The impact of SnMnO2 TCO and Cu2O as an HTL on CIGSSe solar cell performance improvement

doi:10.21203/rs.3.rs-2503598/v1

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Research Article

The impact of SnMnO₂ TCO and Cu₂O as an HTL on CIGSSe solar cell performance improvement

https://doi.org/10.21203/rs.3.rs-2503598/v1

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published 14 Jun, 2023

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This paper has simulated two experimental CIGSSe thin-film solar cells (TFSCs) having a high efficiency of 20% and 22.92%. Later validates the photovoltaics results of both devices based on the experiential values of optoelectronics data. After the simulation, a compelling result was confirmed for both the experimental and simulation solar cells. Finally, different designs have also been proposed. The proposed Type-1 solar cell is designed by the addition of low resistivity, wide energy bandgap (E_g), and minimum absorption coefficient (α) based tin-doped manganese oxide (Sn_1 − xMn_xO₂) material in a conventional solar cell instead of ZnO: B and ZnMgO: Al transparent conducting oxides (TCO) layer. Further, by matching the band energy alignment and adjusting the thickness and doping concentration of the TCO, buffer, and absorber layers, the efficiency of the proposed Type-1 TFSC has been increased from 20 to 27.75%. The proposed Type-1 solar cell has some drawbacks, such as the inability to appropriately suppress the photogenerated minority carrier recombination losses due to the absence of a hole transport layer (HTL), and the EQE is relatively lesser than the conventional solar cell. Furthermore, wide band energy and a high ‘α’ based on cuprous oxide (Cu₂O) as a HTL are added between the absorber and the back ohmic contact layers in the proposed Type-1 solar cell. Then the structure becomes a proposed Type-2 TFSC. The proposed Type-2 TFSC absorbs more blue light, instantly suppressing the recombination losses and enhancing efficiency (29.01%) and EQE (97%).

Thin-film solar cells

Tin-doped manganese oxide

External quantum efficiency (EQE)

Hole transport layer

cuprous oxide

Sunlight is an environmentally friendly, non-depleting, renewable energy source. It can readily replace non-renewable energy sources like fossil fuels and subterranean petroleum deposits [1]. Solar cells are one of the most common renewable energy sources, and they provide a more practical option [1–3]. Solar cell production has advanced significantly from one generation to another. At present, the development of good solar cells based on the efficiency, cost, and toxicity of materials are the key challenges [4–6]. Thus, for the previous few decades, researchers have been working very hard on various p and n-type semiconductor materials to meet the ever-increasing requirements for the three essential aspects (i.e., cost, efficiency, and durability). There are several p-type absorber layer semiconductor materials such as cadmium telluride (CdTe), amorphous silicon (a-Si), copper indium selenide (CIS), and copper-indium-gallium-selenide (CIGS) that are currently used in thin-film solar cells. These materials are lower in cost and less toxic to improve efficiency and deplete the cost of the solar cell [7–9]. Among all the absorber materials used in solar cells, p-type polycrystalline CIGSSe semiconductor material-based solar cells have excellent conversion efficiencies greater than 23.35% (experimentally) [10] and greater than 27.48% (theoretically) [11]. There are several good advantages of CISSSe-based solar cells, such as less material consumption, flexibility, and long-term stability, and they pursue high-speed, low-cost roll-to-roll manufacturing [1, 4–5, 7–8, 12]. In addition, CIGSSe materials have several other advantage, including higher p-type conductivity [13], high ‘α’ in the order of 1×10⁴ to 1.45×10⁵ cm^− 1, springiness, and admirable stability [14–15], insubstantial, lesser developed cost, and greater power conversion efficiency [1, 12, 16–18]. To attain the better efficiency, some techniques like,sol-gel, pulsed laser deposition, thermal evaporation, and spray pyrolysis techniques [19] are used to synthesize the CIGSSe materials. The efficiency of CIGSSe-based solar cells has been improved by adjusting the bandgap, electron affinity, and device thickness. The bandgap and electron affinity of the Cu (In_1 − AGa_A) (S_B Se_1 − B) material is changed with a change in the mole fraction of gallium (G-composition) and sulfur (S-composition) where A = G/(G + In) and B = S/(S + Se). They can be changed by changing the combination of A (0.00 to 0.36) and B (0.00 to 0.40) composition [20–23]. In addition to the p-type CIGSSe absorber layer material's exceptional optoelectrical capabilities, the buffer, TCO, and HTL layer materials are crucial to modelling the high-efficiency solar cell. Researchers have conducted many experimental and analytical studies on TCO and buffer layer-based CIGSSe TFSCs. The different types of TCO/buffer materials like ZnO:Al/CdS [24], ZnO:Al/ZnO, ZnO:Al/ZnMgO, ZnMgO:Al/ZnMgO, ZnO:B/ZnMgO, ZnO:B/CdS, ZnO:B/ZnO, ZnO:Al/i-ZnO, SnO₂/CdS, i-ZnO/WS₂ and ZnO:B/ZnMnO are applied to design TFSCs and attained the efficiency of 16.8% [24], 18.6% [24], 19.6% [24], 20% [25], 20.8% [26], 22.92% [27], 23.35% [10] (experimental), 26.05% [7], 26.30% [28], 26.40% [29], and 28.86% (simulation) [30]. However, it is observed that several drawbacks (details are mentioned in Table 1) of the TCO and buffer layer materials, such as lower E_g (2.42 eV, 3.3 eV, 3.35 eV, 3.54 eV, 3.6 eV, 3.72 eV and 3.76 eV for CdS, ZnO, ZnO: B, ZnO:Al, SnO₂, ZnMgO:Al and ZnMgO), high (α = 1×10⁴ to 1.2×10⁵ cm^− 1), high resistivity, high cost, and low transparency (80 to 90%), are present [31–35].

Further, a modern eco-friendly Sn_1 − xMn_xO₂ TCO layer and ZnMgO buffer layer materials are employed for better consideration, which is more suitable for improving the performance of classic CIGSSe TFSCs, based on ZnO:B [27] and ZnMgO:Al [25] TCO layers. There are several solid optical and electrical features of the Sn_1 − xMn_xO₂ TCO layer, including low α = 10⁴-10⁶ cm^− 1), high transparency (80%) [36–37], and low resistivity (0.84×10^− 4 Ω-cm) [38], great conductivity, small sunlight reflection, large blue light absorption, economical, high refractive index, minimum dielectric loss, and adjustable broad E_g (up to 3.91eV) [37, 39–41]. The sol-gel technique is often used to synthesise Mn-substitute SnO₂ samples with various Mn compositions (x = 0 to 7), resulting in the making of a broad E_g and low-cost Sn_1 − xMn_xO₂ material [39, 42]. However, using several good properties of the SnMnO₂ material, a different high-efficiency SnMnO₂ TCO and ZnMgO buffer layer-based CIGSSe proposed Type-1 TFSC is simulated. After simulation, better results are seen compared to ZnO:B/CdS and ZnMgO:Al/ ZnMgO TCO and buffer layer-based CIGSSe conventional TFSCs. As a result, the proposed Type-1 solar cell's open-circuit voltage (V_OC), short-circuit current density (J_SC), fill-factor (FF), and efficiency (η) are getting increased. However, these devices are not generalised to suppress the recombination loss developed due to photogenerated minority carriers as the structure has no back surface field (BSF) as HTL. Furthermore, an earth-abundant, non-toxic, low-cost, high work function (4.8 to 5.2 eV), high (α = 1×10⁴ to 1.6×10⁵ cm^− 1), hole mobility of 40 to 120 cm²/(V-s), and a broad E_g (1.2 to 2.1 eV) p-type Cu₂O as a HTL were inserted in the proposed Type-1 TFSC between the CIGSSe and the back ohmic metal contact layer to make a proposed Type-2 TFSC [43–46]. As a result, the proposed Type-2 TFSCs are more capable of reducing recombination losses and also enhancing the cell's efficiency and EQE. The better efficiency is attributed to the following reasons: First, the E_g of the ZnO:B (3.35 eV) [27] and ZnMgO:Al (3.8 eV) [25] is lower than the Sn_1 − xMn_xO₂ TCO layer (3.71 to 4.4 eV), and hence a large amount of blue light approaches the CIGSSe absorber layer, resulting in an increase in the J_SC. After this, the E_g alignment was then properly co-ordinated between the absorber, TCO, and buffer layers. Therefore, recombination rates are inhibited in the ZnMgO/CIGSSe layer. Last, using back-grading Cu₂O as a HTL material decreases the reverse saturation current density (J_O) of the photogenerated minority carrier. Besides these, the EQE of the structures is also enhanced due to the absorption of ultra-violet photons in the TCO-SnMnO₂, the buffer-ZnMgO layer regions, and low-energy visible range photons absorbed in the CIGSSe region.

