Tandem and multi-junction solar cells represent a significant advancement in photovoltaic technology by stacking multiple semiconductor layers to capture a broader range of the solar spectrum. Unlike single-junction solar cells, which are limited by the bandgap of a single material, these architectures use complementary absorbers to improve efficiency. The design principles involve careful consideration of lattice matching, tunnel junctions, and current balancing to maximize performance in both terrestrial and space applications.

The fundamental principle behind tandem and multi-junction solar cells is the division of the solar spectrum into segments, each absorbed by a material with an optimal bandgap. A typical tandem cell consists of two junctions, while multi-junction designs may incorporate three or more. The top subcell, made of a wider bandgap material, absorbs high-energy photons, while the lower subcells capture lower-energy photons transmitted through the upper layers. This cascading absorption minimizes thermalization and transmission losses, leading to higher theoretical efficiency limits compared to single-junction cells.

III-V semiconductors, such as GaAs, InP, and their alloys, are commonly used in high-efficiency multi-junction designs due to their tunable bandgaps and high carrier mobilities. Lattice matching is critical when integrating III-V materials to minimize defects at heterointerfaces. For example, a lattice-matched triple-junction cell might consist of GaInP (1.9 eV), GaAs (1.4 eV), and Ge (0.7 eV), where each layer is grown epitaxially to maintain crystal continuity. Mismatched systems, however, require buffer layers or metamorphic growth techniques to accommodate strain, though this introduces additional complexity.

Silicon, while cost-effective and abundant, has a bandgap of 1.1 eV, making it less efficient for high-energy photon absorption. However, silicon-based tandem cells leverage perovskite or III-V top cells to enhance performance. Perovskite materials, such as methylammonium lead iodide (CH3NH3PbI3), offer tunable bandgaps (1.5–2.3 eV) and high absorption coefficients, making them ideal partners for silicon in two-junction configurations. The perovskite/silicon tandem architecture has demonstrated efficiencies exceeding 30%, surpassing the practical limits of single-junction silicon cells.

Tunnel junctions are essential for electrically connecting subcells in a multi-junction stack. These highly doped, thin layers allow carriers to recombine and pass between junctions without significant resistive losses. A typical tunnel junction consists of p++/n++ GaAs or AlGaAs, engineered to minimize voltage drops while maintaining optical transparency. Poorly designed tunnel junctions can lead to series resistance issues, reducing fill factor and overall efficiency.

Current matching is another critical requirement in tandem and multi-junction designs. Since the subcells are connected in series, the current output is limited by the weakest junction. Designers must adjust the thickness and bandgap of each layer to ensure that each subcell generates a similar photocurrent under standard illumination conditions. Spectral variations, such as changes in solar irradiance due to atmospheric conditions, can disrupt current balance, necessitating adaptive designs for real-world operation.

In space applications, multi-junction III-V solar cells dominate due to their high radiation resistance and efficiency. The absence of atmospheric filtering allows these cells to exploit the full solar spectrum, achieving efficiencies above 40% under concentrated sunlight. Terrestrial applications, however, prioritize cost-effectiveness, leading to increased interest in perovskite-silicon tandems. Perovskite layers can be solution-processed at low temperatures, reducing manufacturing costs compared to epitaxial III-V growth.

Compared to single-junction technologies, tandem and multi-junction cells face additional challenges in scalability and stability. Perovskite materials, while promising, suffer from degradation under moisture, heat, and prolonged illumination. Encapsulation techniques and interface engineering are being developed to improve longevity. III-V multi-junction cells, though stable, remain expensive due to complex fabrication processes, limiting their use to niche applications like satellites and concentrator photovoltaics.

The performance of these stacked architectures is often evaluated using metrics such as external quantum efficiency (EQE) and current-voltage characteristics. EQE measurements reveal how efficiently each subcell converts photons of different wavelengths into electrons, highlighting spectral utilization. Meanwhile, current-voltage curves under AM1.5G illumination provide insights into open-circuit voltage, short-circuit current, and fill factor, which collectively determine the overall power conversion efficiency.

Recent research has explored four-junction and even six-junction designs, pushing efficiencies closer to the thermodynamic limits. These advanced structures incorporate materials like GaInAsP and AlGaInSb to further segment the solar spectrum. However, the added complexity increases manufacturing challenges, requiring precise control over doping profiles and interfacial quality.

In summary, tandem and multi-junction solar cells offer a pathway to higher efficiencies by intelligently combining multiple absorbers. Lattice matching, tunnel junctions, and current balancing are key considerations in their design. While III-V multi-junction cells excel in space applications, perovskite-silicon tandems present a compelling option for terrestrial use. Continued advancements in material stability and scalable fabrication will determine their broader adoption in the renewable energy landscape.