Thermocleavage of Partial Side Chains in Polythiophenes Offers Appreciable Photovoltaic Efficiency and Significant Morphological Stability

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 (T-g) of the materials used in PSCs can largely mitigate the thermal instability, yet most CPs used in high-efficiency PSCs only show low T-g values, mainly due to the long and bulky side chains needed for solution processing of such polymers. Thermally removing cleavable side chains is an effective approach to regain the high T-g 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 degrees C) treatment to remove all side chains. To better understand and utilize thermocleavable side chains (TCSs), we explore a series of regioregular polythiophenes having TCSs and hexyl side chains by varying the ratio of different side chains, from 0 mol % TCSs to 100 mol % TCSs at an increment of 20 mol %. Through a systematic investigation, we find that the polymers with more TCSs than hexyl side chains exhibit sufficient stability under a rather harsh condition (100 degrees C, in air and under continuous ambient light). While a complete removal of alkyl chains might offer a higher stability, the device efficiency was very low (similar to 0.14%); by contrast, the polymer having similar to 70 mol % of TCSs achieved the highest efficiency (similar to 1.5%) after alkyl chain cleavage at 200 degrees C and significant morphological stability. Under our stability test (150 degrees C, 24 h and ambient light), these specific polymer:PC61BM based solar cells were able to retain 90% of the original efficiency. These key findings, together with mechanistic understanding of the thermocleavage process, provide valuable insight into the impact of TCS and present a new design rationale to achieve PSCs with both high efficiency and improved stability.