Entropy-Driven Crystallization Behavior in DNA-Mediated Nanoparticle Assembly

Herein, we report an example of entropy-driven crystallization behavior in DNA-nanoparticle superlattice assembly, marking a divergence from the well-established enthalpic driving force of maximizing nearest-neighbor hybridization connections. Such behavior is manifested in the observation of a non-close-packed, body-centered cubic (bcc) superlattice when using a system with self-complementary DNA linkers that would be predicted to form a close-packed, face-centered cubic (fcc) structure based solely on enthalpic considerations and previous design rules for DNA-linked particle assembly. Notably, this unexpected phase behavior is only observed when employing long DNA linkers with unpaired “flexor” bases positioned along the length of the DNA linker that increase the number of microstates available to the DNA ligands. A range of design conditions are tested showing sudden onsets of this behavior, and these experiments are coupled with coarse-grained molecular dynamics simulations to show that this entropy-driven crystallization behavior is due to the accessibility of additional microstates afforded by using long and flexible linkers

All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers

The influence of the number-average molecular weight (<i>M</i>_n) on the blend film morphology and photovoltaic performance of all-polymer solar cells (APSCs) fabricated with the donor polymer poly­[5-(2-hexyldodecyl)-1,3-thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione-<i>alt</i>-5,5-(2,5-bis­(3-dodecylthiophen-2-yl)­thiophene)] (<b>PTPD3T</b>) and acceptor polymer poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (P­(NDI2OD-T2); <b>N2200</b>) is systematically investigated. The <i>M</i>_n effect analysis of <i>both</i> <b>PTPD3T</b> and <b>N2200</b> is enabled by implementing a polymerization strategy which produces conjugated polymers with tunable <i>M</i>_ns. Experimental and coarse-grain modeling results reveal that systematic <i>M</i>_n variation greatly influences both intrachain and interchain interactions and ultimately the degree of phase separation and morphology evolution. Specifically, increasing <i>M</i>_n for both polymers shrinks blend film domain sizes and enhances donor–acceptor polymer–polymer interfacial areas, affording increased short-circuit current densities (<i>J</i>_sc). However, the greater disorder and intermixed feature proliferation accompanying increasing <i>M</i>_n promotes charge carrier recombination, reducing cell fill factors (<i>FF</i>). The optimized photoactive layers exhibit well-balanced exciton dissociation and charge transport characteristics, ultimately providing solar cells with a 2-fold PCE enhancement versus devices with nonoptimal <i>M</i>_ns. Overall, it is shown that proper and precise tuning of both donor and acceptor polymer <i>M</i>_ns is critical for optimizing APSC performance. In contrast to reports where maximum power conversion efficiencies (PCEs) are achieved for the highest <i>M</i>_ns, the present two-dimensional <i>M</i>_n optimization matrix strategy locates a PCE “sweet spot” at intermediate <i>M</i>_ns of both donor and acceptor polymers. This study provides synthetic methodologies to predictably access conjugated polymers with desired <i>M</i>_n and highlights the importance of optimizing <i>M</i>_n for <i>both</i> polymer components to realize the full potential of APSC performance