Space Photovoltaics: Materials, Device Concepts and Operational Challenges

Abstract

Space photovoltaics remains the primary power source for satellites and spacecraft, where high efficiency, radiation resistance, and low mass are essential requirements. While conventional III–V multijunction solar cells currently represent the technological benchmark, recent advances in materials science and device architectures have significantly expanded the design space of space photovoltaic systems. This review provides a comprehensive overview of the fundamental physical principles, material platforms, and device concepts relevant to photovoltaic operation under space conditions, with particular emphasis on the AM0 spectrum, radiation effects, and thermal cycling. Special attention is devoted to advanced architectures, including inverted metamorphic multijunction solar cells, concentrator photovoltaic systems, and emerging tandem concepts such as perovskite/silicon and all-perovskite devices. The review highlights the growing importance of system-level metrics, particularly specific power and integration flexibility, which increasingly complement efficiency as key performance indicators. Although emerging technologies offer unprecedented opportunities for lightweight and high-efficiency photovoltaic systems, challenges related to long-term stability, defect control, and scalability remain critical for their practical implementation. Overall, the future of space photovoltaics lies in the development of application-specific solutions that balance efficiency, durability, mass, and cost, enabling next-generation space missions and energy systems.

1. Introduction to Space Photovoltaics

Solar cells are the primary source of electrical power for most satellites and spacecraft, enabling long-term, maintenance-free operation in orbit and beyond [1,2]. Although the fundamental photovoltaic effect is identical in terrestrial and space applications, the design priorities and performance requirements of space photovoltaics differ significantly from those of ground-based systems. Terrestrial solar panels operate under the AM1.5 spectrum, within a narrow temperature window and in a protected environment where atmospheric filtering, convection and shielding significantly moderate operating conditions [3]. In contrast, space photovoltaics function under the AM0 solar spectrum which delivers a higher photon flux—particularly in the ultraviolet range—and lacks any atmospheric attenuation [4].

The orbital environment imposes additional constraints that strongly affect material selection and device architecture. Solar cells in space must withstand a combination of extreme environmental factors, including intense proton and electron radiation, high vacuum, ultraviolet exposure, atomic oxygen erosion, and severe thermal cycling between approximately −150 °C and +120 °C [5,6]. In addition to charged particle irradiation, space photovoltaic devices are also exposed to high-energy electromagnetic radiation, including X-rays and gamma rays, which contribute to ionization processes within semiconductor materials. These forms of radiation can induce charge accumulation in dielectric layers, modify interface states and degrade passivation quality, ultimately affecting device stability and performance [7,8]. Moreover, heavy ions originating from galactic cosmic rays (GCRs) and solar energetic particles (SEPs) events can cause displacement damage through collision cascades leading to the formation of deep-level defects and extended structural disorder. These effects are particularly detrimental to minority carrier lifetime and junction integrity [9,10,11]. In addition to radiation-induced degradation, microdebris and micrometeoroids represent a non-negligible source of mechanical damage in space environments. High-velocity impacts can result in surface erosion, localized defects or even catastrophic failure of photovoltaic elements, especially in lightweight and thin-film architectures [12,13].

The severity and nature of environmental stress factors in space are strongly dependent on the orbital regime and mission profile. In low Earth orbit (LEO), devices are exposed to moderate radiation levels, significant atomic oxygen flux, and frequent thermal cycling due to rapid transitions between sunlight and eclipse [14,15]. In contrast, geostationary orbit (GEO) is characterized by prolonged exposure to high-energy particles trapped in the Earth’s radiation belts, leading to increased cumulative radiation damage [16,17]. For deep-space missions, the absence of geomagnetic shielding results in continuous exposure to GCRs and SEPs, imposing stringent requirements on radiation hardness and long-term stability [18,19]. Consequently, the design and material selection of space photovoltaic systems must be tailored to mission-specific environmental conditions.

The combined stress factors, schematically summarized in Figure 1, define the operational boundaries for space photovoltaic technologies.

These stressors accelerate defect formation, induce charge-trapping processes and degrade electrical performance over mission lifetimes making radiation hardness, thermal stability and robust passivation essential technological requirements [20]. Radiation-induced displacement damage in particular leads to reductions in minority carrier lifetime and increased non-radiative recombination, which directly impact open-circuit voltage and short-circuit current [21]. Furthermore, the continuous high-energy proton flux in certain orbits, especially within the Van Allen belts, imposes reliability constraints that cannot be mitigated through shielding alone due to mass limitations [22].

