Pyrene- benzo[1,2,5]thiadiazole based conjugated polymers for application in BHJ solar cells

Ethylhexyloxy-functionalised pyrene (PEH) was prepared and copolymerised with both dithienyl-benzo[c]-[1], [2], [5]thiadiazole and dibithiophenyl-benzo[c]-[1], [2], [5]thiadiazole via a Stille coupling polymerisation method to yield PPEH-DTBT-8 and PPEH-DT2BT-8, respectively. A comparative study was conducted to assess the impact of substituting thiophene for bithiophene repeat units upon the resulting properties of the conjugated polymers. PPEH-DT2BT-8 which has bithiophene spacers between pyrene and benzothiadiazole repeat units, exhibited a narrower optical and electrochemical band gap relative to PPEH-DTBT-8; a consequence of the incorporating bithiophene spacer units which promote intramolecular charge transfer between the electron donating and electron accepting moieties. Both PPEH-DTBT-8 and PPEH-DT2BT-8 showed deep HOMO levels of -5.54 and -5.50 eV, respectively. The polymers possess good thermal stabilities with degradation temperatures in excess of 310 °C. The photovoltaic performance of the two polymers was studied by fabricating bulk heterojunction (BHJ) photovoltaic devices using PC70BM as the acceptor. PPEH-DTBT-8 and PPEH-DT2BT-8 demonstrated efficiencies of 0.33 and 1.83%, respectively. The higher efficiency of PPEH-DT2BT-8 can be attributed to vastly improved FF and Jsc values

Pyrene–benzothiadiazole-based copolymers for application in photovoltaic devices

The preparation and characterisation of four narrow band gap pyrene-benzothiadiazole based alternating copolymers is presented. An investigation of the impact of attaching different solubilising 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 (PPEH-DTBT and PPEH-DTffBT) displayed lower molecular weights relative to polymers with larger 2-hexyldecyl substituents (PPHD-DTBT and PPHD-DTffBT). Despite this, the 2-ethylhexyl substituted polymers displayed narrower optical band gaps relative to their analogous 2-hexyldecyl substituted polymers. Of all polymers synthesised, PPEH-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 HOMO 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 ITO/PEDOT:PSS/Polymer:PC70BM/Ca/Al were undertaken. Polymer/PC70BM blend ratios of 1/3 were used in these studies and have indicated that PPEH-DTBT displayed the highest efficiency with a PCE of 1.86 %

Preparation and photovoltaic properties of pyrene-thieno[3,4-c]pyrrole-4,6-dione-based donor-acceptor polymers

Four new donor-acceptor conjugated copolymers, containing pyrene moieties flanked by thienyl or bithienyl groups as a donor units and thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptor units, were successfully prepared via a direct arylation polymerisation method. While all polymers prepared had 2-ethylhexyloxy-substituents on the pyrene repeat units, two different alkylsusbtituents (octyl or 4-hexylphenyl groups) were attached to their TPD moieties. The influence of these different substituents as well as the number of thienyl units linking the pyrene and TPD units along polymer chains on the photophysical, electronic and photovoltaic properties of these materials was investigated. All polymers displayed good thermal stability up to 315°C. The optical band gap of the four polymers, PPEHDT-TPDO, PPEHDT-TPDHP, PPEHDT2-TPDO and PPEHDT2-TPDHP, were estimated to be 2.00, 2.06, 1.94 and 1.91 eV, respectively. Polymers that possessed a single thiophene unit attached to the pyrene unit, PPEHDT-TPDO and PPEHDTTPDHP, displayed deeper HOMO levels compared to those with bithiophene units, PPEHDT2- TPDO and PPEHDT2-TPDHP. Photovoltaic devices were fabricated from all polymers. PPEHDT2-TPDO boasted the highest efficiency with a PCE (2.06 %), a FF of 53.07 %, a Jsc of 4.66 mA/cm2 and a Voc of 0.83 V

Preparation and characterization of quinoxaline-pyrene-based conjugated copolymers for organic photovoltaic devices

In this study, two novel conjugated polymers, poly(4,5,9,10-tetrakis((2-ethylhexyl)oxy]pyrene-alt-2,3-bis(3-(octyloxy)phenyl)-5,8-di(2-thienyl)-6,7-difluoroquinoxaline) (PPyQxff) and poly(4,5,9,10-tetrakis((2-ethylhexyl)oxy)pyren-alt-2,3-bis(3-(octyloxy)phenyl)-5,8-di(2-thienyl)quinoxaline) (PPyQx), consisting of quinoxaline units with and without fluorine substituents, as electron-accepting moieties and pyrene flanked with dithienyl units as electron-donating moieties were prepared via Stille polymerization reactions for use as electron donor materials in bulk heterojunction (BHJ) solar cells. PPyQxff and PPyQx were characterized by X-ray powder diffraction (XRD), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), UV−VIS absorption, and nuclear magnetic resonance (NMR) spectroscopy. PPyQxff and PPyQx revealed excellent solution processability in common organic solvents. PPyQxff and PPyQx presented decomposition temperatures above 300 °C. The inclusion of F atoms to the quinoxaline moiety made a slight reduction in the highest occupied molecular orbital (HOMO) level, relative to the unfluorinated polymer, but had no impact on the lowest unoccupied molecular orbital (LUMO) level. PPyQxff and PPyQx exhibited similar physical properties with strong and broad absorbance from 400 to 700 nm and an optical band-gap energy of 1.77 eV. The X-ray powder diffraction study indicated that PPyQxff possessed a reduced π–π stacking distance relative to PPyQx

High-Performance Multilayer Encapsulation for Perovskite Photovoltaics

An encapsulation system comprising of a UV‐curable epoxy, a solution processed polymer interlayer, and a glass cover‐slip, is used to increase the stability of methylammonium lead triiodide (CH3NH3PbI3) perovskite planar inverted architecture photovoltaic (PV) devices. It is found this encapsulation system acts as an efficient barrier to extrinsic degradation processes (ingress of moisture and oxygen), and that the polymer acts as a barrier that protects the PV device from the epoxy before it is fully cured. This results in devices that maintain 80% of their initial power conversion efficiency after 1000 h of AM1.5 irradiation. Such devices are used as a benchmark and are compared with devices having initially enhanced efficiency as a result of a solvent annealing process. It is found that such solvent‐annealed devices undergo enhanced burn‐in and have a reduced long‐term efficiency, a result demonstrating that initially enhanced device efficiency does not necessarily result in long‐term stability