Title: A Beatific Vision Towards Organic Semiconducting Polymer Crystals: To Be, or Not To Be – That Is The Question!

Abstract: Have you got the latest iPhone X or, Samsung Galaxy A9 or, Pixel 3 in your pocket? Those touchscreens are OLEDs, utilizing semiconducting molecules that deliver many advantages over other existing display technologies such as drastic color contrast. When it comes to the semiconducting polymers, however, we remain behind in commercializing such electronic devices including, but not limited to, displays, solar cells, and sensors. With all the advantages that polymeric semiconductors offer such as solution casting, flexible integration, low cost, and low-temperature processibility etc., there remain specific challenges that need to be addressed, should we envision to assimilate them into our everyday reality. For example, a number of state-of-the-art high performing polymers are now being synthesized with high-yield and controlled polydispersity e.g. P(NDI2OD-T2). These polymers are comprised of mainly two parts: (i) conjugated backbones i.e. main-chains that ensure all the desired optoelectronic properties, and (ii) non-conjugated, often branched, carbon chains i.e. side-chains that ensure the solution processibility. Once these polymers are processed into solid-state films, main-chains are desired to form connected pathways (from electrode to electrode) to realize the best performance e.g. charge transport. To one’s surprise, charge transport in a single main-chain can be as high as 600 cm2/Vs when measured electrode-less; meaning, all we need is a perfectly formed pathway of the polymer main-chains; that is, a device-scaled perfect polymer crystal without any domain-ends, grain-boundaries, defects, and traps! Irritatingly, however, the side-chains are undesirable in the solid state, since they are insulating, remain mostly amorphous, hamper charge transport and upset long-range ordering and perhaps, the perfect crystal morphology of the main-chains. But these side-chains do not act the same way when they remain un-constrained i.e. not attached to any main-chain, rather they naturally form solid state crystals! To that end, we ask these open questions: why cannot these side-chains form crystals in a constrained system i.e. when attached to main-chains? What role do the main-chains play in that? Can we rationally design a polymeric semiconductor where this antagonistic side-chains vs. main-chains duel can be well-tuned into a synergistic scenario? Can those rationally designed main-chains conform into a perfect crystal following the un-constrained crystal structure of the side-chains? All in all, to conform, or not to conform: that is the question we investigate! In this vision-talk, we explore this idea in more of a commentary style: we address these questions, present our latest polymer design-rationales and discuss the way forward towards this beatific vision.