The strategic importance of photovoltaics in space stems from their role as the only continuous, renewable and mass-efficient power source for satellites, scientific instruments, communication platforms, navigation systems and deep-space missions [23,24]. As payload mass and launch costs are tightly constrained, space solar cells must maximize efficiency while minimizing areal density, ensuring both high power output and low mass per watt delivered. For modern constellations and small satellites, power-to-weight ratio becomes a defining metric, motivating ongoing research into ultrathin and flexible photovoltaic structures. Increasing mission duration—from a few years in early satellites to over two decades in current geostationary platforms—further amplifies the need for materials with predictable long-term degradation behavior [25].

Historically, space missions relied on monocrystalline silicon solar cells, which dominated early satellite programs due to their maturity and availability. However, the introduction of III–V semiconductors in the 1990s—particularly GaAs and later multijunction InGaP/GaAs/Ge structures—marked a breakthrough in efficiency, durability and radiation resistance [26]. The ability to engineer multiple bandgaps enabled more efficient utilization of the AM0 spectrum propelling multijunction III–V devices to efficiencies exceeding 30% and establishing them as the industry standard for high-performance space photovoltaics. Today, nearly all high-power satellites rely on III–V multijunction architectures and ongoing research explores the integration of quantum wells, superlattices and graded buffers to enhance radiation tolerance and minimize defect propagation [25,27,28].

At the same time, new material classes are emerging as promising candidates for next-generation, ultralight and radiation-hard solar technologies. Wide-bandgap semiconductors such as GaN and InGaN offer intrinsic radiation resistance, high thermal stability and tunable bandgaps suitable for operation in extreme environments [29,30]. Hybrid perovskites have demonstrated remarkable radiation tolerance along with ultralow areal mass in recent studies hence challenges in stability and encapsulation remain active research topics. The rapid expansion of the commercial space market continues to drive innovation toward photovoltaic (PV) systems that combine high efficiency, low weight and long-term durability [31,32].

In contrast to terrestrial photovoltaic systems, space photovoltaics operate under significantly different environmental and operational conditions, which strongly influence device design, material selection, and performance. In space, solar cells are exposed to a vacuum environment, extreme thermal cycling, high-energy radiation (including protons, electrons, and heavy ions) and the absence of atmospheric filtering, resulting in a different solar spectrum (AM0). These factors lead to accelerated degradation mechanisms including defect generation, carrier lifetime reduction and increased recombination. Moreover, space photovoltaic systems must be optimized for high specific power (W/kg), mechanical robustness, and long-term stability, rather than cost efficiency, which is typically a dominant factor in terrestrial applications. In contrast, terrestrial photovoltaics operate under relatively stable temperatures, atmospheric conditions, and filtered solar spectra (AM1.5), where efficiency, scalability, and cost-effectiveness are the primary design drivers. As a result, materials and device architectures used in space photovoltaics—such as III–V multijunction solar cells—differ significantly from those commonly employed in terrestrial systems, reflecting the need to balance radiation resistance, thermal stability, and performance under extreme conditions.

This review provides a comprehensive and application-oriented perspective on space photovoltaics by combining fundamental physical principles, material systems, and advanced device architectures with mission-specific constraints. While several recent reviews have addressed photovoltaic technologies in a broader context—focusing on efficiency trends, perovskite materials, or general aerospace applications—they typically consider these aspects separately [33,34,35]. In contrast, this work emphasizes the interplay between environmental stress factors, device physics and mission-dependent requirements. Particular attention is given to radiation-induced degradation, advanced multijunction and tandem concepts and system-level metrics such as specific power and integration flexibility, providing an integrated perspective relevant for next-generation space photovoltaic systems.

2. Market and Technology Drivers of Space Photovoltaics

The use of photovoltaic cells in space has been a primary energy source for satellites, probes, and space stations for decades, powering support systems, propulsion systems, and all kinds of research equipment. The first mentions of using small panels to support satellites date back to the 1950s and 1960s, and the International Space Station has extensive photovoltaic wings to ensure its operation. In recent years, the market has experienced steady growth driven by increasing space activity and technological development, with global players competing to develop material and functional innovations, such as wireless energy transmission [36], foldable and expandable panels [37] and ongoing research into energy transmission via lasers and microwaves [38,39]. The space photovoltaic market is therefore facing two key challenges: (1) powering and propelling space and satellite missions, (2) constructing PV farms in space to mitigate threats posed by the energy crisis on Earth, the climate crisis, and the growing pressure to transform energy from fossil fuels (conventional) to renewable energy sources, given the limitations of solar electricity production resulting from the technology’s inefficiency and its limitations in atmospheric conditions (compared to those in space) [40]. The Space-Based Solar Power (SBSP) concept, as discussed in the second challenge, is based on the harvesting of solar radiation by installations placed in geostationary orbit. Their 24/7 exposure to radiation is intended to eliminate the limitations of ground-based installations and technologies, primarily by marginalizing losses caused by:

