Study Effects of Illumination and Temperature on Performance of (pn-Si) Device using Simulation Program SCAPS-1D

Abstract The current research included obtaining the best performance specifications for a silicon device with a mono-crystalline type pn junction (pn–Si). A simulation of the device was performed by the use of a computer program in one dimension SCAPS1D in order to reach the optimum thickness for both p and n layers and to obtain the best efficiency in performance of the pn-Si junction. The optimum device efficiency was eta (η) = 12.4236 % when the ideal thickness for the p and n layers was 5μm and 1.175μm, respectively (p=5 μm and n=1.75μm). The research included studying the effects of different spectra of solar illumination using simulation of the device; the usual solar spectrum AM1_5 G1 sun. Spectrum, Black body spectrum, White spectrum constant photon flux, White spectrum constant photon power, Monochromatic spectrum constant flux, and Monochromatic spectrum constant power. The highest efficiency was obtained from the monochrome spectrum with constant power (eta (η) =22.4338 %). The effects of different temperatures on the device was studied on 250K, 300K, 350K, 400K, and 450K. The highest efficiency was revealed for Monochromatic spectrum constant power (eta (η) =24.5381 %) when the temperature was 250K.


Introduction
The primary structure and the processes of manufacturing of dual device, including light-emitting diodes are similar to those of the solar cells manufactured on a semi-conductive silicon floor. Adding atoms of an impure doped material to silicon with a small concentration can modify the electrical properties of the crystal, allowing the formation of the p-n junction. The material doped with the donor material is called the n-type material, if it has valence electrons more than the four electrons that silicon possesses in its outer orbit; such as a phosphorus (P) atom that has five valence electrons, making covalent bonds with the four adjacent silicon atoms while the fifth electron becomes a conducting electron, as it doped to the conduction band; then silicon become of an n-type [1]. The doped material is known as accepter if the number of valence electrons is less than that in the outer orbit, as compared to silicon, since when replacing a silicon atom with a three-valence Boron atom (B) it can accept an additional electron to form four covalent bonds with silicon atoms. As a result, it generates a positive charge hole (in the valence band), where the semiconductor in this case is called P-type. The p-n junction consists of contacting the n-type semiconductor with the p-type semiconductor, which represents the basic structure of each two devices and solar cells. Many articles and books have been published on the physics of the junction p-n [1][2][3], hence only the basic model is used here to describe the response of current -voltages in solar cells. When the p-n junction is lighted, the current can flow through it, and the junction can act as a solar cell [4].
Current arises from the internal electric field generated at the silicon interface for both the n-type and p-type. Diffusion current arises from the variance in the charge carrier concentrations in the n-type and p-type region. Electrons tend to diffuse from a highly concentrated region of n-type silicon towards a lower concentration region of p-type silicon. The spread of the holes is in the opposite direction, i.e. from the high concentration region of the holes in the p-type semiconductor to the lower concentration region of the holes in the semiconductor n-type [1]. The drift current is greater than the diffusion current when the solar cell is illuminated, therefore, it creates an opposite current in the direction of the current that flows in the standard diode. According to the band theory model, the absorbed photons excite more charge carriers to the conduction band, which in turn are affected by the electric field, which causes them to drift faster. When the cell is not lighted, the current does not flow through it, as a result of the balance of the drift current and the diffusion current with each other, which flow in opposite directions [5]. In this study, the simulation program SCAPS-1D (Solar Cell Capacitance Simulator) was used to perform electrical and optical properties of solar cells with a p-n junction in one dimension using numerical simulations [6]. This program helps to simulate and analyze curves of the properties of current-voltage (I-V) and to analyze AC properties for current-frequency (I-F) and frequencycapacitance (C-F). Each of these measurements can be calculated under conditions of light and dark and it is a function of temperature [7]. The spectral response of the Quantum Energy (QE), Fill Factor (FF), short-circuit current (Jsc), open circuit voltage (Voc), energy beams of materials used in solar cells, and the concentrations of different materials, can also be analyzed by solving basic semiconductor equations. Therefore, continuity equation and the Poisson's equation are both applied for electrons and hole [8,9]. Theoretical simulation of the solar cell and the main factors affecting it In this study, the simulation program SCAPS-1D (Version 3307) was used to solve equations in onedimension using basic semiconductor equations to simulate solar cells. The main equation is that of Poisson's, which connects the charge with electrostatic voltage and is given by the following relationship [9,10]: where is the semiconductor permittivity, is the charge density, is the electric charge, n and p are the concentrations of both electrons and holes, respectively, and and are the concentrations of both donors and accepters, respectively. The continuity equation for both electrons and holes is given by the following relationship [11] where and represent the current density of the electrons and holes, are the diffusion coefficients for electrons and holes, and are the motilities of electrons and holes, respectively. The solar cell of length L can be divided into N of time periods. Values are formed of the electrostatic voltage and the concentrations of electrons and holes the and form each of these periods the unknowns which can be found numerically by solving 3N of nonlinear equations, because the continuity equation contains the recombination term, which is nonlinear for both electrons and holes; that is, the basic equations for each of the periods . , and can be chosen as independent variables instead of , and , which represent Quasi-Fermi energy levels for both electrons and holes. The most important factors that affect the properties of solar cells are [10]: 1-Short circuit current density : it represents the current density that flows through the junction when it is lighted, and there is no applied bias (i.e. the applied bias is zero), while the short circuit current density is in an ideal case that is equal to the photo that generated the current density. Thus, the short circuit current is the highest current that flows through the solar cell when the load resistance is zero, meaning that the solar cell conducts in a short circuit.

