Currently, Sn–Bi solders have been singled out by PV manufacturers as the only possible Pb-free replacement in response to possible updates on RoHs and WEEE regulations due to its low temperature and low cost. Application of Sn–Bi solders on PV modules, however, led to cell breakages due to the inherent brittleness of coarse Bi-containing phases. The novelty of this research lies in the successfully reduced brittleness of Sn–Bi solder through reinforcement with a plant-based compound. Various weight percentages of activated bamboo charcoal (ABC) were added to the Sn–Bi solder to create single lap joints which underwent accelerated aging at 120 °C for 21 and 28 days, respectively. The predominant failure mode indicated that intermetallic compound layer (IMC) failure modes occurred for all Sn–Bi samples, while only samples containing at least 0.75wt.% ABC failed via solder failure mode. This indicates that brittleness of Sn–Bi solder is reduced with increasing ABC content and is reflected in the improvement of shear strengths between 34 and 65%. Presence of blocky Bi-containing phases along the IMC layer in Sn–Bi samples increased the stress concentration on the IMC layer which led to brittle failure. ABC was homogenously distributed in the solder and caused pinning effects resulting in refined morphology of the Bi-containing phases, thereby reducing the brittleness. This effect is more pronounced when at least 0.75wt.% ABC was added.