Low bandgap small molecular donors containing naphtho- [1,2-c:5,6-c’]bis[1,2,5]thiadiazole (NT) as the second acceptor block were designed and synthesized for fullerene solar cells. Based on the tactful material design and device engineering, the resulting small molecular solar cells (SM-OSCs) processed with a halogen-free solvent of CS2 exhibited a high power conversion efficiency (PCE) of 11.53% with a very small energy loss (Eloss) of 0.57 eV. To our best knowledge, such PCE and Eloss values are the best records for single-junction SM-OSCs reported so far.