All-polymer solar cells (all-PSCs) is a promising next-generation photovoltaic technology due to its unique advantages, including good mechanical flexibility, excellent thermal/mechanical durability, high visible transparency, and great potential for large-scale manufacture with low production cost. I will present our study of the effects of polymer number-average molecular weight (Mn) on the performance of all-PSCs. We used a series of biselenophene-napthalene diimide (PNDIBS) acceptor polymers synthesized by direct heteroarylation polymerization method with six Mn values spanning 20 – 130kDa and the donor polymer PBDB-T. We found that the short-circuit current and the power conversion efficiency (PCE) of all-PSC composed of PNDIBS/PBDB-T binary blends were maximized as Mn approached a critical value (Mn = 55kDa) but subsequently declined with further increase beyond the critical value. The blend photophysics, blend charge transport properties, and blend morphology were concurrently optimized at the critical Mn value. The underlying mechanism for the observed trends was explained by the efficient charge photogeneration rate, suppressed space-charge formation, superior and symmetric charge transport, and a bi-continuous network of ordered crystalline domains with predominant face-on molecular orientations at the optimal Mn. Our findings provide important structure-property relationship insights while providing design guidelines that are broadly applicable to the development of high-performance semiconducting polymers with optimal properties and enhanced photovoltaic properties.

Understanding the molecular weight dependence of photophysics, charge transport, morphology, and performance of all-polymer solar cells

ACS Spring Meeting 2021

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