2-Open circuit voltage
: it represents the maximum voltage that can be obtained from the solar cell when the current that is passing through the solar cell junction is zero and the solar cell is an open circuit (i.e. the load resistance is infinite). The open circuit voltage can be expressed by the following equation: where represents the current generated by the incident light at p-n junction, is the saturation current in the absence of light, n is the ideal factor, k is Boltzmann constant, and T is the absolute temperature. It can be noted from eq.(4) that the open circuit voltage depends on the current generated by the incident light and that the saturation current depends on the processes of recombination in the solar cell. Therefore, the open circuit voltage is a measure of the amount of recombination in the solar cell [12,13]. Short circuit current and open circuit voltage are the maximum current and voltage of the solar cell, respectively. However, the power obtained from the solar cell is zero at both operating points. The point on the J-V curve that produces the maximum power is indicated as the maximum power point, where the current density represents the maximum current J max and the applied voltages represent the maximum voltage V max . 3-Fill factor FF: is the measurement of quality or performance of the solar cell. It determines the maximum power of the solar cell and is defined as the ratio between the maximum powers generated from the solar cell to the theoretical value of the power i.e. the product of the short circuit current to the open circuit voltage, which is given by the following relationship: ( ) 4-Conversion Efficiency: this factor depends on the type and intensity of the solar spectrum as well as the temperature of the solar cell. It is defined as the ratio between the maximum powers coming out of the solar cell to the input power , as given by the following equation [14]: ( )

Current -Voltage Characteristics of the Solar Cell
The current-voltage properties of the photovoltaic device can represent graphically the operation of the solar cell and consist of the matching of the current-voltage properties of the solar cell in dark (in the absence of light) with the current generated under lighting state. As the solar cell in the dark state possesses the same properties of the dual device, then when light falls on it, the electrical power can 485: be extracted from the bottom of the current-voltage properties curve, in the fourth quarter, where the voltage is positive and the current is negative. Thus, the lighting current is added to the dark current in the solar cell, where the current density of the total photodiode is given as follows [15]= ( ) where and represent the short-circuit current and the dark current, respectively, as given by the following equation= ( ⁄ ) ( ) By substituting the value of the dark current, the total current density of the diode represents the following: ( ⁄ ) ( ) is the saturation current density in the dark state [16].

Materials and methods
The experimental arrangement of the exposed (Si) p-n junction apparatus is illustrated in Fig. 1.
Different spectra of light were used for solar radiation falling to the Earth surface. Using the data for the SCAPS-1D simulation program, those spectra were drawn as shown in Fig. 2, as follows: 1-The normal solar radiation (AM 1.5) standard spectrum is shown in Fig. 2. Its wavelengths range 250 -2500 nm, whereas the maximum intensity of solar radiation at wavelength 500 nm has a maximum of 2.00E-5 W/m 2 /nm. The figure indicates two spectra of solar radiation, which are the vertical direct solar radiation and the total solar radiation falling on the hemispherical Earth surface 2π on inclined solar panels at an angle of 38 o [7].   2-Black body spectrum, which is shown in Fig. 3-a, is similar to the limit of the normal solar spectrum. Its parameters indicate wavelengths range of 300-4000 nm and a maximum intensity, at wavelength 500 nm, of up to 1.43E-04 W/cm 2 /nm. 3-White spectrum constant photon flux: as shown in Fig. 3-b, this region included all the solar spectra, indicating wavelengths range of 400-800 nm, while the maximum intensity of solar radiation at wavelength 400 nm is up to 7.75 E-05 W/cm 2 /nm. However, the area of illumination intensity was greater than the area of black-body radiation spectrum, and the total sum of incident photons was 6.242E-16 photons/cm 2 /s. 4-White spectrum constant photon power: as shown in Fig. 3-c, this region included all solar spectra between 400-800 nm. The maximum intensity of solar radiation at this region was 2.50E-04W/cm 2 /nm and the area of the intensity of illumination was greater than the area of the spectrum of white light fixed photon flood. 5-Monochromatic spectrum constant flux: as shown in Fig. 3-d, this region included all the solar spectra between 594.8-605.1 nm, while the maximum intensity of solar radiation was about 2.07E-03 W/cm 2 /nm, which had a higher illumination than previous spectra. 6-Monochromatic spectrum constant power spectrum: as shown in Fig. 3-e, this region included all the solar spectra between 594.8-605.1 nm, while the maximum intensity of solar radiation was about 1.00E-02 W/cm 2 /nm, which was the highest illumination intensity of the fixed-wave monochromatic spectrum.  After entering the special parameters for both the p-Si and n-Si layers of the dual device diode, shown in Fig. 4. The SCAPS-1D computer program was applied to simulate the thickness of the p-Si and n-Si layers to obtain the ideal thickness of the pn-Si dual device using the bath set-up package gate, as shown in Figure (5).

