Tandem Solar Cells breaking the SQ barrier

by Waaree Energies (R&D Dept)

The rapid expansion of solar photovoltaics has been driven largely by improvements in efficiency, reliability, and manufacturing scale. Conventional single-junction silicon solar cells have dominated the market for decades and continue to improve incrementally through advanced architectures such as PERC, TOPCon, and HJT. However, all single-junction solar cells are fundamentally constrained by a theoretical efficiency ceiling known as the Shockley–Queisser (SQ) limit. As silicon cells approach practical efficiencies above 26%, further gains using single absorbers become increasingly difficult.

Tandem solar cells have emerged as a powerful solution to overcome this bottleneck. By stacking multiple semiconductor absorbers with different bandgaps, tandem devices are designed to harvest a broader portion of the solar spectrum more efficiently. This approach allows conversion efficiencies that exceed the SQ limit of single-junction cells, opening a new pathway for high-performance photovoltaic technologies.

The architecture of tandem solar cells plays a critical role in their performance and manufacturability. Tandem devices are commonly fabricated in either two-terminal (monolithic) or four-terminal (mechanically stacked) configurations. In two-terminal tandems, the sub-cells are electrically connected in series, and the same current flows through all layers. This design offers simpler module integration and reduced wiring complexity but requires precise current matching between the top and bottom cells. Four-terminal tandems, on the other hand, allow each sub-cell to operate independently, eliminating current-matching constraints but introducing additional optical and electrical complexity. For large-scale commercial deployment, two-terminal tandem architectures are generally preferred due to their simpler system design and lower cost potential.

Among various tandem concepts, perovskite–silicon tandem solar cells have gained the most attention due to their strong efficiency potential and compatibility with existing silicon manufacturing infrastructure. In this configuration, a wide-bandgap perovskite solar cell is deposited on top of a crystalline silicon bottom cell. The perovskite layer efficiently absorbs high-energy photons, while the silicon cell converts the remaining lower-energy photons. One of the key advantages of perovskite materials is their tunable bandgap, which can be adjusted through compositional engineering to optimize tandem performance. In addition, perovskite layers can be processed at relatively low temperatures, making them suitable for integration on finished silicon cells without causing thermal damage.

The efficiency advantage of tandem solar cells arises from improved spectral utilization. By allowing each absorber layer to operate closer to its optimal bandgap, energy losses associated with heat dissipation and unabsorbed photons are significantly reduced. As a result, tandem devices exhibit higher open-circuit voltage and increased power output per unit area. Laboratory-scale perovskite–silicon tandem solar cells have already demonstrated efficiencies exceeding 33%, clearly surpassing the practical efficiency limits of single-junction silicon cells. These gains are particularly attractive for applications where space is limited and maximum power density is required.

Despite their high efficiency potential, tandem solar cells face several technical challenges that must be overcome for large-scale commercialization. Material stability, particularly for perovskite layers under heat, moisture, and ultraviolet exposure, remains a key concern. In monolithic tandem structures, precise control of current matching between sub-cells is essential to avoid performance losses. Additionally, interconnection layers between sub-cells must provide excellent electrical conductivity while remaining optically transparent and chemically stable. Achieving uniform performance over large-area devices is another critical requirement for industrial manufacturing.

From a manufacturing perspective, tandem solar cells offer a strong value proposition because they can be designed as an extension of existing silicon production lines. In many industrial concepts, the tandem structure is realized by adding a perovskite top cell as an additional process step on a finished silicon cell. This approach minimizes capital investment while enabling a significant increase in module power output. Higher module efficiency directly reduces balance-of-system costs, including land usage, mounting structures, and cabling, making tandem technology attractive at the system level.

Tandem solar cells represent a shift in photovoltaic development from material optimization to architectural innovation. By combining multiple absorber layers within a single device, they provide a clear pathway to ultra-high-efficiency solar modules that go beyond the capabilities of conventional silicon technology. As research continues to improve stability, scalability, and manufacturing yield, tandem solar cells are expected to transition from pilot-scale demonstrations to commercial production in the coming years.

In a nutshell, tandem solar cells offer a practical and scalable approach to achieving higher photovoltaic efficiency by intelligently combining materials with complementary optical properties. Perovskite–silicon tandems stand out as the most promising option due to their efficiency gains, processing flexibility, and compatibility with existing silicon infrastructure. With continued technological progress, tandem solar cells are poised to play a central role in the next generation of high-performance photovoltaic modules.