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Abstract
This paper gives a synopsis of a CZTS-based n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell that has been designed and explored by the simulation technique with the help of a solar cell capacitance simulator (SCAPS-1D). The design utilizes CdS as the window layer, CZTS as the first absorber layer, AgGaTe2 as the second absorber layer, and MoS2 as the BSF layer. The influencing parameters of these materials such as thickness, doping concentration, and defect density have been adjusted to achieve the right balance between the proposed structure and to see the changes that affect the device's overall performance. In ideal condition, the single n-CdS/p-CZTS heterojunction structure shows power conversion efficiency (PCE) of 17.75% with short circuit current, JSC of 24.82 mA/cm2, open circuit voltage, VOC of 0.88 V and fill factor (FF) of 81.3%. But, with the inclusion of MoS2 as the BSF, the overall PCE is elevated to 25.84% with VOC of 1.09 V, JSC of 26.96 mA/cm2 and FF of 87.64%. Finally, with the fusion of AgGaTe2 as a current augmenting layer the JSC gets a huge boost and is enhanced to 34.7 mA/cm2 with a PCE of 33.89%. These simulation findings unveil the potential of the proposed solar cell structure with CZTS as the absorber layer and AgGaTe2 as the current boosting layer in creating an environment-friendly, affordable and highly efficient thin film solar cell.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
CZTS is a greenish-black crystal whose chemical formula is Cu2ZnSnS4. Its intrinsic characteristics are the molar mass of 439.471 g mol−1, the density of 4.59 g/cm3, the dielectric constant of 13.65 and the relative permittivity of 10 [1]. However, this material has proven itself one of the best absorber materials not only for its abundance in the territory and the benign nature to the environment but also the possession of a bandgap of 1.5 eV that is very much near to the ideal bandgap of 1.35 eV that is considered to be the best for solar cell application [2]. Interestingly, the bandgap of this material is tuneable and the value varies from 1.5 to 2.1 eV, which leads to its usability for bandgap-graded devices and multijunction solar cells [3–5]. Another thing that is necessary for a material acting as an authentic candidate to be an absorber layer is a high absorption coefficient, while CZTS material is not unlike as it owns an absorption coefficient of 104 cm-1 [2,6]. The conventional fabrication processes to produce this material include evaporation, sputtering, spray pyrolysis, solution-based hydrazine process and ink-based approaches [7,8]. However, at the time of its production, the crystallinity and the grain size of CZTS may not be as expected as it is doped with bigger atoms such as Na, Sb, and Se [9]. The highest recorded solar energy conversion efficiency for pure CZTS solar cells was 9.2% in 2016, but it later increased to 11% in 2018 [10,11]. To enhance the conversion efficiency of CZTS-based solar cells, it is crucial to gather more comprehensive information about the optoelectronic properties of CZTS-based materials and identify the key factors that constrain the performance of CZTS-based solar cells [8,11].
The most common factors that are mainly responsible for the path of attaining high efficiency are the following: grain boundaries in CZTS-based absorbers are detrimental to solar cell efficiency due to structural defects and recombination [12]. Minimizing grain boundaries or passivating the defects associated with them is beneficial. Sodium (Na) at grain boundaries has been shown to enhance the photovoltaic efficiency of CZTS by passivating the grain boundary defect states [13]. Bandgap tailing, which reduces the effective bandgap of the semiconductor, can contribute to the VOC deficiency in CZTS solar cells [14]. The presence of a high density of tail states caused by bandgap tailing results in limited VOC production [12]. Large amounts of Cu/Zn antisite defects can cause band tailing, but the incorporation of silver (Ag) as an alloying element may help mitigate this issue by reducing the number of Cu/Zn antisite defects and decreasing band tailing and non-radiative recombination [15]. A high density of charged defects worsens band-gap tailing leading to a larger VOC deficiency. It also increases recombination probability and contributes to a short minority carrier lifetime in CZTS-based solar cells. The minority carrier diffusion length (Ld) is an important parameter that characterizes the quality of the absorber material. Short Ld negatively affects both the short-circuit current (JSC) and VOC [11,12]. The formation of secondary phases such as ZnS, CuxS, SnSx, and Cu2SnS3 in CZTS-thin films has negative effects on solar cell performance. Secondary phases with a smaller bandgap than the absorber introduce electronic states within the bandgap acting as traps and recombination centers, which leads to a larger deficiency in the open circuit voltage (VOC) and increased recombination rates. On the other hand, secondary phases with a larger bandgap restrict charge carrier transport, increase series resistance (RS) and lower the fill factor (FF) of the solar cell [11,12].
