The electrospray (ES) process has emerged as a scalable and efficient fabrication method for organic photovoltaic cells (OPVs). However, the ES process could often involve numerous parameters, which impose a major challenge on uncovering the interplay among process, morphology of active layers, and device performance. This work attempts to reduce the parameter space and capture the essence of the ES process using the Damkhöler (Da) number of evaporation, which is the ratio of the droplet residence time over evaporation time. We first derived an explicit equation for Da that links nine different parameters affecting the process. Experimental results indicate that Da number shows strong effect on morphology and crystallinity of the active layer composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Most remarkably, the power conversion efficiency exhibits monotonically dependence on Da values spanning more than one order of magnitude, e.g., from 0.13 to 1.52. This finding suggests that Da number analysis can reduce the large parameter space in electrospray deposition, thus provides a simple way to control and predict the morphology of active layer and the performance of the solar cells.