In the context of increasing global energy demands and environmental awareness, the development of efficient and eco-friendly thin film solar panels has become a major research focus worldwide. Among various materials, copper zinc tin sulfide (Cu2ZnSnS4, CZTS) stands out as a promising absorber for thin film solar panels due to its optimal direct bandgap (1.4–1.5 eV) and high optical absorption coefficient exceeding 104 cm−1. However, conventional CZTS-based thin film solar panels typically employ cadmium sulfide (CdS) as a buffer layer, which poses environmental toxicity concerns and limits performance due to its narrow bandgap and unfavorable band alignment at the CdS/CZTS interface. To address these issues, we explore zinc magnesium oxide (Zn1-xMgxO) as an alternative buffer material. Zn1-xMgxO offers tunable bandgaps (3.37–7.80 eV) and is environmentally benign, making it suitable for enhancing the efficiency of cadmium-free thin film solar panels. This study employs numerical simulation to investigate the performance of Zn1-xMgxO/CZTS heterojunction thin film solar panels, focusing on the effects of heterointerface band offset, buffer layer carrier concentration, and thickness.
The simulation is conducted using the one-dimensional solar cell simulation software SCAPS-1D, which solves the fundamental semiconductor equations under specified boundary conditions. The device structure for the Zn1-xMgxO/CZTS thin film solar panel is modeled as a multilayer stack: back contact electrode / CZTS absorber layer / Zn1-xMgxO buffer layer / ZnO window layer / front metal electrode. The key equations governing the simulation are the Poisson equation and the continuity equations for electrons and holes, expressed as:
$$
\nabla \cdot \epsilon \nabla \phi = -q (p – n + N_D – N_A)
$$
$$
\nabla \cdot J_n = q (R – G) + q \frac{\partial n}{\partial t}
$$
$$
-\nabla \cdot J_p = q (R – G) + q \frac{\partial p}{\partial t}
$$
where $\epsilon$ is the dielectric constant, $\phi$ is the electric potential, $q$ is the electron charge, $p$ and $n$ are the hole and electron concentrations, $N_D$ and $N_A$ are the ionized donor and acceptor concentrations, $J_n$ and $J_p$ are the electron and hole current densities, and $R$ and $G$ are the recombination and generation rates, respectively. The material parameters used in the simulation are summarized in Table 1, derived from theoretical calculations and literature to ensure realistic modeling of thin film solar panels.
| Parameter | ZnO (Window) | Zn0.75Mg0.25O (Buffer) | CZTS (Absorber) |
|---|---|---|---|
| Bandgap, $E_g$ (eV) | 3.3 | 4.3 | 1.5 |
| Electron affinity, $\chi$ (eV) | 4.3 | 4.4 | 4.2 |
| Relative permittivity, $\epsilon_r$ | 9 | 9 | 10 |
| Effective density of states in conduction band, $N_C$ (cm−3) | 2.2 × 1018 | 2.0 × 1018 | 2.2 × 1018 |
| Effective density of states in valence band, $N_V$ (cm−3) | 1.8 × 1019 | 1.8 × 1019 | 1.8 × 1019 |
| Donor concentration, $N_D$ (cm−3) | 1.0 × 1018 | 1.0 × 1015 | – |
| Acceptor concentration, $N_A$ (cm−3) | – | – | 2.0 × 1015 |
| Electron mobility, $\mu_n$ (cm2 V−1 s−1) | 100 | 100 | 100 |
| Hole mobility, $\mu_p$ (cm2 V−1 s−1) | 25 | 25 | 25 |
| Thickness, $d$ (nm) | 200 | 50 | 3000 |
| Defect density (cm−3) | 1.7 × 1016 | 1.7 × 1016 | 2.7 × 1017 |
The band alignment at the Zn1-xMgxO/CZTS heterointerface is critical for the performance of thin film solar panels. The conduction band offset ($\Delta E_c$) is defined as the difference between the conduction band minima of Zn1-xMgxO and CZTS, calculated using the electron affinities. For Zn1-xMgxO, the bandgap $E_g$ varies with magnesium composition $x$, following the relation $E_g(x) = 3.37 + 2.51x + 1.23x^2$ eV (based on empirical data), and the electron affinity $\chi(x)$ decreases as $x$ increases, approximately as $\chi(x) = 4.3 – 1.8x$ eV. This tunability allows for optimizing the interface properties. Table 2 lists the bandgaps, electron affinities, and predicted conduction band offsets for different $x$ values in Zn1-xMgxO relative to CZTS.
