Efficient collection of incoming photons and photo-generated
carriers in Si nanostructure array solar cells
Yunae Cho, Eunah Kim, and Dong-Wook Kim*
Department of Physics, Ewha Womans University, Korea
*
corresponding author: dwkim@ewha.ac.kr
Joondong Kim
Department of Electrical Engineering, Incheon National University, Korea
Abstract- Si nanostructure array enables much improved broadband and omnidirectional optical
absorption, compared with conventional light trapping strategies. We achieved a very high
photocurrent density of 36.94 mAcm-2 from our nanoconical frustum array crystalline Si (c-Si) solar
cells. Optical simulation studies showed that the expected photocurrent of 10-m-thick
nanostructured cells could slightly exceed the Lambertian limit with the help of remarkable
antireflection (AR) effects and efficient carrier collection capability.
There have been intensive research efforts to realize high efficiency Si nanostructure array solar cells, since
the nanostructured Si wafers can significantly lower the optical reflection in broad wavelength range. The
remarkable AR effects are originated from graded refractive index, multiple reflection, and Mie resonance.1,2
The measured photovoltaic (PV) performance of nanostructured cells, however, often cannot surpass that of
conventional cells, despite their substantially enhanced optical absorption. This suggests that the collection
capability of photo-generated carriers as well as the absorption of incoming light should be carefully considered
for the nanostructured solar cells.
In this work, we performed comparative experimental and simulation studies of hexagonal nanoconical
frustum array solar cells to provide insight into improving performance of the nanostructured Si PV devices. The
power conversion efficiency of the nano-patterned cell was 15.8%, much higher than that of the flat cell, 14.3%.
The optical simulations showed that the optical gain of the nano-patterned cells maintained, as the absorber
thickness was reduced to 10 m. The expected photocurrent of the 10 μm thick cell even slightly exceeded the
Lambertian limit, due to the increase in absorption for light having a wavelength of >1 μm. The optical benefits
of the nanostructure array, in particular the substrate-coupled Mie resonance, also caused strong concentration of
light near the top surface of the nano-patterned cells. As a result, the highest photon density was expected to
reside near the p–n junction, where the built-in potential readily separates the electron–hole pairs. This would
enable very efficient carrier collection, only if proper surface passivation could suppress electrical loss.
REFERENCES
1. Kim, J. et al., “Transparent conductor-embedding nanocones for selective emitters: optical and electrical
improvements of Si solar cells,” Sci. Rep. Vol. 5, 9256, 2015.
2. Cho, Y. et al., “Wafer-scale nanoconical frustum array crystalline silicon solar cells: promising candidates
for ultrathin device applications,” Nanoscale Vol. 6, 9568-9573, 2014.