Are these sentences ok? All-small-molecule organic solar cells (SM-OSCs) have attracted considerable attention owing to the merits of small molecules, such as easy purification, well-defined chemical structure and less batch-to-batch variation. To achieve high-performance SM-OSCs, the rational design of well-matched donor and acceptor materials to reduce energy losses is extremely essential. In this work, we developed a novel wide-bandgap small molecule donor, namely BTTzR, by firstly introducing the thiazolo[5,4-d]thiazole (TTz) unit as the building block. The optimized SM-OSCs based on BTTzR and Y6 exhibited an outstanding power conversion efficiency (PCE) of 13.9%. More importantly, the devices demonstrated very small non-radiative energy losses of 0.18 eV, which are similar to that of inorganic counterparts. This work indicates that BTTzR is a promising small molecule donor material for high-performance SM-OSCs application and provides a new sight of material design to reduce the non-radiative energy losses in the OSCs.