The main goal of the research is to simulate and validate the two solar cells that have been constructed and recommended by other researchers: ZnO:B/CdS/CIGSSe [27] and ZnMgO:Al/ZnMgO/CIGSSe [25]. Both device structures have been investigated and simulated to validate (using experimental data) the photovoltaic results using the AFORS-HET tool. Consequently, a high E_g SnMnO₂ as a TCO layer applies to the conventional device instead of ZnO:B and ZnMgO:Al TCO layers, to proposed Type-1 solar cell. Optimization of the layer parameters of the ZnO:B/ZnMgO, ZnMgO:Al/ZnMgO, and SnMnO₂/ZnMgO TCO and buffer layers based on the proposed Type-1 TFSCs has been carried out to enhance the efficiency and EQE. Furthermore, Cu₂O as a HTL is used between the CIGSSe absorber layer and the back metal contact layer in the proposed Type-1 solar cells to create the eco-friendly and highly efficient proposed Type-2 TFSCs. A detailed analysis has been done to achieve the performance in the matching of bandgap alignment, electron affinity, optimizing the layer parameters, and quantum efficiency for the proposed Type-1 and proposed Type-2 TFSCs using the AFORS-HET simulation tool.

The AFORS-HET [47–48] (Automat for heterostructure simulation) numerical simulation tool simulates the ZnO:B, ZnMgO:Al, and SnMnO₂ TCO layers based on the proposed Type-1 and Type-2 CIGSSe TFSCs. This simulation tool solves the electrostatic Poisson Eqs. (1)–(3) and the carrier continuity equation for both electrons and holes. The optical intensity and optical generation rate in a planar layered medium were calculated using the transfer matrix method (TMM). SRH recombination models, radiative recombination, and auger combinations are all considered [49–50]. To obtain the voltage-current density diagram (J-V) under illumination, the standard AM 1.5G solar spectrum is employed.

$\frac{{{\partial ^2}\varphi }}{{\partial {x^2}}}=\left( {\frac{q}{{{\varepsilon _0}\,{\varepsilon _r}}}} \right)\,\left( {p - n+N_{d}^{+}\, - N_{a}^{ - }+\sum\limits_{{defects}}^{{}} {{\rho _t}} } \right)$ (1)

$- \frac{1}{q}\frac{{\partial {J_n}\,}}{{\partial x}}=\left( {{G_n}\, - {R_n}\, - \frac{\partial }{{\partial t}}n\,} \right)$ (2)

$\frac{1}{q}\frac{{\partial {J_p}\,}}{{\partial x}}=\left( {{G_p}\, - {R_p}\, - \frac{\partial }{{\partial t}}p\,} \right)$ (3)

Here, φ, the electric potential, q, the electron charge, ε_o and ε_r, the absolute/relative dielectric constant, p/n, the hole/electron densities, ${\text{N}}_{\text{d}}^{+}$, the ionised donor-like doping density, ${\text{N}}_{\text{a}}^{-}$, the ionised acceptor-like doping density, the defected density, J_n and J_p, the electron and hole current density, G_n/p, the electron/hole optical generation rate, and Rn and Rp, the electron/hole recombination rate. To calculate optical energy (E_g), (χ) and refractive index (n), we use the subsequent arithmetic equations, which take into account the experimental value of the bending parameter;

The E_g and electron affinity of Mg-doped ZnO are defined as [51]:

$${E_g}(eV){\text{ }}of{\text{ Z}}{n_{1 - x}}M{g_x}O(x)=x{E_g}(MgO)+\left( {1 - x} \right){E_g}(ZnO) - b\,x(1 - x)=3.47{x^2}+1.03x+3.3$$

$$\chi \,of\,ZnMgO\,(eV)=7.85\,eV - {E_g}(ZnMgO)$$

Where E_g =7.8 eV of MgO [52] and E_g =3.3 eV of ZnO [32], and b = 3.47 eV [51, 53] is the bowing constant. The E_g and electron affinity of (G, S)-compositions of CIGSSe material are defined as [20, 22, 54–55]:

$${E_g}(eV){\text{ }}of{\text{ }}\,Cu\left( {I{n_{1 - A}}{G_A}} \right)\left( {{S_B}S{e_{1 - B}}} \right)=1+0.13{A^2}+0.08{A^2}B+0.13AB+0.55A+0.54B\,$$

$$\chi (eV){\text{ }}of{\text{ }}CIGSSe\,(A,B)={\chi ^{CIGSe}} - \beta \,\,\left[ {0.13{A^2} - 0.08{A^2}B - 0.13AB - 0.55A - 0.54B\,} \right]$$

Where A = G/(G + In) and B = S/(S + Se) are varies from A (0.00 to 0.36) and B (0.00 to 0.40), bowing constant (b) = 0.13 [56], β = 0.94, and the E_g= 1.69 eV for CIGS [22], and E_g= 1.11 Ev for CIGSe [21].

The E_g, electron affinity, and refractive index of Mg-doped ZnMgO:Al are defined as [57]:

$${E_g}(eV){\text{ }}of{\text{ Z}}{n_{1 - x}}M{g_x}O:Al(x)=x{E_g}(MgO)+\left( {1 - x} \right){E_g}(ZnO:Al) - b\,x(1 - x)=3.47{x^2}+0.89x+3.54$$

$$\chi \,of\,ZnMgO:Al\,(eV)=8.04\,eV - {E_g}(ZnMgO:Al)$$

$$n=4.084 - 0.62{E_g}(ZnMgO:Al)$$

Here is the 3.54 eV [34] E_g of ZnO:Al used in Eq. (8). The bandgap energy, electron affinity, and refractive index of Mn-doped SnO₂ are defined as [35, 58]:

$${E_g}(eV){\text{ of }}S{n_{1 - x}}M{n_x}{O_2}(x)=x{E_g}(MnO)+\left( {1 - x} \right){E_g}(Sn{O_2}) - b\,x(1 - x)=0.79{x^2}+0.05x+3.6$$

$$\chi (SnMn{O_2})(eV)=8.1\,eV - {E_g}(SnMn{O_2})$$

$$n=4.084 - 0.62{E_g}(SnMn{O_2})$$

Where x = Mn/Mn + SnO₂ is the Mn-composition ratio of the SnMnO₂ material, it changed to 0 to 0.7, b = 0.79 is the bowing constant, and E_g= 3.6 eV [59] for SnO₂ material and E_g=4.44 eV [60] for MnO material respectively.

