Modelling and Simulation Single Layer Anti-Reflective Coating of ZnO and ZnS for Silicon Solar Cells Using Silvaco Software
In this study, simulated single layer Anti-Reflective
Coating (ARC) on silicon solar cell that based on the refractive index
limit of silicon dioxide (SiO2), zinc oxide (ZnO) and zinc
sulphide (ZnS) are presented. Two simulations of ZnO and ZnS coating were
simulated to compare with SiO2 ARC on silicon solar cell surface.
These simulations carried out with variable coating thickness that is
50, 60, 70 and 80 nm by using ATLAS simulator. From the simulation obtained,
it was found that the value of Voc and Jsc are 397.69
mV and 15.646 mA cm-2, respectively, from silicon solar cell
with 0.05 μm SiO2 coating. For the Fill Factor (FF) and
power conversion efficiency (η) of this solar cell is 0.758 and 4.72%
were computed. As for the ARC simulation, the spectral response of ZnO
and ZnS coating was increased around 600 and 700 nm, respectively, which
are capable of reducing the refractivity over a wide range of wavelengths
compared to SiO2 increased around 400 nm wavelength. This can
be concluded that when the refractive index value is high, the available
photocurrent also can be high in wide range wavelength and more reducing
the refractivity. In ARC analysis, the ZnS coating could perform more
efficiency on wide range of wavelength compared to SiO2 and
Silicon is a semi conductor optical material with relatively high refractive
index (Kavakli and Kantarli, 2002). It is an ideal material
for solar cell nowadays. Not only is silicon non-toxic, but it is also the second
most abundant element in the Earths crust (after oxygen) posing minimal
environmental or resource depletion issues if used on a large scale. Silicon
solar cells have attracted considerable attention as low cost and high efficiency
solar cells (Minemoto et al., 2007). The material
of pure silicon is shiny. When the light hit the silicon wafer, it can reflect
up to 35% of the sunlight. To reduce the amount of sunlight lost, an Anti-Reflective
Coating (ARC) is put on the silicon wafer. The most commonly used coatings are
titanium dioxide (TiO2) and silicon oxide (SiO), though others are
used. This case studied of zinc oxide (ZnO) and zinc sulphide (ZnS) material
as an ARC and to be compared to silicon dioxide coating, SiO2. A
good ARC is vital for solar cell performance as it ensures a high photocurrent
by minimizing reflectance (Wright et al., 2005).
Unlike many other optoelectronic devices, solar cells operate at a range of
wavelengths, from 300-1200 nm, which means they need a broadband ARC. To enhance
the short circuit current density (Jsc) of solar cells, an ARC is
fabricated on the surface of the solar cells. Si and Si-related materials (such
as SiO2) are of interest for solar cell work because they can possibly
be used as a surface passivating ARC or as the top cell in an all-silicon tandem
solar cell (Cho et al., 2002). Zinc oxide (ZnO)
has recently received growing attention, as this material can be produced with
superior electrical conductivity and optical transparency (Ellmer
et al., 2008). Meanwhile the zinc sulfide (ZnS) is a wide band gap
semiconductor with high refractive index and hence a promising material for
ARC over commercial silicon solar cells (Gangopadhyay et
al., 2004; Nasr et al., 2008).
Traditionally, the simulated electronic devices were microscopic while solar
cells had always been too big to be modeled (Baudrit et
al., 2005). The Silvaco software package is a simulation software tool
targeting the area of electronic design (ATLAS Users
manual et al., 1998).
One of its major products is the Virtual Wafer Fabrication (VWF) package. This
is a large suite of highly sophisticated tools that aid in the design and development
of all types of semiconductor and VLSI devices. The Silvaco ATLAS software is
already studied in solar cell application (Micheal et
al., 2005). ATLAS predicts the electrical characteristics of physical
structures by simulating the transport of carriers through a two-dimensional
grid. The accuracy of this physically based simulation tool depends greatly
on the accuracy of the material parameters used in constructing the solar cell
model. Important parameters needed for solar cell modelling in ATLAS include
band gap energy, electron and hole state densities, electron and hole mobilities,
permittivity, electron affinity, radiative recombination rate and optical parameters.
One of the most critical parameters for advanced solar cell modelling is the
correct definition of the refractive index, n and the extinction coefficient,
k, for a material. Once a solar cell is simulated in ATLAS, it may be illuminated
with a constant wavelength of light or a complex spectrum such as Air Mass Zero
(AM0) spectrum, which represents the solar spectrum for earth orbiting spacecrafts.
