Narrow Energy Gap Polymers with Absorptions up to 1 200 nm and their Photovoltaic Properties

A series of donor/acceptor carbazole copolymers comprising alternating 6,7‐diphenyl‐4,9‐bis‐(thiophen‐2‐yl)‐[1,2,5]thiadiazolo[3,4‐g]quinoxaline and 3,6‐dimethyl‐9‐alkyl‐9H‐carbazole repeat units ( P1 ), or 3,6‐dimethyl‐9‐triarylamino‐9H‐carbazole repeat units ( P2 ), or 9‐triarylamino‐9H‐carbazole repeat units ( P3 ) has been prepared following Suzuki polymerization procedures. P3 absorbs light up to 1 200 nm and has an energy gap of 1.1 eV, while P1 and P2 have energy gaps of 1.3 and 1.25 eV, respectively. Photovoltaic cells with ITO/PEDOT:PSS/ P3 :PCBM (1:1 w/w)/Ca showed an open‐circuit voltage of 0.4 V under white light illumination, power conversion efficiency of 0.61%, and short‐circuit current of 5.2 mA · cm⁻².

     

Pyrene–benzothiadiazole‐based copolymers for application in photovoltaic devices

The preparation and characterization of four narrow band gap pyrene–benzothiadiazole‐based alternating copolymers are presented. An investigation of the impact of attaching different solubilizing groups to the pyrene repeat units on the optical, electrochemical, and thermal properties of the resulting materials was undertaken along with studies on the aggregation of polymer chains in the solid state. Unsurprisingly, polymers which had the smaller 2‐ethylhexyl chains attached to the pyrene units (PP_EH‐DTBT and PP_EH‐DTffBT) displayed lower molecular weights relative to polymers with larger 2‐hexyldecyl substituents (PP_HD‐DTBT and PP_HD‐DTffBT). Despite this, the 2‐ethylhexyl substituted polymers displayed narrower optical band gaps relative to their analogous 2‐hexyldecyl substituted polymers. Of all polymers synthesized, PP_EH‐DTBT displayed the lowest optical band gap (1.76 eV) in the series. All polymers display degradation temperatures in excess of 300°C. Polymers with smaller alkyl chains on the pyrene units display shallower highest occupied molecular orbital levels, which could be due to increased intramolecular charge transfer between the donor and acceptor units. Preliminary investigations on bulk heterojunction solar cells with a device structure indium tin oxide/poly(3,4‐ethylenedioxythiophene) : polystyrene sulfonate /Polymer : PC₇₀BM/Ca/Al were undertaken. Polymer/PC₇₀BM blend ratios of one third were used in these studies and have indicated that PP_EH‐DTBT displayed the highest efficiency with a power conversion efficiency of 1.86%. Copyright © 2016 John Wiley & Sons, Ltd.     

Time-resolved and cw photoluminescence from strongly coupled organic microcavities

The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates.     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).