Results and discussion 1-Thickness test
The thickness of each of the two layers was tested after choosing thickness of the p-Si layer between of 1-5 µm, while the thickness of the n-Si layer was between 0.1-0.2 µm. The results obtained are shown in Fig. 6, which shows the relationship of the layer change p-Si (2-5)µm, after fixing the thickness of X layer to (n-Si) = 1.75µm, with {V OC ،J SC, FF%, η%, V MPP ،J MPP }.  At a thickness of X (p-Si) = 5µm and when X (n-Si) = 1.75µm, by using normal solar spectrum at temperature 300K, so the final results getting are {V OC = 0.73V, J SC = 19.96mA/cm 2 , FF = 84.93%, eta(η) =12.42%, V MPP = 0.6482V and J MPP = 19.1598mA/cm 2 }.This results agrees with the recent application requirements for solar cells [1], whose efficiency must be higher than eta(η) =12.42%, after testing the optimum thickness of the dual p-Si devices. When compared with other thicknesses usered. After fixing both layers X (p-Si) = 5µm and X (n-Si) = 1.75µm, we show the results in Fig. 7.

2-Testing the working solar spectrum
Solar spectrum models were applied using the SCAPS-1D computer program to simulate the effects of the solar spectrum on both p-Si and n-Si layers, when projected temperature was fixed at

3-Testing the effects of temperature on the device
Solar spectrum models, as shown in Table 1, and using the SCAPS-1D computer program, were applied in simulating the effects of temperature values of 250K, 300K, 350K and 400K in p-Si and n-Si. The models were adopted to obtain output values of the special characteristics of the voltagecurrent (I-V Characteristics), which are V OC ،J SC, FF%, Efficiency eta (η%), V MPP ، and J MPP , where V MPP and J MPP represent maximum power voltage and maximum power current, respectively, as shown in Fig's. (8,9,10,11,12,13).   Table 1. Fill Factor FF(%) Spectrum No. Figure 9-Results obtained from applying the effect of temperature on p-Si and n-Si layers to obtain the filling factor (FF%) of the device at different solar spectra shown in Table 1.    Figure 11-Results obtained from applying the effect of temperature for p-Si and n-Si layers to obtain the saturation reverse current (J SC ) of the device and at different solar spectrum as shown in Table 1.  [14].

4-Effects of both Front and Back bias on the device
Solar spectrum models were applied using the SCAPS-1D computer program to simulate the effects of both right contact (Front) and left contact (Back) on both p-Si and n-Si layers. The aim was to obtain the output values of the IV Characteristics, which are V OC ،J SC, FF%, η%, V MPP ، and J MPP as in Fig's.(14, 15, 16, 17, 18, 19).   Table 1.   Table 1.   Table 1, on p-Si and n-Si layers to obtain the Fill Factor (FF%) of the device at the forward and backward bias.  Table 1, on p-Si and n-Si layers to obtain the open circuit voltage (V oc ) of the device at the forward and backward bias.
As shown in all the previous figures, all the IV Characteristics have relatively higher values at the forward bias right contact (Front), than those of the left contact (Back).

Conclusions
In this study, a simulation was performed on a pn-Si device using one dimension computer program SCAPS-1D. It was found that the best efficiency of the device is obtained eta (η) = 12.4% by using the normal solar spectrum M1_5G 1 at 300 K, when the optimum thickness of the p-layer is 5 μm and that for the n-layer is 1.75 μm.
It was also found from studying the effects of different solar spectra that the highest values of all I-V characteristics were obtained at the monochromatic spectrum of constant power, where the optimum efficiency of the pn-Si was eta (η) = 22.4 %.
When we studied the effects of different temperatures on the six solar spectra models, and after performing a simulation on the device, it was found that the highest values for both efficiency and fill factor were eta (η)=24.54% and FF=87.26%, respectively, at a monochromatic spectrum of constant power, when the temperature was 250K.
Also, we found that the forward bias gave the best results obtained from the monochromatic spectrum of constant power as compared to those from the reverse bias in all of the IV characteristics , where (η) = 22 % at the forward bias and (η) =16 % at the reverse bias.  Table  1, on p-Si and n-Si layers for Maximum Potential Voltage (V MPP ) for the device at the forward and backward bias.  Table 1, on p-Si and n-Si layers for maximum current power capacity (J MPP ) for the front and back bias.