To enter the structure and composition of a solar cell into ATLAS, several parameters
must be defined. The most important physical parameters in ARC design are the
refractive index (n) and film thickness (d) (Minemoto et
al., 2007). By using a numerical analysis, we could calculate the External
Quantum Efficiency (EQE) and the performance of the device taking into account
many material and structure parameters.
MATERIALS AND METHODS
Silvaco ATLAS is a physically based device simulator that predicts the electrical
characteristics that are associated with specified physical design and bias
conditions (Baudrit et al., 2005). Physically
based simulation is very different from analytical modeling that provides efficient
approximation and interpolation but does not provide insight, or predictive
capabilities, or encapsulation of the theoretical knowledge.
Modelling solar cell structure: The solar cell that has been chosen
for test is made in usual method in VLSI. An orientation silicon wafer
of <111> with 50 μm thickness and 1x1014 atom cm-2
boron concentration was chosen. The 1 μm p-n junction was developed
by implant of phosphorus with 1x1015 atom cm-2 and
energy is 110 ev. The diffuse time 300 min and the temperature 900 °C
are constant. The ATHENA software is used to design the solar cell structure
with the area 50×50 μm2.
||Silicon solar cell structure with SiO2 coating
when applying 90° incident light
The next process is applying
voltage by using ATLAS simulator to compute open circuit voltage, Voc and short circuit current density, Jsc. Then the 90° incident
light angle is applying on the top of silicon solar cell to trace the
reflectance in silicon wafer. The complete virtual solar cell structure
shows in Fig. 1.
Simulation of anti-reflective coating: The second step is to simulate
the spectral response of variable coating thickness by using ATLAS simulator.
The mesh of solar cell has been created to 50x50 μm2,
then the substrate material is setting up to silicon. The doping concentration
of boron is fixed to 1x1014 atom cm-2 of p-type.
In this analysis, the beam is fixed to 90° angle and the ARC layer
is setting up to variable thickness (50 to 80 nm) to plot the spectral
response graph. Table 1 shows the input data used for
ARC analysis simulated by ATLAS simulator. Only using ATLAS simulator
can do the ARC simulation by producing spectral response graph to calculate
the quantum efficiency of each ARC.
RESULTS AND DISCUSSION
Electrical properties of silicon solar cell: It is described earlier,
the solar cell 2D devices have been modelled to compare the electrical
properties between silicon solar cell with no SiO2 and 0.05
μm SiO2 coating. Figure 1 shows the
basic structure of silicon solar cell deposited 0.05 μm SiO2
layer when applying 90° incident light.
||Graph of J-V characteristics of silicon solar cell with
0.05 μm SiO2 and no SiO2 coating
|| The electrical properties of virtual silicon solar
Figure 1 shows the direction of incident light go straight into substrate without
reflecting because the surface structure are even or flat, but it does
not mean all the photons were fully absorbed. If the surface structure
is textured or different angle of incident light before entering the substrate,
the direction of light might be reflected.
For the solar cell simulation, the J-V characteristic is shown in Fig.
2. As for the result of Voc and Jsc for silicon solar
cell structure with SiO2 and no SiO2 coating, it is shown
in Table 2. In Table 2 shows the Voc
from both is 0.05 μm SiO2 and no SiO2 coating too
small referred to the best Voc of silicon solar cell, Voc
= 0.706 V (Markvart and Castaner, 2003), possibility the
area of structure is small, which is 50x50 μm2. It shows the
values of Voc and Isc for silicon substrate is higher
than 0.05 μm SiO2 coating and the efficiency of virtual cell
is decrease about 13.6% when deposited 0.05 μm SiO2 coating.
This might be happened the refractive index of SiO2 is lower than
2.0. Figure 2 shows the result computed from solar cell simulation
Anti-reflective coating simulation result: In this study, the
ARC analysis has been used four variables thickness of single layer coating
with the p-n junction of 1 μm could be simulated. The ARC simulation
result is shown in Fig. 3-5, which
shows the photocurrent effects of the wavelength on silicon solar cell.
