Palladium-Assisted Reaction of 2,2-Dialkylbenzimidazole and Its Implication on Organic Solar Cell Performances

Abstract

A 2,2-dimethyl-2H-benzimidazole (22MBI) pulling unit has been synthesized as a potential high-solubility substitute for benzothiadiazole and incorporated into a push−pull-type copolymer used for decent-efficiency (∼3%) organic photovoltaic devices. We herein replace the two methyl side groups of 22MBI by longer alkyl (ethyl, butyl, and hexyl) side chains to further improve the solubility. However, the copolymers replaced by the new pulling units, 2,2-diethyl/dibutyl/dihexl-2H-benzimidazole (22EBI/22BBI/22HBI), lose favorable optical characteristics and exhibit negligible (<0.5%) power conversion efficiency. Intrigued by this anomalous side-group effect of 2,2-dialkyl-2H-benzimidazole (22BI), we carry out time-dependent density functional theory calculations on a series of 22BI-based copolymers with various lengths of 2,2-dialkyl side chains (methyl, ethyl, butyl, and hexyl), but no discernible difference in equilibrium structure nor in electronic structure is found between them. We hence formulate a hypothesis that 22BI may either isomerize into 1,2-dialkylbenzimidazole (12BI) or lose one of its alkyl (≥ethyl) chains as olefin to become 2-dialkylbenzimidazole (2BI) because these aromatic products, unlike the quinoidtype 22BI, would exhibit unfavorable electronic structure for organic photovoltaic (OPV) applications. Indeed, the absorption spectra measured for the 22BI-based copolymers with long dialkyl side chains are best reproduced by the calculations on 2BI-and 12BI-based copolymers, and the two side products are calculated to be more stable than 22BI, indicating the spontaneity of the proposed reactions. The activation barriers are prohibitively high (>29 kcal/mol) but could be reduced down to 8 kcal/mol in the presence of palladium-based polymerization catalysts. Indeed, the presence of the predicted 12BI-containing side products is confirmed by NMR spectra. A temperature-dependent polymerization experiment shows that 22MBI is in fact subject to the same type of isomerization when the temperature is raised to 150°C above the original polymerization temperature (90−110°C ), further supporting the hypothesis from our calculations and explaining the observed anomalous side-group effect.

Ionic Liquid Designed for PEDOT:PSS Conductivity Enhancement

“…The rotational and translational contributions to ΔΔ G 0→298K are estimated by the equipartition theorem and the ideal gas approximation. , Such an approach has been shown suitable to describe PEDOT-complex conformations . We choose B3LYP despite concerns about its reliability in describing long-range charge transfer because our previous B3LYP calculations have reproduced surprisingly well the UV/visible absorption spectra of charge-transfer (donor–acceptor) compounds − and also because the binding energies and the ion exchange energies calculated with B3LYP are within 4% of error (see Supporting Information) from those calculated with a long-range-corrected (or range-separated hybrid) LC-ωPBE functional , after tuning the range-separation parameter ω to 0.17 au –1 using Gaussian09 . The final geometries, where the PTS and TCB anions are bound to tri-EDOT mostly from the side (Figure and Supporting Information), capture the binding geometries observed by X-ray crystallography on PEDOT:PTS crystals ,, and MD simulations on PEDOT:TCB clusters (Section and Supporting Information).…”

N-Alkylthienopyrroledione versus benzothiadiazole pulling units in push–pull copolymers used for photovoltaic applications: density functional theory study

“…1) in the same manner as done on 1-3 in our previous study 24 (but including the solvent effect in the current study), expecting more favorable backbone structure, electronic structure, optical properties, and PCE values for 4-6 than for 1-3. Our calculation scheme has been carefully validated against various polymers of such kind and proved reliable for predicting PCE, 24,25,56,57 but we are surprised to find essentially the same electronic structure and PCE predicted for both classes of polymers in contrast to the observation (Sections 2.1 and 3.1). Intrigued by this discrepancy, we subsequently perform DFT calculations on their p-stack-paired monomer models (n = 1, Fig.…”

“…The same type of calculation as done in our previous studies 24,25,56,57 is carried out using Jaguar v6.5 58,59 and Gaussian09. 60 The optimized structures of the monomer and dimer models of BT-based 1-3 are taken from our previous study 24 to build the monomer and dimer models of the TPD-based 4-6 (n = 1-2; Fig.…”

Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values

“…OSCs prepared using low molecular weight molecules are attracting interest due to their small losses in V oc in comparison with other organic semiconductor materials . Indeed, compared to the their organic counterparts, the semiconductor polymers, the so-called “ small molecules (SMs) ” have the advantage of not being dependent on regioregularity issues, differences in molecular weight, and difficult purification processes for cleaning the metal catalysts used in the cross-coupling chemical reactions. − In fact, it is of utmost importance that the synthesis of the small molecules is straightforward with only a few synthetic steps and an easy to scale process. Moreover, to achieve efficient light harvesting, it is also desirable that the processed organic molecules extend their absorbance as far into the IR as possible .…”