However, the short circuit current limit of CZTS is 26 mA/cm2 due to its wide bandgap that hinders the production of high efficiency in CZTS-based thin film solar devices [16]. To enhance the efficiency of the device we need to raise the current in the device. To fulfill this desire, we may use a compatible absorber layer that can work as the current augmenter with a CZTS absorber layer and AgGaTe2 material can act as that augmenter layer. AgGaTe2 is a black-colored, tetragonal chalcopyrite semiconductor material that belongs to the I-III-VI2 family and I42d space group and it melts at the temperature of 724 °C [17,18]. Moreover, at a low hole doping density of 1016 cm-3 and at temperature 850 K it exhibits a thermoelectric figure of merit, ZT of 0.8. However, in thermoelectric, to get the optimum performance, this material should be doped at the range between 1019 and 1021 cm-3 [19]. The characteristics that have certified the AgGaTe2 material as an authentic candidate in the field of solar cells are a direct bandgap of 1.32 eV, elevated absorption coefficient and huge Seebeck coefficient of 873 µVK-1 [18,20]. Furthermore, the bandgap of this material can be tuned from 1.3 to 2.3 eV, while it is noticeable that the appropriate bandgap for a single heterojunction solar cell is almost between 1.45 and 1.5 eV and for the top cell of two junction tandem solar cell is about 1.6 to 1.7 eV. That means this material is not only a perfect postulant of conventional solar cells but also choiceable in tandem solar cells [21]. Besides, the electronic band structure calculations demonstrate that the valence-band electronic configuration of AgGaTe2 comprises heavy and light valence bands. These bands can be adjusted and brought together by alloying with other ternary chalcopyrites and adopting a pseudo-cubic structure. Efforts to enhance the carrier concentration of AgGaTe2 by inducing Ag deficiency resulted in a notable improvement in the power factor [20].
In the realm of solar cell application, it is crucial to reduce the undesired setbacks that occur at the interface between air and the uppermost layer, caused by the reflective behavior akin to a Fresnel surface. This is done by electing a suitable window material like CdS to amplify the efficiency of capturing light [1,22]. In recent years, there has been a significant interest in cadmium sulfide (CdS) semiconductor. Its potential applications span various fields, including flat panel displays where it can be used in light-emitting diodes (LEDs), transistors, electrochemiluminescence, optical waveguides, non-linear integrated optical devices, second harmonic generation, a device for memory elements and photocatalysis [23–27]. This attention can be attributed to desirable characteristics of CdS, such as its high refractive index, excellent transport properties and outstanding chemical and thermal stability. Due to desirable properties, CdS is considered a top contender for serving as an n-type window material in heterojunction solar cells, including cadmium telluride (CdTe), copper indium gallium diselenide (CuInGaSe2), and copper zinc tin sulfide (CZTS) solar cells [28–31]. To achieve high-efficiency heterojunction solar cells, it is crucial to have a thin film window layer that exhibits excellent transmittance, particularly in the blue region. However, pure CdS thin films possess a bandgap of approximately 2.42 eV. To address this limitation, several elements, such as Ga, Cu, B, In, Sn, and Zn have been incorporated via doping or alloying into CdS to showcase tuneable optical and electrical properties [32]. Furthermore, a lot of fabrication methods are available to fabricate CdS material which include thermal evaporation, electrochemical growth method, successive ion layer adsorption and reaction, atomic layer deposition, molecular beam epitaxy (MBE), metal-organic vapor phase epitaxy (MOVPE), close spaced sublimation (CSS), spray pyrolysis, chemical bath deposition (CBD) and chemical vapor deposition [33–39].
