A new series of metal-free organic chromophores (TPA-TTAR-A (1), TPA-T-TTAR-A (2), TPA-TTAR-T-A (3), and TPA-T-TTAR-T-A (4)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D-π-A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5-5.2 × 10(4) M(-1) cm(-1)). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO2 surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye 3) promotes more vertical dye orientation and denser packing on TiO2 (molecular footprint = 79 Å(2)), thus enabling optimal dye loading. Using dye 3, a DSSC power conversion efficiency (PCE) of 10.1% with Voc = 0.833 V, Jsc = 16.5 mA/cm(2), and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I(-)/I3(-) redox shuttle. Photophysical measurements on dye-grafted TiO2 films reveal that the additional thiophene unit in dye 3 enhances the electron injection efficiency, in agreement with the high quantum efficiency.

Organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs) have drawn great interest from both academics and industry, due to the possibility of low-cost conversion of photovoltaic energy at reasonable efficiencies. This review focuses on recent progress in molecular engineering and technological aspects of fused-thiophene-based organic dye molecules for applications in solar cells. Particular attention has been paid to the design principles and stability of these dye molecules, as well as on the effects of various electrolyte systems for DSSCs. Importantly, it has been found that incorporation of a fused-thiophene unit into the sensitizer has several advantages, such as red-shift of the intramolecular charge transfer band, tuning of the frontier molecular energy level, and improvements in both photovoltaic performance and stability. This work also examines the correlation between the physical properties and placement of fused-thiophene in the molecular structure with regard to their performance in OPVs and DSSCs. OPEN ACCESSPolymers 2014, 6 2646

New branched alkyl tetrathienothiophene (TTAR)-based organic sensitizers with power conversion efficiency up to 11%.

ARTICLE This journal isTwo novel π-conjugated small molecules based on the electron-deficient diketopyrrolopyrrole (DPP) and the electron-rich fused tetrathienoacene (TTA) frameworks are synthesized and characterized. As verified in the bandgap compression of these chromophores by electrochemistry and density functional theory (DFT) computation, these DPP-TAA derivatives exhibit substantial conjugation and ideal MO energetics for light absorption. The large fused TTA core and strong intermolecular S···S interactions enforce excellent molecular planarity, favoring a close-packed thin film morphologies for efficient charge transport, as indicated by grazing incidence wide angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analysis. Top-gate/bottomcontact thin film transistors based on these systems exhibit hole mobilities approaching 0.1 cm 2 V -1 s -1 . Organic photovoltaic cells based on DDPP-TTAR:PC 71 BM blends achieve power conversion efficiencies (PCE) > 4% by systematic morphology tuning and judicious solvent additive selection. typically susceptible to batch-to-batch variations, small molecule syntheses generally provide excellent reproducibility, purity, and yields, leading for far more uniform materials properties. 3, 22-25 For these Scheme 1. Examples of DDP-fused thiophene organic semiconductors.reasons, solution-processable small molecules offer great potential for applications in technologically relevant organic electronic devices. Furthermore, small molecules can achieve precisely controlled molecular packing and ordered nanophase morphologies, via either self-assembly or controlled solution coating techniques. 26, 27 Thus, the design and implementation of small molecules also opens new possibilities for understanding fundamental organic semiconductor structure-property relationships and further optimizing device parameters. Nevertheless, to date only relatively low p-type mobilities and PCEs have been achieved using DPP-based small molecules. 10 For example, our team first reported NDT(TDPP) 2 (Scheme 1) exhibiting OPV PCEs of up to 4%, 28 followed by alkyne functionalized (TDPP) 2 -EBT exhibiting a mobility of 0.17 cm 2 V -1 s -1 and a PCE of 1.7%. 29 More recently, Huang et.al. 30 and Lin et al. 14 combined DPP with the widely implemented benzodithiophene (BDT) moiety and demonstrated OSC PCEs of >5%. While these results are promising, more detailed investigations will be necessary to better understand and further enhance the TFT and OPV device performance of DPP-based small molecules.Motivated by the potential of introducing fused thiophene units into electronic polymers, here we investigate fused thiophenes systems for the design of high performance small molecule materials. From our previous results, fused thiophene motifs enhance structural planarity and strong intermolecular S···S interactions. 31-33 These properties promote extensive intramolecular π-conjugation and close intermolecular π-π stacking, thereby enhancing charge transpor...

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Three new fused thiophene semiconductors, end‐capped with diperfluorophenylthien‐2‐yl (DFPT) groups (DFPT‐thieno[2′,3′:4,5]thieno[3,2‐b]thieno[2,3‐d]thiophene (TTA), DFPT‐dithieno[2,3‐b:3′,2′‐d]thiophenes (DTT), and DFPT‐thieno[3,2‐b]thiophene (TT)), are synthesized and characterized in organic thin film transistors. Good environmental stability of the newly developed materials is demonstrated via thermal analysis as well as degradation tests under white light. The molecular structures of all three perfluorophenylthien‐2‐yl end‐functionalized derivatives are determined by single crystal X‐ray diffraction. DFPT‐TTA and DFPT‐TT exhibit good n‐type TFT performance, with mobilities up to 0.43 and 0.33 cm2 V−1 s−1, respectively. These are among the best performing n‐type materials of all fused thiophenes reported to date. The best thin film transistor device performance is achieved via an n‐octadecyltrichlorosilane dielectric surface treatment on the thermally grown Si/SiO2 substrates prior to vapor‐phase semiconductor deposition. Within the DFPT series, carrier mobility magnitudes depend strongly on the semiconductor growth conditions and the gate dielectric surface treatment.

A new type of carbene-based ruthenium sensitizer, CB104, with a highly conjugated ancillary ligand, diphenylvinylthiophene-substituted benzimidazolepyridine, was designed and developed for dye-sensitized solar cell applications. The influence of the thiophene antenna on the performance of the cell anchored with CB104 was investigated. Compared with the dye CBTR, the conjugated thiophene in the ancillary ligand of CB104 enhanced the molar extinction coefficient of the intraligand π-π* transition and the intensity of the lower energy metal-to-ligand charge-transfer band. However, the incident photon-to-current conversion efficiency spectrum of the cell anchored with CB104 (0.15 mM) showed a maximum of 63 % at 420 nm. The cell sensitized with the dye CB104 attained a power conversion efficiency of 7.30 %, which was lower than that of the cell with nonconjugated sensitizer CBTR (8.92 %) under the same fabrication conditions. The variation in the performance of these two dyes demonstrated that elongating the conjugated light-harvesting antenna resulted in the reduction of short-circuit photocurrent density, which might have been due to the aggregation of dye molecules. In the presence of a coabsorbate, chenodeoxycholic acid, the CB104-sensitized cell exhibited an enhanced photocurrent density and achieved a photovoltaic efficiency of 8.36 %.

This review highlights our recent efforts in the development of organic semiconductors based on anthradithiophene (ADT), dithienothiophene (DTT), tetrathienoacene (TTA), benzothienodithiophene (BTDT), benzothienothiophene (BTT), chalcogen-planarized BT, and some quinoidal oligothiophenes for the