Photo-Cross-Linkable Azide-FunctionalizedPolythiophenefor Thermally Stable Bulk Heterojunction Solar Cells.

We have synthesized photo-cross-linkable azide-functionalizedpoly(3-hexylthiophene)to explore improvements in the thermal stability of bulk heterojunctionsolar cells. Exposing blends of photo-cross-linkable polythiopheneand [6,6]-phenyl-C61-butyric acid methyl ester to ultravioletlight preferentially cross-linked the polythiophene without degradingits optical or electrical properties. X-ray scattering measurementsshowed that cross-linking slightly compacted the polythiophene chainlamellar stacking while increasing the polymer crystal coherence lengthby 20%. Optimized solar cells having cross-linked active blend layersretained 65% of their initial photovoltaic power conversion efficiencyafter 40 h of thermal annealing at 110 °C, while devices usingun-cross-linked commercial polythiophene underwent significant phaseseparation and retained less than 30% of their initial efficiencyafter annealing. [ABSTRACT FROM AUTHOR]