Sureshraju Vegiraju

Metal-Free Tetrathienoacene Sensitizers for High-Performance Dye-Sensitized Solar Cells

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.

Asymmetric fused thiophenes for field-effect transistors: crystal structure–film microstructure–transistor performance correlations

New asymmetric phenyl and perfluorophenyl end-functionalized dithienothiophene (DTT)- and bisdithienothiophene (BDTT)-based fused-thiophene derivatives (FPP-DTT; 1 and FPP-BDTT; 3) were synthesized and characterized for organic thin-film transistor (OTFT) applications. For comparison, symmetric phenyl end-capped dithienothiophene and bisdithienothiophene derivatives DP-DTT (2) and DP-BDTT (4) were also explored in parallel. The crystal structures of all four molecules were determined via single-crystal X-ray diffraction. Asymmetric compounds 1 and 3 exhibit face-to-face π–π stacking, while symmetric 2 and 4 show herringbone stacking. Single-crystal and thin-film transistors based on these four materials were fabricated. For single-crystal transistors, asymmetric FPP-DTT and FPP-BDTT gave high p-channel mobilities of 0.74 and 0.73 cm2 V−1 s−1, respectively, as well as current on/off ratios of ∼105. Symmetric DP-DTT and DP-BDTT gave relatively lower p-channel mobilities of 0.36 and 0.41 cm2 V−1 s−1, respectively. For thin-film transistors, FPP-DTT and DP-DTT films deposited at 25 °C exhibited decent p-channel characteristics with a carrier mobility as high as 0.15 and 0.20 cm2 V−1 s−1, respectively for top-contact/bottom-gate OTFT devices. The device characteristics on various gate dielectrics have been correlated with the film morphologies and microstructures of the corresponding compounds.

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTLs suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL material in low-cost high-performance tin-based perovskite solar cells.

Functionalized benzothieno[3,2 b]thiophenes (BTTs) for high performance organic thin-film transistors (OTFTs)

New benzothieno[3,2-b]thiophene (BTT) derivatives, end-functionalized with biphenyl (Bp-BTT), naphthalenyl (Np-BTT), and benzothieno[3,2-b]thiophenyl (BBTT; dimer of BTT) moieties, were synthesized and characterized for bottom-gate/top-contact organic thin-film transistors (OTFTs). All three materials exhibit good environmental stability as assessed by thermogravimetric analysis, and no decomposition after extended light exposure, due to their wide band gaps and low-lying HOMOs. The single crystal structures of Bp-BTT and BBTT reveal flat molecular geometries, close π–π stacking, and short sulfur-to-sulfur distances, suggesting an ideal arrangement for charge transport. X-ray diffraction (XRD) measurements verify that the bulk crystal structures are preserved in the polycrystalline thin films. As a consequence, Bp-BTT and BBTT exhibit good OTFT performance, with µ = 0.34 cm2 V−1 s−1 (max) and Ion/Ioff = (3.3 ± 1.6) × 108 for Bp-BTT, and µ = 0.12 cm2 V−1 s−1 (max) and Ion/Ioff = (2.4 ± 0.9) × 107 for BBTT; whereas Np-BTT gives lower device performance with µ = 0.055 cm2 V−1 s−1 (max) and Ion/Ioff = (6.7 ± 3.4) × 108. In addition, octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) treatment of the SiO2 gate dielectric is found to be effective in enhancing the OTFT performance for all three BTT derivatives, by improving the interfacial semiconductor film morphology and in-plane crystallinity.

Metal-free branched alkyl tetrathienoacene (TTAR)-based sensitizers for high-performance dye-sensitized solar cells

A new series of metal-free alkylated tetrathienoacene (TTAR)-based organic chromophores, TPA–TTAR–TA (R = branched-C8H17, 1, TTAR-b8; R = C15H31, 2, TTAR-15; R = C9H19, 3, TTAR-9), are synthesized for application in dye-sensitized solar cells (DSSCs). Due to the extensively conjugated TTAR π-bridge, all three dyes exhibit high extinction coefficients (1 × 105 M−1 cm−1). By systematically exploring the effects of the TTAR alkyl chain substituents, a significant influence of the dye coverage (orientation) on the TiO2 surfaces is observed. The branched-alkyl TTAR-b8 (1) promotes significant tilting and packing distortion on TiO2 in comparison to more ordered monolayers of linear long alkyls TTAR-15 (2) and TTAR-9 (3). Photophysical measurements on the dye-grafted TiO2 films reveal that the branched-alkylated TTA unit in 1 enhances the electron injection efficiency, in agreement with the high quantum efficiency. Notably, by utilizing a three-dimensional (3D) photonic crystal (PhC) layer to enhance the coherent scattering an increase the light absorption, TTAR-b8 exhibits higher short-circuit current densities and achieved a high PCE of 11.18%. TTAR-b8 is thus the best performing fused-thiophene-based organic DSSC dye reported to date.

Synthesis and characterization of novel symmetrical two-photon chromophores derived from bis(triphenylaminotetrathienoacenyl) and fused-thiophene units

Four new donor–π–donor (D–π1–π2–π1–D) fused-thiophene-based chromophores, end-functionalized with electron-donating triphenylamine (TPA) groups, were developed and characterized for a two-photon absorption study. Within this series, tetrathienoacene (thieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]thiophene; TTA) moieties were employed as side-conjugated (π1) units, and the central conjugated core (π2) units were altered with thiophene (T), bithiophene (bT), thienothiophene (TT), and dithienothiophene (DTT) for chromophores 1–4, respectively. The structural and photophysical relationships of the four compounds were compared, and all four chromophores showed strong fluorescence with good thermal stability. The energy gap compression of these chromophores was verified by electrochemistry and density functional theory (DFT) calculations. The two-photon-related properties of 1–4 were examined using femtosecond laser pulses as the probing tool. The magnitude of the two-photon absorptivity was found to be strongly dependent on the molecular conjugation length and the center fused-thiophene unit. Within the family, the most conjugated DTT-centered chromophore (4) exhibits the strongest and the most widely dispersed two-photon absorption cross-section value up to 3000 GM. To the best of our knowledge, this is the highest 2PA cross section value reported to date among the studied fused thiophene-based chromophores.

Low-voltage-driven organic phototransistors based on a solution-processed organic semiconductor channel and high k hybrid gate dielectric

Solution-processed micro-ribbon shaped organic semiconductors and high k hybrid gate dielectric were fabricated for low-voltage-driven organic phototransistors, which integrate the photodetection and memory properties within one single device.