The pink line on the top shows the photocurrent from the source and the
other lines are the available photocurrent can be absorbed by variable
ARC thickness. In Fig. 3 shows there has a empty space
or gap between the available photocurrent and source photocurrent lines
of SiO2 ARC and this situation might cause of the SiO2
refractive index is lower than 2.0.
||Graph of spectral response of variable thickness SiO2
ARC (d = 50 ~ 80 nm)
||Graph of spectral response of variable thickness ZnO
ARC (d = 50 ~ 80 nm)
||Graph of spectral response of variable thickness ZnS
ARC (d = 50 ~ 80 nm)
When the refractive index value is
higher, the available photocurrent can be higher in wide range wavelength
and more reducing the refractivity. The spectral response of ZnO and ZnS
coating are capable of reducing the refractivity over a wide range of
wavelengths increase around 600 and 700 nm, respectively (Fig.
The Quantum Efficiency (QE) measurement is one of the most significant characterization
tools for solar cells (Schadel et al., 2006).
The source photocurrent is the amount of current generated by the light source
(ATLAS Users Manual, 1998).
||Graph of External Quantum Efficiency (EQE) for single-layer
ARC with variable thickness (a) 50, (b) 60 (c) 70 and (d) 80 nm, respectively
on silicon solar cell
is the amount of current absorbed by the semiconductor. Differences between
these two photocurrents are due to reflection, transmission or absorption is
non-semiconductor materials. The ratio of available and source photocurrents
is often known as external quantum efficiency. Figure 6a-d shows the graph of External Quantum Efficiency (EQE) for single-layer ARC with
variable thickness (50, 60, 70 and 80, respectively) on silicon solar cell.
In overall, the graph shows the SiO2 is the lowest EQE lines compared
to ZnO and ZnS coating. The EQE of ZnO is high increase around 400-700 nm wavelength.
Meanwhile, the EQE of ZnS is increasing higher in wide range of wavelength,
which is around 600-800 nm wavelength. And this mean the ZnS coating could perform
more efficiency on wide range of wavelength. This can be concluded the ZnO layer
is the optimum to be a first layer for fabricating multilayer coating solar
cell. From the results obtained, it is found that the maximum percentage of
EQE, which nearly 99.99% is on 60-80 nm thickness ZnO coating.
A theoretical study of the ARC on silicon solar cells is made (Bouhafs
et al., 1998). In this study, presented a simulation of single layer
ARC on silicon solar cells, based on silicon dioxide (SiO2), zinc
oxide (ZnO) and zinc sulphide (ZnS) coatings by using ATLAS simulator. The ability
of the ATLAS device simulator (Micheal and Bates, 2005;
Micheal et al., 2005), to accurately a model solar
cell characteristic has been shown. The detailed outputs available to the solar
cell designer allow for efficient and effective simulation and optimization
of even most advanced solar cell designs. Using these tools, the basic of silicon
solar cell structure was designed by using ATHENA device simulator, meanwhile
the J-V characteristics and spectral response for ARC analysis were showing
by ATLAS simulator. From the solar cell analysis, it is found the Voc
is 397.69 mV and Jsc is 15.646 mA cm-2, meanwhile the
FF and solar cell efficiency is 0.758 and 4.72%, respectively, from 0.05 μm
SiO2 coating. In ARC simulation, the ZnO and ZnS ARC are capable
to reduce the refractivity because the available photocurrent is increase around
600 and 700 nm for ZnO and ZnS, respectively. And this can be concluded when
the refractive index value become higher, the available photocurrent also can
be higher in wide range wavelength and capable reducing more refractivity on
solar cell. For the ARC analysis, the spectral response graph was plotted to
evaluate the external quantum efficiency. The percentage of EQE is calculated
to compare the differences between coatings thickness. From the results obtained
the maximum percentage of EQE, which nearly 99.99% is on 60-80 nm thickness
ZnO coating. Meanwhile the EQE of ZnS is increasing around 600-800 nm of broad
range wavelength. Solar cell simulation could be useful for time saving and
cost consumption. This method also cheaper and faster compared to experimental.
So, the simulation has some advantages than physical experimental to made decision
to fabricate a solar cell.
The authors would like to thank the Malaysian Ministry of Science, Technology
and Environment for sponsoring this research under project Science Fund
03-01-02-SF0385. Also thanks to Laboratory of Advanced Semiconductor Packaging
(ASPAC), Faculty of Engineering and Built Environment, Universiti Kebangsaan
Malaysia for providing the facilities.
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