Certain solvent additives significantly affect the morphology of the active layer in bulk heterojunction (BHJ) conjugated polymer solar cells and improve the device performance. Previous examinations of the BHJ films have shown that the best additives are characterized by higher boiling points than the host solvent and are poorer solvents for the conjugated host polymer than for the fullerene acceptor; however, little in the way of a mechanistic explanation has been presented, particularly on the dynamics of the transition from solution to the bulk material. This article combines spectroscopic analysis in various solvent mixtures and during solvent evaporation to show that a key feature of the film growth concerns aggregation of polymer chains into more ordered supramolecular structures prior to complete drying. The driving force for aggregation occurs in a medium that (a) is more fluid and allows chains to find optimal registry or conformations and (b) can solvate the fullerenes. We propose that when a single solvent that is good for the two components of the BHJ blend is used, the chains remain solvated up to a point where viscosity inhibits their motion and they are unable to attain similar packing characteristics.

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A new POSS (polyhedral oligomeric silsesquioxane)-substituted polyfluorene was synthesized from the nickel-catalyzed Yamamoto coupling reaction. The synthesized polymers could be well characterized by 1 H NMR, FT-IR, and elemental analysis. PL spectra of PFPOSSs on the quartz film showed reduced aggregation/excimer formation because the bulky POSS group prohibited interchain interactions. This effective dilution effect and the high thermal stability of the POSS unit also improved the color stability of PFPOSSs blue emission even after thermal annealing at 150 °C. The fluorescence quantum yields (Φ FL) of PFPOSSs in both solution and solid state were higher than those of poly(dialkylfluorene)s. Moreover, the ITO/PEDOT-PSS/polymer/Ca/Al LED device using this polymer as emitting layer showed a very stable blue light emission. LED devices of PFPOSSs showed a low turn-on voltage of 3.7-4.4 V, high brightness of 350-1010 cd/m 2 , and external efficiencies of 0.11-0.36%.

The first novel polyfluorene copolymers containing siloxane bridges were synthesized by Ni(0)-mediated copolymerization between 2,7-dibromo-9,9′-dihexylfluorene and a bridged fluorene monomer containing siloxane linkages. Two such bridged copolymers were prepared: PSiloBg1 (containing 1 mol % siloxane-bridged fluorene unit) and PSiloBg3 (containing 3 mol % siloxane-bridged fluorene unit). PSiloBg1 and PSiloBg3 exhibited good solubility in common organic solvents, thermal stability up to 420°C, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110°C , respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3 exhibited a polymerization yield (96%) and a molecular weight (MW ) 185 000) higher than those of PDHF (polymerization yield ) 62%, MW ) 82 000). Interestingly, after the bridged polymers had been annealed at 150°C for 4 h in air, their PL spectra showed no significant increase in vibronic structures at 450 and 475 nm and no evidence of aggregation formation and excimers at wavelengths of >500 nm. In addition, the full width at half-maximum (fwhm) of the bridged polymers was very small (fwhm ) 51-52 nm) compared to that of PDHF (fwhm ) 85 nm). Collectively, the results show that the polyfluorenes with siloxane bridges exhibit thermally stable almost pure blue emission, making these polymers promising candidate materials for device applications.

A series of copolymers, poly{9,9-bis(2‘-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanovinyl)-1-(2‘-ethylhexyloxy)-4-methoxybenzene-5‘ ‘,5‘ ‘‘-diyl} (PFTCVB), were synthesized from the monomers 2,7-dibromo-9,9-bis(2‘-ethylhexyl)fluorene and 2,5-bis(2-(5‘-bromothienyl)-1-cyanovinyl)-1-(2‘ ‘-ethylhexyloxy)-4-methoxybenzene (BTCVB) through the Ni(0)-mediated polymerization. The copolymers were characterized using FT-IR spectroscopy, UV−vis spectroscopy, TGA, photoluminescence (PL) and electroluminescence (EL) spectroscopy, elemental analysis, and molecular weight studies. The synthesized copolymers showed an absorption maximum at about 380 nm, and between 425 and 600 nm the absorption increased with increasing fraction of thiophene-containing monomer 4 (BTCVB). In PL, the emission maxima of the copolymers were red-shifted as the fraction of BTCVB increases, despite the copolymers showing little variation in UV−vis absorption characteristics. Light-emitting devices were fabricated in an ITO (indium−tin oxide)/PEDOT/polymer/LiF/Al configuration. The EL spectra showed similar emissions to the PL results, and the copolymer containing 15 mol % of BTCVB showed bright-red emission.