• partial absorption of light by the atmosphere;
• Earth’s diurnal and annual cycles (day/night, seasons);
• lower and less stable radiation intensity;
• atmospheric conditions (cloud cover, number of sunny days per year);
• exposure to extreme weather conditions (rain, hail, floods, earthquakes).

This strategy also develops the concept of in-orbit manufacturing, i.e., the production of cells in space, for example, through 3D printing, which can eliminate the risk of damage during cell transport from Earth to orbit. This concept supports the market in the search for lightweight and flexible materials to maximize production efficiency, minimize cargo weight, accelerate the scaling of space operations and missions, and create larger structures that cannot be assembled, for example, in a rocket’s payload [41].

Both trends contribute to the intensification of work and investment by governments, companies, and funds to find technological solutions that will drive the number and effectiveness of all space missions. Therefore, the market for materials for cell production in space should be analysed based on the current state and trends.

As mentioned above, the leading driver of the growth in the market value of photovoltaic cell production materials for space (satellite) applications is the growing wave of space exploration and the search for innovative energy solutions, including SBSP. The market for these materials is further stimulated by investments from governments of the world’s largest economies, as well as the dynamic growth in the number of companies involved in the production of modern satellites for communication, security, military and defence purposes, and, above all, Earth observation and specialized navigation services.

Satellite solar cell materials are specialized substances that harness solar energy to power satellites and spacecraft, including their propulsion systems. The global materials market is generally divided into four main groups:

• silicon;
• GaAs;
• copper indium gallium selenide (CIGS);
• other materials, such as InP, InGaP, etc.

The global market for satellite solar cell materials was valued at USD 41.7 million in 2023 and USD 44.0 million in 2024. Thanks to the aforementioned growth drivers, the market value is expected to increase to USD 96 million in 2030 and to USD 116.3 million in 2032, at a weighted average annual growth rate (CAGR) of 12–13%.

These projections are consistent with recent industry analyses and space economy reports, which indicate sustained growth driven by the rapid expansion of satellite constellations increasing demand for high-performance communication systems and ongoing investments in space infrastructure by both governmental agencies and private companies [42,43,44]. At the same time, it should be noted that the space photovoltaic market remains relatively niche compared to terrestrial photovoltaics with growth strongly dependent on launch rates, mission types and technological readiness levels of advanced photovoltaic concepts.

In this context, emerging photovoltaic technologies, particularly perovskite-based solar cells and perovskite-based tandem architectures, are attracting increasing attention due to their unique combination of high absorption coefficients, tunable bandgaps and compatibility with low-temperature and relatively easy fabrication routes. These features make them promising candidates for applications where high specific power and mechanical flexibility are critical. Recent studies have also demonstrated that perovskite materials can exhibit a certain degree of radiation tolerance, partly attributed to their defect-tolerant electronic structure and self-healing effects, which may mitigate performance degradation under irradiation. At the same time, their practical implementation in space remains limited by challenges related to long-term stability, environmental sensitivity and encapsulation under vacuum and thermal cycling conditions. As a result, perovskite-based systems are currently considered as emerging, high-potential solutions rather than fully mature technologies [45,46,47]. Consequently, while the market demonstrates a clear upward trend, its development is closely coupled with broader space industry dynamics, highlighting the importance of application-specific optimization of photovoltaic materials and device architectures [48].

To provide a clearer comparison between established and emerging photovoltaic technologies for space applications,

Table 1. Comparison of photovoltaic technologies for space applications under AM0 conditions, including market status, efficiency and specific power.

Technology	Status	Efficiency (AM0)	Specific Power (W/kg)	Refs.
Si (space-grade)	Limited/legacy	~14–18%	~20–80	[6]
III–V triple-junction (GaInP/GaAs/Ge)	Dominant (market)	~28–32% (BOL)	~150–300	[6,49]
Advanced III–V (IMM, 4–6 junction)	Advanced/early deployment	~32–35% (AM0)	~200–400	[50]
Thin-film (CIGS, flexible)	Emerging	~18–22% (AM0 equiv.)	~300–1000	[51,52,53]
Perovskite/tandem	Research	~20–30% (projected)	~500–1500 (projected)	[54]

summarizes their typical performance under AM0 conditions, including efficiency, specific power, and current development status.