Moreover, it is noticeable that all of the factors are associated with the abatement of the open circuit voltage that leads to poor efficiency performance of any solar cell. However, a problem always takes a solution with itself and in the case of solar cells, it is the back surface field layer. A back surface field (BSF) layer in a solar cell offers several advantages to improve the open circuit voltage (VOC) [40]. The insertion of a BSF layer with the back surface of the absorber layer leads to build-up potential at the junction which is not only responsible for the movement of electrons towards the cathode but also confines the holes [41]. The concept of the addition of BSF layer can facilitate the separation and diffusion of the minority charge carriers that eventually result in the huge production of photocurrent [41]. MoS2 BSF can be introduced with the AgGaTe2 layer to complete the expected task. MoS2 is a promising semiconductor material for its usefulness in various energy band devices, for instance, transistors, batteries, chemical sensors, hydrogen evolution reaction (HER) electrocatalysts and photovoltaics [42]. The characteristics that convey its potential as a back surface field layer of solar cells are healthy optical absorption in the order of 105 cm-1, carrier mobility between 200 and 350 cm2 /V·s, variable energy bandgap from 1.2 to 1.8 eV and excellent catalytic activities [42,43]. However, due to its extremely thin thickness, the inherent light absorption of monolayer MoS2 on a silica substrate is limited to approximately 5-10% within the wavelength range of 400 to 700 nm [44]. To enable the practical application of this material in absorption-based photonic devices, several approaches involving photonic cavities and plasmonic structures have been explored to enhance the light absorption of monolayer MoS2. Zheng et al. conducted numerical simulations and demonstrated that the absorption efficiency of monolayer MoS2 can be increased to 33%, while the external quantum efficiency of an atomically thin monolayer MoS2 solar cell can be enhanced to 7.09% by utilizing a chirped cavity [45]. MoS2 can be synthesized using various versatile techniques, including exfoliation, liquid-phase exfoliation, chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Exfoliation is a commonly employed method for MoS2 synthesis. However, there is a growing interest in using metal-organic chemical vapor deposition (MOCVD) for MoS2 synthesis due to its advantages of high reproducibility and controlled growth [46].
In this article, we design and simulate CZTS-based n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell with AGT and MoS2 as the current booster and BSF layer, respectively. The simulation results reveal that the PCE of the CZTS solar cell can be enhanced with the AGT layer. This indicates the future fabrication of efficient CZTS solar cells.
2. Device architecture and numerical simulation
Figure 1(a) illustrates the schematic model of the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell and the corresponding energy diagram is displayed in Fig. 1(b). For the device architecture n-type material, CdS has been utilized as the window layer with an optical bandgap of 2.4 eV, electron affinity of 4.4 eV and the thickness of CdS was set to 0.1 µm. The sunlight penetrates through the window layer and gets absorbed by the CZTS active absorber layer, which is a p-type material with an optical bandgap of 1.5 eV, electron affinity of 4.4 eV and breadth of 0.7 µm. The AgGaTe2 is a compound p-type material that is used as a second absorber layer with a bandgap, electron affinity and thickness of 1.2 eV, 4.4 eV and 0.3 µm, respectively. On the contrary, 0.1 µm thin p-type MoS2 has been utilized as the BSF layer with optical bandgap and electron affinity values of 1.62 eV and 3.8 eV respectively. We have opted for the Ti as the front contact layer with a work function of 3.84 eV and the Ni with a work function of 5.35 eV has been utilized as the rear contact layer for the effective collection of charge.