Valance band offset (VOB) and conduction band offset (CBO) of ZnMgO/CIGSSe interface:

$$\underbrace {{\vartriangle {E_V}\,(eV)}}_{{VOB}}=E_{g}^{{ZnMgO}} - E_{g}^{{CIGSSe}} - \left( {{\chi _{ZnMgO}} - {\chi _{CIGSSe}}} \right)$$

$$\underbrace {{\vartriangle {E_C}\,(eV)}}_{{CBO}}={\chi _{ZnMgO}} - {\chi _{CIGSSe}}$$

VOB and CBO of ZnO:B/CIGSSe interface:

$$\underbrace {{\vartriangle {E_V}\,(eV)}}_{{VOB}}=E_{g}^{{ZnO:B}} - E_{g}^{{CIGSSe}} - \left( {{\chi _{ZnO:B}} - {\chi _{CIGSSe}}} \right)$$

$$\underbrace {{\vartriangle {E_C}\,(eV)}}_{{CBO}}={\chi _{ZnO:B}} - {\chi _{CIGSSe}}$$

VOB and CBO of ZnMgO:Al/CIGSSe interface:

$$\underbrace {{\vartriangle {E_V}\,(eV)}}_{{VOB}}=E_{g}^{{ZnMgO:Al}} - E_{g}^{{CIGSSe}} - \left( {{\chi _{ZnMgO:Al}} - {\chi _{CIGSSe}}} \right)$$

$$\underbrace {{\vartriangle {E_C}\,(eV)}}_{{CBO}}={\chi _{ZnMgO:Al}} - {\chi _{CIGSSe}}$$

VOB and CBO of SnMnO₂/CIGSSe interface:

$$\underbrace {{\vartriangle {E_V}\,(eV)}}_{{VOB}}=E_{g}^{{SnMn{O_2}}} - E_{g}^{{CIGSSe}} - \left( {{\chi _{SnMn{O_2}}} - {\chi _{CIGSSe}}} \right)$$

$$\underbrace {{\vartriangle {E_C}\,(eV)}}_{{CBO}}={\chi _{SnMn{O_2}}} - {\chi _{CIGSSe}}$$

In this paper, the photovoltaic results of two conventional (experimental) CIGSSe TFSCs are verified by applying the experimental values of the photovoltaic parameters (listed in Table 2 and the data is taken from different references) in the AFORS-HET numerical analysis tool [25, 27]. After simulation, we get almost similar results (listed in Table 3) in terms of experimental and simulated solar cells. The designed cell structures, E_g-diagram, J-V characteristic, and EQE-wavelength spectra of the two conventional solar cells are shown in Fig. 1a–d and Fig. 2a–d.

Further, a wide E_g (up to 3.91 eV), high transparency (80%), low (α = 1×10⁴ to 1×10⁶ cm^-1), and high conductivity (which is listed in Table 4) SnMnO₂ as a TCO layer material applied to the basic solar cell instead of ZnO:B and ZnMgO:Al TCO layers, to proposed a Type-1 solar cell. Optimization of the layer parameters of the ZnO:B/ZnMgO, ZnMgO:Al/ZnMgO, and SnMnO₂/ZnMgO TCO and buffer layers based on the proposed Type-1 TFSCs has been done to enhance the efficiency and EQE. Furthermore, a low-cost, eco-friendly, tunable E_g-based Cu₂O is used as a HTL inserted between the CIGSSe layer and the back metal contact layer in the proposed Type-1 photovoltaic cells to make the eco-friendly and highly efficient proposed Type-2 TFSCs. The proposed Type-2 photovoltaic cell comprises the following cells: Al/MgF₂/SnMnO₂/ ZnMgO/CIGSSe/Cu₂O/Mo, aluminium (Al)/nickel (Ni), and molybdenum (Mo) metal ohmic contacts are electrically connected to the front n-type and back p-type semiconductors. A layer of anti-reflection coated (ARC) material, magnesium fluoride (MgF₂) is put on the upper side of the TCO layer to prevent sunlight reflection. To reduce the recombination losses and boost blue light collecting, an n-type SnMnO₂ TCO layer is placed on top of the buffer-ZnMgO layer. This layer has a large electric field (E_g), a small a low ‘α’, and a high resistivity. The heterojunction is made when the lattices of the buffer, TCO, and absorber layers are the same. Afterwards, an n-type ZnMgO buffer layer is added on top of the p-type CIGSSe absorber layer. The ZnMgO buffer layer's primary responsibility is to shield the CIGSSe absorber layer from damage while still allowing as many photons to reach it as possible [61]. As a subsequent HTL, a p-type CIGSSe absorber layer is formed on top of the Cu₂O. When light strikes the solar cells, the CIGSSe layer separates photogenerated electron-hole (E-H) sets. In addition, a HTL made of Cu₂O is sandwiched between the CIGSSe absorber and the rear metal contact layers. Cu₂O HTL is main principal role is to redirect the minority carrier electron away from the space charge region (SCR) and transport the hole toward the back metal contact layer.

Table 1

A detailed investigation of high-efficiency experimental CIGSSe thin-film photovoltaic cells was summarised.
Publication year	2017	2018	2019
Device structure and	ZnO:Al/ZnMgO/CdS/CIGSSe [24]	ZnMgO:Al/ZnMgO/CIGSSe [25]	ZnO:B/CdS/CIGSSe [27]
Layer thickness (nm)	300/60/50/2200/10 [24]	120/700/2200/10 [25]	120/500/20/2500/10 [27]
V_OC (mV)	683.0 [24]	695.0 [25]	746.0 [27]
J_SC (mA/cm²)	38.17 [24]	39.30 [25]	38.50 [27]
FF (%)	74.98 [24]	73.10 [25]	79.70 [27]
Efficiency (%)	19.60 [24]	20.00 [25]	22.92 [27]
R_S (Ω cm²)	0.72 [24]	0.37 [25]	0.20 [27]
R_SH (Ω cm²)	1380 [24]	250 [25]	1312 [27]
α (cm^− 1)	1.2×10⁵ [62] /2.2×10⁵ [63]/2×10⁴ [64]/1.45×10⁵ [14]	1.2×10⁵ [25]/2.2×10⁵/1.45×10⁵	1.2×10⁵ [69]/2×10⁴/1.45×10⁵
Resistivity of each layer ρ (Ω cm)	1.3×10^− 3 [64]/1.82×10^− 4 [65]/2.1×10^− 4 [66]/3 ×10⁶ [67]	1.3×10^− 3 [68]/1.82×10^− 4/3 ×10⁶	2.1×10^− 3 [70]/2.1×10^− 4/3 ×10⁶
Types of deposition used	The sputter-deposition method used for Cu–In–Ga, Selenizanition and sulfurization was used for the generation of Mo. CBD for CdS layer [24]	Sputter-deposition method, K-treatment followed by post-treatment called heat-light soaking (HLS)	Selenizanition and sulfurization were used for the generation of Mo. CBD for CdS layer [27]. Sol-gel dip-coating method used for ZnO: B material
Composition of CIGSSe	Cu/Ga + In = 0.90 Ga/Ga + In = 0.32	Cu/Ga + In = 0.90 Ga/Ga + In = 0.32	Cu/Ga + In = 0.80 Ga/Ga + In = 0.40
Merit	Small size, low cost, minimum leakage current (1.9 ×10^− 9 mA/cm²) flow through the device [24].	Small size, low cost, minimum leakage current (1.69 ×10^− 9 mA/cm²) flow through the device [25].	Small size, low cost, minimum leakage current (3.26 ×10^− 11 mA/cm²) flow through the device [27].
Demerit	Structure design by low energy bandgap and toxic CdS buffer-2 layer.	Absence of BSF layer and more optimization needed.	Structure design by low energy bandgap and toxic CdS buffer layer

