Conjugated polymers have a long history of exploration and use in organic solar cells, and over the last twenty-five years, marked increases in the solar cell efficiency have been achieved. However, the synthetic complexity of these materials has also drastically increased, which makes the scalability of the highestefficiency materials difficult. If conjugated polymers could be designed to exhibit both high efficiency and straightforward synthesis, the road to commercial reality would be more achievable. For that reason, a new synthetic approach was designed towards PTQ10 (= poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)]). The new synthetic approach to make PTQ10 brought a significant reduction in cost (1/7th the original) and could also easily accommodate different side chains to move towards green processing solvents. Furthermore, high-efficiency organic solar cells were demonstrated with a PTQ10:Y6 blend exhibiting approximately 15 % efficiency.

Conjugated polymers (CPs) are promising semiconductors for intrinsically stretchable electronic devices. Ideally, such CPs should exhibit high charge mobility, excellent stability, and high stretchability. However, converging all these desirable properties in CPs has not been achieved via molecular design and/or device engineering. This work details the design, synthesis and characterization of a random polythiophene (RP-T50) containing ~50 mol% of thiophene units with a thermocleavable tertiary ester side chain and ~50 mol% of unsubstituted thiophene units, which, upon thermocleavage of alkyl chains, shows significant improvement of charge mobility and stability. Thermal annealing a RP-T50 film coated on a stretchable polydimethylsiloxane substrate spontaneously generates wrinkling in the polymer film, which effectively enhances the stretchability of the polymer film. The wrinkled RP-T50-based stretchable sensors can effectively detect humidity, ethanol, temperature and light even under 50% uniaxial and 30% biaxial strains. Our discoveries offer new design rationale of strategically applying CPs to intrinsically stretchable electronic systems.

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 PC 61 BM) after a high temperature (300 °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 °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 (∼0.14%); by contrast, the polymer having ∼70 mol % of TCSs achieved the highest efficiency (∼1.5%) after alkyl chain cleavage at 200 °C and significant morphological stability. Under our stability test (150 °C, 24 h and ambient light), these specific polymer:PC 61 BM 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.