Effects of alkyl side chains of double-cable conjugated polymers on the photovoltaic performance of single-component organic solar cells

In this work, the effects of alkyl side units of double-cable conjugated polymers on the photovoltaic performance of single-component organic solar cells (SCOSCs) were systematically studied. Four double-cable conjugated polymers with the same conjugated backbone, linker and naphthalene diimide (NDI) side units, but with distinct alkyl units from ethylhexyl, butyloctyl, hexyldecyl to octyldodecyl were designed and synthesized. These polymers exhibit similar absorption spectra and frontier energy levels, but show distinct crystallinity, charge carrier mobilities and photovoltaic performance. Several advanced techniques, including differential scanning calorimetry, atomic force microscopy and grazing-incidence wide-angle X-ray, reveal that double-cable conjugated polymers with longer side units exhibit high solubility, resulting in large fibrillar structures and low crystallinity. Therefore, double-cable polymers with longer alkyl side units exhibit low charge carrier mobilities and low efficiencies in SCOSCs. Our universal studies provide a good example to show the effect of alkyl side units in double-cable polymers on the performance of SCOSCs, which is useful for designing new double-cable conjugated polymers.