The pursuit of high-efficiency, low-cost photovoltaic devices has led to significant research into advanced light-trapping strategies for thin film solar panels. A primary challenge for crystalline silicon (c-Si) based thin film solar panels is their weak absorption of photons in the near-infrared region (700–1100 nm), which constitutes a substantial portion of the solar spectrum. While reducing the thickness of the silicon absorber layer lowers material costs and mitigates issues like light-induced degradation, it inherently diminishes light absorption. Nanophotonic structures, particularly metallic and dielectric gratings, have emerged as powerful tools to enhance light trapping by coupling incident light into guided modes within the thin film, thereby increasing the effective optical path length. Most research has focused on uniform, periodic gratings with single profiles such as triangular, rectangular, or parabolic shapes. However, the potential of combining different grating profiles, especially in an anisotropic or hybrid arrangement, to further optimize the broadband optical absorption in thin film solar panels remains a promising yet less explored avenue. This study investigates the impact of such hybrid grating configurations on the photocurrent generation of c-Si thin film solar panels, revealing that anisotropically mixed grating structures offer superior performance.
The core structure of the simulated thin film solar panel consists of three layers: a 100 nm thick aluminum-doped zinc oxide (AZO) top layer acting as a transparent conductive oxide, a 350 nm thick c-Si absorber layer, and a back reflector. In the proposed designs, the standard flat silver (Ag) back reflector is replaced by a periodic Ag grating. To ensure a fair comparison of optical performance, the total volume of the silicon absorber layer is kept constant across all grating configurations. The grating profiles under investigation are convex triangle (VT), concave triangle (CT), convex parabola (VP), and concave parabola (CP). These profiles are combined to create seven distinct structures, categorized into three groups: mono-gratings (single profile), syntropy hybrid gratings (mixing convex or concave profiles of the same type, e.g., VT+VP), and anisotropy hybrid gratings (mixing convex and concave profiles, e.g., VT+CP). A representative unit cell of an anisotropy hybrid structure, comprising one convex triangle and one concave parabola, is defined by its period P, individual grating widths W1 and W2, and heights H1 and H2. For initial optimization, the ratios are set as RW = W1/W2 = 1 and RH = H1/H2 = 1.
The optical performance is evaluated using the Finite-Difference Time-Domain (FDTD) method. A plane wave source with the standard AM 1.5 solar spectrum in the 300–1100 nm wavelength range is used under normal incidence. Periodic boundary conditions are applied in the horizontal direction, and perfectly matched layers are used in the vertical direction. The key figure of merit is the photocurrent density Jph, which assumes an ideal internal quantum efficiency of 100% and is calculated as:
$$J_{ph} = e \times \int_{300}^{1100} \frac{P_{abs}(\lambda) \times SP(\lambda)}{E_e(\lambda)} d\lambda$$
where \(e\) is the electron charge, \(P_{abs}(\lambda)\) is the wavelength-dependent absorption in the c-Si layer, \(SP(\lambda)\) is the AM 1.5 spectral power density, and \(E_e(\lambda)\) is the energy of a single photon at wavelength \(\lambda\). An extensive parameter sweep is performed for each structure to find the optimal grating height (H, from 40 to 80 nm) and width (W, from 150 to 350 nm) that maximizes \(J_{ph}\).
| Group | Structure | Ag Grating Profile | Optimal H (nm) | Optimal W (nm) | Max Jph (mA/cm²) |
|---|---|---|---|---|---|
| Mono-grating | T | Convex Triangle | 70 | 350 | 15.43 |
| VP | Convex Parabola | 70 | 350 | 15.00 | |
| CP | Concave Parabola | 80 | 350 | 15.40 | |
| Syntropy Hybrid Grating | VTVP | Convex Triangle + Convex Parabola | 80 | 300 | 15.66 |
| CTCP | Concave Triangle + Concave Parabola | 80 | 300 | 16.29 | |
| Anisotropy Hybrid Grating | VTCP | Convex Triangle + Concave Parabola | 80 | 300 | 17.54 |
| CTVW | Concave Triangle + Convex Parabola | 80 | 300 | 17.36 |
The optimization results, summarized in the table above, reveal clear trends. First, the anisotropy hybrid grating structures (VTCP and CTVP) yield the highest photocurrent densities, significantly outperforming both mono-grating and syntropy hybrid designs. For instance, the VTCP structure achieves a Jph of 17.54 mA/cm², which is approximately 14% higher than the best mono-grating (T at 15.43 mA/cm²) and 7.7% higher than the best syntropy hybrid grating (CTCP at 16.29 mA/cm²). Second, the optimal dimensions shift from wider gratings (W=350 nm) for mono-gratings to narrower ones (W=300 nm) for hybrid structures, indicating a different mode coupling regime. The flat thin film solar panel reference yields a Jph of only 10.38 mA/cm², underscoring the dramatic enhancement (>62.9% for VTCP) provided by the grating structures.
