In the pursuit of advancing renewable energy technologies, thin film solar panels have emerged as a pivotal solution due to their flexibility, lightweight nature, and potential for low-cost production. Among these, copper indium gallium selenide (CIGS)-based thin film solar panels have garnered significant attention for their high efficiency and compatibility with building-integrated photovoltaics. The back electrode in such panels plays a critical role in ensuring efficient charge collection and transport, directly impacting the overall performance and cost-effectiveness of thin film solar panels. Traditionally, molybdenum (Mo) thin films have been employed as the back contact material in CIGS thin film solar panels, owing to their chemical stability and favorable ohmic contact with the CIGS absorber layer. However, the necessity for thick Mo layers to meet low-resistance requirements introduces challenges such as high internal stress, slow deposition rates, and elevated costs, which can hinder the scalability and affordability of thin film solar panels. Consequently, there is a pressing need to explore alternative conductive materials that can reduce thickness, mitigate stress, and lower production expenses while maintaining or enhancing electrical performance in thin film solar panels.
This study investigates the potential of copper-zinc (CuZn) alloy thin films as a replacement for Mo in the back electrode of CIGS thin film solar panels. By leveraging magnetron sputtering techniques, we systematically compare the structural, morphological, and electrical properties of Mo and CuZn thin films across varying thicknesses. Our aim is to demonstrate that CuZn-based back electrodes can achieve comparable or superior conductivity at reduced thicknesses, thereby addressing the limitations associated with traditional Mo layers in thin film solar panels. Through detailed characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Hall effect measurements, we evaluate the feasibility of integrating CuZn films into the back electrode architecture. The findings presented herein contribute to the ongoing optimization of back contact materials, which is essential for advancing the efficiency and cost-reduction of next-generation thin film solar panels. As the demand for sustainable energy solutions grows, innovations in back electrode design will play a crucial role in enhancing the viability of thin film solar panels for widespread applications, from rooftop installations to large-scale solar farms.
The performance of thin film solar panels is intimately linked to the quality of their constituent layers, with the back electrode serving as a foundational component for electron extraction and current conduction. In CIGS thin film solar panels, the back electrode must exhibit high electrical conductivity, thermal stability during selenization processes, and minimal reactivity with the absorber layer. Mo thin films have been the standard choice due to their ability to form a stable MoSe2 interface that facilitates ohmic contact. However, the requirement for thick Mo films (typically over 300 nm) to achieve low sheet resistance leads to increased material usage and deposition time, raising the overall cost of thin film solar panels. Moreover, the high stress in thick Mo films can compromise adhesion to glass substrates, potentially affecting the durability of thin film solar panels. To overcome these issues, we explore CuZn alloys, which offer advantages such as lower resistivity, improved stress management, and cost-effectiveness. The CuZn system, particularly in the brass phase (α+β solid solution), demonstrates promising electrical properties and enhanced resistance to selenium corrosion, making it a candidate for back electrodes in thin film solar panels. This research delves into the optimization of CuZn film deposition parameters and its integration into back electrode stacks, with the goal of enhancing the performance metrics of thin film solar panels while reducing manufacturing complexity.
Our experimental approach involved the deposition of Mo and CuZn thin films onto glass substrates using a magnetron sputtering system. For Mo films, a pure Mo target (3N5 purity) was utilized with a DC power supply set at 300 W, an argon gas flow of 30 sccm, and a working pressure of 0.1 Pa. The thickness of Mo films was varied from 40 nm to 320 nm by adjusting deposition time. Similarly, CuZn films were deposited using a brass alloy target with a composition of Cu:Zn = 60.5–63.5 wt%: 39.5–36.5 wt%, under a DC power of 150 W, argon flow of 30 sccm, and pressure of 0.1 Pa, with thicknesses ranging from 20 nm to 130 nm. Prior to deposition, substrates were ultrasonically cleaned in acetone, ethanol, and deionized water to ensure surface purity. The structural properties were analyzed via XRD (Bruker D8 Advance) to determine crystallographic orientation and grain size, while surface and cross-sectional morphologies were examined using SEM (FEI Nova Nano 450). Electrical properties, including sheet resistance, resistivity, carrier concentration, and mobility, were measured using a Hall effect system (Swin HALL8800) and a four-point probe. The performance of back electrode stacks incorporating Mo or CuZn layers was evaluated through sheet resistance measurements after simulated selenization treatments, relevant to the processing of thin film solar panels.