Table 2

Experimental data of optoelectronics parameters utilized in the conventional CIGSSe photovoltaics cells.
Material Parameters	n-ZnO:B	n-CdS	p-CIGSSe	n-ZnMgO:Al	n-ZnMgO
Thickness (nm)	500 [27]	20 [27]	2500 [27]/2200 [25]	700 [25]	124 [25]
Energy bandgap E_g (eV)	3.309 [71]	2.42 [31, 75]	1.15 [27]	3.8 [25]	3.75 [25]
Electron affinity χ (eV)	4.5 [72]	4.2 [75]	4.1[14]	4.5 [57]	4.2 [57]
Relative permittivity ε_r	9 [72]	10 [75]	13.6 [14]	10.5 [57]	8 [57]
Conduction band effective density of states N_C (cm^− 3)	1.1×10¹⁹ [71]	1×10¹⁸ [14]	6.7×10¹⁸ [14]	1.8×10¹⁸ [57]	3×10¹⁸ [81]
Valance band effective density of states N_V (cm^− 3)	1.8×10¹⁹ [71]	2×10¹⁹ [14]	1.7×10¹⁹ [14]	2.2×10¹⁹ [57]	2×10¹⁹ [81]
Electron mobility µ_e (cm² V^− 1 s^− 1)	100 [73]	100 [18]	100 [14]	100 [57]	50 [57]
Hole mobility µ_h (cm² V^− 1 s^− 1)	22 [33, 74]	25 [18]	50 [14]	10 [57]	10 [57]
Donor doping concentration N_d (cm^− 3)	1×10¹⁸	1×10¹⁸	0	1×10¹⁸ [25]	1×10¹⁹ [57]
Acceptor doping concentration N_A (cm^− 3)	0	0	2.6×10¹⁶ [75]	0	0
Electron thermal velocity V_e (cm²s^− 1)	2.40×10⁷ [73]	2.6×10⁷ [76]	3.6×10⁶ [77]	1×10⁷ [57]	1×10⁷ [57]
Hole thermal velocity V_h (cm²s^− 1)	1.30×10⁷ [73]	1.4×10⁷	1×10⁷ [14]	1×10⁷ [57]	1×10⁷ [57]
Auger coefficient of electron R_ae (cm⁶s^− 1)	5×10^− 30	1×10^− 31 [76]	1×10^− 28 [14]	2.2×10–³⁴[57]	1.5×10^− 32 [81]
Auger coefficient of hole R_ah (cm⁶s^− 1)	5×10^− 30	1.2×10^− 31[76]	1×10^− 28 [14]	1.1×10^− 34 [57]	1.2×10^− 32 [81]
Band to band recombination coefficient R_bb (cm³s^− 1)	1.2×10^− 11	2×10^− 13 [14]	8×10^− 11 [14]	1.5×10^− 11 [57]	2×10^− 11 [82]
Total trap density N_ttr (cm^− 3)	1×10¹⁷ [72]	3.4×10¹⁸ [76]	2.4×10¹⁴ [78]	7×10¹⁹ [57]	1×10¹⁶ [83]
capture cross-section of electron σ_te (cm²)	1×10^− 12 [72]	1×10^− 12 [76]	1×10^− 15 [78]	7×10^− 16 [57]	5×10^− 13 [83]
capture cross-section of hole σ_th (cm²)	1×10^− 15 [72]	1×10^− 20 [76]	6×10^− 17 [78]	1×10^− 16 [57]	1×10^− 15 [83]
Urbach energy E_urbach (eV)	0.14 [71]	0.0344 [11]	0.014 [79]	0.18 [68]	0.63 [63]
Sheet resistances R_Sheet (Ω/square)	2.5×10³ [74]	0.72 [77]	1.15×10³ [67]	18.7 [25]	3.7×10⁶ [25]
Carrier concentration η_i (cm^− 3)	2×10²⁰ [33]	1.5×10¹⁹ [76]	2×10¹⁶ [80]	2.43×10²⁰[66]	2.64×10¹⁹[53]

Table 3

Simulation and experimental output results of the conventional CIGSSe TFSCs.
Structure of solar cells	Type of structure	V_OC (mV)	J_SC (mA/cm²)	FF (%)	η (%)
(Al/Ni)/MgF₂/ZnMgO:Al/ZnMgO/CIGSSe/Mo [25]	Experimental results	695.00	39.30	73.16	20.00
(Al/Ni)/MgF₂/ZnMgO:Al/ZnMgO/CIGSSe/Mo [25]	Simulation results	695.40	39.37	73.41	20.10
Al/SLG/MgF₂/ZnO:B/CdS/CIGSSe/Mo [27]	Experimental results	746.00	38.50	79.70	22.92
Al/SLG/MgF₂/ZnO:B/CdS/CIGSSe/Mo [27]	Simulation results	746.40	38.52	79.77	22.93

Table 4

The optimized parameters applied to the suggested TFSCs.
Electrical Parameters	n-ZnO:B	n-ZnMgO:Al	n-Sn_1 − xMn_xO₂	n-ZnMgO	p-CIGSSe	p-Cu₂O
Thickness (nm)	40 to 400	40 to 400	40 to 400	50 to 150	1000 to 3000	10 to 50
E_g (eV)	3.429 to 3.256	3.54 to 3.929	3.6 to 4.022	3.3 to 3.694	1.11 to 1.69	1.2 to 2.1 [44]
χ (eV)	4.45 to 46.73	4.5 to 4.111	4.45 to 4.078	4.55 to 4.156	4.14 to 4.685	4.4 [85]
ε_r	9	10.5	9	9	13.6	10
N_C (cm^− 3)	3×10¹⁸	2.2×10¹⁸	2.2×10¹⁸	2.2×10¹⁸	1.2×10¹⁸	2.2×10¹⁸
N_V (cm^− 3)	1.8x10¹⁹	1.8×10¹⁹	1.8×10¹⁹	1.8×10¹⁹	1.8×10¹⁹	1.8×10¹⁹
µ_e (cm²V^− 1s^− 1)	100	100	74.7 [38]	100	100	100
µ_h (cm²V^− 1s^− 1)	31	10	10	25	25	40 [43]
N_d (cm^− 3)	1×10¹⁶ to 1×10²²	1×10¹⁶ to 1×10²²	1×10¹⁶ to 1×10²²	1×10¹⁶ to 1×10²²	0	0
N_A (cm^− 3)	0	0	0	0	1×10¹⁵ to 1×10¹⁸	1×10¹⁵ to 1×10¹⁷ [86]
V_e (cm/s)	2.4×10⁷	1×10⁷	1×10⁷	3×10⁶	3.9×10⁷	1×10⁷
V_h (cm/s)	1.3×10⁷	1×10⁷	1×10⁷	1.5×10⁶	1.4×10⁷	1×10⁷
R_ae (cm⁶s^− 1)	1.12×10^− 30	2.2×10–³⁴	7.2×10^− 31 [90]	2.8×10–³⁴	1×10^− 28	3×10^− 27 [87]
R_ae (cm⁶s^− 1)	1.12×10^− 30	1.1×10^− 34	3.2×10^− 31	1.5×10^− 34	1×10^− 28	2×10^− 27 [87]
R_bb (cm³s^− 1)	1.2×10^− 12	1.5×10^− 11	1.4×10^− 10	2×10^− 11	8×10^− 11	9×10^− 15 [87]
α (cm^− 1)	1.2×10⁵ [69]	1.2×10⁵ [25]	1×10⁴ [37]	2.2×10⁵ [63]	1.45×10⁵ [14]	1×10⁴ to 1.6×10⁵ cm^− 1 [45]
ρ (Ω cm)	2.1×10^− 3 [70]	1.3×10^− 3 [68]	0.381×10^− 4 [38]	1.82×10^− 4 [65]	3 ×10⁶ [67]	3×10⁴ [88]
R_Sheet (Ω/sq.)	2.5×10³ [74]	18.7 [25]	0.4047×10³ [38]	3.7×10⁶ [25]	1.15×10³ [67]	20.4 ×10⁶ [88]
η_i (cm^− 3)	2×10²⁰ [33]	2.43×10²⁰ [66]	2.54×10²⁰ [38]	2.64×10¹⁹[53]	2×10¹⁶ [80]	1×10¹⁶
CBT/VBT N_ttr (cm^− 3)	1×10¹⁷	1×10¹⁶	1.8×10¹⁵	1×10¹⁵	1×10¹⁴	1×10¹⁴
CBT/VBT σ_te (cm²)	5×10^− 13	1×10^− 15	1×10^− 14	1×10^− 13	5×10^− 13	1×10^− 14
CBT/VBT σ_th (cm²)	1×10^− 15	5×10^− 15	1×10^− 15	1×10^− 14	1×10^− 15	5×10^− 14
E_urbach (eV)	0.2	0.18 [68]	0.439 to 0.837 [37]	0.63 [63]	0.014 [79]	0..709 [89]
GDAL N_ttr (cm^− 3)	1×10¹⁶	1×10¹⁵	1×10¹⁵	1×10¹⁵	3.5×10¹³	1×10¹³
GDAL σ_te (cm²)	1×10^− 15	3×10^− 15	1×10^− 15	1×10^− 14	1×10^− 15	5×10^− 14
GDAL σ_th (cm²)	1×10^− 12	3×10^− 14	1×10^− 14	1×10^− 13	5×10^− 13	1×10^− 14
GDDL N_ttr (cm^− 3)	1×10¹⁶	1×10¹⁵	1×10¹⁵	1×10¹⁵	3.5×10¹³	1×10¹³
GDDL σ_te (cm²)	1×10^− 12	3×10^− 14	1×10^− 14	1×10^− 13	1×10^− 13	1×10^− 14
GDDL σ_th (cm²)	1×10^− 15	3×10^− 15	1×10^− 15	1×10^− 14	5×10^− 15	5×10^− 14

