Junction Configuration Effects on the photovoltaic parameters of a-Si/CZTS solar cells

Due to increased energy intensive human activities resulting accelerated demand for electric power coupled with occurrence of natural disasters with increased frequency, intensity, and duration, it becomes essential to explore and advance renewable energy technology for sustainability of the society. Addressing the stated problem and providing a radical solution has been attempted in this study. To harvest the renewable energy, among variety of solar cells reported, a composite a-Si/CZTS photovoltaic devices has not yet been investigated. The calculated parameters for solar cell based on the new array of layers consisting of a-Si/CZTS are reported in this study. The variation of i) solar cell efficiency as a function of CZTS layer thickness, temperature, acceptor, and donor defect concentration; ii) variation of the open circuit current density as a function of temperature, open circuit voltage; iii) variation of open circuit voltage as a function of the thickness of the CZTS layer has been determined. There has been no reported study on a-Si/CZTS configuration-based solar cell, analysis of the parameters, and study to address the challenges imped efficiency of the photovoltaic device and the same has been discussed in this work. The value of the SnO 2 /a-Si/CZTS solar cells obtained from the simulation is 23.9 %.


Introduction
Due to depleting non-renewable energy resources around the world [1], climate and sustainability concerns [2], demand for efficient and high-performance consumer electronics [2,3], and rapid industrial growth [3], finding an innovative solution for energy generation becomes a significant concern and needs urgent attention.Solar cell technology has been one of the innovative solutions as an alternative to fossil fuels and nuclear energy [1][2][3][4].A solar cell is a simple device consisting of p and n-type layers to form a p-n junction that converts solar energy to electricity [5][6][7].However, there are numerous challenges in current solar cell technology that needs to be addressed to achieve high efficiency.Currently, 90% of solar cell market belongs to silicon (Si) based solar cells due to various advantages.The theoretical efficiency of solar cells e.g., dye-sensitized solar cells, traditional Si-based solar cells and other types of the solar cells is limited to approximately 30% [8,[9][10].This is mainly due to i) poor absorption of incident light primarily because of completely transparent surface [5], ii) inappropriate material bandgap [8], and iii) low reflection within the device causing faster recombination of emitted electrons [8,11].As a result, a tandem cell is constructed from single-junction gallium arsenide (GaAs) and multi-junction (up to three p-n junctions) concentrators with different materials to absorb a larger spectrum of incident radiation [12].
However, the reported theoretical efficiency is 40.8% under radiation concentrations of 326 suns which is not possible for the practical condition [13].The efficiency drops to 33.8% with one sun, and the process itself is not matured as in case of Si-based technology [8,12].To increase the efficiency, the probability of utilizing and integrating a-Si/CZTS based solar cells as a prospective material for constructing the solar cell is promising and have several advantages in the race for large-scale solar module production [14][15][16].In this context, this Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 12 January 2022 study was designed with an objective to simulate and optimize the various physical and electrical parameters of a solar cell prepared from thin layers CZTS (Cu2ZnSnS4) to obtain good photovoltaic performance [17].

Description of simulation parameters
In this study, the physical and electrical parameters of a-Si/CZTS solar cell structure was simulated by the SCAPS software [18][19].The details of the software can be found in the literature [20].
SCAPS is based upon solving of basic semiconductor equations including Poisson's equation (1) and continuity equations: The electron and hole current density is given by: where, φ, electrostatic potential; q, electric charge; ε, permitivity of the free space; n, concentration of the free electrons; p, concentration of the free holes; + The main parameters critical for the simulation of the CZTS based solar cell are listed as a site in Table 1.

Results and discussion
The variation of the efficiency and short circuit current (Jsc) as a function of the thickness of the CZTS layer in the range studied from 0.2 to 1.0 µm is presented in  The photovoltaic parameters for SnO2/a-Si/CZTS solar cell configuration calculated from the (I-V) characteristics are: Voc (0.95 Volt), Jsc (30.02 mA/cm 2 ), FF (85.81 %) and ƞ (23.9 %).
The variation of efficiency (η) and short-circuit current density (Jsc) as a function of temperature in the range from 280 to 320 K is presented in in figure 4.There is a decrease in

Figure 2 .
The value of efficiency of the solar cell increases continuously with the increase in the thickness of the CZTS layer indicating the increased absorption of the light by the active layer.The maximum value of the efficiency obtained is 23.79 at the 1 µm thickness.The increase in efficiency is rapid initially and thereafter it increases gradually in decreasing the saturation.efficiency increases with the increase in the thickness From the figure 2 it is also observed that, the shortcircuit current (Jsc) increases logarithmically to a maximum value of 30.02 mA/cm 2 at 10. µm thickness.The values of open circuit current reported in the literature[12] are lower than that obtained in the present study.The value of other material parameters of different layers such as the doping of the absorbent layer, the thickness and the doping of the buffer layer (a-Si) were constant.

Figure 2 :Figure 3 :
Figure 2: The variation of the efficiency and open circuit current as a function of thickness of

Preprints
(www.preprints.org)| NOT PEER-REVIEWED | Posted: 12 January 2022 the values of short-circuit current density (Jsc) and efficiency (η) of the solar cell with increasing temperature.The variation of open circuit voltage and efficiency with temperature is compared with the values given in the literature [22].The value of efficiency decreases gradually from 23.90 up to 300 K and rapidly thereafter till the 320 K to a value of 23.10.The trend representing the variation of efficiency is similar to as available in the literature but the values are significantly low.The open circuit voltage and short circuit current decreases almost linearly as a function of the temperature in the rage studied.The values of open circuit voltagereported in the literature[23] is higher with similar trend with the increase in temperature.

Figure 4 :
Figure 4: Effect of temperature on the of short-circuit current density (Jsc) and efficiency (η)

Table 1 :
Values of various material parameters used in the simulation