Fig. 1. (a) Schematic model and (b) corresponding energy band diagram of the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell.
To simulate the designed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell, a one-dimensional solar cell capacitance (SCAPS-1D) simulator was utilized. The simulation was carried out under the illumination of 1 sun with a power density of 1000 W/m2 at the temperature of 300 K with the global air mass of 1.5 G spectrum to investigate the effects in each layer properties such as thickness, doping level, and defect density. These properties were varied to observe the impacts in the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell output performance. Now to find out the device characteristics; J-V, C-V, C-f, and QE, SCAPS-1D was launched to solve the Poisson equation correlated to electrons and holes [47–49]. The impact of defects on carrier transport and recombination is considered in the relevant equations.
The following equations can be employed to relate how electrons and holes are transferred through the semiconductor by drift and diffusion [50]:
EFp and EFn are the quasi-Fermi levels of p-type and n-type carriers, respectively. µp is the hole mobility and µn is the electron mobility.
The physical parameters of CdS, CZTS, AGT and MoS2 used in the computation were collected from the literature as shown in Table 1 [1,51,52]. The optical absorption data were taken from the SCAPS optical model with default settings, specifically employing the sqrt()-Eg configuration. The following equation relates the absorption coefficient α with the wavelength that is used by the SCAPS model:
where, A and B denote the model parameters, while h and v stand for plank constant and the frequency.
Table 1. Simulation parameters utilized for different active layers of proposed CZTS-based PV solar cell
3. Results and discussion
In this section, the PV performance of the CZTS device has been discussed by varying the thickness, doping concentration and defect density of the absorber, augmenter, window and BSF layer.
3.1 Impact of CZTS absorber layer on PV performance
Figure 2(a) demonstrates the PV response of the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 solar cell depending on the thickness of the CZTS absorber layer within the range of 0.3 to 1.3 µm while the acceptor density and total bulk defect density are kept constant at 1016 and 1014 cm-3, respectively. It is seen in the figure that the short circuit current, JSC increases to 34.6 mA/cm2 from 34.2 mA/cm2 with the increment in thickness. This is because of the greater absorption of light and as a result huge number of electron and hole pairs are generated [53–55]. The value of JSC remains constant at 34.7 mA/cm2 from the range of 0.7 to 1.1 µm and beyond that the JSC exhibits e negative change because at a higher absorber layer thickness, the lifetime of the minority charge carrier is decreased [56]. On the other hand, the open circuit voltage, VOC depicts a downward zigzag movement which decreases from the value of 1.12 to 1.09 V. The negative change in the value of VOC is for the rise in reverse saturation current as the thickness increases [57]. The fill factor (FF) also experiences a downward movement with a decrement from the value of 88.9 to 88.2%. Therefore, the power conversion efficiency (PCE) decreases with a rapid downfall from the value of 34.1% to 33.2% depending upon the VOC, JSC and FF.
Fig. 2. Variation on PV performances of n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell according to CZTS absorber layer (a) Thickness (b) Doping concentration and (c) Defect density.
Figure 2(b) shows the impact of the carrier density of the CZTS layer on the output PV parameters of the designed solar cell. The doping concentration has been varied in the limit of 1014 to 1019 cm-3, keeping the thickness of 0.7 µm and the defect density value of 1014 cm-3. From the graph, it is noticed that the value of JSC remains constant at 34.8 mA/cm2 with the increase of doping density to 1015 cm-3 and then slightly drops to 34.7 mA/cm2 at 1016 cm-3. But as the value of doping concentration advances, the JSC experiences a sheer downfall from 32.9 to 13.1 mA/cm2. The reason behind this is the rise of recombination rate at a higher doping density and because of that higher doping density the minority charge carriers have greater chances to recombine, and the probability of recombination is increased [58,59]. On the contrary, the VOC shows an upliftment from the value of 1.1 to 1.13 V. VOC increases as the density of holes rises at higher doping concentrations [60]. The FF shows a modest movement in the upward direction and rises from 87.9 to 88.2%. FF is increased because of the reduction in series resistance at a higher doping concentration [61]. The PCE shows a drastic drop off from 33.6% to 13% and the peak value is 33.9% at the doping concentration of 1016 and beyond that range the efficiency falls to almost two times that of the peak value. This happens because of the effect of disruptive change in the short circuit current density.
