Although the solar-thermal technology has opened up a potential green energy harvesting method, it is challenging to suppress the non-negligible energy dissipation while maintaining a high absorbance. Most disordered organic polymers are almost incapable of limiting the absorption in the desired cutoff wavelength range, which is detrimental to the design of selective absorbers. Moreover, the development of absorbers with a periodic plasmonic nanostructure is always lacking in cost-effective scalability. Herein, we report a scalable selective absorber with a quasiperiodic nanostructure composed by an economical widespread surface self-assembly of densely arranged Fe3O4 nano-particles, possessing a high-performance energy conversion for low-grade solar energy. By investigating the scale effect of the quasiperiodic densely arranged plasmonic nanostructure, a significant solar absorption >94% and ideal passive suppression of thermal emissivity <0.2 can be obtained simultaneously. With the synergy of material properties, thermal management, and environmental effect, a flexible planar solar thermoelectric harvester is demonstrated under natural sunlight (AM1.5G), reaching a significant sustaining open-circuit voltage of >20 mV/cm2, without a heat sink. This highly versatile strategy is expected to lead the exploration of energy evolution in fundamental research and pioneer next-generation, high-performance, economical, and practical solar co-harvesting systems.