4.1. Photovoltaic parameters examination of ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe and SnMnO ₂ /CIGSSe TCO and absorber layer based proposed Type-1 TFSCs

In this section, an attempt is made to improve the performance of the proposed Type-1 TFSCs by optimizing the cell parameters such as thickness and doping concentration and synchronization of E_g orientation of the TCO, buffer, and absorber layers. The proposed Type-1 solar cell optimized photovoltaic device model and thickness versus E_g-band illustration are presented in Fig. 3a-d.

4.1.1. Matching of E_g-orientation of the ZnMgO/CIGSSe interface layer based proposed Type-1 TFSCs

At the beginning of the simulation, the E_g-orientation between the buffer ZnMgO and the absorber CIGSSe layers was looked at for different amounts of magnesium, gallium, and sulphur. At the same time, the ZnMgO and CIGSSe layers' thicknesses are adjusted to 50 nm and 2000 nm, respectively, with N_d and N_a of 1×10²⁰ cm^− 3 and 1×10¹⁷ cm^− 3. Table 5 depicts the changes in performance characteristics caused by changing the Mg-fraction of the ZnMgO buffer layer from 0 to 0.22, the G-fraction (0 to 0.36), and the S-fraction (0 to 0.40) of the CIGSSe layer. According to Eqs. (4) and (5), as the CIGSSe material's (G, S)-fractions rise, E_g and electron affinity increase from 1 to 1.453 eV and 4.14 to 4.533 eV, respectively. The ZnMgO material exhibits a similar trend, with an increase from 0 to 0.22 in the Mg-fraction, an increase from 3.3 to 3.694 eV in E_g, and a reduction from 4.55 to 4.156 eV in electron affinity. It is supported by Eqs. (6) and (7). The values of CBO fall from 0.410 to -0.377 eV while the values of VBO rise from 1.89 to 2.241 eV, respectively, when using Eqs. (14) and (15). An increase in E_g and a subsequent rise in V_OC occur with increasing Mg-fraction of Zn_1 − xMg_xO and increasing the compositions of G and S of the CIGSSe material. This is achievable since the E_g is openly related to the V_OC (V_OC=E_g/q). A rise in the band energy of the ZnMgO buffer and CIGSSe absorber layer, on the other hand, results in a recombination rate that limits the V_OC of CIGSSe solar cells by increasing the electron barrier height via an rise in the E_g in the SCR. The high concentration of E_g caused an increase in V_OC, which was in turn caused by the lowered recombination rate brought about by SCR. In contrast, FF and efficiency began to rise [22]. However, the J_SC is lowered when the band energy of the both layers are increased [90]. This is because the passage of electrons is impeded and the probability of photon absorption is decreased in the SCR. The Mg-fraction of Zn_1 − xMg_xO and G = 0.28, S = 0.32 fractions of the Cu(In_1 − AGa_A) (S_B Se_1 − B)₂ absorber layer based proposed-1 TFSC achieve the best performance at x = 0.18.

Table 5

The proposed Type-1 TFSC's performance parameters variation with several changed in E_g of ZnMgO/CIGSSe interface layer.
A	B	E_g of Cu(In_1 − AGa_A) (S_B Se_1 − B)₂ (eV)	χ of CIGSSe (eV)	x	E_g of Zn_1 − xMg_xO (eV)	χ of Zn_1 − xMg_xO (eV)	CBO (eV)	VBO (eV)	V_OC (mV)	J_SC (mA/cm²)	FF (%)	η (%)
0.00	0.00	1.000	4.140	0.00	3.300	4.550	0.410	1.890	524.6	42.93	71.89	16.19
0.12	0.16	1.156	4.284	0.10	3.437	4.412	0.128	2.153	677.4	42.58	78.57	22.66
0.16	0.20	1.203	4.325	0.12	3.473	4.377	0.052	2.270	724.4	41.91	79.35	24.09
0.20	0.24	1.251	4.366	0.14	3.512	4.338	-0.028	2.289	771.3	40.96	79.94	25.25
0.24	0.28	1.300	4.408	0.16	3.553	4.297	-0111	2.364	819.3	40.04	80.61	26.44
0.28	0.32	1.350	4.450	0.18	3.597	4.253	-0.197	2.247	869.2	39.41	81.01	27.75
0.32	0.36	1.401	4.492	0.20	3.645	4.205	-0.287	2.244	909.2	37.41	81.34	27.67
0.36	0.40	1.453	4.533	0.22	3.694	4.156	-0.377	2.241	952.2	35.58	81.64	27.66

4.1.2. Matching of E_g-orientation of theTCO ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe, SnMnO₂/CIGSSe interface layer of the proposed Type-1 TFSCs

Table 6 displays the deviation of photovoltaic parameters by changing the fraction of the B-ZnO:B, Mg-ZnMgO:Al, Mn-SnMnO₂ as TCO layer materials, and G, S-CIGSSe as absorber layer materials based on solar cells. At the same time, 3.597 eV E_g and 10²⁰ cm^− 3 N_d are taken from the buffer-ZnMgO layer for the analysis. As the B, Mg and Mn compositions of the ZnO:B, ZnMgO:Al and SnMnO₂ TCO layers are increased, leading to an increase in E_g of all TCO layer materials. This may be verified by Eqs. (8) and (11). But the electron affinity and refractive index of the ZnO:B, ZnMgO:Al and SnMnO₂ TCO layer materials decrease as justified by Eqs. (9) and (12) and Eqs. (10) and (13). Similarly, the values of CBO and VBO of the ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe, and SnMnO₂/CIGSSe interface solar cells are calculated according to Eqs. (16)-(21).

