We introduce a cost-effective method of forming size-tunable arrays of nanocones to act as a three-dimensional (3D) substrate for hydrogenated amorphous silicon (a-Si:H) solar cells. The method is based on self-assembled tin nanospheres with sizes in the range of 20 nm to 1.2 μm. By depositing these spheres on glass substrates and using them as an etch mask, we demonstrate the formation of glass nanopillars or nanocones, depending on process conditions. After deposition of 150 nm thick a-Si:H solar cell p-i-n stacks on the glass nanocones, we show an output efficiency of 7.6% with a record fill factor of ∼69% for a nanopillar-based 3D solar cell. This represents up to 40% enhanced efficiency compared to planar solar cells and, to the best of our knowledge, is the first demonstration of nanostructured p-i-n a-Si:H solar cells on glass that is textured without optical lithography patterning methods.