As shown in Table 1, III–V multijunction solar cells remain the dominant technology for space applications due to their superior efficiency and well-established reliability under AM0 conditions. In contrast, emerging technologies such as thin-film and perovskite-based systems offer significantly higher specific power, highlighting their potential for future lightweight and flexible space platforms. However, their long-term stability and radiation resistance remain key challenges that must be addressed before widespread deployment.

3. Basic Operating Principles of Solar Cells

Solar cells generate electrical power by converting incident photons into separated charge carriers that can be extracted through an external circuit. Although this process appears conceptually simple, the underlying physics involves a sequence of intricately coupled mechanisms governed by semiconductor band structure, carrier generation and recombination, electrostatics of the p–n junction and the external current–voltage characteristics. For space photovoltaics, these principles acquire additional layers of complexity due to the AM0 spectrum, irradiation by high-energy particles and extreme thermal cycling, all of which directly affect carrier lifetimes, voltage losses (defined as the reduction of the open-circuit voltage (V_OC) from its theoretical limit due to recombination and other non-ideal effects) and overall device stability [33,55,56].

3.1. The Photovoltaic Effect and the Electronic Band Structure

The photovoltaic effect originates from the excitation of electrons across the semiconductor bandgap ($E_g$), where absorption of a photon with energy equal to or exceeding $E_g$ produces an electron–hole pair. In Figure 2, the photovoltaic effect has been schematically demonstrated. As demonstrated, photon absorption with energy equal to or exceeding the bandgap leads to the generation of electron–hole pairs and their separation under the influence of the internal electric field.

The efficiency with which materials absorb light depends strongly on the nature of their bandgap. Direct-bandgap semiconductors such as GaAs, InGaP or InGaN exhibit large absorption coefficients, enabling the use of thin absorbers with high specific power—an essential requirement in mass-constrained space missions. Indirect-bandgap materials, including crystalline silicon, require significantly larger thicknesses to achieve comparable absorption. This, in turn, affects device mass, rigidity and radiation susceptibility [57,58,59]. When the device is illuminated, electrons and holes occupy nonequilibrium distributions described by separate quasi-Fermi levels. The splitting between these quasi-Fermi levels defines the maximum achievable open-circuit voltage, making $V_{OC}$ inherently sensitive to all recombination mechanisms present in the material [60,61]. At a deeper thermodynamic level, the Shockley–Queisser limit establishes the ultimate efficiency ceiling for a single-junction device by accounting for both absorption and radiative emission processes. Voltage losses arise from entropy generation whenever recombination deviates from the ideal radiative pathway, providing a framework that links optical design, material quality and recombination physics to the attainable $V_{OC}$ [62,63].

3.2. Carrier Generation, Recombination and Transport

Once photogenerated, carriers traverse a complex landscape defined by generation profiles, recombination processes and transport regimes. The spatial distribution of carrier generation depends on photon energy and optical design features such as antireflection coatings, textured surfaces and reflective back contacts. In space photovoltaics, these optical elements must remain stable under ultraviolet irradiation, atomic oxygen exposure and displacement damage caused by charged particles [64,65]. These processes, including carrier generation, recombination and transport, are schematically summarized in Figure 2.

Carrier recombination fundamentally limits the voltage and current of a solar cell. In high-radiation environments, the dominant mechanism is Shockley–Read–Hall (SRH) recombination, which proceeds via defect states within the bandgap. Radiation induces displacement defects that act as efficient non-radiative recombination centers, significantly reducing minority carrier lifetimes and diffusion lengths [66,67,68]. The main recombination mechanisms in photovoltaic materials, their physical origins and their relevance under space operating conditions are summarized in

Table 2. Summary of dominant recombination mechanisms in photovoltaic materials and their relevance for space applications.