Figure 2(c) displays the consequences of the increase in total bulk defect density of the CZTS absorber layer of the designed solar cell. The defect density has been varied from 1012 to 1017 cm-3. The thickness and doping concentration are kept constant at 0.7 µm and 1014 cm-3, respectively. From the graph, it can be seen that all the output parameters are almost constant throughout the defect density range between 1012 to 1014 cm-3 and beyond that limit, all the output parameters experience a rapid downfall depending on the increasing defect density. The JSC steeply falls from 34.7 to 31.9 mA/cm2 with the advancement in defect density because of the increased imperfection level that prevents the production of more electron-hole pairs and enhances the recombination current [49]. Both the VOC and FF display the same manner just like the JSC and show a downward movement. The VOC drops from 1.1 to 0.96 V and the FF drops from 88.8 to 75.8%. The reason behind that is the bulk defects which can raise the reverse saturation current and reduce the mobility of the carriers. Decrement in these output parameters is caused by the Shockley-Read-Hall (SRH) recombination process which is paramount at a higher defects [62]. As a result, The PCE also decreases and shows a drastic decrement from 34 to 23.2%.
3.2 Effect of the AGT absorber layer on PV performance
Figure 3(a) displays the influence of the thickness of the AgGaTe2 (AGT) absorber layer of the designed solar cell. The thickness has been varied from 0.1 to 0.6 µm while the acceptor density and defect density are kept constant at 5 × 1019 and 1014 cm-3, respectively. From the figure, it can be seen that the JSC increases throughout the range of thickness and rises from 30.6 to 37.1 mA/cm2. As the breadth of the absorber layer advances more photons of light are absorbed which increase the production of the electron-hole pair and the JSC increases. The value of VOC shows a constant behavior and remains constant at the value of 1.1 V. That means it is unaffected by the increasing breadth of the AGT absorber layer. The dominant factors dictating VOC appear to be strongly governed by properties inherent to the CZTS absorber layer, where the role of AGT remains secondary and less impactful within the explored thickness range. The FF shows the same manner just like the VOC and is constant at the value of 88.6%. The PCE shows a gradual increment and rises from the value of 29.8 to 36.3%.
Fig. 3. Variation on PV performances of n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell according to AGT absorber layer (a) Thickness, (b) Doping concentration and (c) Defect density.
Figure 3(b) expresses the impact of the carrier concentration of the AGT absorber layer on the output parameters of the designed cell. The doping concentration has been varied within the limit of 1017 to 1022 cm-3. The JSC shows a lean increment at first and beyond the range of 1018 cm-3 it is constant at the value of 34.7 mA/cm2. The JSC increases because of the enhancement in doping concentration and as a result the mobility of the carrier is also increased. The VOC is almost constant throughout the range of doping concentration and maintains a constant value of 1.1 V. The FF shows a movement in the upward direction and increases from 83.7 to 88.6% just because at a higher doping concentration the diode properties improve [48,51]. The PCE also shows an upliftment and rises from 30.8 to 33.9%.
