We report three DTBDT-based small molecules with various end groups and the effects of end-group tuning on their photovoltaic properties. We designed and synthesized DTBDT-Rho, DTBDT-EC, and DTBDT-CN, which have 3-ethylrhodanine, dicyanovinyl, and ethyl cyanoacetate, respectively, as end groups. DTBDT-EC and DTBDT-CN have much deeper LUMO and HOMO energy levels than DTBDT-Rho due to the stronger electron withdrawal of their end groups. As expected, the three DTBDT-based small molecules exhibit disparate bulk heterojunction blend morphologies and photovoltaic properties. DTBDT-Rho exhibits superior miscibility with PC₇₁BM in blend films as well as crystalline structure within DTBDT-Rho domains, which result in solar cell devices with outstanding photovoltaic performances, namely PCEs above 7%. In contrast, DTBDT-CN gives rise to devices with poor performance of PCEs near 2%. It exhibits excessive molecular aggregation with less favorable molecular packing. Our study of end group tuning and morphological analysis provides valuable guidelines for the design of new small molecules and insight into their photovoltaic performances.