Mechanism	Physical Origin	Dominant Conditions	Impact on Device Performance	Relevance in Space	Typical τrec in Terrestrial PV *	Typical τrec in Space PV/Irradiated Devices *
SRH	Defect states within the bandgap	Low–moderate carrier density; defect-rich materials	Strong reduction of carrier lifetime and diffusion length	High (radiation-sensitive)	~100 ns–1 µs	~1–100 ns
Radiative	Band-to-band recombination	Direct bandgap semiconductors	Intrinsic recombination; preserves thermodynamic efficiency limits	Neutral/beneficial	~1 µs–1 ms (Si)/~1–100 ns (III–V)	similar order, may decrease due to defect-assisted recombination
Auger	Carrier–carrier interaction	High carrier density (e.g., multijunction devices, high injection)	Efficiency loss at high injection levels	Relevant in MJ cells	~10–100 ns	~1–50 ns (enhanced under radiation and high injection)
Surface/interface	Surface states, dangling bonds, interface defects	High surface-to-volume ratio; thin films and nanostructures	Reduced carrier collection; strongly dependent on passivation quality	High (especially for nanostructures)	~10 ns–1 µs	~1–100 ns (degraded passivation under irradiation)

* The values represent typical orders of magnitude reported in the literature [73,74,75,76,77]. The characteristic recombination lifetime (τ_(rec)) strongly depends on material system (e.g., Si vs. III–V), doping level, carrier density, irradiation dose, defect density and device architecture.

, based on established semiconductor physics models and experimental studies [66,67,68,69,70,71,72].

Radiative recombination, although intrinsic, constitutes a desirable loss channel because it preserves the thermodynamic limits of energy conversion efficiency [69,70]. In contrast, Auger recombination becomes significant at high carrier densities, particularly in multijunction devices operating under intense illumination or concentration conditions [71]. Additionally, surface and interface recombination play an increasingly important role in thin-film and nanostructured devices, where the large surface-to-volume ratio amplifies sensitivity to surface passivation quality [72]. Carrier transport occurs through a combination of drift in the electric field of the depletion region and diffusion driven by concentration gradients in quasi-neutral regions. For efficient charge collection, minority carrier diffusion lengths must exceed the distance required to reach the junction. Otherwise, premature recombination limits photocurrent generation [69]. Radiation exposure progressively introduces scattering centers and traps, leading to a reduction in carrier mobility and lifetime. This results in a measurable decrease in diffusion length and overall transport efficiency [66,67]. In high-quality III–V materials, carrier motion may initially exhibit quasi-ballistic behavior; however, this behavior rapidly degrades with increasing radiation-induced disorder [71,72].

3.3. The p–n Junction: Charge Separation and Current Extraction

The p–n junction lies at the heart of photovoltaic operation providing the intrinsic electric field required to separate photogenerated carriers. When p-type and n-type semiconductors are brought into contact, differences in chemical potential drive diffusion of electrons and holes resulting in a depletion region characterized by a built-in electric field. This field efficiently sweeps electrons toward the n-side and holes toward the p-side, ensuring carrier separation even when generation occurs close to the junction [3]. This mechanism is illustrated in Figure 2, where the built-in electric field drives electrons and holes toward the respective electrodes. Under illumination, the interplay between photogenerated carriers and the diode’s recombination current determines the shape of the current–voltage (I–V) curve. At open circuit, the quasi-Fermi level splitting is maximized and generation is balanced by recombination resulting in zero net current. At short circuit, extraction dominates and the current approaches the maximum photogenerated value [78]. Practical devices deviate from ideal behavior due to resistive losses: series resistance reduces the FF by limiting current flow at high voltage, while shunt pathways introduce leakage currents that degrade performance at low bias. Both types of losses become more pronounced in irradiated devices due to increased defect concentrations and degradation of contact interfaces [79]. In multijunction architectures, the junction physics becomes more complex. Interfaces between different materials introduce band offsets that influence carrier transport, recombination and tunneling behavior. Achieving optimal alignment of conduction and valence bands is essential for maximizing carrier extraction and ensuring current matching across subcells under AM0 illumination [80].

3.4. Key Performance Parameters: V_OC, J_SC, FF and Efficiency

The performance of a photovoltaic device is typically described by four key parameters. The short-circuit current density J_SC reflects the ability of the device to absorb photons and collect the resulting carriers. Under AM0 illumination, $J_{SC}$ is generally higher than under AM1.5 due to the increased flux of ultraviolet and visible photons [80]. The open-circuit voltage $V_{OC}$ depends on the quasi-Fermi level splitting and declines logarithmically with increasing recombination. Hence, even small increases in defect density caused by proton irradiation can lead to significant voltage losses [81]. The FF quantifies the “squareness” of the I–V curve and is sensitive to resistive losses and recombination pathways. High-quality devices typically achieve FF values exceeding 80%, although radiation-induced shunt pathways or mobility degradation may reduce this value over time [5]. The corresponding current–voltage (I–V) characteristic and key parameters such as V_OC and FF are shown in Figure 2. Efficiency, defined as the ratio of maximum power output to incident optical power, integrates all three preceding parameters and serves as the primary figure of merit for terrestrial photovoltaics. For space applications, however, additional metrics such as specific power (W/kg) and areal power density (W/m²) are equally important, as they directly influence spacecraft mass, payload constraints and launch costs [82].

