Lowering the quantum defect by tandem pumping with fiber lasers at 1018nm was critical for achieving the record 10kW single-mode ytterbium fiber laser. Here we report the demonstration of an efficient directly-diode-pumped single-mode ytterbium fiber laser with 240W at 1018nm. The key for the combination of high efficiency, high power and single-mode at 1018nm is an ytterbium-doped 50μm/400μm all-solid photonic bandgap fiber, which has a practical all-solid design and a pump cladding much larger than those used in previous demonstrations of single-mode 1018nm ytterbium fiber lasers, enabling higher pump powers. Efficient high-power single-mode 1018nm fiber laser is critical for further power scaling of fiber lasers and the all-solid photonic bandgap fiber can potentially be a significant enabling technology.