When the E_g of ZnO:B, ZnMgO:Al, SnMnO₂ TCO, and CIGSSe absorber layers are increased, With a increased in electron barrier height resulting from a larger bandgap in the SCR, the rate of recombination that limits the V_OC of CIGSSe solar cells is reduced. Due to a decrease in recombination rate caused by the barrier height generated by the insertion of high E_g content surrounding the SCR, V_OC was found to be increased. In contrast, FF and efficiency begin to increase up to E_g=1.35 eV of the CIGSSe layer. But, at a higher value of energy bandgap (E_g > 1.35 eV) of the CIGSSe absorber layer, the efficiency of ZnO:B, ZnMgO:Al and SnMnO₂ TCO layer-based solar cells decreases due to the fewer photons reaching towards the CIGSSe absorber layer. On the other hand, as the E_g of the ZnO:B, ZnMgO:Al, SnMnO₂, and CIGSSe layers increases, leads to a decrease in the electron concentration conduction and also reduces the possibility of photon absorption rate in the SCR, resulting in a decrease in the J_SC, as shown in Table 6. The bandgap energy of 1.35 eV of the absorber layer and the 3.822 eV E_g of the SnMnO₂ TCO cell-based photovoltaic cells give superior performance compared to ZnO:B and ZnMgO:Al TCO layer-based photovoltaic cells.

Table 6

Development of the V_OC, J_SC, FF, and η of the proposed Type-1 solar cell by varying the E_g of different TCO and CIGSSe layer.
TCO and absorber layer interface materials	Interface parameters	Results of different interface solar cells
ZnO:B/CIGSSe	E_g of CIGSSe (eV)	1.0	1.156	1.203	1.251	1.300	1.350	1.401	1.453
	χ of CIGSSe (eV)	4.140	4.284	4.325	4.366	4.408	4.450	4.492	4.533
	B-O fraction (ZnO:B)	0.00	0.01	0.03	0.05	0.07	0.10	NA	NA
	E_g of ZnO:B (eV)	3.429	3.392	3.309	3.287	3.278	3.256	NA	NA
	χ of ZnO:B (eV)	4.500	4.537	4.620	4.642	4.651	4.673	NA	NA
	Refractive index of ZnO:B (n)	1.958	1.980	2.032	2.046	2.051	2.065	NA	NA
	CBO (eV)	0.360	0.253	0.295	0.276	0.243	0.223	NA	NA
	VBO (eV)	2.069	1.983	1.811	1.760	1.735	1.683	NA	NA
	V_oc (mV)	520.6	675.4	721.4	769.3	817.3	866.2	NA	NA
	J_sc (mA/cm²)	42.48	41.96	41.34	40.45	39.61	38.99	NA	NA
	FF (%)	72.43	78.01	78.98	79.57	80.39	80.39	NA	NA
	η (%)	16.02	22.11	23.55	24.76	26.03	27.36	NA	NA
ZnMgO:Al/CIGSSe	Mg composition (ZnMgO:Al)	0.00	0.06	0.09	0.12	0.15	0.18	0.21	0.24
	E_g of ZnMgO:Al (eV)	3.540	3.600	3.639	3.684	3.736	3.794	3.858	3.929
	χ of ZnMgO:Al (eV)	4.500	4.440	4.401	4.356	4.304	4.246	4.182	4.111
	Refractive index of ZnMgO:Al (n)	1.889	1.852	1.827	1.799	1.767	1.731	1.692	1.648
	CBO (eV)	0.360	0.156	0.076	-0.010	-0.104	-0.204	-0.268	-0.422
	VBO (eV)	2.180	2.288	2.360	2.443	2.540	2.648	2.725	2.898
	V_oc (mV)	522.5	676.4	722.4	770.3	818.3	868.2	908.2	951.2
	J_sc (mA/cm²)	42.50	42.02	41.42	40.52	39.69	39.05	37.11	35.29
	FF (%)	72.30	78.05	79.01	79.61	80.42	80.95	81.29	81.57
	η (%)	16.05	22.18	23.64	24.85	26.12	27.44	27.40	27.38
Sn_1 − xMn_xO₂/CIGSSe	Mn composition (SnMnO₂)	0.0	0.1	0.2	0.3	0.4	0.5	0.6	0.7
	E_g of Sn_1 − xMn_xO₂ (eV)	3.600	3.613	3.641	3.686	3.746	3.822	3.914	4.022
	χ of Sn_1 − xMn_xO₂ (eV)	4.500	4.487	4.459	4.414	4.354	4.287	4.186	4.078
	Refractive index of SnMnO₂ (n)	1.852	1.844	1.826	1.798	1.761	1.714	1.657	1.590
	CBO	0.360	0.203	0.134	0.048	-0.054	-0.163	-0.306	-0.511
	VBO	2.240	2.254	2.304	2.387	2.500	2.635	2.819	3.080
	V_oc (mV)	523.5	677.4	723.4	771.3	820.3	869.2	909.2	952.2
	J_sc (mA/cm²)	42.90	42.40	41.75	40.88	40.05	39.41	37.44	35.46
	FF (%)	72.31	78.12	78.84	79.69	80.38	81.01	81.31	81.61
	η (%)	16.24	22.44	23.81	25.13	26.40	27.75	27.68	27.55
*NA (not applicable)

4.1.3. Effect of thickness and N_d density of TCO layer

The efficiency and durability of solar cells are highly dependent on the TCO layer's thickness and N_d. The thickness and N_d of the TCO ZnO:B, ZnMgO:Al, and SnMnO₂ layers are plotted against photovoltaic parameters in Fig. 4a-b. The thickness of the ZnMgO and CIGSSe layers is adjusted to 50 nm and 2000 nm in the simulation tool at the commencement of the simulation analysis. The photovoltaic characteristics decrease when the thickness of the ZnO:B, ZnMgO:Al, and SnMnO₂ TCO cells increases from 40 to 400 nm. Because of the thicker TCO layer, fewer photons reach the absorber, causing a decrease in solar absorption and the creation of E-H pairs in the SCR, resulting in a fall in photovoltaic characteristics, as shown in Fig. 4a. In other words, a thicker TCO layer degrades cell efficiency because its light absorption produces optical losses in the absorber layer and thus current losses [11, 91]. Similarly, the simulation tool allows the user to adjust the N_d of the ZnO:B, ZnMgO:Al, and SnMnO₂ TCO layers from 1×10¹⁶ to 1×10²² cm^− 3, while keeping the buffer- ZnMgO layer N_d= 10²⁰ cm^− 3 and absorber CIGSSe layer N_a=10¹⁷ cm^− 3, respectively. The photovoltaic parameters (V_OC, J_SC, FF, and ) of the ZnO:B, ZnMgO:Al, and SnMnO₂ TCO films are displayed versus the N_d in Fig. 4b. N_d values ranged from 1×10¹⁶ to 1×10²² cm^− 3. The performance of V_OC, J_SC, FF, and all other compounds improved exponentially up to 1×10²⁰ cm^− 3. They then remained continual, as presented in Fig. 4b. The greater the N_d of the TCO layer, the lower the recombination loss caused by the photogenerated minority carrier, which is related to the device's lower reverse saturation current density [30, 57]. The reverse saturation current (J_O) drops as the N_d increases, increasing the V_OC and hence the cell performance parameters. The SnMnO₂ TCO layer's 40 nm thickness and 1×10²⁰ cm^− 3 N_d of the proposed Type-1 photovoltaic cells outperform ZnO:B and ZnMgO:Al TCO layer-based photovoltaic cells.

