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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-DCVMe)2 (a solubility of 5 mg mL-1 ) system exhibits both high short circuit current density (Jsc) 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. [ABSTRACT FROM AUTHOR]

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 ( Jsc) 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. [ABSTRACT FROM AUTHOR]

The synthesis of a series of novel A-π-D-π-A oligomers bearing either electron-donating dithieno[3,2- b :2′,3′- d ]silole (DTS) or dithieno[3,2- b :2′,3′- d ]pyrrole (DTP) units linked through a bithiophene π-bridge with the electron-withdrawing methyldicyanovinyl (DCV) or N-ethylrhodanine (Rh) groups is described. In order to evaluate the effects of different donor-acceptor combinations on various physical properties of the oligomers they are comprehensively studied by UV–Vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and cyclic voltammetry (CV). Replacing the DTS by DTP as well as the DCV by Rh leads to increasing of HOMO energy levels and slight decreasing of LUMO energy levels, respectively. The DTP-based oligomers have a red shift in absorption spectra and an increased solubility as compared to the DTS-based analogs. Replacing the acceptor DCV unit by Rh also results in significantly higher solubility, but the Rh-based oligomers exhibit weaker intermolecular interactions, poorer optical absorption of sun light as well as lower charge carrier mobility in blends. Altogether, the structural improvement of the DTS-DCV and DTP-DCV blends upon annealing correlates well with the observed photovoltaic performances in organic solar cells. These results give more insight how to fine-tune and predict physical properties and photovoltaic performance of small A-π-D-π-A molecules having different donor-acceptor combinations in their chemical structures and thus providing a molecular design guideline for the next generation of high-performance photovoltaic materials. [ABSTRACT FROM AUTHOR]