The thickness of the absorber layer is one of the most important parameters for increasing the performance of solar cells [33] . In this section, we firstly study the effect of absorber layer thickness of the proposed structure for achieving optimized V o c , J s c , F F and η . To demonstrate this, the performance characteristics of the thin film solar cell have been visualized as a function of thickness of absorber layer CZTS with three buffer layer materials as shown in Figure 3. For each simulation, the thickness of CZTS was varied from 100 to 2000 nm with a fixed buffer layer thickness of 50 nm at a temperature of 300 K. A single donor-type defect with a defect density of 1 × 10¹³cm⁻³ was introduced in the CZTS layer. By investigating the curves of J s c , V o c , F F and η as a function thickness of absorber layer as shown in Figure 3(a)-(d) respectively, it has been found that all performance parameters except V o c showed similar changing tendency with changing absorber layer thickness in our study. The open circuit voltage shows a different approach.

As can be seen from the Figure 3(b), Figure 3(d) and Figure 3(a), performance parameters J s c , F F and η upsurges linearly with rising thickness of CZTS absorber layer in the lower range from nearly 100 to 900 nm while V o c is found to be saturated shown in Figure 3(c). The results can be clarified by the facts that as the thickness of absorber layer increases, the photons absorption of incident light in this region becomes more which leads to excess electron-hole

pair generation, and consequently promotes the short circuit current density in Figure 3(b). The behavior of the F F is likely similar to that of V o c and J s c [34] while that of the efficiency can be illustrated by the behavior of V o c , J s c and F F [35] . Owing to the high-absorption coefficient of CZTS [15] , a few hundred nanometer film is capable of absorbing enough sunlight. Therefore, the increment in J s c , F F and η is found more significant at the thickness increment in the lower range from 100 nm to 900 nm of the absorber layer. In addition to the improvement of light absorption, the chance of carrier recombination also increases simultaneously with increasing thickness of absorber layer because the charge carriers have to travel longer distances for diffusion in a thicker absorber layer. Therefore, the rise in thickness above 900 nm leads the nearly saturated photo current density, fill-factor and efficiency. On the other hand, the open circuit voltage remained approximately constant since the increasing thickness of absorber layer results in the increment of carrier diffusion length, the probability of recombination rate of photo-generated carrier and life-time of photo-generated carrier have been affected, the amount of carrier collection at the electrodes decreases and consequently saturation in V o c with increasing thickness. They all increase up to 900 nm and then possesses a slow increase in the efficiency up to almost 1800 nm and then shows a stiff straight line up to 2000 nm with really less comparable efficiency for all three materials.

In addition to the results, the numerical values of V o c , J s c , F F and η are comparable for the three buffer layer materials. For examples, the best effi­ciencies for ZnS/CZTS, CdS/CZTS, and Cd₀.₄Zn₀₆S/CZTS structures were found as 26.71% at 0.9 µm, 26.72% at 1.3 µm, and 27.00% at 1.7 µm, respectively as can be seen from Figure 3(a). As can be seen from Table 1, the highest electron mobility of Cd₀.₄Zn₀₆S among the three materials would result the highest photo-voltaic conversion efficiency of the solar cell. For CdS, ZnS, and Cd₀.₄Zn₀₆S buffer layers, the upper limit of the CZTS absorber layer thickness was discovered at 900 nm, 1300 nm, and 1700 nm, respectively. With the Cd₀₄Zn_{0 6}S buffer layer, the greatest efficiency (27.0%) was discovered for the 1700 nm thick CZTS layer. Therefore, we have chosen the optimal thickness of CZTS absorber layer as around 2000 nm for further simulation because of optimized values of performances parameters.

In this section, the buffer layer thickness was varied from 50 to 150 nm with a fixed absorber layer thickness of 2000 nm for simulation. A single acceptor-type defect with defect density of 6 × 10¹⁶ cm⁻³ was introduced in the buffer layer. Figure 4 shows the variation of solar cell parameters as a function of buffer layer thickness.