4.1.4. Effect of ZnMgO buffer layer thickness and N_d on the performance of the TCO ZnO:B, ZnMgO:Al and SnMnO₂ layers based solar cells

The thickness of the ZnMgO buffer layer and N_d are increased from 50 to 150 nm and from 1×10¹⁶ to 1×10²² cm^− 3, respectively. The layer's other parameters remain unchanged. Figure 5a illustrates photovoltaic characteristics vs ZnMgO buffer layer thickness for several types of TCO cell-based photovoltaic cells. The performance of ZnO:B, ZnMgO:Al, and SnMnO₂ TCO layer solar cells diminishes when the thickness of the ZnMgO cell layer grows from 50 to 150 nm. The increased resistivity and suppression of photon absorption and carrier production in the SCR results in a decrease in the photovoltaic characteristics of the proposed Type-1 solar cell [92]. Similar performance parameter patterns are reported in [11, 93]. In Fig. 5b, we can see that the photovoltaic parameters (V_OC, J_SC, FF, and ) of the ZnMgO buffer layer improve exponentially up to N_d= 10²⁰ cm^− 3. As the ZnMgO N_d density increases, the photovoltaic parameters stay constant. While raising the N_d of the ZnMgO buffer layer increases electron transport towards the ARC layer. It also raised the photon capture rate, which increased charge carrier formation in the SCR and hence improved V_OC, J_SC, FF, etc. At 10²⁰ cm^− 3 N_d and 50 nm thickness of the buffer layer, the proposed Type-1 solar cell with the SnMnO₂ TCO layer is highest efficiency.

4.1.5. Impact of thickness and N_a of the CIGSSe absorber layer on the performance of the proposed Type-1 solar cells

The critical elements for constructing TFSCs are the CIGSSe cell thickness and the N_a, as they have a considerable impact on sunlight absorption-recombination, material consumption, and cost. To improve cell performance, the thickness and N_a of the CIGSSe absorber layer must be adjusted. The performance of photovoltaic parameters vs CIGSSe absorber layer thickness and N_a of the proposed Type-1 solar cells' is shown in Fig. 6a-h. The performance of V_OC, J_SC, and efficiency rise up to a thickness of the CIGSSe layer of 2000 nm and then begin to drop due to some absorption of light-generated photons and less formation of E-H pairs in the area of SCR. The FF, on the other hand, continually declines when the thickness of the CIGSSe absorber layer increases from 500 to 3000 nm. To put it another way, the thicker CIGSSe absorber layer absorbs more photons, resulting in more E-H pairs. These E-H pairings boost the electric field and thus the built-in potential, increasing charge carrier separation in the SCR. As a result, the proposed Type 1 TFSCs' efficiency, V_OC, and J_SC are enhanced, as illustrated in Fig. 6a. Similarly, [11, 91, 93, 94] observed similar behaviour.

The built-in electric field (V_bi) across the solar device grows when the N_a of the CIGSSe absorber layer increases from 1×10¹⁵ to 1×10¹⁸ cm^− 3. As shown in Fig. 6b, increasing V_bi increased E-H pair separation and device performance characteristics such as V_OC, FF, and efficiency up to 1×10¹⁷ cm^− 3 N_a of the CIGSSe layer [94]. However, as Na (1×10¹⁷ to 1×10¹⁸ cm^− 3) increases, the performance of both FF and efficiency degrades. This occurs as a result of more widespread recombination of photogenerated carriers in the bulk region [92]. Conversely, J_SC typically falls when the concentration of N_a is changed from 1×10¹⁵ to 1×10¹⁷cm⁻³. Furthermore, hole transport is impeded at the CIGSSe absorber layer. As a result, the resistivity and photogenerated reverser saturation current density rise. As can be seen in the J-V characteristics in Fig. 6c-e, [95] this effect causes a continual drop in the J_SC. As N_a rises, the reverse saturation current (J_O) rises, resulting in a drop in the J_SC. J-V characteristics exhibited nearly identical behaviour in [94, 96].

Similarly, The effects of varying the N_a of the CIGSSe absorber layer on the wavelength versus EQE spectra are validated in Fig. 6f-h. The EQE is describe by the number of electrons generated in the external circuit by a light-generated photon of a particular wavelength incident on a solar cell. The performance of EQE dramatically decreases with an increase in N_a from 1×10¹⁵ to 1×10¹⁸ cm^− 3 of a CIGSSe absorber layer. As the N_a of the absorber layer increases, the recombination process reduces the number of collected electrons. Due to J_SC lessening, attributed to a drop in the EQE [30, 97]. The SnMnO₂ TCO cell-based photovoltaic cells give superior conversion efficiency at N_a= 10¹⁷ cm^− 3 and a 2000 nm thickness of the CIGSSe absorber layer.

4.1.6. Impact of E_g of the TCO and CIGSSe layer on the performance of the characteristics of J-V and wavelength versus EQE spectra

Altering the E_g of the SnMnO₂, ZnMgO:Al, ZnO:B TCO layers and the absorber CIGSSe layer causes a shift in the J-V characteristics and wavelength versus EQE spectra, as shown in Fig. 7a-f. Other parameters of different layers are fixed. When the E_g of ZnMgO:Al, ZnO:B, SnMnO₂ TCO, and CIGSSe layer increase from 3.6 to 4.022 eV (SnMnO₂), 3.54 to 3.929 eV (ZnMgO:Al), 3.429 to 3.256 eV (ZnO:B), and 1 to 1.453 eV (CIGSSe), the electron concentration increases in the SCR and reduces the percentage of photons recombination in that region. A reduction in the recombination percentage will raise the V_OC and decrease the J_SC, as presented in Fig. 7a-c. In contrast, the wavelength versus EQE of the SnMnO₂, ZnMgO:Al, and ZnO:B TCO layers based on photovoltaic cells is continuously depleting with the increment in the E_g of the TCO and the CIGSSe layer shown in Fig. 7d-f. The larger E_g of the TCO and absorber layer, fewer absorption photons, and less generation of the charge carrier of the SCR result in a decrease in the J_SC and EQE.

4.2. Photovoltaic parameters exploration of ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe and SnMnO₂ /CIGSSe TCO and absorber layer based proposed Type-2 TFSCs

The proposed Type-1 photovoltaic device construction in the previous section was not universal in order to overcome the photogenerated minority carriers' (i.e., electrons) recombination loss due to the absence of a BSF as an HTL. Grading is one strategy for reducing photogenerated carriers' (i.e., electrons') recombination loss in the proposed Type-1 solar cell by inserting a wide E_g of Cu₂O as an HTL material between the CIGSSe absorber and the back metal contact layer. The structure is transformed into a proposed Type-2 solar cell after the addition of Cu₂O as an HTL. Type-2 solar cells can reduce minority carrier recombination losses and thereby improve the photovoltaic characteristics of ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe, and SnMnO₂/CIGSSe solar cells. As illustrated in Fig. 8a-b, the proposed Type-2 device architecture and energy band visualisation justify minimising photogenerated recombination losses.