| Mg Composition, $x$ | Bandgap, $E_g$ (eV) | Electron Affinity, $\chi$ (eV) | Conduction Band Offset, $\Delta E_c$ (eV) |
|---|---|---|---|
| 0.125 | 3.90 | 4.80 | -0.64 |
| 0.250 | 4.34 | 4.40 | -0.20 |
| 0.375 | 4.86 | 3.90 | 0.32 |
| 0.500 | 5.35 | 3.35 | 0.81 |
The performance of thin film solar panels is evaluated through key output parameters: open-circuit voltage ($V_{oc}$), short-circuit current density ($J_{sc}$), fill factor ($FF$), and power conversion efficiency ($\eta$). These are derived from the current-voltage ($J$-$V$) characteristics under standard AM1.5G illumination. The efficiency is calculated as:
$$
\eta = \frac{V_{oc} \times J_{sc} \times FF}{P_{in}} \times 100\%
$$
where $P_{in}$ is the incident power density (1000 W/m2). To benchmark our simulation, we first modeled a conventional CdS/CZTS thin film solar panel with the same absorber parameters, obtaining an efficiency of 10.26%, which aligns with experimental reports and validates our defect settings for CZTS (defect density of 2.7 × 1017 cm−3). This provides a baseline for comparing the Zn1-xMgxO-based thin film solar panels.
We systematically investigate the impact of the conduction band offset $\Delta E_c$ at the Zn1-xMgxO/CZTS interface on the thin film solar panel performance. By varying the Mg composition $x$ from 0.125 to 0.5, $\Delta E_c$ ranges from -0.64 eV to 0.81 eV. The simulation results, summarized in Table 3, show that for $\Delta E_c < 0$ eV (i.e., when the Zn1-xMgxO conduction band is below that of CZTS), $V_{oc}$, $J_{sc}$, and $FF$ are significantly reduced due to enhanced interface recombination, which acts as a sink for photogenerated carriers. This recombination current $J_{rec}$ can be described by the Shockley-Read-Hall model:
$$
J_{rec} = q \frac{n_i^2}{N_D} \frac{v_{th} \sigma N_t}{\sqrt{1 + \frac{4 \Delta E_c}{kT}}} \exp\left(-\frac{E_a}{kT}\right)
$$
where $n_i$ is the intrinsic carrier concentration, $v_{th}$ is the thermal velocity, $\sigma$ is the capture cross-section, $N_t$ is the interface trap density, $E_a$ is the activation energy, $k$ is Boltzmann’s constant, and $T$ is temperature. As $\Delta E_c$ increases to positive values, the interface recombination suppresses, leading to improved performance. However, when $\Delta E_c$ exceeds approximately 0.2 eV, a spike-like barrier forms at the heterointerface, hindering electron transport from the CZTS absorber to the buffer layer. This barrier increases the series resistance $R_s$, reducing $J_{sc}$ and $FF$. The optimal $\Delta E_c$ is found to be around 0.1 eV, yielding the highest efficiency of 10.68% for the Zn1-xMgxO/CZTS thin film solar panel. This highlights the importance of precise band engineering in designing high-efficiency thin film solar panels.
| $\Delta E_c$ (eV) | $V_{oc}$ (mV) | $J_{sc}$ (mA/cm2) | $FF$ (%) | $\eta$ (%) |
|---|---|---|---|---|
| -0.3 | 691.3 | 22.54 | 49.15 | 7.66 |
| -0.2 | 704.4 | 22.98 | 56.46 | 9.14 |
| -0.1 | 714.7 | 23.27 | 61.42 | 10.22 |
| 0.1 | 718.5 | 23.37 | 63.58 | 10.68 |
| 0.3 | 718.5 | 23.14 | 56.50 | 9.39 |
| 0.4 | 718.2 | 22.12 | 33.61 | 5.34 |
Next, we examine the influence of the Zn1-xMgxO buffer layer donor concentration $N_D$ on the thin film solar panel characteristics. Varying $N_D$ from 1014 to 1020 cm−3 while keeping other parameters constant (with $\Delta E_c = 0.1$ eV and buffer thickness of 50 nm), we observe that the device performance improves with increasing $N_D$ up to around 1018 cm−3. This is attributed to the enhanced built-in potential $V_{bi}$, which widens the depletion region and improves carrier collection. The built-in potential can be expressed as:
$$
V_{bi} = \frac{kT}{q} \ln\left(\frac{N_A N_D}{n_i^2}\right)
$$
where $n_i$ is the intrinsic carrier concentration of the buffer layer. For $N_D < 10^{16}$ cm−3, $V_{oc}$, $J_{sc}$, $FF$, and $\eta$ remain nearly constant due to limited field-assisted drift. As $N_D$ increases beyond 1016 cm−3, these parameters rise significantly, with saturation occurring above 1020 cm−3 as the depletion width $W$ becomes too narrow, given by:
$$
W = \sqrt{\frac{2\epsilon_r \epsilon_0 V_{bi}}{q} \left(\frac{1}{N_A} + \frac{1}{N_D}\right)}
$$
where $\epsilon_0$ is the vacuum permittivity. The optimal $N_D$ is identified as 1018 cm−3, balancing the trade-off between high $V_{bi}$ and sufficient carrier collection. This optimization is crucial for maximizing the efficiency of thin film solar panels based on Zn1-xMgxO buffers.
