Silicon tandem solar cells represent a promising pathway to surpass the efficiency limits of single-junction photovoltaics by combining silicon with another high-performance absorber material. The approach leverages silicon as the bottom cell due to its mature technology, cost-effectiveness, and near-ideal bandgap for the infrared portion of the solar spectrum. The top cell, typically made of III-V compounds or perovskite materials, captures higher-energy photons, enabling more efficient utilization of sunlight. Key challenges in developing these tandem devices include spectrum splitting, current matching between subcells, and interfacial engineering to minimize losses.

The concept of spectrum splitting in tandem cells relies on the complementary absorption characteristics of the top and bottom cells. Silicon, with a bandgap of approximately 1.1 eV, efficiently absorbs light in the near-infrared range but is transparent to higher-energy photons. A top cell with a wider bandgap, such as perovskite (1.6–1.8 eV) or III-V materials like GaAs (1.4 eV), absorbs the visible and ultraviolet portions of the spectrum. The combination allows the tandem device to cover a broader range of wavelengths, reducing thermalization losses that occur when high-energy photons generate excess heat in a single-junction cell. For optimal performance, the top cell must be designed to transmit photons with energies below its bandgap to the silicon bottom cell, ensuring minimal parasitic absorption.

Current matching is critical to maximize the efficiency of tandem solar cells. Since the subcells are connected in series in a two-terminal configuration, the current output is limited by the lower-performing cell. Mismatched photocurrents lead to efficiency losses, as the excess current generated by one subcell cannot be utilized. Achieving current balance requires careful tuning of the thickness and bandgap of the top cell. For example, a perovskite top cell with a bandgap of 1.7 eV paired with silicon typically requires a thickness of 300–400 nm to generate a current density of around 19–20 mA/cm², closely matching the silicon bottom cell’s output under standard test conditions. In III-V/Si tandems, the current matching challenge is more pronounced due to the lower bandgap of III-V materials like GaAs, necessitating thicker top cells or advanced light-trapping structures.

Interfacial engineering plays a pivotal role in minimizing optical and electrical losses at the junction between the top and bottom cells. In perovskite/silicon tandems, the recombination layer must facilitate efficient charge transfer while maintaining high transparency. Commonly used interlayers include thin metal oxides like indium tin oxide (ITO) or zinc oxide (ZnO), which provide good conductivity and optical properties. However, parasitic absorption in these layers can reduce the overall efficiency, prompting research into ultra-thin or nanostructured alternatives. For III-V/Si tandems, the interface must address lattice mismatch and thermal expansion differences, often requiring buffer layers or wafer bonding techniques to ensure high-quality heterojunctions.

The choice of top cell material significantly impacts the tandem device’s performance and manufacturability. III-V materials, particularly GaAs and InP, offer high efficiencies exceeding 30% in laboratory settings but face scalability challenges due to high production costs and complex epitaxial growth processes. Perovskites, on the other hand, provide a cost-effective alternative with tunable bandgaps and solution-processability, enabling large-area deposition techniques like spin-coating or inkjet printing. Recent advances in perovskite stability and defect passivation have narrowed the gap with III-V materials, making perovskite/silicon tandems a commercially viable option.

One of the primary advantages of silicon-based tandems is their potential compatibility with existing photovoltaic manufacturing infrastructure. Perovskite top cells can be deposited directly onto textured silicon wafers, preserving the light-trapping benefits of conventional silicon solar cells. This integration strategy minimizes additional processing steps and capital expenditures, facilitating a smoother transition from single-junction to tandem production lines. In contrast, III-V/Si tandems often require separate growth of III-V layers followed by bonding or direct epitaxy, increasing complexity and cost.

Thermal management is another consideration in tandem cell design, as the operating temperature can influence performance and longevity. Silicon solar cells typically exhibit a negative temperature coefficient, with efficiency decreasing as temperature rises. The top cell’s thermal behavior must be compatible with silicon to avoid delamination or degradation under real-world conditions. Perovskites are particularly sensitive to heat and moisture, necessitating robust encapsulation schemes to ensure long-term stability. III-V materials, while more thermally stable, may introduce additional stress due to differences in thermal expansion coefficients.

Recent research has demonstrated remarkable progress in silicon tandem cell efficiencies. Perovskite/silicon tandems have achieved certified efficiencies above 29%, while III-V/Si devices have surpassed 35% under concentrated sunlight. These results highlight the potential of tandem architectures to outperform single-junction silicon cells, which are approaching their practical efficiency limit of around 27%. Further improvements are expected through advanced light management techniques, such as nanostructured anti-reflection coatings and bifacial designs, which enhance photon absorption in both subcells.

Despite these advancements, several challenges remain before silicon tandem cells can achieve widespread commercialization. For perovskite/silicon tandems, long-term stability and scalability are critical hurdles. Encapsulation methods must prevent moisture ingress and ion migration, which can degrade perovskite layers over time. III-V/Si tandems face cost barriers, as the expense of III-V materials and growth techniques outweighs the efficiency gains in most terrestrial applications. Efforts to reduce costs through substrate reuse or thin-film approaches are ongoing but require further development.

The environmental impact of tandem cell production also warrants consideration. While silicon is abundant and relatively benign, some top cell materials, such as lead-based perovskites or rare-earth-containing III-V compounds, raise concerns about toxicity and resource availability. Research into lead-free perovskites and recycling strategies for III-V materials is essential to ensure sustainable adoption of tandem technologies.

In summary, silicon tandem solar cells offer a compelling route to higher photovoltaic efficiencies by combining silicon’s strengths with complementary absorber materials. Spectrum splitting, current matching, and interfacial engineering are key areas of focus to optimize performance. While perovskite/silicon tandems present a near-term opportunity due to their cost and processing advantages, III-V/Si devices remain the benchmark for high-efficiency applications. Overcoming stability, cost, and environmental challenges will be crucial for realizing the full potential of these technologies in the global energy landscape. Continued innovation in materials science and device engineering will drive progress toward commercially viable, high-performance tandem solar cells.