It was noticeably found that all fundamentals parameters of solar cell remain same in their respective scales and independent of thickness of buffer layer in the relatively lower range for the three buffer layer materials. It is observed from Figure 4(c) that no significant variation on V o c was found with increasing thickness of buffer layer for all buffer layer materials used in this study. This result has a good agreement with the previous reported result [36] [37] . As can be seen from Figure 4(b), the short circuit density shows slightly degradation tendency with increasing of buffer layer thickness throughout the whole range for CdS and Cd₀₄Zn₀₆S. It finally becomes lowest at 150 nm. However, the parameter J s c remains nearly independent of buffer layer thickness in case of ZnS material. Thus, although J s c for three buffer layer materials possess approximately

same value at lower thickness of buffer layer, it is possible to obtain higher short circuit density even at higher thickness of buffer layer, such as 150 nm by using ZnS instead of CdS and Cd₀₄Zn₀₆S as buffer layer material. These obtained results can be explained by the facts that as the thickness of buffer layer increases, the diffusion length of carriers exceeds, this may lead to the inefficient photo-generated carrier’s separation and collection, thereby may cause reduction in short circuit density [38] . Since the donor density of ZnS is much higher and uniformly distributed than that of CdS and Cd₀₄Zn₀₆S as seen from Table 1, the reduction in J s c was occurred less or negligible in ZnS than either CdS or Cd₀₄Zn₀₆S at higher thickness. The fill factor for CdS and Cd_0.4Zn_0.6S do not show any change with increasing buffer layer thickness. It has also been investigated that although F F and η seen from Figure 4(d) and Figure 4(a) respectively remained unchanged with increasing buffer layer thickness in the lower range from 50 nm to about 90 nm for ZnS buffer layer like CdS and Cd₀₄Zn₀₆S, after 90 nm thickness of buffer layer, both parameters of the solar cell for ZnS based buffer layer start decreasing dramatically. The decreasing behavior in F F for ZnS has been seen from around 65 nm which becomes more significant at 150 nm while η falls to almost 15% at 150 nm thickness, which is dissimilar to CdS and Cd₀.₄Zn₀₆S. Since CB effective density of states of ZnS is lower than that of CdS and Cd₀₄Zn₀₆S, thereby carrier concentrations and energy distributions of carriers are relatively lower. In addition, as the increasing buffer layer thickness results in longer diffusion length, higher series resistance and consequently increases the probability of carrier recombination rate. By these ways, the carriers separation becomes limited more in ZnS based buffer layer due to those facts at higher thickness of buffer layer [39] and thereby reduction in F F and finally fall in η as well. The obtained optimum efficiencies for CdS, ZnS, and Cd₀₄Zn₀₆S buffers are 27.21%, 27.11%, and 27.11%, respectively which is shown in Figure 4(a). Since band gap energy of ZnS is the highest than either CdS or Cd_0.4Zn_0.6S as shown in Table 1, free electron is only generated by solar radiation having photon energy equal or greater than 3.5 eV. On the other hand, photon energy lower than 3.5 eV can also contribute to generate free electron in either CdS or Cd_0.4Zn_0.6S-based buffer layer since band gap energy of either CdS or Cd_0.4Zn_0.6S is lower than 3.5 eV. This is one of the possible reasons for lowering the performance efficiency of ZnS based solar cell. From this study, we clarify that either CdS or Cd_0.4Zn_0.6S is suitable as a buffer layer materials for any thickness of that layer in the range of 50-150 nm while the range is limited to 50-75 nm for ZnS.

The doping concentration of the absorber layer plays a significant role on the performance of solar cells. In this section, the doping concentration of the absorber layer material of the proposed structure has been evaluated for achieving optimized V o c , J s c , F F and η for three buffer layer materials. To demonstrate this, the performance characteristics have been visualized as a function of doping concentration ranging from 1.00 E + 14 cm⁻¹ to 1.00 E + 18 cm⁻¹ of the absorber layer material CZTS with three buffer layer materials as shown in Figure 5. It is clearly found that all fundamentals parameters of solar cell remain almost constant with increasing acceptor doping concentration up to 1.00 E + 18 cm⁻¹ for the two buffer layer materials ZnS and CdS. Solar cell with Cd₀₄Zn₀₆S based buffer layer showed distinct behaviors in J s c , F F η in the lower range of doping concentration of absorber layer which made it apart from ZnS and CdS. ZnS and CdS show almost stable J s c values while J s c of Cd_0.4Zn_0.6S tends to decrease as the doping concentration is increased from 1.0 E + 14 cm⁻¹ to 3.33 E + 17 cm⁻¹ as shown in Figure 5(b). After 3.33 E + 17 cm⁻¹, J s c of Cd_0.4Zn_0.6S becomes saturated. Since the electron mobility of Cd_0.4Zn_0.6S is relatively higher than that of ZnS and CdS, it is apparent that the reduction in the width of depletion layer becomes sufficient while doping concentration increases in absorber layer with fixed thickness, thus much more photo-generated carriers rapidly recombine with holes, there is a weaken the carrier collection and finally reduction tendency in J s c [40] . As the acceptor carrier concentration increases, open circuit voltage V o c for all the buffer layer materials tend to maintain an almost stable voltage of 1.10 V which can be seen from Figure 5(c). This can be explained by the facts that with increasing doping concentration at a constant thickness of absorber layer, the carrier collection probability becomes immediately saturated for the fixed collection length, thus additional charge carriers vanish by the recombination. This explanation has good agreement with previous report [41] .

