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  1. Abstract

    Although cyclic voltammetry (CV) measurements in solution have been widely used to determine the highest occupied molecular orbital energy (EHOMO) of semiconducting organic molecules, an understanding of the experimentally observed discrepancies due to the solvent used is lacking. To explain these differences, we investigate the solvent effects onEHOMOby combining density functional theory and molecular dynamics calculations for four donor molecules with a common backbone moiety. We compare the experimentalEHOMOvalues to the calculated values obtained from either implicit or first solvation shell theories. We find that the first solvation shell method can capture theEHOMOvariation arising from the functional groups in solution, unlike the implicit method. We further applied the first solvation shell method to other semiconducting organic molecules measured in solutions for different solvents. We find that theEHOMOobtained using an implicit method is insensitive to solvent choice. The first solvation shell, however, producesEHOMOvalues that are sensitive to solvent choices and agrees with published experimental results. The solvent sensitivity arises from a hierarchy of three effects: (1) the solute electronic state within a surrounding dielectric continuum, (2) ambient temperature or solvent atoms changing the solute geometry, and (3) electronic interactions between the solute and solvents. The implicit method, on the other hand, only captures the effect of a dielectric environment. Our findings suggest thatEHOMOobtained by CV measurements should account for the influence of solvent when the results are reported, interpreted, or compared to other molecules.

     
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  3. Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n‐type electrical doping in the poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density–voltage behaviors and external quantum efficiency (EQE) spectra. Using drift‐diffusion modeling, Hall effect, and capacitance–voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Factors that influence electrical doping and device behaviors, such as difference between PEI and another polymeric ETL, polyethylenimine ethoxylated, and the active layer thickness and energy of negative integer charge‐transfer state are elucidated. These findings provide insight into material selection and device design for organic electronic devices.

     
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