylporphyrin for organic solar cells

A catalytic system using Pd₂(dba)₃·(C₆H₆)/PPh₃/CuI for Sonogashira coupling was demonstrated to synthesize a selenophene-conjugated magnesium tetraethynylporphyrin Mg-TEP-(Se-DPP)₄ (2a). The catalytic system enabled four-fold cross-coupling of the four terminal alkynes of magnesium tetraethynylporphyrin with bromoselenophene-tethered diketopyrrolopyrroles (DPPs) to produce the desired star-shaped 2a in 80% yield. This molecule shows higher solubility in organic solvents, more efficient visible and near-infrared region absorption, and a narrower band gap compared with reference thiophene-conjugated congeners. Two strategies, namely, selenium substitution and end-capping, were investigated to optimize bulk heterojunction structures in the active layers of organic solar cells. The optimized device based on 2a:PC₆₁BM exhibited the highest PCE of 6.09% among the tested devices after solvent vapor annealing, owing to efficient exciton dissociation, balanced carrier mobility, and suppressed carrier recombination in the film's ordered morphology.

A catalytic system using Pd₂(dba)₃·(C₆H₆)/PPh₃/CuI for Sonogashira coupling was demonstrated to synthesize a selenophene-conjugated magnesium tetraethynylporphyrin Mg-TEP-(Se-DPP)₄ (2a).

Effective Molecular Engineering Approach for Employing a Halogen-Free Solvent for the Fabrication of Solution-Processed Small-Molecule Solar Cells

To utilize the potential of small-molecule-based organic solar cells, proper designs of the photoactive materials which result in reasonable performance in a halogen-free solvent system and thickness tolerance over a range are required. One of the best approaches to achieve these requirements is via the molecular engineering of small-molecule electron donors. Here, we have modified a previously reported dithienobenzodithiophene (DTBDT)-based small molecule (SM1) via the dimerization approach, that is, the insertion of an additional DTBDT into the main backbone of the small molecule (SM2). An SM1-based photoactive film showed severe pinhole formation throughout the film when processed with a halogen-free o-xylene solvent. On the other hand, the modified small-molecule SM2 formed an excellent pinhole-free film when processed with the o-xylene solvent. Because of the dimerization of the DTBDT in the SM2 core, highly crystalline films with compact lamellae and an enhanced donor/acceptor interdigitation were formed, and all of these factors led to a high efficiency of 8.64% with chloroform and 8.37% with the o-xylene solvent systems. To the best of our knowledge, this study represents one of the best results with the SM donor and fullerene derivative acceptor materials that have shown the device performance with halogen-free solvents.

Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor

To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two dithieno[2,3- d:2',3'- d']-benzo[1,2- b:4,5- b']dithiophene (DTBDT)-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) fullerene acceptor and the O-IDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor-acceptor interface, whereas blends with PC₇₁BM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased J_sc and fill factor values and unusual decrease in V_oc values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared with that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.

Three-Dimensional Observation of a Light-Soaked Photoreactant Layer in BTR:PCBM Solar Cells Treated with/without Solvent Vapor Annealing

A key challenge to the commercialization of solution-processed solar cells is a proper understanding of the morphological variations during long periods, particularly under light-soaking conditions. Many research groups have competitively reported solvent vapor annealing (SVA)-treated small-molecule devices with efficiency rates exceeding 11%; however, their light-soaking effects have been rarely studied. Here, we investigate the morphological changes in the light-soaked devices with/without SVA treatments depending on the illumination time via three-dimensional observations. From the results, we found that the trends of morphological variations differ in the surface and bulk parts of the active film and that the difference is closely related to the device performance capabilities. Therefore, our research will enhance the underlying knowledge of the light-soaking effect on active morphologies over long term.

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).

Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells

A porphyrin-based molecule DPPEZnP-BzTBO with bulky benzothiophene groups was designed and synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. The optimized devices under thermal annealing (TA) and then chloroform solvent vapor anneanling (SVA) for 80 s exhibited an outstanding power conversion efficiencie (PCE) of 9.08%. Contrasted with the smaller thienyl substituted analogues we reported previously, DPPEZnP-BzTBO-based BHJ solar cells exhibited a higher open circuit voltage due to the lower highest occupied molecular orbital energy level. The TA post-treatment of the active layers induced the formation of more crystallized components, and the subsequent SVA provided a driving force for the domain growth, resulting in more obvious phase segregation between the donor and the acceptor in nanoscale. Furthermore, the PCEs kept above 95% upon the further SVA treatment within the time range of 60 to 95 s probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication.