InGaP window layer for Gallium Arsenide (GaAs) based solar cell using PC1D simulation

Abstract

Improving the overall performance of the PV cell can play a crucial role in the total generated photovoltaic power worldwide. An efficient window layer is essential to check the front surface recombination in the solar cell. In this paper, we explored the InGaP window layer for GaAs solar cells and analyzed performance with the help of PC1D simulation software. For this, we have varied thickness and doping levels of the InGaP window layer, and the performance of the solar cell has been examined with the help of current-voltage (I-V) characteristics. We also reviewed the effect of temperature on the performance of the solar cell. It has been found that the short circuit current 3.192 A, open-circuit voltage 0.8959 V, and power conversion efficiency 25.78% of InGaP/GaAs solar cell at window layer thickness 30 nm with doping level 1.00E+17cm 3 .

Full text

Jour. of Adv Research in Dynamical & Control Systems, Vol. 12, 07-Special Issue, 2020
2878
DOI: 10.5373/JARDCS/V12SP7/20202430 ISSN 1943-023X
2878 Received: 27 May 2020/Accepted: 25 June 2020
InGaP window layer for Gallium Arsenide (GaAs)
based solar cell using PC1D simulation
K. C. Devendra1, D. K. Shah2, R. Wagle3, A. Shrivastava4 and D. Parajuli5,
*
1Lebesby Oppvekstsenter School, 9740 Lebesby Norway
2School of Semiconductor &Chemical Eng. & Solar Energy Research Center, JNU, Jeonju, Korea
3Pokhara University, Pokhara, Nepal
4Naval Material Research Laboratory, Maharastra, India
5Department of Physics, Andhra University, Visakhapatnam, India.
Abstract
Improving the overall performance of the PV cell can play a crucial role in the total generated
photovoltaic power worldwide. An efficient window layer is essential to check the front surface
recombination in the solar cell. In this paper, we explored the InGaP window layer for GaAs solar cells
and analyzed performance with the help of PC1D simulation software. For this, we have varied
thickness and doping levels of the InGaP window layer, and the performance of the solar cell has been
examined with the help of current-voltage (I-V) characteristics. We also reviewed the effect of
temperature on the performance of the solar cell. It has been found that the short circuit current 3.192
A, open-circuit voltage 0.8959 V, and power conversion efficiency 25.78% of InGaP/GaAs solar cell at
window layer thickness 30 nm with doping level 1.00E+17cm3.
Keywords: Solar Cell, Window Layer, lnGaP, PC1D, Power Conversion Efficiency.
1. Introduction
Solar energy is a prime renewable energy resource because it is highly safe to use, clean, non-
polluted, abundant, and inexhaustible. Scientists and researchers are uninterruptedly concentrating
their efforts to explore a low-cost and ecofriendly technology [1]. Crystalline silicon PV cell has
laboratory energy conversion efficiency over 26.7% for single-crystal cells and over 22.3% for multi-
crystalline cells [2]. Gallium Arsenide (GaAs) based solar cells are used over Si-based solar cells these
days. GaAs is a direct bandgap semiconductor of 1.42 eV, and it has a higher absorption coefficient
than Silicon which encourages the researchers to use GaAs as the best absorber material. However,
their higher recombination rate is still a major problem [3] and limits their efficiency around 10% [4].
This can partially be solved by using a window layer on the surface having a higher bandgap [5]. The
window layer changes the electrical and optical properties thereby reducing the reflectance and
generating a difference in refractive index across window and absorber layer [6,7]. AlGaAs is an
effective thin material for the window layer [8]. Similarly, InAlGaP and InGaP are promising back
surface materials to design an efficient multijunction GaAs solar cell [9, 10]. Moreover, GaAs and InGaP
(III-V) semiconductors have a bandgap value close to optimum absorption value and have remarkable
physical properties. They are mostly used in the optoelectronics field and play a major role in
microwave applications and power electronics [11]. The properties such as types of substrate, doping
concentration, window layer thickness, the thickness of antireflection materials, depth factor, etc. play
important role in the efficiency of the solar cell. These properties of the solar cell can be analyzed by
using several simulation software [12-14]. In this work, PCID simulation is used for the study of
different window layers for GaAs-based solar cell and their efficiency, fill factor, short circuit current,
output voltage. The effect of thickness and doping concentration study of InGaP window layers helped
in the optimization of their efficiency.
*
This article is revised and posted by this author.

Jour. of Adv Research in Dynamical & Control Systems, Vol. 12, 07-Special Issue, 2020
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DOI: 10.5373/JARDCS/V12SP7/20202430 ISSN 1943-023X
2878 Received: 27 May 2020/Accepted: 25 June 2020
2. Device Structure and Simulation Parameters
The schematic device structure of the GaAs-based solar cell for investigation is shown in Figure
1. In this model, four layers back surface field (BSF) layer, base layer, emitter layer, and window layer
have been sandwiched each other. The layers in this solar cell are fixed according to the bandgap, the
layer having a lower bandgap fixed on the bottom, and the layer having a higher bandgap on the top
surface. The reason behind this is the layer having a higher bandgap absorbs the shorter wavelength.
GaAs has a higher bandgap than InGaP. PC1D is one of the free software for solar cell modeling and
simulation. The PC-1D contains mainly four sections which are device, region, excitation, and result. In
the device section, we get information about solar cells which one is supposed to be simulated. In the
region, we introduced parameters like thickness, doping, mobility, dielectric constant, energy band
gap, refractive index [15].
Table 1: Solar Cell parameters
Figure 1. Device structure
In this work, a highly doped GaAs layer has been used as a back-surface field (BSF) to reduce
the surface recombination losses. The thickness of the base layer performs a critical role to improve
Parameters
Value
Device area
100 cm2
Thickness of Window layer
30 nm
Thickness of Emitter layer
50 nm
Thickness of baselayer
2.2 micrometer
Dielectric constant
11.8
Energy bandgap
1.9 ev
p-type doping base layer
1x1016 cm-3
n-type doping emitter layer
1x1017 cm-3
n-type doping window layer
1x1017 cm-3
Excitation mode
Transient
Temperature
25 °C
Electron affinity
1945 eV
Hole affinity
141 eV
Bulk recombination
10 μs
Constant intensity
0.1W/cm2
Primary light source
AM 1.5D spectrum
Other parameters
Internal model of PC1D

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the efficiency of the solar cell. Here, the thickness of the base layer is 2.2 m [8] and the emitter layer
is 50 nm. The window layers are in the range of 30 nm to 100 nm and have been optimized by
simulation. All the parameters of the proposed solar cell in the device structure are carefully selected
from the experimental data and other literature [8-10]. The simulations were accomplished under AM
1.5 solar radiation, constant intensity 0.1 W/cm2 at 300 K temperature, and bulk recombination 100
μs in this model. The p-type background doping of the base layer was set 1 x 1016 cm-3 and n-type
doping for emitter later was set to 1x1017 cm-3. The doping concentration of the window layer has been
varied from 1 x1013 cm-3 to 1 x1020 cm-3 and optimized at 1 x1017 cm-3. Other parameters were chosen
to be the internal model of the PC1D simulator.
3. Result and Discussion
It has been simulated the solar cell using the parameters shown in Table 1 with the help of
PC1D software. The I-V curve in Figure 2 has been obtained by optimizing thickness and doping. From
the I-V characteristics, short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF) and
efficiency (η) of solar cell has been computed.
3.1 Effect of window layer thickness on InGaP/GaAs solar cell
A short circuit current is defined as the maximum current from the solar cell when the voltage
across the cell is zero. The short-circuit current is due to the generation and collection of the light-
generated carrier. The short-circuit current of the solar cell depends on the area of the solar cell, the
number of photons, the spectrum of the light incident, and the optical properties of the material [16].
The effect of thickness of window layer on short circuit current and Open circuit voltage is shown in
Figure 2. The thickness of the InGaP window layer was varied from 30 nm to 100 nm. At 30 nm thickness
of InGaP, Isc has been obtained about 3.193 A and increased up to 3.195 A at thickness 100 nm.
Figure 2. Thickness versus Short circuit current and Open circuit voltage
The open-circuit voltage is a measure of recombination of electron-hole pairs in a solar cell.
The recombination of carriers affects the open-circuit voltage and as recombination increases the
open-circuit voltage decreases. It depends on the saturation current of the solar cell and the light
generated current [17]. The open-circuit voltage has been obtained the maximum at net current
through the solar cell is zero. The open-circuit voltage has been decreased at 30 nm thickness as shown
in Figure 2. The open-circuit voltage of the window layer is about 0.8959 V and decreased to 0.8788 V
at a thickness of 100 nm.
3.2 Effect of thickness of window layer on Efficiency of the solar cell.
As the thickness of the InGaP window layer increases from 30 nm to 100 nm, the efficiency of
the reported solar cell decreased. At 30 nm thickness, the efficiency of solar cell has been exhibited

