Regioregular Low Bandgap Polymer with Controlled Thieno[3,4-b]thiophene Orientation for High-Efficiency Polymer Solar Cells

The regioregular p-type copolymer PBDTTT-C-T composed of TT-BDT-TT-BDT repeating units (TT = thieno[3,4-b]thiophene, BDT = benzo[1,2-b:4,5-b']dithoiphene) and perfectly controlled TT orientation was synthesized. The optical, thermal, and electrochemical properties of the regioregular PBDTTT-C-T were characterized and compared with the random PBDTTT-C-T without structural regioregularity. The regioregular PBDTTT-C-T showed lower optical bandgap (1.55 eV) and higher degree of crystallinity compared to the random PBDTTT-C-T. The inverted bulk heterojunction PSCs based on the regioregular PBDTTT-C-T exhibited a power conversion efficiency as high as 7.79%, which is 19% higher than the random PBDTTT-C-T-based PSCs. It was found that the improved photoabsorption and increase in charge carrier mobility due to high regioregularity of conjugated polymer backbones and effective ordering between polymer chains are the most likely reasons for enhancement of power conversion efficiency in PSCs. © 2015 American Chemical Society.

Large Work Function Modulation of Monolayer MoS2 by Ambient Gases

Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors. © 2016 American Chemical Society127311sciescopu

Regioregular D-1-A-D-2-A Terpolymer with Controlled Thieno[3,4-b]thiophene Orientation for High-Efficiency Polymer Solar Cells Processed with Nonhalogenated Solvents

A regioregular D1-A-D2-A terpolymer PDTSTTBDT incorporating dithieno[3,2-b:2′,3′-d]silole (DTS, D1) and benzo[1,2-b:4,5-b]dithiophene (BDT, D2) units with perfectly controlled thieno[3,4-b]thiophene (TT, A) orientation was synthesized for the first time. The thermal, optical, and electrochemical properties of the regioregular PDTSTTBDT were characterized and compared with the random PDTSTTBDT without structural regioregularity. The regioregular PDTSTTBDT showed ideal optical bandgap (1.45 eV), lower lying HOMO energy level, and higher degree of crystallinity compared to the random PDTSTTBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The inverted bulk-heterojunction polymer solar cells (PSCs) based on the regioregular PDTSTTBDT and o-xylene process solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs. It was found that the remarkable enhancement of photovoltaic performance in regioregular PDTSTTBDT-based PSCs is mainly due to improved light absorption, effective polymer ordering, and high charge carrier mobility. © 2016 American Chemical Society.

Donor acceptor polymers with a regioregularly incorporated thieno [3,4-b]thiophene segment as a pi-bridge for organic photovoltaic devices

A series of donor-acceptor (D-A) polymers with regioregularly incorporated a thieno[3,4-b]thiophene as a π-bridge between electron donor and electron acceptor segments were successfully synthesized for the first time. The optical, thermal, and electrochemical properties of the D-A polymers with the thieno[3,4-b]thiophene as a π-bridge were characterized by UV/vis spectroscopy, thermogravimetric analysis, and cyclic voltammetry measurements. They exhibited low energy bandgap because of extended π-conjugation length and increased electron density through conjugated polymer backbones. Moreover, they showed suitable HOMO and LUMO energy levels for photovoltaic applications. The bulk heterojunction organic photovoltaic devices based on the D-A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment exhibited about 10% higher power conversion efficiency (PCE) than the D-A polymer with thienyl segment as a π-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D-A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge can achieve high PCE through structural modification of the polymers. © 2015 Elsevier B.V.