How Can Heterojunction Technology Break the “Deadly Triangle” of Space Photovoltaics?

As global low-Earth orbit (LEO) satellite constellations enter a phase of large-scale deployment, the space photovoltaics industry is facing unprecedented opportunities alongside formidable challenges. Under the “first come, first served” rules of the International Telecommunication Union (ITU), major players such as SpaceX, China SatNet, and Kuiper are accelerating efforts to secure limited orbital and spectrum resources.

Taking Starlink as an example, its planned constellation reaches up to 42,000 satellites, with nearly 10,000 already launched. However, the rapid advancement of satellite internet technologies is pushing spacecraft power systems to their limits. In extreme space environments characterized by intense radiation, atomic oxygen erosion, and drastic temperature fluctuations ranging from -180°C to +100°C, space photovoltaic technology is trapped in a “deadly triangle” of efficiency degradation, weight constraints, and high costs—becoming a core bottleneck for constellation economics and reliability.

Against this backdrop, any technological breakthrough that can simultaneously improve efficiency, weight, and cost will draw significant attention across the industry chain. Yet, a key question must first be addressed: why do n-type technologies, which dominate and rapidly evolve in terrestrial photovoltaic markets, encounter fundamental barriers in space applications?

The root cause lies in the extreme high-energy particle radiation environment in space. Continuous proton and electron irradiation penetrates solar cells, causing atomic displacement in the crystal lattice and generating numerous defects. Under this displacement-damage-dominated degradation mechanism, n-type silicon substrates are highly sensitive to radiation-induced defects. Their minority carrier lifetime drops sharply after irradiation, leading to significantly higher efficiency degradation rates.

More critically, under ultra-high radiation fluence accumulation, n-type silicon may even undergo catastrophic type inversion, where its electrical characteristics shift from n-type to p-type. This results in the failure of the PN junction based on n-type design, ultimately causing complete device breakdown. This fundamental physical limitation implies that directly transferring successful terrestrial n-type technologies into space applications carries substantial long-term reliability risks.

Figure: Schematic Illustrations of Different Structures

As a result, the industry is turning its focus toward p-type silicon substrates, which exhibit greater stability and more predictable degradation behavior under radiation. In p-type silicon, defects have relatively weaker trapping effects on minority carriers, allowing carrier lifetime to remain at a higher level post-irradiation. This establishes a stronger physical foundation for radiation resistance.

Figure: Radiation-Induced Lattice Defects and Resulting Solar Cell Degradation

However, conventional p-type solar cells suffer from limited efficiency ceilings. This is where p-type heterojunction (HJT) technology demonstrates its unique value—leveraging the radiation stability of p-type silicon while benefiting from the excellent surface passivation properties of amorphous silicon thin films.

P-type HJT technology uses mature p-type crystalline silicon as the base and deposits ultra-thin amorphous silicon layers on its surface to form a heterojunction. This structure integrates dual advantages: on one hand, it inherits the slow degradation and immunity to type inversion of p-type silicon under space radiation; on the other hand, the superior passivation of amorphous silicon minimizes voltage loss and significantly enhances conversion efficiency. It effectively unifies reliability and high performance.

Compared to the two dominant technology pathways currently used in space, the balanced characteristics of p-type HJT stand out. Against high-end gallium arsenide (GaAs) solutions, while triple-junction GaAs solar cells remain the benchmark in initial efficiency and radiation resistance, they are extremely expensive, rely on scarce raw materials, and involve complex manufacturing processes—limiting their application in large-scale, cost-sensitive constellations.

In contrast, p-type HJT builds upon a mature, cost-effective crystalline silicon supply chain, enabling substantial cost reductions while maintaining sufficient reliability. Compared to traditional space-grade crystalline silicon solutions such as PERC, although lower in cost, these conventional technologies still lag in post-irradiation efficiency and radiation resistance. P-type HJT, with its symmetrical double-sided structure, achieves significantly higher efficiency while delivering comparable or superior radiation performance.

Recently, Risen Energy announced a series of breakthroughs in its n-type heterojunction (HJT) technology. Its single-junction cells have achieved a conversion efficiency of 27.03% under AM1.5 conditions, verified by an authoritative third-party institution, while its silicon-based HJT tandem cells have reached 31.95%.

Building on its extensive R&D experience in heterojunction technology, the company has also made notable progress in p-type HJT development. According to third-party testing data, its p-type silicon-based HJT cells achieve an initial efficiency of 22% under AM0 spectrum while reaching an ultra-thin wafer thickness of 50 μm. After undergoing accelerated irradiation equivalent to a fluence of 1E14 (1 MeV), the cells still maintain a conversion efficiency of 19.4%.

This end-of-life (EOL) performance, compared to the industry baseline of 15%–17% for commercial space-grade PERC cells used to balance cost, translates into up to 20% weight reduction for the same power output. This can reduce launch costs per satellite by hundreds of thousands to even millions of dollars. For constellations planning tens of thousands of satellites, total savings could reach billions of dollars—making it a decisive factor in commercial viability.

Figure: AM0 Conversion Efficiency Comparison Under 1 MeV Irradiation

Beyond static efficiency metrics, a more forward-looking discovery may signal the evolution of next-generation space solar cells. Researchers have observed a self-healing thermal annealing effect in heterojunction cells under simulated space conditions. After sustaining radiation damage, periodic temperature fluctuations experienced during orbital operation may induce partial self-repair of lattice defects.

This mechanism has become a key research focus for NASA, the European Space Agency (ESA), and leading institutions in China. Although its microscopic mechanisms and engineering potential require further study, it opens up new possibilities for developing long-lifetime, highly reliable space photovoltaic cells and may fundamentally address radiation damage challenges.

From a global perspective, the technological race in space photovoltaics is accelerating on multiple fronts. The United States has achieved engineering deployment of multi-junction GaAs arrays in flexible roll-out solar wings. China has realized in-orbit operation of flexible triple-junction GaAs cells. Europe has established a standardized engineering system centered on GaAs technologies, while Japan primarily adopts rigid deployable solar array structures.

Despite divergent technological paths, the core objective remains the same: achieving higher power-to-weight ratios and lower cost per watt while ensuring reliability in extreme environments. Heterojunction technology—especially when combined with flexible substrates and perovskite tandem structures—is attracting increasing strategic investment due to its comprehensive advantages in efficiency, weight reduction, cost, and manufacturing compatibility.

As megawatt-scale energy demands for space-based data centers and lunar research stations transition from concept to reality, the ability to deliver lighter, more powerful, and cost-effective energy solutions will define leadership in the next phase of deep-space exploration.