Jun 22, 2010
Mass-produced nanopatterns upgrade photodiodes
The definition of well arranged sub-wavelength nanopatterns on optoelectronic devices can be a complicated or expensive task. Now, a new nanopattern definition technique using spin-coated nanosphere lithography has been proposed by researchers in Taiwan and applied to Si based solar cells and photodetectors. The photodiodes fabricated by the team demonstrate excellent light absorption efficiency (photoresponsivity). Moreover, the nanostructured devices have much higher acceptance angles than samples with a planar surface.
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 nanotechweb.org.
More details can be found in the journal Nanotechnology.
About the author
The research was conducted by Dr Cheng-Pin Chen (now at TSMC), Miss Pei-Hsuan Lin (now at TSMC), Mr Yen-Jen Hung and Mr Shao-Sun Hsu under the instruction of Prof. JianJang Huang at the Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taiwan. Prof. Huang's group is focused on applying nanotechnology to optoelectronics and electronics.