Multijunction Solar CellEdit

A multijunction solar cell is a photovoltaic device that stacks two or more p-n junctions, each tuned to a different segment of the solar spectrum. By using materials with different bandgaps, these devices extract more energy from the same incident light than a single-junction cell can. This spectrum-splitting approach makes multijunction cells especially attractive in applications where high efficiency is paramount, such as space-based photovoltaics and high-concentration terrestrial systems. See photovoltaics and solar cell for broader context, and note that multijunction concepts are a core part of the broader field of tandem solar cell research.

In practice, multijunction devices arrange subcells so that the same electrical current flows through all junctions in series, while the total output voltage adds up from the individual junctions. The efficiency gains come from better utilization of both high-energy photons and low-energy photons. The top subcell (with a larger bandgap) absorbs the high-energy portion of the spectrum, the middle subcell absorbs photons with intermediate energies, and the bottom subcell (with a smaller bandgap) captures the low-energy photons that would otherwise pass through. This approach is complementary to strategies such as spectral filtering and light concentration, and it dovetails with other high-efficiency concepts in CPV and space photovoltaics.

Principles

Bandgap engineering

Each junction in a multijunction stack is built from a semiconductor with a specific bandgap. By choosing bandgaps that partition the spectrum efficiently, the stack can convert a larger fraction of the incident solar power into electricity. The design challenge is to balance absorption depth, open-circuit voltage, and current collection across all junctions.

Current matching and series connection

Because the subcells are typically connected in series, the overall current is limited by the smallest current among the subcells. Designers must ensure near-ideal current matching under the expected illumination to maximize power output. This constraint influences the number of junctions, their materials, and the layout of the stack.

Monolithic versus stacked approaches

Multijunction cells can be built monolithically by growing all subcells on a common substrate, or by mechanically stacking and bonding separately fabricated cells. Monolithic, lattice-matched designs tend to be more robust for space and certain terrestrial uses, but they require careful materials engineering to manage lattice constants and thermal expansion. Stacked approaches offer flexibility in combining very different materials but introduce bonding and contact-management challenges.

Materials systems and integration

The most mature and widely deployed multijunction systems use III–V semiconductors (such as GaInP, GaInAs, and Ge) because of their favorable bandgaps, high minority-carrier lifetimes, and strong radiative efficiency. Other material combinations, including InP-based or metamorphic structures, expand the design space but can introduce manufacturing complexity. See GaInP/GaInAs/Ge for a common triple-junction example and III-V solar cell for broader material context.

Technologies and design

Common architectures

Triple-junction (3J) structures are among the most common in high-performance cells. A typical terrestrial or space-oriented triple junction uses a high-bandgap top cell, a mid-bandgap cell, and a bottom cell with the lowest bandgap.
Four-junction (4J) and higher-order stacks are explored to push efficiencies further, especially under concentrated light conditions.
Monolithic, lattice-matched configurations aim to minimize mechanical stress and optical losses, while metamorphic or mechanically stacked designs trade some simplicity for broader material choices.

Material systems

GaInP/GaInAs/Ge: A widely studied and deployed triple-junction combination that has served as a workhorse for space solar arrays.
InP- and GaAs-based stacks: Variants that target specific spectral regions or fabrication constraints.
Bottom cells such as Ge or low-bandgap III–V materials: Bottom subcells often provide the infrared portion of the spectrum, helping to maximize overall output.

Applications and settings

Space photovoltaics: Multijunction cells dominate satellite and deep-space missions where efficiency and power-to-weight ratios are critical. See space and satellite for related topics.
Concentrator photovoltaics (CPV): In terrestrial CPV, optical concentrators raise the light intensity, allowing smaller, higher-efficiency stacks to produce more power per unit area, albeit with added cooling and tracking requirements.

Performance and applications

Efficiency under different illumination

Under standard test conditions with one-sun illumination, state-of-the-art monolithic multijunction cells can exceed roughly the 40% efficiency mark, depending on material choices and fabrication quality. When illuminated with higher concentrations of sunlight, efficiencies can rise further, with tens of percent improvements driven by the higher current in the middle and bottom junctions. See power conversion efficiency and concentrator photovoltaics for related performance metrics.

Space and terrestrial use

In space, the priority is often weight, resilience, and efficiency, making multijunction III–V devices highly attractive. In terrestrial settings, the higher cost of III–V materials has limited widespread adoption to niche markets such as CPV installations or specialized high-value deployments, though ongoing research seeks to reduce cost and increase manufacturability. See satellite and CPV for connected topics.

Manufacturing, economics, and controversies

Cost drivers and scale

III–V multijunction devices offer high efficiency but come with higher materials and processing costs relative to conventional silicon cells. The manufacturing complexity, especially for lattice-matched stacks, raises capital expenditure and affects yield. As a result, multijunction cells have historically found their strongest value in applications where efficiency per area and power density justify the cost, such as in space or high-concentration terrestrial systems.

Material supply and robustness

The reliance on specific materials (for example, gallium, indium, germanium) raises concerns about supply chain security and price volatility. These materials can be sensitive to geopolitical and market dynamics, which informs debates about diversification and strategic stockpiling or substitution with alternative materials. See supply chain and critical materials for related discussions.

Policy, subsidies, and market dynamics

Government subsidies and renewable-energy incentives can accelerate the adoption of high-efficiency technologies, but they also attract debate about resource allocation and market distortion. Proponents argue that high upfront costs and long learning curves justify targeted support to accelerate commercialization, while critics contend that subsidies should favor more mature, cost-competitive technologies or broader-based energy strategies. In the broader energy policy discourse, multijunction cells illustrate tensions between frontier research and scalable, affordable electricity. See energy policy and subsidy for broader context.

Emerging pathways

Advances in tandem configurations that couple multijunction cells with less expensive materials (for example, silicon or perovskites in tandem stacks) aim to blend high efficiency with lower production costs. Such approaches are part of ongoing research in the wider photovoltaics field and may influence future market dynamics and licensing landscapes. See perovskite solar cell and silicon solar cell for related developments.