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.
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, PP EH DT-TPD O , PP EH DT-TPD HP , PP EH DT2-TPD O and PP EH DT2-TPD HP , 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, PP EH DT-TPD O and PP EH DT-TPD HP , displayed deeper HOMO levels compared to those with bithiophene units, PP EH DT2-TPD O a n d P P EH DT2-TPD HP. Photovoltaic devices were fabricated from all polymers. PP EH DT2-TPD O boasted the highest efficiency with a PCE (2.06 %), a FF of 53.07 %, a J sc of 4.66 mA/cm 2 and a V oc of 0.83 V.
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