In this research, n-dodecylthiol was added to P3HT/PC70BM polymer solar cells (PSCs) to improve the crystallinity of P3HT and enhance the phase separation of P3HT/PC70BM. Crystallinity of P3HT:PC70BM doped with 0–5% by volume of n-dodecylthiol was measured using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. Both methods showed improvement in crystallinity, which resulted in improving the power conversion efficiency (PCE) of polymer solar cells by 33%. In addition, annealing at 150 °C for 30 min showed further improvement in crystallinity with n-dodecylthiol concentration up to 2%. The highest power conversion efficiency of 3.21% was achieved with polymer crystallites size L of 11.2 nm, after annealing at 150 °C for 30 min under a vacuum atmosphere. The smaller crystallite size suggests a shorter path of the charge carriers between P3HT backbones, which could be beneficial to getting a higher short circuit current in the devices made with the additive. Kinetics study of P3HT:PC70BM crystallinity using Avrami model showed a faster crystallization rate (1/t0.5) at higher temperatures.