Solar cells operate on the principle of the photoelectric effect. The incident light from the sun carries photon packets with an energy equal to hf. Photovoltage establishes when photon energy is greater enough to ignite electrons from the semiconductor valence band to the conduction band. Moreover, the nature of semiconductor material also affects the overall performance of the solar cell.
3.1. Optimizing C2N thickness
The thickness of the absorber layer in solar cells contributes to overall device performance [22, 24, 33]. Beer-Lambert law states that the absorbed light intensity increases exponentially when the length of the path decreases [34]. Therefore, a thickness of about a few hundred nanometers is enough to absorb and cover sunlight spectra for efficient photocurrent generation in solar cell devices. The performance of the C2N based solar cell gets better as we increase the thickness of the absorber layer. It is shown in Fig. 4 (a-d) that the Key parameters such as Voc, Jsc, and PCE enhanced as we raised the thickness value from 300 nm to 1000 nm.. The charge carriers partially recombine in the absorber layer below 500 nm, resulting in the saturation trend of efficiency. Whereas, at the thickness of 1000 nm, the cell delivers the highest efficiency rate of 19.01% with supporting Jsc= 18.257244 mA/cm2, Voc= 1.2257 V, and FF = 84.96%
In Fig. 4 (c), the fill factor (FF) shows anomalous behaviour with the increased thickness of the C2N layer. It started decreasing beyond 300 nm and then start increasing after 800 nm thickness. Below 800 nm the diffusion length is smaller than the thickness of the absorber layer, with an active recombination rate leading to increased series resistance and power depletion, resulting in increased fill factor [24].
The increase in Voc and Jsc is because as we increase the thickness of the absorber layer the more photons with longer wavelengths contribute to the generation of electron-hole pairs [35]. The enhancement in absorber layer thickness contributes to the improvement of solar cell performance. As demonstrated in Fig. 4 (e), and (f) the larger the thickness of the absorber layer, the more light contributes to the absorption process.
3.2. Influence of C2N defect density
C2N is a material related to the graphene family, therefore there are chances of existing impurities, interstitial atoms, and vacancies [36]. Most of the deep defect points in C2N limit the bandgap and increase the light absorption, and some have obstructive influence due to deep gap states in the C2N band structure [37]. The photovoltaic device efficiency is affected badly by the deep defect points because it acts as the trapping edges for generated electron-hole pairs in the active layer. In the case of C2N as the absorber layer, there must be some amount of defect density that could exist. As C2N is graphene-like material and graphene defect density is reported to cover the value from 1010 cm− 3 to 1014 cm− 3 [38, 39]. Moreover, the defect density in the range of 1013 cm− 3 to 1020 cm− 3 using SCAPS simulation has been reported in [40, 41]. We performed simulations covering low, average, and deep effect levels for C2N based solar cells ranging from 1013 cm− 3 to 1017 cm− 3. The defect density behaviour for the Zn1-xMgxO/C2N solar cell is shown in Fig. 5 (a-d). Defect density above 1015 cm− 3 has drawn the efficiency down sharply and also lowered short circuit density. Increasing defect density results in increased series resistance and poor fill factor because those deep-level defects act as a recombination centre, leading to lower Voc and Jsc [22].
The increase in C2N defect density has a negative influence on solar cell performance. Figure 5 (e) and (f) depict that Jsc and QE degenerate when the defect density of the active layer increases. The tuning of the bandgap structure occurs by the increase in defect density which inversely contributes to the light absorption process [24]. In this study, the value for simulation is kept at 1014 cm-3 with the following output parameters: Voc of 1.2257, Jsc about 18.257244 mA/cm2, FF of 84.96%, and PCE of 19.01%.
3.3. Influence of C2N doping density
Doping concentration is an essential step in increasing the device's performance. The solar cell efficiency to doping rate has been reported in many studies [22, 24, 42, 43]. However, doping concentration could be harmful to the device if exceeds a certain limit [42]. The depletion region in the structure could be built by a high doping rate which is due to built-in voltage. This depletion region is a potential centre for trapping free charge carriers and changing the electronic behaviour of the material and consequently degrading the performance of the solar cell. To find this behaviour, the doping level of C2N was set in the range of 1 x 1013 cm-3 to 1 x 1017 cm-3. The changing pattern of various parameters with an increase in doping level can be visible in Fig. 6 (a-d). Voc increases exponentially with an increase in the doping value, whereas Jsc starts decreasing abruptly after 1015 cm-3 due to the high recombination rate. The FF and PCE shows maximum value at 1015 cm-3 but decreases after this optimum value due to the rise in series resistance and high recombination rate in the space charge region of the cell.