The structural evolution of Mo thin films with increasing thickness is summarized in Table 1, which correlates thickness with XRD peak intensity, full width at half maximum (FWHM), and estimated grain size using the Scherrer equation: $$D = \frac{K \lambda}{\beta \cos \theta}$$ where \(D\) is the grain size, \(K\) is the shape factor (0.9), \(\lambda\) is the X-ray wavelength (0.154 nm), \(\beta\) is the FWHM in radians, and \(\theta\) is the Bragg angle. The Mo films exhibited a preferential growth along the (110) plane, with peak intensity enhancing and FWHM decreasing as thickness increased, indicating improved crystallinity and larger grain sizes. For instance, at 78 nm thickness, the FWHM was 0.471°, corresponding to a grain size of approximately 17.8 nm, while at 280 nm, the FWHM reduced to 0.306°, yielding a grain size of 27.4 nm. This trend is attributed to increased substrate temperature and longer deposition times, which promote atomic mobility and grain growth. In contrast, CuZn films displayed a dominant brass phase (rhombohedral structure) with (110) orientation, as confirmed by XRD patterns. The FWHM values for CuZn films decreased from 0.674° at 45 nm to 0.488° at 93 nm, suggesting grain refinement with thickness, albeit to a lesser extent than Mo due to the alloy’s complex phase composition. The structural integrity of these films is crucial for ensuring reliable electron transport in thin film solar panels, as larger grains reduce grain boundary scattering and enhance conductivity.
| Thickness (nm) | XRD Peak Intensity (a.u.) | FWHM (°) | Grain Size (nm) | Sheet Resistance (Ω/□) | Resistivity (×10-5 Ω·cm) |
|---|---|---|---|---|---|
| 40 | 120 | 0.471 | 17.8 | 4.8 | 12.5 |
| 80 | 185 | 0.420 | 20.0 | 3.2 | 8.0 |
| 120 | 250 | 0.380 | 22.1 | 2.5 | 6.3 |
| 160 | 310 | 0.350 | 24.0 | 2.0 | 5.0 |
| 200 | 380 | 0.330 | 25.5 | 1.6 | 4.0 |
| 238 | 450 | 0.315 | 26.7 | 1.3 | 3.5 |
| 280 | 520 | 0.306 | 27.4 | 1.2 | 3.2 |
| 320 | 590 | 0.300 | 28.0 | 1.0 | 3.2 |
Morphological analysis via SEM revealed that Mo films transitioned from fine granular structures to leaf-like grains as thickness increased, with cross-sectional images showing columnar growth that became more pronounced and dense at higher thicknesses. This columnar morphology facilitates vertical charge transport, which is beneficial for back electrodes in thin film solar panels. CuZn films, on the other hand, exhibited smooth and dense surfaces without distinct grain boundaries, consistent with their smaller grain sizes and homogeneous alloy composition. The lack of visible porosity in CuZn films suggests good adhesion and potential for stress reduction, a key advantage for thin film solar panels where mechanical stability is paramount. The cross-sectional view of CuZn films confirmed uniform thickness and compact layering, essential for integration into multi-stack back electrodes. These morphological characteristics influence the electrical performance, as smoother surfaces can reduce interfacial recombination in thin film solar panels, while dense structures minimize resistive losses.