3.5. The AM0 Solar Spectrum and Implications for Space Solar Cell Engineering

The solar spectrum incident on a photovoltaic device is strongly influenced by the optical path length through the Earth’s atmosphere, commonly described by the air mass (AM) parameter. In space, solar cells operate under AM0 conditions, where the radiation reaches the device without atmospheric attenuation. Under terrestrial conditions, however, the spectrum is progressively modified as sunlight passes through the atmosphere, leading to AM1, AM1.5, and higher values depending on the solar zenith angle. These variations result in significant changes in spectral distribution and intensity, directly affecting photovoltaic performance and device design. The schematic in Figure 3a illustrates the concept of air mass and the differences between AM0, AM1 and AM1.5 conditions, while Figure 3b presents the corresponding spectral irradiance distributions for AM0 and AM1.5.

As shown in the schematic, the AM parameter quantifies the effective optical path length of sunlight through the atmosphere relative to its shortest path when the Sun is at the zenith (AM1). As the solar zenith angle increases, the path length through the atmosphere becomes longer, leading to enhanced absorption and scattering processes. This results in a progressive attenuation of the solar spectrum, particularly in the ultraviolet and infrared regions, due to absorption by atmospheric constituents such as ozone, water vapor and carbon dioxide. Consequently, the AM1.5 spectrum, commonly used as a terrestrial standard, differs significantly from the AM0 spectrum encountered in space, where no atmospheric filtering occurs. These spectral differences have a direct impact on photon flux, carrier generation rates and current matching in multijunction solar cells, making spectral conditions a critical factor in photovoltaic device design and optimization [83,84,85].

4. Inverted Metamorphic Multi-Junction Solar Cells

4.1. Concepts and Device Physics

Inverted metamorphic multi-junction (IMM) solar cells constitute one of the most advanced III–V photovoltaic architectures currently implemented in space applications. They are based on monolithic stacks of series-connected p–n junctions with different bandgaps, designed to partition the solar spectrum and reduce thermalization losses. By spectrally distributing photon absorption across multiple subcells, multijunction devices significantly exceed the efficiency limits of single-junction solar cells under AM0 illumination. Quantitatively, the advantage of multijunction architectures is substantial: state-of-the-art one-sun single-junction cells reach about 27–29% efficiency (depending on absorber with GaAs among the best-performing single-junction technologies), while triple-junction devices optimized for the space spectrum have already shown 34% under AM0 and fabrication-grade space multijunction cells are typically around 30% at beginning of life [86,87].

The defining feature of the IMM architecture is the combination of metamorphic growth and an inverted epitaxial sequence. In conventional lattice-matched multijunction devices, all subcells are grown on a substrate with closely matched lattice constants, typically GaAs or Ge [88]. While this provides high crystalline quality, it limits the available bandgap combinations. In contrast, metamorphic designs intentionally introduce lattice mismatch and accommodate it using compositionally graded buffer layers. This approach enables integration of absorbers with independently optimized bandgaps, significantly increasing design flexibility. In the inverted configuration, the highest-bandgap junction is grown first on the substrate, followed by progressively lower-bandgap subcells. After epitaxial growth is completed, the structure is processed and flipped into its final orientation, and the original growth substrate is removed. This sequence allows the upper, voltage-dominant subcells to be grown under high crystalline quality before strain-relaxed metamorphic layers are introduced. Dislocations generated during lattice relaxation are therefore spatially separated from the top junctions, reducing their impact on device performance [89,90]. A schematic representation of IMM solar cells, including the epitaxial growth sequence and substrate removal process, is presented in Figure 4.

From a device physics perspective, IMM solar cells operate as series-connected stacks, meaning that the operating current is limited by the lowest-current subcell. Consequently, precise bandgap selection and thickness optimization are required to achieve current matching under AM0 conditions. The total open-circuit voltage is the sum of the voltages of individual subcells. Therefore, improvements in minority carrier lifetime and recombination control directly increase overall efficiency [91,92]. Tunnel junctions provide low-resistance electrical interconnection between adjacent subcells. In IMM devices, however, heavily doped tunnel junction layers are grown early in the stack and are subsequently exposed to the full thermal budget of later epitaxial growth. This inverted thermal history can enhance dopant diffusion and point-defect-mediated intermixing, making tunnel junction design and dopant control critical technological aspects of IMM development [93,94].