3.4. Optimization of buffer layers for C2N
The buffer layer is one of the important layers in any solar cell device. The interfacial strain and defects in the window layer could be reduced by the use of a buffer layer. Moreover, for band alignment buffer layer appears to be instrumental [44]. Band alignment is one of the key factors to determine solar cell performance. In this study, for electron transport layer (ETL), zinc magnesium oxide (Zn1-xMgxO) with four different ‘x’ concentration values were used in C2N based solar cell simulations. Table 3 shows the output parameters of the device to different Mg(x) concentrations and the Quantum efficiencies for incident light spectrum are shown in Fig. 7.
Table 3. C2N based solar cell responses to various Mg(x) concentrations
C2N layer coupling with buffer layer is crucial to solar cell performance. In this case, C2N based solar cell was examined with four buffer layers, among which ZMO (x = 0.25) shows the lowest PCE of 16.46% while ZMO (x = 0.1875) yields the highest PCE of 19.01%.
Figure 7 shows the response of the buffer layer to the incident light spectrum. For the incident light, a wavelength less than 700 shows a noticeable variation in the QE in the absorption of light for different buffer layers. After simulations, ZMO with x = 0.25 shows an insufficient absorption of light, contributing to low QE. Moreover, the energy spike at the interface for x = 0.0625, 0.125, and 0.1875 is relatively small than that of x = 0.25 because their CB is close to that of C2N.
3.5. Thickness Optimization of the buffer layer
In this study, buffer layer thickness was examined within the range of 10 nm to 80 nm, and it was noticed that from 10 nm to 30 nm the performance of the solar cell gets better. Beyond 30 nm up to 80 nm, the efficiency started decreasing due to the decrease in recombination of charge carriers at the surface and bulk recombination at the interface of the layers and also an increase in the series resistance. Near the surface, mostly short-wavelength photons are absorbed. From our simulations, the optimum PCE was recorded at the ETL thickness of 30 nm as shown in Fig. 9.
3.6. Doping density Optimization for the buffer layer
In this section, the doping density for the buffer layer was kept in the range of 1014 cm-3 to 1022 cm-3. As shown in Fig. 10 (a-d), the performance of the solar cell is impacted by the doping of the buffer layer. The solar cell shows high performance at doping density ND= 1 x 1019 cm-3 contributing to PCE = 19.01% having Nt = 1 x 1014 cm-3, which is due to the happening of saturation after the increase in conductivity according to the Moss-Burstein effect [43].
3.7. Effect of varying temperature on the solar cell performance
Temperature is an important factor in solar cell performance. High operating temperature negatively impacts the solar cell performance and distorted the layers in the solar cell structure [45–48]. To study the influence of temperature on the solar cell performance the range of temperature is set from 280 k to 480 k. In Fig. 11, it is shown that there is a drop in PCE with the increase in temperature. At, the given operating temperature, the PCE decreased from 19.85–12.82% at 280 k and 480 k, respectively. This is because for the reason that photogenerated charge carriers' diffusion length decreased, and so the simulated parameters overall decreased. As the temperature rise there happens a deformation between layers which causes a reduction in the diffusion length and results in poor connectivity between the layers. As a result, the series resistance and rate of recombination increases which could resist the charge carriers to reach the back contact and had a bad impact on the performance of the solar cell.
Table 4
C2N based solar cell simulated parameters from previous and current studies
Ref
|
Framework
|
Absorber
|
Method
|
η
|
Outcome
|
[18]
|
ZnO/CdS/CZTS
|
Single
|
Computational
|
11.50
|
↑, Enhancement
|
[19]
|
Glass/FTO/ZnMgO/CdTe:Au
|
Single
|
Experimental
|
15.70
|
↑, Enhancement
|
[20]
|
Glass/FTO/ ZnMgO /CdTe/:Cu
|
Single
|
Experimental
|
16.76
|
↑, Enhancement
|
[21]
|
CZTSSe/CZTS/ZnMgO
|
Single
|
Computational
|
17.05
|
↑, Enhancement
|
[22]
|
FTO/CdS/C2N/Au
|
Single
|
Computational
|
17.40
|
↑, Enhancement
|
[23]
|
FTO/GaS/C2N
|
Single
|
Computational
|
17.80
|
↑, Enhancement
|
[24]
|
Glass substrate/TCO/IGZO/C2N/Metal back contact
|
Single
|
Computational
|
18.57
|
↑, Enhancement
|
[49]
|
Substrate/ITO/PEDOT:PSS/P3HT:PCBM/TiOx/Al
|
Single
|
Computational
|
5.14
|
↑, Enhancement
|
[50]
|
AZO/ZnO/CdS/Cu2ZnSn(SxSe1 − x)4
|
Single
|
Computational
|
15.3
|
↑, Enhancement
|
Present Work
|
Glass/TCO/ZnMgO/C2N/Metal back contact
|
Single
|
Computational
|
19.01
|
↑, Enhancement
|