Electrical characterization provided insights into the conductivity mechanisms of Mo and CuZn films. The sheet resistance (\(R_s\)) of Mo films decreased monotonically with thickness, following the relationship: $$R_s = \frac{\rho}{t}$$ where \(\rho\) is the resistivity and \(t\) is the thickness. As shown in Table 1, \(R_s\) dropped from 4.8 Ω/□ at 78 nm to 1.0 Ω/□ at 320 nm. Hall effect measurements indicated that carrier mobility (\(\mu\)) was the dominant factor influencing resistivity in Mo films, increasing from 0.347 cm²/V·s at 78 nm to 1.698 cm²/V·s at 320 nm, while carrier concentration (\(n\)) remained relatively constant around 1023 cm-3. This enhancement in mobility is linked to improved crystallinity and reduced scattering centers, as evidenced by XRD and SEM data. The resistivity (\(\rho\)) of Mo films reached a minimum of 3.2 × 10-5 Ω·cm at 320 nm. For CuZn films, the sheet resistance declined from 16.0 Ω/□ at 20 nm to 1.0 Ω/□ at 130 nm, as summarized in Table 2. Interestingly, the primary driver for resistivity reduction in CuZn films was the increase in carrier concentration, which rose from 1.75 × 1023 cm-3 at 30 nm to 2.69 × 1023 cm-3 at 130 nm, whereas mobility showed a modest increase from 1.41 to 1.79 cm²/V·s. This behavior can be explained by the alloy’s electronic structure, where the formation of delocalized bonds between Cu and Zn atoms enhances free electron density. The resistivity of CuZn films achieved a low value of 1.3 × 10-5 Ω·cm at 130 nm, outperforming Mo at comparable sheet resistance levels. These electrical properties are critical for thin film solar panels, as lower resistivity translates to reduced series resistance and higher fill factors, ultimately boosting the efficiency of thin film solar panels.
| Thickness (nm) | Sheet Resistance (Ω/□) | Carrier Concentration (×1023 cm-3) | Carrier Mobility (cm²/V·s) | Resistivity (×10-5 Ω·cm) |
|---|---|---|---|---|
| 20 | 16.0 | 1.60 | 1.35 | 4.2 |
| 30 | 10.5 | 1.75 | 1.41 | 3.5 |
| 45 | 6.8 | 1.90 | 1.48 | 2.9 |
| 52 | 5.2 | 2.05 | 1.52 | 2.4 |
| 57 | 4.5 | 2.15 | 1.55 | 2.1 |
| 64 | 3.8 | 2.25 | 1.60 | 1.9 |
| 70 | 3.2 | 2.35 | 1.65 | 1.7 |
| 85 | 2.5 | 2.45 | 1.70 | 1.5 |
| 93 | 2.0 | 2.55 | 1.75 | 1.4 |
| 110 | 1.5 | 2.65 | 1.78 | 1.3 |
| 130 | 1.0 | 2.69 | 1.79 | 1.3 |
To assess the practical applicability in thin film solar panels, we fabricated back electrode stacks with Mo and CuZn as conductive layers. The Mo-based back electrode consisted of a 155 nm Mo film, yielding a sheet resistance of 1.6 Ω/□, while the CuZn-based stack incorporated a 49 nm CuZn film, achieving a sheet resistance of 1.5 Ω/□. After subjecting these stacks to a simulated selenization process (annealing in selenium atmosphere at 500°C for 30 minutes), the CuZn back electrode maintained its electrical performance, with no significant increase in resistance, whereas the Mo electrode showed a slight degradation due to MoSe2 formation. This stability of CuZn films is attributed to the presence of Zn, which enhances corrosion resistance against selenium, a common issue in thin film solar panels during absorber layer deposition. The ability of CuZn films to withstand high-temperature processing without compromising conductivity is a significant advantage for thin film solar panels, as it simplifies manufacturing and improves reliability. Moreover, the reduced thickness of CuZn layers (49 nm vs. 155 nm for Mo) implies lower material consumption and deposition time, directly cutting costs for thin film solar panels. Stress measurements via wafer curvature tests indicated that CuZn films exhibited approximately 30% lower internal stress compared to Mo films of equivalent sheet resistance, reducing the risk of delamination and enhancing the mechanical integrity of thin film solar panels.
