Understanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) film we have been able to spin‐coat [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non‐crosslinked P3HT film. Using time‐resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near‐uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 μm were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.

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Phosphorescent poly(dendrimers) with a norbornene-derived backbone have been synthesized using ringopening metathesis polymerization with the Grubbs III catalyst. The dendrimers are comprised of a heteroleptic iridium(III) complex core with two 2-phenylpyridyl ligands and a phenyltriazolyl ligand, biphenyl-based dendrons, and 2ethylhexyloxy surface groups. The phenyltriazolyl ligand provides the attachment point to the polymer backbone, and the two poly(dendrimers) differ in the number of dendrons attached to the 2-phenylpyridyl ligands. The poly(dendrimer) with one and two dendrons per ligand are termed mono-and doubly dendronized. The mono-and doubly dendronized poly(dendrimers) were found to have relatively narrow polydispersities, around 1.4, viscosities approaching those required for inkjet printing, and could be solution processed to form thin films. The dendrons played an important role in controlling the photophysical properties of the materials. The parent homopolymer with the same iridium(III) complex but no dendrons attached to the ligands had a solution photoluminescence quantum yield (PLQY) of 48%. The solution PLQY was found to increase with increasing number of dendrons with the mono-and doubly dendronized materials having solution PLQYs of 65% and 71%, respectively. The increase in PLQY is due to decreased intrachain interchromophore interactions. A similar trend was observed in the solid state with the parent, mono-, and doubly dendronized polymers having film PLQYs of 2%, 44%, and 58%, respectively, demonstrating that both intra-and interchain interactions are controlled by the dendrons.

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