The dependency of n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell on the bulk defects of the AGT absorber layer has been shown in Fig. 3(c). The defect density has been varied from 1012 to 1017 cm-3. The thickness and the doping concentration of AGT are kept constant at 0.3 µm and 5 × 1019 cm-3, respectively. All the output PV parameters are almost constant with the increase in bulk defects up to 1014 cm-3 but beyond that range all the parameters exhibit a downward movement. The JSC falls from 34.7 to 30.8 mA/cm2 and the VOC drops from 1.1 to 1.02. The FF and PCE also experience the same manner like the JSC and VOC. The FF and PCE drop from 88.6 to 82.5% and 33.9 to 26%, respectively. The cause of this negative change in all these output parameters is the Shockley-Read-Hall (SRH) recombination which is predominant at a higher defect density. Because at a higher defect density, the reverse saturation current rises and decreases the mobility of carriers.
3.3 Impression of the CdS window layer on the PV performance
Figure 4(a) expresses the impact of the thickness of the CdS window layer on the output PV parameters of the proposed cell. The thickness of the CdS layer has been varied in the range of 0.05 to 0.3 µm while the donor density and defect density are kept constant at 1018 and 1014 cm-3, respectively. In the figure, the PV parameters (JSC, VOC, FF, and PCE) exhibit nearly constant behavior and are unaffected by the change in the width of the CdS layer. The CdS window layer has a sufficiently high carrier lifetime and diffusion length, even with a defect density of 1014 cm-3. This allows electrons to efficiently reach the cathode before the possibility of being lost through recombination [52]. Consequently, the performance of the device remains relatively consistent. The value of JSC stands at 34.7 mA/cm2 and the VOC also shows the same manner with a constant value of 1.1 V. FF is constant at a value of 88.6%. Therefore, the resultant PCE is showing a constant value of 33.9%.
Fig. 4. Variation on PV performances of n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell according to CdS window layer (a) Thickness, (b) Doping concentration and (c) Defect density.
Figure 4(b) demonstrates the PV response of the designed cell by the doping concentration of the CdS layer within the range of 1016 to 1021 cm-3. The values of thickness and defect density of the layer are kept constant at 0.1 µm and 1014 cm-3, respectively. Here, the JSC and VOC are uninfluenced by the carrier density and are constant at 34.7 mA/cm2 and 1.1 V, respectively. The output parameters FF and PCE depict nearly the same nature. The FF increases slightly from 88.5 to 88.6% while donor density is raised from 1016 to 1017 cm-3 and is constant at 88.6% within the limit of 1017 to 1021 cm-3. The increment in the FF is for the reduction in diode idealistic factor [48]. The PCE is slightly increased from 33.8 to 33.9% within the range of 1016 to 1017 cm-3 and is constant at a value of 33.9% within the range of 1017 to 1021 cm-3.
Figure 4(c) depicts the consequences of the increase in bulk defect density of the CdS layer within the limit of 1012 to 1017 cm-3. The value of thickness and donor density have been kept at values of 0.1 µm and 1018 cm-3, respectively. All the output parameters are nearly constant throughout the defect density range of the window layer. The value of JSC remains constant at 34.7 mA/cm2 with the increase in defects to 1016 cm-3 and beyond that range it experiences a weeny change in the downward direction and drops to 34.5 mA/cm2. The reason behind this downfall is the increment in dark current which may furiously influence the other PV parameters as well as the device's overall efficiency [63]. The VOC and FF have exhibited the same nature while remained constant at 1.1 V and 88.6%, respectively. The PCE is constant at the value of 33.9% with the increase of defect density to 1016 cm-3 and then slightly drops to 33.7% while the defect density was 1017 cm-3. Hence, the bulk defect density of the window layer has negligible impact on the output parameters.
3.4 Influence of MoS2 BSF layer on PV performance
The impact of MoS2 as a back surface field (BSF) on the proposed cell’s performance has been observed in this section.