To understand the physical mechanisms behind this enhancement, the absorption spectra for both transverse-electric (TE) and transverse-magnetic (TM) polarized light are analyzed. For TE polarization, the anisotropy hybrid gratings show a pronounced absorption boost across a broad range, particularly in the 500–700 nm region, compared to syntropy hybrids. For TM polarization, the anisotropy structures also demonstrate superior absorption. Analysis of the electric and magnetic field distributions at specific resonance wavelengths provides insight. Under TE illumination, strong field localization and enhancement within the silicon layer are observed, characteristic of excited waveguide modes that effectively trap light. Under TM illumination, in addition to waveguide modes, localized surface plasmon resonances (LSPRs) are excited at the interfaces of the Ag grating. The combination of these two mechanisms—waveguide modes and LSPRs—enables the anisotropy hybrid gratings to more efficiently capture and dissipate incident light across the spectrum compared to structures that primarily leverage only one type of resonance, leading to superior performance in thin film solar panels.
Given its top performance, the VTCP (convex triangle + concave parabola) anisotropy hybrid structure is selected for further detailed investigation. A critical question is the optimal number of each grating type within a fixed period. Structures are denoted as VTn1CPn2, where n1 and n2 are the numbers of convex triangle and concave parabola units per period P=500 nm. The total grating height sum is fixed at H1+H2=100 nm, and individual widths are determined by W1 = W2 = P/(n1+n2). The photocurrent density is calculated for various combinations.
| Structure (VTn1CPn2) | n1, n2 | Width W1, W2 (nm) | Jph (mA/cm²) |
|---|---|---|---|
| VT1CP1 | 1, 1 | 250, 250 | 16.91 |
| VT1CP2 | 1, 2 | 167, 167 | 15.78 |
| VT1CP4 | 1, 4 | 100, 100 | 14.95 |
| VT2CP1 | 2, 1 | 167, 167 | 15.80 |
| VT4CP1 | 4, 1 | 100, 100 | 14.90 |
The results clearly show that the simple combination of one convex triangle and one concave parabola (VT1CP1) per period yields the highest photocurrent density (16.91 mA/cm²). To quantitatively compare the light-trapping effectiveness, an Absorption Enhancement Factor \(F_{cs}\) relative to other VTn1CPn2 structures is defined:
$$F_{cs} = \frac{A_{s11} – A_{cs}}{A_{cs}}$$
where \(A_{s11}\) and \(A_{cs}\) are the integrated absorption enhancements for the VT1CP1 structure and a comparison structure, respectively. For both TE and TM polarizations, \(F_{cs}\) is consistently positive for all other VTn1CPn2 combinations, confirming that the 1:1 unit count is the optimal configuration for this anisotropy hybrid design in thin film solar panels.
Finally, the sensitivity of the optimal VT1CP1 structure to variations in the relative dimensions of its two components is examined. The width ratio RW = W1/W2 and height ratio RH = H1/H2 are systematically varied while keeping the period constant (W1+W2=500 nm) and the total grating height constant (H1+H2=100 nm). The photocurrent density \(J_{ph}\) is calculated for each combination.
The analysis reveals that the performance of this anisotropy hybrid thin film solar panel is robust within a certain parameter window. The maximum \(J_{ph}\) is achieved when the width ratio is close to unity (RW ≈ 1), specifically at RW = 1.08, yielding a \(J_{ph}\) of 17.13 mA/cm². When the width ratio is fixed at 1:1, the photocurrent density remains high over a range of height ratios. The performance is optimal when RH is between approximately 0.67 and 1.86. This indicates that the anisotropy hybrid grating thin film solar panel does not require extremely precise control over the individual feature sizes, as long as they are roughly comparable, which is advantageous for practical fabrication.
In conclusion, this investigation demonstrates that anisotropic hybrid gratings, which combine convex and concave profiles, are a highly effective strategy for enhancing light absorption in ultrathin crystalline silicon solar panels. By breaking symmetry, these structures more efficiently excite a combination of waveguide modes and localized surface plasmon resonances across the solar spectrum. Among various profiles, the combination of a convex triangle and a concave parabola (VTCP) proves most effective. The optimal configuration within a period is a simple 1:1 pairing of these two grating units. This design boosts the photocurrent density by 62.9% compared to a flat reference cell and shows tolerance to small variations in the relative dimensions of its components. These findings provide a valuable theoretical foundation and design guideline for developing next-generation, high-efficiency thin film solar panels with advanced nanophotonic light-trapping architectures. The proposed anisotropy hybrid grating approach represents a significant step forward in minimizing material use while maximizing photon capture in photovoltaic devices.