The impact of alkyl side‐chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star‐shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyanovinyl units derivatized with various alkyl end‐capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure‐properties relationships. UV‐vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influences the photovoltaic properties of these small‐molecule‐based BHJ devices. Solution‐processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current (Jsc) and fill factor (FF), with an efficiency as high as 4.76% without any post‐treatments; these are among the highest reported for solution‐processed OSCs based on star‐shaped molecules.

Length of the terminal alkyl chains at dicyanovinyl (DCV) groups of two dithienosilole (DTS) containing small molecules (DTS(Oct)2‐(2T‐DCV‐Me)2 and DTS(Oct)2‐(2T‐DCV‐Hex)2 ) is investigated to evaluate how this affects the molecular solubility and blend morphology as well as their performance in bulk heterojunction organic solar cells (OSCs). While the DTS(Oct)2‐(2T‐DCV‐Me)2 (a solubility of 5 mg mL−1) system exhibits both high short circuit current density (J sc) and high fill factor, the DTS(Oct)2‐(2T‐DCV‐Hex)2 (a solubility of 24 mg mL−1) system in contrast suffers from a poor blend morphology as examined by atomic force morphology and grazing incidence X‐ray scattering measurements, which limit the photovoltaic properties. The charge generation, transport, and recombination dynamics associated with the limited device performance are investigated for both systems. Nongeminate recombination losses in DTS(Oct)2‐(2T‐DCV‐Hex)2 system are demonstrated to be significant by combining space charge limited current analysis and light intensity dependence of current–voltage characteristics in combination with photogenerated charge carrier extraction by linearly increasing voltage and transient photovoltage measurements. DTS(Oct)2‐(2T‐DCV‐Me)2 in contrast performs nearly ideal with no evidence of nongeminate recombination, space charge effects, or mobility limitation. These results demonstrate the importance of alkyl chain engineering for solution‐processed OSCs based on small molecules as an essential design tool to overcome transport limitations.

X-ray methods have been used to analyze the supramolecular tubular structures in the hexagonal solid state and columnar hexagonal liquid crystalline ( &, ) phases of a polymethacrylate with tapered side groups: specific~y,poly(2-(2-[2-(2-(methacryloyloxy)ethoxy)ethoxylethoxy)ethyl3,4,5-tris(@-(dodecyloxy)-benzyl)oxy)benzoate), abbreviated as 12-ABG-4EO-PMA. Oriented fibers were drawn from the liquid crystalline phase at -60 O C and annealed for several days at 4 "C. The X-ray pattern for these fibers recorded at room temperature (25 "C) contained eight equatorial reflections that are orders predicted for a hexagonal unit cell with a = b = 60.4 A. The similarity of the data to those for unoriented specimens in the & phase suggests that both phases contain the same type of supramolecular cylindrical moieties. In addition, wideangle maxima are observed on the equator and two layer lines that suggest formation of a three-dimensionallyordered structure at room temperature. These data give the first available information on the supramolecular structure within the columns. The layer line spacings define a repeat of c = 5.03 A along the column axis, containing eight monomeric units based on the observed density. Strong off-meridional maxima at d = 4.30 and 3.84 A on the first layer line suggest that the "planes" of the aromatic moieties are tilted rather than perpendicular to the cylinder axis. Possible "pine tree" models are discussed in which the tapered side groups are stacked in 8-fold layers or form &fold helices within the columns. Increasing the temperature to 60 O C leads to a reduction in the cylinder diameter to 58.0 A. Stacking correlations remain along the fiber axis direction, but otherwise the internal structure of the cylinder is muchmore disordered than at room temperature. On cooling, the three-dimensional order is restored.

X-ray methods have been used to investigate the changes in structure with temperature for the supramolecular structure formed by a poly(methacry1ate) with tapered side chains. The polymer is poly[2-[2-[2-[2-(methacryloyloxy)ethoxylethoxylethoxyle~yl 3,4,5-tris[[p-(dodecyloxy)benzylloxylbenzoate]. Oriented fibers drawn from the columnar hexagonal (&J liquid crystalline phase at -60 "C adopt an ordered structure at room temperature, in which ordered cylindrical moieties are packed on a hexagonal lattice with a x 60 A. The internal structure of the cylinder has an axial repeat of c = 5.03 A, which contains eight monomer units based on the density. Integral half-widths for the small-angle and wideangle maxima yield lateral crystallite widths of 820 and S50 A, respectively. These data show that there is a high degree of perfection in the packing of the cylinders, but there is little or no correlation of the internal structure from one cylinder to the next. At the transition to the &, phase at -40 "C there is a loss of order in the structure within the cylinder: all that remains are limited correlations due to stacking of the side chains along the axial direction. The separation between the stacked units increases with temperature, while the cylinder diameter decreases steadily from 60.4 A at 39.9 "C to -54 A at 95 "C.These structural changes are correlated with striking changes in the dimension of the fibers with temperature. The dimensions show little change with temperature in the ordered solid state, but after passing through the transition to the & phase there is a -28% increase in length and -14% decrease in width on increasing the temperature from 40 to 95 "C, just below the &-to-isotropic transition, pointing to a progressive rearrangement of the supramolecular organization.