Design, Fabrication and Characterization of n-Si Columnar Structures for Solar Cell Applications
Authors: Develioglu Aysegul , Trabzon Levent , Alphan Yunus 
Published: October 2018

Background:

Glancing Angle Deposition (GLAD) provides oblique deposition and substrate
motion to engineer thin film microstructures in three dimensions on nano scale. Using this technique zigzag,
chevrons, staircase, post, helical and various type of nanostructures including 3-D multilayers can be
obtained from various metals with controllable morphologies. The aim of the study is to increase surface
porosity and junction using GLAD method area for thin film solar cells and therefore to increase p-n junction
area. This provides efficient charge separation and strong light absorption.

Methods:

Glancing angle deposition using e-beam evaporation technique has been employed to create 3-
D silicon nano-structures on the surface. Al and Ag contact layers were deposited by thermal evaporation
technique. Hole-conductor polymer PEDOT: PSS was spin coated onto n type silicon thin film. Reflectance
spectra were measured using UV-VIS spectroscopy. Scanning electron microscopy was used to image
surface and cross-section with and without PEDOT: PSS. Also, transmission spectra of PEDOT: PSS
was measured using UV-VIS spectroscopy. Surface wettability properties and contact angles of silicon
samples were measured by contact angle measurement with water.

Results:

Columnar structures possess less reflection compared to the flat surface depending on surface porosity.
This phenomenon shows that these structures can be used as anti-reflection coatings for solar cells
and optical devices to decrease reflectivity and increase light harvesting with higher efficiency. Contact
angle decreases when surface roughness increases therefore we can see that columnar structures are more
hydrophilic compared to dense films. Flat silicon has 98° contact angle while columnar structures have
71° and 61°. PEDOT: PSS exhibits high transparency in the range from 200 to 1100 nm of wavelength of
light, which resembles to solar radiation inside the atmosphere. Also, SEM images of the samples show
that silicon columnar structures form better contact with PEDOT: PSS than flat surface.

Conclusion:

GLAD technique has been used to achieve homogenous rough surface by e-beam evaporation.
Both cross-sectional and top-view SEM images show that columnar structures have higher porosity
than flat surfaces. The response of UV-VIS spectroscopy shows that columnar structures have less
reflection due to highly porous surface. With increasing incident flux angle, antireflection property of the
surfaces was enhanced by surpassing the surface reflection. Due to the reduced hydrophobicity of porous
structures, organic polymer can be distributed homogenously in between the columnar structures with
increased p-n junction interface area. PEDOT: PSS is highly conductive, and it is highly transparent
material in the range of the wavelength typically seen in the solar radiation. This makes it easier for light
to reach to Si interface to generate electrons and holes. These results provide better understanding of
Si- based heterojunction solar cells efficiency improvement with surface modification. This study also
shows dependency of optical and electrical activity to surface geometry and surface porosity.