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25.78%, and further increasing thickness, the efficiency has been decreased as shown in Figure 3. At
100 nm thickness, the efficiency of solar cells has been 25.32%.
Figure 3. Variation of efficiency with the thickness
3.3 Studies of Current-voltage and Power voltage curve:
The current-voltage and power voltage characteristics of the solar cell are a vital part of the
investigation of the performance of the solar cell. The I-V curve is used to evaluate short-circuit current
(Isc), open-circuit voltage (Voc), fill factor, and efficiency of the solar cell [18]. It has been examined
the structure under a solar spectrum AM 1.5 and at room temperature, T = 300 K. The electrical
properties of the GaAs-based cells with and without window layer have been shown in Figure 4 and
Figure 5.
Figure 4. Current-voltage and Power- voltage characteristics of GaAs solar cell
The power generated by the solar cell is the product of current and voltage, power curve is
obtained by the multiplication of point to point for all voltage from short-circuit current to open-circuit
voltage [19]. It has been found that the short-circuit current of the solar cell without window layer 1.98
A and short-circuit current 1.01 V open-circuit voltage and 1.76 W maximum output power.

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Figure 5. Current-voltage and Power- voltage characteristics of InGaP/GaAs solar cell
The window layer affects the short-circuit current due to the increment of the effective photon
absorbed from the incident light [20]. The performance of the GaAs solar cell has been increased by
adding window layers. It has been found that the short-circuit current of the InGap window layer solar
cell has been found to be 3.192 A and open-circuit voltage 1.01 V and maximum power 2.578 W as
shown in Figure 6.
3.4 Impact of Short-circuit Current and Open-circuit voltage by Varying Doping Concentrations
The doping concentration of the window layer plays an important role in the performance of
the solar cell [21]. It influences short-circuit current and open-circuit voltage, maximum output power,
fill factor, and efficiency of the solar cell [22]. It has been varied the doping concentration from
1.00E+13 to 1.00E+20 in reported work. It has been described that the short-circuit current has been
constant 1.00E+19 at certain doping concentrations and after that short-circuit current has been
decreased, but the open-circuit voltage slightly increased by increased doping concentration, at
1.00E+17 it has been reached a maximum value as shown in Figure 6.
Figure 6. Doping concentration versus Short circuit current and open-circuit voltage

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3.5 Effect on Efficiency by Varying Doping Concentration of InGaP/GaAs Solar Cell
The role of doping concentration in the solar cell on the efficiency of the solar cell is very
important for an efficient solar cell. The higher level of doping causes damages to the crystal structure
of the solar cell which is the main reason behind the drop in efficiency of the solar cell [23]. By varying
the doping levels of the InGaP window layer, the efficiency of the solar cell is being changed. The
doping concentration of the InGaP layer has been varied from 1.00E+13 cm-3 to 1.00E+20 cm-3. The
maximum efficiency has been achieved in window layer doping at 1.00E+17, after that the efficiency
has been decreased as shown in Figure 7. It has been achieved the highest level of conversion efficiency
of 25.79% at the doping levels 1.00E+17cm-3.
Figure 7. Variation of efficiency with doping concentration
3.6 Energy Band Diagram of the Solar Cell
The lower energy level of the semiconductor is called the valance band and the energy level
where energy can be considered free is called the conduction band [24]. The energy band diagram of
the solar cell describes the amount of energy required from the sun for conduction and generated
energy from the solar cell. An energy bandgap is a minimum energy required by the electrons in the
outermost shells of a material to be able to jump free of the parent atoms [25].
Figure 8. Energy band diagram of InGaP/GaAs solar cell

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In this diagram, the blue-colored line, which is located at the top, is the conduction band edge
(0.4964 eV) and the pink-colored line at the bottom represents the valance band edge (1.451 eV) as
shown in Figure 8. Between conduction band edge and valance band edge, there in Fermi-level band
edge, the green-colored line indicates the electron quasi-Fermi level and the light pink colored line
indicates hole quasi-Fermi level.
3.7 Effect of Temperature on Solar Cell
The performance of the solar cell depends on the temperature [26]. In this reported work, the
performance of the solar cell is reviewed, it has been shown that at higher operating temperature, it
affects the electron-hole mobility carrier concentration bandgap and the density of the state [27]. It
has been varied the temperature from 25°C up to 55°C, the performance of the cell has been decreased
with increasing temperature as shown in Figure 9. It has been confirmed that the efficiency of the solar
cell highest at 25°C and decreased with increased temperature.
Figure 9. Effect of temperature on the efficiency of solar cell
4. Conclusion
GaAs solar cell with InGaP window layer has been theoretically designed and simulated by
using PC1D simulation software. The GaAs solar has been optimized by varying thickness and doping
concentration of the InGaP window layer. The simulation has been confirmed the performance of GaAs
solar cell is optimal by using window layer and increased to 25.78% from 17.50% of without layer. The
efficiency of the solar cell has been decreased with increased the temperature of the reported solar
cell. The conversion efficiency of 25.78% of the solar cell is highest at thickness 30 nm of window layer,
doping concentration 1e+17 cm-3 and operating temperature 25°C.
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