Nanoscale Ordered Structure Distribution in Thin Solid Film of Conjugated Polymers: Its Significance in Charge Transport Across the Film and in Performance of Electroluminescent Device

We use ultrahigh-vacuum conducting atomic force microscopy to probe the local current distributions in spin-cast thin films from the solutions of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(9,9-di-n-octyl-2,7-fluorene) (PFO). We found that spatially homogeneous distribution of the ordered structures (well-packed chains and/or aggregates) in MEH-PPV can be controlled by the selection of solvent or mixed solvent, by which effects of spatial charge transport distribution in MEH-PPV thin films on the performance of polymer light-emitting diodes (PLEDs) are unambiguously clarified. For PFO thin film, after the treatment by immersing in the mixed nonsolvent composed of a solvent and nonsolvent, the ordered structures (β-phase) are generated; its excess content can result in highly conducting regions. However, the device efficiency can be promoted significantly by optimizing the content of β-phase

Well-Packed Chains and Aggregates in the Emission Mechanism of Conjugated Polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions)

Fine Tuning the Purity of Blue Emission from Polydioctylfluorene by End-Capping with Electron-Deficient Moieties

We propose a simple way to achieve pure blue emission and improved device efficiency via capping poly(9,9-dioctylfluorene) (PFO) with electron-deficient moieties (EDMs, such as oxadiazole (OXD) and triazole (TAZ)), which can induce a minor amount of long conjugating length species (regarded as β phase) to control extents of energy transfer from amorphous matrix to the β phase. The device efficiency of PFO end-capped with TAZ is higher than that with para-tert-butyl phenyl (TBP) by a factor of 2 (with CsF/Al as cathode), and its electroluminescent spectrum remains stable and with pure blue emission during cyclic operations (C.I.E. color coordinates x = 0.165, y = 0.076, independent of operating voltage and within the limit for pure blue emission x + y < 0.30). The improvement of device efficiency is dependent on the structure of EDM, such as size and planarity. The deep blue emission is originated from the incomplete energy transfer from amorphous matrix to the β phase induced by the end-cappers

Structural and Optical Identification of Planar Side-Chain Stacking P3HT Nanowires

Poly­(3-hexylthiophene) (P3HT) is well-recognized for forming π–π stacked crystalline films and nanowires. In this study, we identified a new planar crystalline P3HT architecture assembled by hexyl side chains. The atomic force microscopy results indicated that both the π–π stacking and side-chain stacking nanowires were crystalline ribbons decorated with amorphous fringes. The different stacking structures of the nanowires led to different interchain coupling strengths and hence spectroscopic properties. Both types of nanowires were characterized using confocal microscopy and scanning near-field optical microscopy. The coexistence of the planar side-chain stacking structure in P3HT films and nanowires along with the π–π stacking architecture leads to multiplicity in their optical properties