Solution-processing has the potential to reduce the cost and energy requirements in the fabrication of organic light-emitting diode (OLED) devices. In non-doped emissive layers, polymers are commonly used but often suffer from batch-to-batch variation. Oligomers, which have precise molecular structures, avoid this problem but maintain the favourable film-forming properties that polymers possess. We present three benzothiadiazole-containing fluorene-based oligomers, BT-F3₂, BT-F4₂ and BT-F5₂, which have terfluorene, quaterfluorene and pentafluorene arms, respectively, attached to a benzothiadiazole core. The materials show very high photoluminescence quantum yields, with BT-F4₂ exhibiting the highest PLQY of 0.96 in the thin film. Although there is little change in the electroluminescence spectra, the oligomer size influences device performance. BT-F3₂ had an external quantum efficiency of 1.9 % and high luminance whilst BT-F4₂ reached an external quantum efficiency of 2.3 %. Furthermore, the stability of the OLED devices was evaluated and showed that the device lifetime decays with increasing molecular length.