Prof. JianJang Huang and his group from National Taiwan University have developed a spin-coated nanosphere lithography method to fabricate nanopatterns on optoelectronic devices such as light-emitting diodes (LEDs), nanorod LED arrays, photodiodes and solar cells. The nanopatterns are realized by first spin-coating a monolayer of silica nanoparticles, which are resolved in IPA (isopropyl alcohol) solvent, on the sample and then etching. The silica nanoparticles with a diameter of 100 ± 10 nm act as a hard mask during the pattern definition.

In their first application, the scientists simply spin-coated nanoparticles on the n-type ZnO/p-type Si photodiode surface. The patterning step caused a dramatic increase in the acceptance angle from 23° for the planar structure to 50° for the device with the nanosphere coating (at the wavelength 550 nm).

In the second approach, the group fabricated a photodiode with a nanocone profile by depositing n-type ZnO and intrinsic amorphous Si(a-Si) layers on the nanopatterned p+-Si substrate. The sample demonstrates a 36.2% enhancement in photoresponsivity compared with a planar device. The results show a main absorption peak at the green wavelength range that corresponds to a-Si absorption. Furthermore, the acceptance angle measurement reveals that nanostructured photodiodes have larger acceptance angles than planar samples. The study also shows that the nanocone structure has higher acceptance angle than a nanorod structure.

The above results indicate that spin-coated nanosphere lithography has the potential to be applied to solar cells. To follow this up, the researchers now plan to improve the efficiency of thin film solar cells on a glass substrate in the lab. "We will integrate the concept of nanotechnology and graded bandgap structure into solar cells," Prof. J J Huang of National Taiwan University told

More details can be found in the journal Nanotechnology.