The intrinsic degradation of conjugated polymer (CP) based solar cells (PSCs) due to morphological change by heat is an outstanding challenge. Increasing the glass transition temperature (Tg) of the materials used in PSCs can largely mitigate the thermal instability, yet most CPs used in high-efficiency PSCs only show low Tg values, mainly due the long and bulky side chains needed for solution processing of such polymers. Thermally removing cleavable side chains is an effective approach to re-gain the high Tg of CPs after the film formation, thereby achieving higher stability; however, previous results using polythiophenes only achieved moderate efficiency (0.8% with PC61BM) after a high temperature (300 °C) treatment to remove all side chains. We find that maintaining ~ 30 mol% hexyl chains and rest (~ 70 mol%) being carboxylic acid (upon alkyl chain cleavage at 200 °C) on polythiophene can achieve much higher efficiencies (~ 1.5% with PC61BM) and significant morphological stability under aggressive testing conditions (150 °C, ambient light, 24 h).

Tackling morphological stability of polymer solar cells with thermocleavable side chains

ACS Fall 2021

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