Figure 5(a) illustrates the effect of the thickness of the MoS2 layer on the cell’s output parameters. The thickness has been altered in the range of 0.05 to 0.3 µm while the acceptor density and defect density are set at 1020 and 1014 cm-3, respectively. From the figure, it is noticed that all the output parameters have depicted no change with the alteration in thickness. All the PV parameters have shown the same manner and maintained a constant value throughout the entire range of the thickness. The JSC is constant at the value of 34.7 mA/cm2 while the VOC also showing the same nature with the constant value of 1.1 V. The FF remains constant at the value of 88.6% as well as the PCE with a constant value of 33.9%. This phenomenon can also be explained by the minimal impact of thickness on hole that crosses the BSF layer to anode as the carrier lifetime and hence diffusion length of holes are high compared to BSF thickness [64].
Fig. 5. Variation on PV performances of n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell according to MoS2 layer (a) Thickness, (b) Doping concentration and (c) Defect density.
Figure 5(b) depicts the influence of the doping concentration of the BSF layer on the cell’s PV parameters. The doping concentration has been varied in the range of 1018 to 1023 cm-3. The values of thickness and defect density of the layer are kept constant at 0.1 µm and 1014 cm-3, respectively. All the output parameters are fairly independent throughout the doping concentration of the BSF layer. All the parameters have showed a constant behaviour and no change with the variation in the carrier density. The values of JSC and VOC are constant at 34.7 mA/cm2 and 1.1 V, respectively. The FF and PCE remain at a fixed value of 88.6 and 33.9%, respectively. However, it is foreseeable that outside the range of the observed doping density, the decrease in photocurrent could result from the absorption of low-energy photons by free carriers, which adversely impacts the overall performance of the solar cell [64].
The impact of defect density on the cell’s PV response of the BSF layer has been displayed in Fig. 5(c). The defect density has been varied within the range of 1012 to 1017 cm-3 while the values of thickness and acceptor density stay at the value of 0.1 µm and 1020 cm-3, respectively. All the performance parameters have maintained a constant value and are unaffected in the entire observed range of the defect density of 1012 to 1017 cm-3. Nevertheless, more than that range the device’s output parameters may experience a downfall because defects can enhance the dark current which might hazardously influence the cell’s performance.
3.5 Impact of operating temperature on PV performance
The effects of temperature on the device performance are displayed in Fig. 6. The working temperature plays a vital role in the cell’s performance and remarkably affects the output PV parameters. The operating temperature has been altered in the range of 250 to 375 K. The JSC exhibits nearly constant behavior and stands at the value of 34.7 mA/cm2. However, the VOC shows a dramatic downward movement and falls from 1.17 V at 250 K to 0.983 V at 375 K. The FF also reduces and drops from 90.1 to 85.5% within the range of the operating temperature from 250 to 375 K. Eventually, as the VOC and FF both decrease with the increase in temperature, the resultant PCE also decreases from 36.6 to 29.2% throughout the range of operating temperature. The heat generated within the photovoltaic (PV) cell can influence its operation by affecting parameters such as VOC and FF. This internal heat can also lead to a reduction in the bandgap. Additionally, unwanted irradiation that is absorbed may have a detrimental impact on the encapsulation coating of the PV cell [65].
3.6 J-V characteristics and QE curve of the designed solar cell
Figure 7(a) expresses the J-V characteristics curves of the n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell. Firstly, the n-CdS/p-CZTS single heterojunction solar cell has shown PV parameters of VOC = 0.88 V, JSC = 24.82 mA/cm2, FF = 81.3% and PCE = 17.75%. But, when the MoS2 layer with a thickness of 0.1 µm is introduced as a BSF layer the VOC drastically improves to a value of 1.09 V. This is due to the establishment of a high built-in potential at the back surface of the proposed cell. The JSC also increases to a value of 25.97 mA/cm2, so does the FF and PCE. But with an AgGaTe2 (AGT) current augmenting layer having a thickness of 0.3 µm added in the cell’s structure, the JSC has taken a leap in a positive manner and significantly rises to a value of 34.7 mA/cm2. The magnificent leap towards a greater value is for the reduction in the velocity of surface recombination. The value of FF and PCE also increases rapidly with a value of 88.58 and 33.89% respectively.