4.2. Materials and Technological Challenges

One of the most important practical advantages of IMM architectures for space photovoltaics is the possibility of replacing the conventional Ge bottom junction with a better current-matched low-bandgap absorber, such as ~1 eV GaInAs. The integration of such absorbers relies on compositionally graded metamorphic buffers, which gradually adjust the lattice constant between layers. Proper buffer design is essential to suppress threading dislocation propagation and to maintain high minority carrier lifetimes in the bottom junction. Because the bottom subcell often dominates radiation-induced degradation in space environments, its crystalline quality has direct mission relevance. The inverted architecture also enables substrate removal and thin-film processing. By incorporating a sacrificial release layer, typically AlAs, epitaxial lift-off allows separation of the active III–V stack from the growth substrate. This approach enables substrate reuse and significantly reduces device mass. For space systems, where specific power (W/kg) is a critical parameter, thin-film IMM cells provide a substantial advantage over conventional Ge-based multijunction devices [95,96,97].

Despite these advantages, IMM solar cells present several technological challenges. The design of metamorphic graded buffers must balance strain relaxation with minimal optical and electrical losses. Threading dislocations, trap formation and interface recombination must be controlled to prevent degradation of voltage and current matching [95,98]. Additionally, dopant redistribution in inverted stacks, particularly involving Zn in heavily doped layers, can alter emitter doping profiles and increase series resistance if not properly managed. These materials-science constraints ultimately determine the achievable performance and reliability of IMM devices in space environments. Due to their combination of high efficiency under AM0, scalable multijunction architecture, compatibility with thin-film processing and radiation-robust III–V material systems, IMM solar cells currently represent the technological benchmark for high-performance satellite power generation [99,100].

4.3. State-of-the-Art IMM Solar Cells

The examples presented below collectively demonstrate how advances in material engineering, device design and processing strategies converge in IMM solar cells. These architectures have emerged as one of the most promising platforms for high-efficiency space photovoltaics, offering a unique combination of bandgap flexibility, high conversion efficiency and low mass. To provide a structured overview of recent developments, representative IMM architectures and their key technological contributions are summarized in

Table 3. Representative state-of-the-art examples of inverted metamorphic solar cells and their technological significance.

Architecture	Key Performance	Main Technological Contribution	Relevance for Space PV	Refs.
IMM3J/IMM4J (AM0)	30.6–30.9% (AM0); ≥3 W/g; projected 35.9% (5 J)	Demonstrates AM0 optimization and high specific power	Direct benchmark for space applications	[101]
~1 eV GaInAs bottom cell	~2% mismatch (CGB)	Strain relaxation and threading dislocation density (TDD) control in metamorphic junctions	Materials limitation of bottom junction	[102]
GaInP/GaAs/GaInAs IMM3J	>40% (500 suns)	Tunnel junction design and Zn diffusion control	Critical for high-efficiency operation	[103]
Flexible IMM4J	25.76% (AM1.5G); 550 W/kg	Epitaxial Lift-Off (ELO) + polyimide transfer	Lightweight deployable systems	[104]
Flexible IMM5J	35.1% (AM1.5G); Voc = 4.73 V	Scaling to 5J architecture	Next-gen high-efficiency devices	[105]
Thin-film IMM	33.13%; <35 µm thickness	Cu-plated flexible substrate	Extreme mass reduction	[106]
Early IMM3J	>33% (1 sun); ~40% (CPV)	Foundational inverted concept + TDD control	Key technological basis	[107]
IMM3J (irradiated)	32.2% initial	Radiation response and current matching	Space durability	[108]
Microscale IMM	system-level	Transferable architectures	Flexible/conformal PV	[109]
Low-bandgap IMM	down to ~0.5 eV	Extreme bandgap engineering	Future MJ architectures	[110,111]

.

