Bithiazole: An Intriguing Electron-Deficient Building for Plastic Electronic Applications

The heterocyclic thiazole unit has been extensively used as electron-deficient building block in π-conjugated materials over the last decade. Its incorporation into organic semiconducting materials is particularly interesting due to its structural resemblance to the more commonly used thiophene building block, thus allowing the optoelectronic properties of a material to be tuned without significantly perturbing its molecular structure. Here, we discuss the structural differences between thiazole- and thiophene-based organic semiconductors, and the effects on the physical properties of the materials. An overview of thiazole-based polymers is provided, which have emerged over the past decade for organic electronic applications and it is discussed how the incorporation of thiazole has affected the device performance of organic solar cells and organic field-effect transistors. Finally, in conclusion, an outlook is presented on how thiazole-based polymers can be incorporated into all-electron deficient polymers in order to obtain high-performance acceptor polymers for use in bulk-heterojunction solar cells and as organic field-effect transistors. Computational methods are used to discuss some newly designed acceptor building blocks that have the potential to be polymerized with a fused bithiazole moiety, hence propelling the advancement of air-stable n-type organic semiconductors.

Photovoltaic Small Molecules of TPA(FxBT-T-Cz)3: Tuning Open-Circuit Voltage over 1.0 V for Their Organic Solar Cells by Increasing Fluorine Substitution

To simultaneously improve both open-circuit voltage (V_oc) and short-circuit current density (J_sc) for organic solar cells, a novel D(A-π-Ar)₃ type of photovoltaic small molecules of TPA(F_xBT-T-3Cz)₃ was designed and synthesized, which contain central triphenylamine (TPA), terminal carbazole (Cz), armed fluorine-substituted benzothiadiazole (F_xBT, where x = 1 or 2), and bridged thiophene (T) units. A narrowed ultraviolet-visible absorption and a decreasing highest occupied molecular orbital energy level were observed from TPA(F₁BT-T-3Cz)₃ to TPA(F₂BT-T-3Cz)₃ with increasing fluorine substitution. However, the TPA(F₂BT-T-3Cz)₃/PC₇₁BM-based solar devices showed a rising V_oc of 1.01 V and an enhanced J_sc of 10.84 mA cm^-2 as well as a comparable power conversion efficiency of 4.81% in comparison to the TPA(F₁BT-T-3Cz)₃/PC₇₁BM-based devices. Furthermore, in comparison to the parent TPA(BT-T-3Cz)₃ molecule without fluorine substitution, the fluorine-substituted TPA(F_xBT-T-3Cz)₃ molecules exhibited significantly incremental V_oc and J_sc values in their bulk heterojunction organic solar cells, owing to fluorine incorporation in the electron-deficient benzothiadiazole unit.

Influence of the fused hetero-aromatic centers on molecular conformation and photovoltaic performance of solution-processed organic solar cells

Tuning the thiophene-fused building centers is a facile way to alter the molecular conformation and enhance photovoltaic performance.

Thiophene-based push–pull chromophores for small molecule organic solar cells (SMOSCs)

A concise review on small molecules organic solar cells based on π-conjugated thiophene scaffolds.

Naphthodithiophenes: emerging building blocks for organic electronics

Linear-fused naphthodithiophenes (NDTs) are emerging building blocks in the development of new semiconducting small molecules, oligomers, and polymers. The promising nature of NDT-based materials as organic semiconductors has been demonstrated by superior device characteristics in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) in the last few years. In particular, it is quite impressive that a power conversion efficiency as high as 8.2% has been achieved for a single-junction OPV cell consisting of NDT-based semiconducting polymers and a fullerene derivative in such a short period of time. Here, we provide an overview of recent synthetic evolutions in NDT chemistry and progress in NDT-based materials, especially conjugated oligomers and polymers and their applications to OFETs and OPVs.

Synthesis, characterization and optoelectronic properties of a new perylene diimide-benzimidazole type solar light harvesting dye

A perylene diimide type small molecule (BI-PDI) has been synthesized through Suzuki coupling reaction between N,N'-bis(2,6-diisopropylphenyl)-1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimide and 2-(2-hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-boronic acid. BI-PDI small molecule has showed an absorption band between 350 and 750 nm on thin films. HOMO and LUMO energy levels of BI-PDI dye have been calculated to be about -5.92 eV and -3.82 eV, respectively. Solution-processed bulk heterojunction (BHJ) solar cells have been constructed using BI-PDI as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor or poly(3-hexylthiophene) (P3HT) as donor and BI-PDI as acceptor. The external quantum efficiencies (EQE) of the devices cover the most of the visible region between 400 and 700 nm for both configurations. Photovoltaic performances of BI-PDI-based organic solar cells are limited by the aggregation tendency of PDI structure and poor hole/electron mobilities of the active layer.

2-Alkyl-5-thienyl-substituted benzo[1,2-b:4,5-b']dithiophene-based donor molecules for solution-processed organic solar cells

In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A-D-A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C61-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (Jsc) of 9.08 mA cm(-2), an open circuit voltage (Voc) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW cm(-2)). The FFs of these solution-processed small-molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.