Fig. 7. (a) J-V characteristics curves and (b) Quantum Efficiency (QE) curves of the optimized n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell.
Figure 7(b) shows the Quantum Efficiency (QE) curves of the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell. The figure also shows the QE contribution of CZTS and AGT separately which has been calculated from the incident solar spectrum in the CZTS layer and transmitted spectrum to the AGT layer according to QE computation in tandem solar cell [66]. The wavelength has been varied in the range of 300 to 1200 nm. From the figure, it can be observed that the QE of the single heterojunction n-CdS/p-CZTS cell falls towards 0% when the wavelength is beyond 824 nm. The QE of the designed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar enhances to 1000 nm due to the addition of AGT layer combined with the action of MoS2 BSF layer by lessening of carrier recombination at the surface. As a consequence, the overall efficiency of the designed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell is improved.
3.7 Overall output of the designed photovoltaic cell
Table 2 displays the outcome of the designed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell where the variation in active layer and the changes in PV parameters can also be perceived. As we can see from the table, with the incorporation of both the p + -AgGaTe2 current augmenting layer and p++-MoS2 BSF layer in the single heterojunction n-CdS/p-CZTS solar cell all the PV parameters increase. The VOC is significantly increased from 0.88 V to 1.1 V with the inclusion of p++-MoS2 BSF layer. Moreover, the JSC is seen to be drastically improved with the addition of the p + -AGT current augmenting layer and rises from 24.82 mA/cm2 to 34.7 mA/cm2. With the increment in JSC and VOC both the FF and PCE also increase, and so does the overall cell’s efficiency and performance.
Table 2. The PV parameters of the n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell without and with p + -AgGaTe2 second absorber layer
However, the performance of the proposed structure compared to analogous systems is shown in Table 3. The table presents a range of PV device structures, with notable examples including n-In2S3/p-CZTS/Au, n-ZnSe/p-CZTS/Mo, Perovskite/CZTS tandem, and n-ZnSe/p-CZTS/p + -WSe2. In the midst of these, the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 structure emerges as a distinctive contender, boasting a remarkable power conversion efficiency (PCE) to 33.89%. This comparison provides insights into the diverse landscape of photovoltaic technologies and highlights the promising attributes of the proposed configuration.
Table 3. The performance of the proposed platform compared with analogous systems
4. Conclusion
This work reports the simulation results of the designed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 thin film solar cell. The influencing parameters like thickness, doping level, and defect density have thoroughly been investigated. At first, the optimized n-CdS/p-CZTS single heterojunction cell offers a PCE of 17.75% with JSC of 24.82 mA/cm2, FF of 81.3% and VOC of 0.88 V. Then, after the insertion of the MoS2 back surface layer, the VOC and JSC are increased by 0.21 V and 2.14 mA/cm2, respectively resulting in an improved PCE of 25.84%. Eventually, with the inclusion of AgGaTe2 as a second absorber layer, the JSC gets a boost with an increment of 7.74 mA/cm2 and goes to the value of 34.7 mA/cm2. Herein, the optimized outcome of the proposed n-CdS/p-CZTS/p + -AgGaTe2/p++-MoS2 device parameters are JSC = 34.7 mA/cm2, VOC = 1.1 V, FF = 88.58% and PCE = 33.89%. Certainly, these numbers are great if we look around at the other thin film solar cells available in the market and might have the potential to meet the world energy crisis. Besides, it opens the door for further research in the field of CZTS-based thin film solar cell in producing sustainable renewable energy and offers the promise of a cleaner and greener earth.
Acknowledgments
The authors are highly indebted to Prof. Marc Burgelman, University of Gent, Belgium, for supplying SCAPS simulation software.
Disclosures
The authors declare no competing financial interest.
Data availability
Simulation details and associated data are available free of charge from authors upon reasonable request.
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