4.2.1. Effects of bandgap energy of the Cu₂O as a HTL

The modification of photovoltaic characteristics such as V_OC, J_SC, FF, and efficiency as a result of varying the E_g of Cu₂O as an HTL is depicted in Fig. 9a-b. At the same time, the software tool sets the 3.597 eV E_g of the ZnMgO buffer and the 1.35 eV E_g of the CIGSSe layer. As the energy bandgap of Cu₂O as an HTL goes from 1.2 eV to 2 eV, the most holes are sent to the back metal contact layer and the fewest photogenerated minority carriers, which are electrons, are sent back to the SCR. As a result, the proposed Type-2 solar cell can achieve an efficiency gain of up to E_g=1.6 eV for the Cu₂O layer. Over (> 1.6 eV) E_g of the Cu₂O layer, the efficiency of proposed Type-2 solar cells begins to deteriorate because a larger barrier height is generated on the BSF layer and fewer holes are carried towards the rear metal contact layer. At 1.6 eV, E_g of the proposed Cu₂O layer-based Type-2 solar cell provides maximum efficiency (29.01%).

4.2.2. Effect of Cu₂ O as a HTL layer thickness and acceptor concentration on the performance of the TCO ZnO:B, ZnMgO:Al and SnMnO₂ layers based on proposed Type-2 solar cell

Figure 10a displays the variations of photovoltaic parameters with the changes in the thickness of the Cu₂O as a HTL. As the thickness of the Cu₂O HTL increases from 10 to 50 nm, a large number of photons are gathered at the SCR, and a huge number of those photons are converted into E-H pairs. As a result, the V_OC, J_SC, FF, and efficiency is raised to 30 nm. With a further increment in the thickness, the device's performance drops due to negligible immersion of light created photons and less generation of E-H pairs in the SCR. In other words, the thicker CIGSSe layer absorbs a more significant number of photons, resulting in an increasing number of E-H pairs. These E-H pairs boost the built-in potential, electrical field, and separation of the charge carrier in the SCR. The fluctuations in photovoltaic parameters with N_a of Cu₂O as an HTL are shown in Fig. 10b. When the N_a of the Cu₂O increases from 10¹⁵ to 10¹⁷cm⁻³, the V_bi across the device increases. Due to the increased E-H separation afforded by the high V_bi, the V_OC, FF, and 10¹⁶ cm^− 3 N_a of Cu₂O all increased. As the N_a of the Cu₂O HTL is more than N_a =10¹⁶ cm^− 3, The more and more carriers re-combine the bulk region. This makes the FF and efficiency go down. In contrast, the J_SC often decreases when the N_a density changes from 10¹⁵ to 10¹⁷ cm^− 3, as the resistivity and photogenerated reverse saturation current density increase. The proposed Type-2 solar cell with the SnMnO₂ TCO layer exceeds the Cu₂O HTL at at N_a=10¹⁶ cm^− 3 and 30 nm thickness.

4.2.3. Effect of the energy bandgap of SnMnO₂ TCO layer material on the performance of particle generation rate of proposed Type-2 and proposed Type-1 solar cells

The particle generation rate versus thickness of proposed Type-2 and proposed Type-1 solar cells with different energy bandgaps of SnMnO₂ TCO layer materials are shown in Fig. 11a-b. The energy bandgap of SnMnO₂ TCO layer materials is increasing, resulting in a rise in the particle generation rate and improved solar cell performance.

4.3. Performance parameters comparisons with Cu₂O and without Cu₂O HTL materials based on proposed solar cells

The characteristics of J-V and wavelength performance versus EQE of proposed Cu₂O HTL material-based ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe, and SnMnO₂/CIGSSe solar cells with and without Cu₂O are shown in Fig. 12. Table 7 compares the photovoltaic properties of the proposed Type-2 TFSCs to those of currently known structures. According to the results, the proposed Type-2 TFSCs outperform all previously reported designs.

Table 7

Photovoltics parameters evaluation of recently works and the proposed Type-1 and 2 TFSCs devices.
TCO layer material/ Thickness(nm)	Buffer-1 layer material/ Thickness(nm)	Absorber layer material/ Thickness(nm)	BSF or HTL/ Thickness(nm)	V_OC (mV)	J_SC (mA/cm²)	FF (%)	η (%)	Year/References
ZnO:Al/200	i-ZnO/100 CdS/50	CIGS/1000	-----------	805.3	34.47	84.11	23.35	2022/[98]
ZnO:Al/300	ZnO/50 ZnMgO/30	CIGS/800	CIGS/1200	900	36.13	81.61	26.78	2022/[99]
ZnO:B/500	ZnMgO/50 Zn (O, S)/50	CIGS/2000	Cu₂O/200	817	40.78	81.85	27.30	2022/[100]
ZnO:B/40	ZnMgO/50	CIGSSe/2000	-----------	866.2	38.99	81.00	27.36	Proposed Type-1 Solar cells
ZnMgO:Al/40	ZnMgO/50	CIGSSe/2000	-----------	868.2	39.05	80.95	27.44
SnMnO₂/40	ZnMgO/50	CIGSSe/2000	-----------	869.2	39.41	81.01	27.75
ZnO:B/40	ZnMgO/50	CIGSSe/2000	Cu₂O/30	870.2	39.48	81.63	28.05	Proposed Type-2 Solar cells
ZnMgO:Al/40	ZnMgO/50	CIGSSe/2000	Cu₂O/30	873.2	40.04	81.84	28.61
SnMnO₂/40	ZnMgO/50	CIGSSe/2000	Cu₂O/30	874.2	40.51	81.91	29.01

The AFORS-HET simulation programme was used in this paper to do theoretical and mathematical analysis for improve the performance of conventional, proposed Type-1, and proposed Type-2 solar cells. The photovoltaic results of two distinct experimental (conventional) CIGSSe-based solar cells were successfully confirmed using the AFORS-HET software tool. Further, instead of ZnO:B and ZnMgO:Al TCO layer materials, a Sn_1-xMn_xO₂ material is used as a TCO layer in a conventional solar cell to develop a SnMnO₂/ZnMgO/CIGSSe-based proposed Type-1 TFSC. By adjusting the energy band alignment and adjusting the thickness and doping density of the TCO, buffer, and absorber layer cells, the proposed Type-1 TFSCs improved the V_OC = 869.2 mV, J_SC = 39.41 mA/cm², FF = 81.01%, and = 27.75%. Furthermore, due to the absence of a BSF as an HTL, the proposed Type-1 photovoltaic cell has limitations such as photogenerated leakage current. To modify the conventional design of Type-1 solar cells, Cu₂O HTL is recommended to be inserted as a BSF material between the rear ohmic contact and the CIGSSe layer. The proposed Type-2 solar cells reduce photogenerated reverse current and improve the photovoltaic characteristics of proposed Type-2 based on ZnO:B/CIGSSe, ZnMgO:Al/CIGSSe, and SnMnO₂/CIGSSe. The thickness, E_g, and doping density of the absorber, TCO, and buffer layers are optimized to improve photovoltaic characteristics. In comparison to existing solar cell types, the proposed Type-2 TFSC based on SnMnO₂/ZnMgO/CIGSSe/Cu₂O has a maximum efficiency of = 29.01%.

There is no conflict of interest

Data availability

Data will be made available on request.

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published 14 Jun, 2023

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Reviewers agreed at journal
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21 Feb, 2023
Editor assigned by journal
23 Jan, 2023
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21 Jan, 2023

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The impact of SnMnO₂ TCO and Cu₂O as an HTL on CIGSSe solar cell performance improvement

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Abstract

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1. Introduction

2. Mathematical Discussions

3. Device Construction And Numerical Simulation Parameters

4. Results And Discussion

5. Conclusion

Declarations

References

Status:

Journal Publication

Version 1