As can be seen from Figure 5(d), the F F of Cd_0.4Zn_0.6S based solar cell increases linearly from very low value with increasing doping concentration from 1.00 E + 14 cm⁻¹ to 3.33 E + 17 cm⁻¹, then stays a constant value till the last. However, ZnS and Cds shows almost constant of F F around 87% throughout the variation in doping concentration shown in Figure 5(d). The behavior of the η could be described by the whole behavior of the V o c , J s c and F F [35] . Similar shape like F F was found in η as shown in Fig. 5(a) for all buffer layer materials. ZnS and Cds shows almost constant of η around 27% throughout the variation in doping concentration while Cd_0.4Zn_0.6S shows a uprising state in η from 1.00 E + 14 cm⁻¹ but after 3.33 E + 17 cm⁻¹, it becomes a stable value of about 25% from up to 1.00 E + 18 cm⁻¹.

Operating temperature has a vital impact on the solar cells performances [42] . All of the simulations in this study were run at 300 K, which is the national average for ambient temperature. In this section, the temperature was raised from 290 K to 350 K to account for how the temperature affects V o c , J s c , F F and η of CZTS based thin-film solar cells for three buffer layer materials as demonstrated in Figure 6. It should be noted that the specified range of temperature considered in this section is consistence with the range of temperature subsists from winter to summer seasons in Bangladesh. It can be seen from the Fig. 6 that all parameters except short circuit current density have been strongly affected by the change in temperature in our study. It is seen from Figure 6(b) that the value of J s c is found nearly temperature independent but strongly dependent and shows three

different straight lines for three different buffer layers. The value of J s c can be restricted by the device ohmic losses such as series and shunt resistances, metal contact and recombination losses. The constant current density with increasing temperature apparently indicates that the combined effects of these mentioned parameters might mitigate the variation in J s c in our simulation. The J s c of Cd_0.4Zn_0.6S is the highest, CdS shows the lowest. ZnS being in the middle of them.

It has been noticed that V o c , F F and η in Figure 6(c), Figure 6(d) and Figure 6(a) respectively showed degradation tendency with increasing temperature from 290 K to 350 K for three novel buffer layer materials. The reasons of such degradation can be explained by the facts that temperature affects solar cell performance negatively through touching several parameters of materials simultaneously. Increasing temperature shifts the band gap energy of the semiconductors to lower energy [43] and also simultaneously increases the velocity-instability of charged particles [44] , reverse saturation current and resistivity of the materials as well. As a result, the probability of recombination rate of charge carriers before reaching the depletion region promotes and eventually degradation of V o c occurs as seen from Figure 6(c). Furthermore, the behavior of F F is dependent on the combined behavior of V o c and J s c [45] [46] . Constant J s c while decrement in V o c jointly led to a decrease in F F of the device for all buffer layer materials. It is found from Figure 6(d) that the F F of ZnS and CdS shows decreasing in parallel line, while CdS showing the higher between the two. Cd₀₄Zn₀₆S shows little different approach than the other two materials during temperature uprising in the lower range. It slightly increases first from 290 K to 310 K, then starts to decrease gradually like to either ZnS or CdS. The value of η reflects the total behavior of V o c , J s c and F F . Constant J s c and decrement in both V o c and F F showed the way to a decrease in the η of the device as shown in Figure 6(a). From the simulation, the best efficiencies for CdS/CZTS, ZnS/CZTS, and Cd_0.4Zn_0.6S/CZTS were found as 27.33%, 27.78%, and 27.54%, respectively, at a temperature of 290 K. In addition, it was also noticeably that Cd₀₄Zn₀₆S offers the most efficient short circuit current density while least F F in this section.

Finally, we have tried to make comparison between our proposed model and the related models of the other researchers reported previously in terms of performance parameters as shown in Table 2.

Our proposed model of thin-film solar cell supports comparatively higher open circuit voltage, short-circuited current density, fill-factor and efficiency successfully with lower toxicity.