Furthermore, the thickness $d$ of the Zn1-xMgxO buffer layer plays a vital role in the performance of thin film solar panels. We simulate the effect by varying $d$ from 10 nm to 200 nm, with $\Delta E_c = 0.1$ eV and $N_D = 10^{18}$ cm−3. The results indicate that thinner buffers (around 50 nm) yield higher efficiencies due to reduced parasitic absorption and improved photon transmission to the CZTS absorber. The photon flux $\Phi(\lambda)$ transmitted through the buffer layer can be modeled using the Beer-Lambert law:
$$
\Phi(\lambda) = \Phi_0(\lambda) \exp(-\alpha(\lambda) d)
$$
where $\Phi_0(\lambda)$ is the incident photon flux, $\alpha(\lambda)$ is the absorption coefficient of Zn1-xMgxO, and $\lambda$ is the wavelength. For $d > 50$ nm, $V_{oc}$, $J_{sc}$, $FF$, and $\eta$ gradually decline because more photons are absorbed in the buffer, reducing the generation rate $G$ in the CZTS layer. However, excessively thin buffers (e.g., below 20 nm) may lead to pinholes and increased shunt resistance $R_{sh}$, causing reliability issues. Thus, a buffer thickness of approximately 50 nm is recommended for optimal performance in Zn1-xMgxO/CZTS thin film solar panels, ensuring both good optical transmission and electrical integrity.
To further elucidate the device physics, we analyze the quantum efficiency (QE) and spectral response of the optimized thin film solar panel. The external quantum efficiency $QE(\lambda)$ is calculated as:
$$
QE(\lambda) = \frac{J_{ph}(\lambda)}{q \Phi_0(\lambda)} \times 100\%
$$
where $J_{ph}(\lambda)$ is the photocurrent density at wavelength $\lambda$. The simulation shows that the Zn1-xMgxO buffer with $\Delta E_c = 0.1$ eV enhances the QE in the short-wavelength region (300–500 nm) compared to CdS buffers, owing to its wider bandgap that minimizes blue photon losses. This contributes to the higher $J_{sc}$ observed. Additionally, the recombination kinetics in the CZTS absorber are examined through the ideality factor $n$ derived from the dark $J$-$V$ curve:
$$
J = J_0 \left[\exp\left(\frac{qV}{nkT}\right) – 1\right]
$$
where $J_0$ is the reverse saturation current. For the optimized device, $n$ approaches 1.5, indicating that recombination is dominated by bulk defects rather than interface states, thanks to the favorable band alignment. This underscores the advantage of using Zn1-xMgxO in reducing interface recombination for thin film solar panels.
In summary, our simulation study demonstrates that Zn1-xMgxO is a promising cadmium-free buffer material for CZTS-based thin film solar panels. By tuning the Mg composition to achieve a conduction band offset of approximately 0.1 eV at the Zn1-xMgxO/CZTS heterointerface, setting the buffer donor concentration to 1018 cm−3, and optimizing the thickness to around 50 nm, we can achieve a power conversion efficiency of up to 10.68%. These findings provide a theoretical framework for designing high-performance, environmentally friendly thin film solar panels. Future work should focus on experimental validation and exploring other alloy buffers like Zn1-xMgxO with graded compositions to further enhance efficiency and stability. The continuous advancement in thin film solar panel technology holds great promise for sustainable energy solutions, and materials like Zn1-xMgxO are key enablers in this journey toward greener photovoltaic systems.
The simulation also highlights the importance of defect management in CZTS absorbers for thin film solar panels. Intrinsic point defects such as copper vacancies ($V_{Cu}$) and antisite defects (e.g., Zn$_{Cu}$) can act as recombination centers, limiting $V_{oc}$. The defect density $N_t$ used in our model (2.7 × 1017 cm−3) is based on typical values for polycrystalline CZTS films. Reducing $N_t$ through improved fabrication processes could push efficiencies beyond 12% for these thin film solar panels. Moreover, the temperature dependence of performance was briefly studied by varying the operating temperature from 25°C to 75°C. The efficiency decreases linearly with temperature, with a temperature coefficient of -0.4%/°C, consistent with other thin film solar panels. This thermal behavior is modeled using the Varshni equation for bandgap narrowing:
$$
E_g(T) = E_g(0) – \frac{\alpha T^2}{T + \beta}
$$
where $\alpha$ and $\beta$ are material-specific constants. Such insights are valuable for real-world deployment of thin film solar panels in diverse climates.
In conclusion, the SCAPS-1D simulation tool proves effective for optimizing Zn1-xMgxO/CZTS heterojunction thin film solar panels. By systematically analyzing band offsets, doping concentrations, and layer thicknesses, we identify key design rules for achieving high efficiency. This work contributes to the ongoing efforts to develop cadmium-free thin film solar panels with competitive performance, paving the way for broader adoption of solar energy technologies. As research progresses, we anticipate further innovations in buffer layers and absorber materials that will drive the efficiency of thin film solar panels closer to theoretical limits, making them a cornerstone of future renewable energy systems.