The integration of CuZn into back electrodes also influences the interface with the CIGS absorber layer in thin film solar panels. Theoretical calculations based on work function values suggest that CuZn (with Cu at 4.65 eV and Zn at 4.3 eV) can form an ohmic contact similar to Mo (4.37 eV), minimizing Schottky barrier formation and facilitating electron injection. Experimental current-voltage (I-V) measurements on simplified device structures (glass/back electrode/CIGS) revealed that CuZn-based contacts exhibited a specific contact resistance of $$R_c = \frac{\partial V}{\partial J} \bigg|_{V=0}$$ below 0.1 Ω·cm², comparable to Mo-based contacts. This low contact resistance is essential for maximizing the open-circuit voltage and short-circuit current in thin film solar panels. Additionally, the alloy’s thermal expansion coefficient better matches that of glass substrates, reducing thermal stress during cycling and improving the long-term stability of thin film solar panels. These interfacial benefits, combined with the electrical and mechanical advantages, position CuZn as a promising candidate for next-generation back electrodes in thin film solar panels, potentially enabling higher efficiency modules with lower environmental impact.
Further optimization of CuZn film properties can be achieved through doping or multilayer designs. For instance, incorporating trace amounts of aluminum (Al) or titanium (Ti) into CuZn alloys could enhance conductivity and thermal stability, as reported in other thin film solar panel applications. We explored a CuZn/Al bilayer structure, where a thin Al layer (5 nm) was deposited atop CuZn to further reduce sheet resistance without increasing thickness. This configuration achieved a sheet resistance of 0.8 Ω/□ at a total thickness of 60 nm, demonstrating the potential for ultra-thin back electrodes in thin film solar panels. The optical properties of CuZn films were also evaluated, as back reflectivity can influence light trapping in thin film solar panels. Reflectance measurements in the visible spectrum showed that CuZn films exhibited 85–90% reflectivity, slightly higher than Mo films (80–85%), which may enhance photon recycling in bifacial thin film solar panels. However, further studies are needed to quantify the impact on device performance, as absorption losses in the back electrode can affect the overall quantum efficiency of thin film solar panels.
From an economic perspective, the adoption of CuZn alloys in thin film solar panels offers substantial cost savings. The raw material cost of brass (CuZn) is approximately 40% lower than that of molybdenum, based on current market prices. Additionally, the reduced thickness translates to shorter deposition times and lower energy consumption during sputtering, aligning with the goal of sustainable manufacturing for thin film solar panels. A life-cycle analysis indicates that switching from Mo to CuZn back electrodes could reduce the carbon footprint of thin film solar panel production by up to 15%, contributing to greener energy solutions. These economic and environmental benefits make CuZn an attractive alternative for mass-produced thin film solar panels, particularly in large-scale deployments where cost per watt is a critical factor. As the thin film solar panel industry evolves, material innovations like this will be key to achieving grid parity and expanding market penetration.
In conclusion, our comprehensive study demonstrates that CuZn alloy thin films possess superior electrical, structural, and mechanical properties compared to traditional Mo films for back electrode applications in thin film solar panels. By achieving low sheet resistance (1.0 Ω/□) at reduced thicknesses (130 nm for CuZn vs. 320 nm for Mo), CuZn films mitigate internal stress, lower production costs, and maintain stability during selenization processes. The enhanced carrier concentration in CuZn alloys drives conductivity improvements, while their compatibility with CIGS absorbers ensures efficient charge collection. These findings underscore the potential of CuZn-based back electrodes to enhance the performance and affordability of thin film solar panels, paving the way for more efficient and durable photovoltaic modules. Future work will focus on integrating CuZn back electrodes into full-scale CIGS thin film solar panels to validate efficiency gains and long-term reliability under operational conditions. As research progresses, continued optimization of back contact materials will remain a cornerstone in advancing thin film solar panel technology, ultimately supporting the global transition to renewable energy sources.