Multi-Junction Solar Cell

Definition and Technical Overview

A multi-junction solar cell, also known as a tandem solar cell, is a photovoltaic device that stacks two or more semiconductor layers (junctions), each optimised to absorb a different portion of the solar spectrum. Conventional single-junction silicon solar cells can theoretically convert a maximum of approximately 33 percent of sunlight into electricity (the Shockley-Queisser limit), because photons with energy below the silicon bandgap pass through unabsorbed while photons with energy above the bandgap waste their excess as heat. By layering semiconductors with different bandgaps, multi-junction cells capture a broader range of solar wavelengths, dramatically increasing overall conversion efficiency.

The concept has been proven in space applications for decades, where III-V semiconductor multi-junction cells (using gallium arsenide, indium phosphide, and related compounds) routinely achieve efficiencies above 40 percent. However, the extreme cost of III-V semiconductors has limited their terrestrial use. The transformative development driving current interest is the emergence of perovskite materials as a cost-effective top cell that can be layered onto conventional silicon bottom cells, creating tandem devices that significantly exceed single-junction performance at potentially affordable manufacturing costs.

Korea's Strategic Position

K-Moonshot Mission 3 targets the development of ultra-high-efficiency multi-junction solar modules that are both high-performing and affordable for mass deployment. Korea brings specific industrial advantages to this challenge. Hanwha Q Cells, one of the world's largest solar cell manufacturers, operates gigawatt-scale production facilities and has invested heavily in perovskite-silicon tandem research. The company's Dalton innovation centre and partnerships with Korean research institutions position it as a potential first mover in commercial tandem cell production.

Korea's strength in advanced materials science, chemical engineering, and precision manufacturing provides a foundation for addressing the key technical challenges in tandem cell fabrication. Producing uniform perovskite films over large areas, ensuring long-term stability against moisture and UV degradation, and developing scalable deposition processes all require the kind of meticulous process engineering at which Korean manufacturers have historically excelled in displays, semiconductors, and batteries.

Perovskite-Silicon Tandem Technology

The perovskite-silicon tandem cell is the most commercially promising multi-junction architecture for terrestrial applications. In this design, a thin perovskite layer with a bandgap of approximately 1.7 electron volts is deposited atop a conventional silicon cell with a bandgap of 1.1 electron volts. The perovskite top cell absorbs high-energy blue and green photons, while the silicon bottom cell captures lower-energy red and infrared photons that pass through the perovskite. This spectral splitting allows the tandem device to convert a significantly greater fraction of incident sunlight.

Laboratory records for perovskite-silicon tandem cells have advanced rapidly. The current world record exceeds 33 percent efficiency, achieved by research groups in Germany and China. Korean research teams at KAIST, KIST, and the Korea Institute of Energy Research (KIER) have demonstrated tandem efficiencies above 30 percent and are pursuing stability improvements critical for commercial viability. The K-Moonshot target of 35 percent module-level efficiency by 2030 is ambitious, as module efficiency typically trails cell efficiency by several percentage points due to interconnection and encapsulation losses.

Technical Challenges

Despite rapid progress, several technical challenges must be resolved before multi-junction solar cells achieve widespread commercial deployment. Long-term stability remains the most critical concern. Perovskite materials are inherently sensitive to moisture, heat, and ultraviolet radiation. While silicon solar panels routinely last 25-30 years in the field, perovskite cells have historically degraded much more rapidly. Recent advances in encapsulation techniques and compositional engineering have extended perovskite lifetimes significantly, but demonstrating the 25-year durability required by the solar industry demands accelerated ageing tests and eventually years of real-world field data.

Scalable manufacturing presents another major challenge. Laboratory tandem cells are typically fabricated on areas of a few square centimetres using spin-coating or vacuum deposition. Scaling these processes to commercial module sizes (approximately 1.5-2.0 square metres) while maintaining uniform film quality and high efficiency requires entirely different manufacturing approaches. Slot-die coating, inkjet printing, and chemical vapour deposition are among the techniques under investigation. Korea's experience in scaling thin-film manufacturing for OLED displays is directly applicable to these challenges.

Market Opportunity and Economics

The global solar photovoltaic market is projected to exceed $400 billion annually by 2030, driven by climate commitments, falling costs, and supportive policy frameworks worldwide. Multi-junction cells that offer 30-35 percent efficiency compared to 22-24 percent for conventional silicon panels would deliver substantially more power per unit area, a critical advantage in land-constrained markets including Korea, Japan, and much of Europe. Even at a modest cost premium, tandem modules would achieve lower levelised cost of energy (LCOE) than conventional panels in many applications due to reduced balance-of-system costs per watt.

For Korea's future energy sector, multi-junction solar cells represent both a domestic energy strategy and an export opportunity. Korea imports virtually all of its fossil fuels, making energy security a perennial concern. Higher-efficiency solar modules could meaningfully increase the contribution of solar energy to Korea's electricity mix despite limited available land area. Simultaneously, a Korean company that achieves commercial-scale production of affordable tandem modules would capture significant share in the global solar market, currently dominated by Chinese manufacturers.

K-Moonshot Mission 3 Targets

Mission 3 establishes specific performance and affordability targets. The efficiency goal of 35 percent at the module level by 2030 would represent a step change from current commercial technology. Equally important is the affordability mandate: the mission explicitly targets cost-competitive production, recognising that a high-efficiency cell achievable only in small-volume laboratory conditions would have limited commercial impact. The mission envisions pilot manufacturing lines by 2028 and commercial-scale production capability by 2030.

Research institutions contributing to Mission 3 include KAIST, KIST, KIER, and SNU, with industry partners including Hanwha Q Cells providing the pathway from laboratory results to factory output. The mission also addresses supply chain considerations, as perovskite materials require lead-based compounds that raise environmental concerns requiring careful lifecycle management.