This work investigates the effects of the solvent additive, 1,8-octanedithiol (1,8-ODT), on solution-processed inverted bulk heterojunction (BHJ) organic solar cells based on a thin film blend of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] (PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). In this work, varying concentrations of 1,8-ODT were added into the polymer solution in order to optimise the device performance. The UV–Vis spectrometer measurements showed that adding 1,8-ODT can improve the magnitude of light absorption in the blend film, as well as inducing an ∼40 nm red shift for long wavelengths. External quantum efficiency (EQE) measurements displayed improvements at all wavelengths for 1–4 vol% 1,8-ODT concentration, when compared to the reference device. Current density–voltage (J–V) measurements confirmed this addition of photocurrent, as the short circuit current density (Jsc) increases by 23%, from 9.89 mA cm−2 for the reference to 12.14 mA cm−2 for the 1 vol% additive cell. The highest average device efficiency of 2.59% was obtained using a 1,8-ODT concentration of 3 vol%. Further increasing additive concentration caused a reduction in efficiency. Additionally, the effect of drying the additive processed films in a vacuum was investigated. Drying in a vacuum had little effect on films processed with low additive concentration. However, the vacuum drying process led to a significant efficiency enhancement for high 1,8-ODT concentrations. The efficiency of the 4 vol% 1,8-ODT devices increased by 25% as a result of drying in a vacuum. This was due to the removal of excess residual additive from the active layer.