• Performance benchmarks and AM0 relevance

A clear demonstration of the suitability of IMM solar cells for space applications is provided by Klitzke et al. [101] who reported IMM3J and IMM4J devices optimized for AM0 operation with efficiencies exceeding 30% and specific power values above 3 W/g. These results highlight the critical advantage of IMM architectures in achieving high power-to-mass ratios which is a key parameter in space systems. Furthermore, projections toward IMM5J designs indicate the scalability of this platform toward efficiencies approaching 36% under AM0 conditions. These findings are consistent with earlier work by Geisz et al. [107] which established the inverted metamorphic concept as a viable alternative to lattice-matched architectures. Their study demonstrated that positioning the metamorphic junction at the end of the growth sequence minimizes defect propagation into upper subcells enabling efficiencies exceeding 33% under one-sun illumination and approaching 40% under concentration. Radiation tolerance, which is essential for long-term space missions, has been further investigated by Xu et al. [108]. Their results demonstrate that although radiation-induced defects degrade all subcells, IMM architectures maintain relatively stable current matching and overall performance thus confirming their robustness under space-relevant irradiation conditions.

• Materials and defect control in metamorphic junctions

Despite their high performance, IMM solar cells are fundamentally limited by materials-related challenges associated with lattice mismatch and defect formation. This is particularly evident in the integration of low-bandgap bottom junctions such as the ~1 eV GaInAs absorber studied in [102]. The required accommodation of lattice mismatch through compositionally graded buffer (CGB) layers introduces complexities related to strain relaxation, TDD and trap formation. These buffer layers play a critical role in gradually accommodating lattice mismatch between materials, thereby reducing strain accumulation and suppressing the formation of threading dislocations that would otherwise degrade device performance. Advanced characterization techniques, including High Resolution X-ray diffraction (HRXRD), Transmission Electron Microscopy (TEM) and cathodoluminescence, reveal that incomplete strain relaxation leads to defect generation that significantly reduces minority carrier lifetime. These findings demonstrate that the metamorphic bottom junction is not only an efficiency-enhancing element but also a critical factor determining device stability and long-term performance. In addition, dopant redistribution effects, particularly Zn diffusion in heavily doped tunnel junctions, represent another key limitation [103]. Such processes can degrade electrical properties by increasing series resistance and compensating intended doping profiles. Strategies such as reducing dopant concentration, introducing diffusion barriers and optimizing epitaxial growth conditions have been shown to effectively mitigate these effects.

• Flexible and low-mass IMM implementations

One of the most significant advantages of IMM architectures is their compatibility with thin-film processing and substrate removal techniques. Huang et al. [104] demonstrated flexible IMM4J solar cells fabricated via ELO and transferred onto polyimide substrates, achieving high specific power values up to 550 W/kg. Similarly, Long et al. [106] reported thin-film IMM devices with efficiencies exceeding 33% and total thicknesses below 35 µm. Such operation was possible due to the use of Cu-plated flexible substrates that simultaneously serve as mechanical support and electrical contact. The integration flexibility of IMM structures is further highlighted by microscale devices demonstrated by Gai et al. [109], where solar cells were transferred onto foreign substrates enabling conformal and distributed photovoltaic systems. These approaches open new possibilities for integrating photovoltaics into lightweight and mechanically adaptable space structures.

• Scaling toward next-generation architectures

The scalability of IMM technology beyond conventional triple- and quadruple-junction devices is demonstrated in recent work on five-junction architectures [105], which achieve efficiencies above 35% while maintaining high material quality through advanced buffer engineering. At the same time, IMM structures provide a unique platform for integrating extremely low-bandgap absorbers extending spectral response into the infrared region. Studies on dual-junction and sub-0.6 eV materials [110,111] highlight both the opportunities and challenges associated with extreme bandgap engineering, including increased defect density and voltage losses due to enhanced recombination. These results indicate that IMM architectures are not only the current state-of-the-art in high-efficiency photovoltaics but also a scalable platform for future developments involving higher junction counts and broader spectral utilization.

5. Ultra-High Efficiency Multijunction Solar Cells with Optical Concentration

5.1. Fundamental Concept of Optical Concentration

Optical concentration constitutes one of the most effective strategies for enhancing the performance of photovoltaic devices, particularly in the case of MJ solar cells. The principle relies on focusing incident solar radiation onto a reduced active area using optical elements such as Fresnel lenses or parabolic mirrors, thereby increasing the photon flux density reaching the absorber layers. As a consequence, the photocurrent increases approximately linearly with the concentration ratio, while the open-circuit voltage exhibits a logarithmic dependence due to its relation to quasi-Fermi level splitting. This dual effect enables significant improvements in power conversion efficiency beyond the limits achievable under one-sun illumination conditions. In practical systems, the effectiveness of optical concentration is strongly governed by the trade-off between concentration ratio and optical efficiency which depends on factors such as optical losses, acceptance angle and uniformity of irradiance distribution across the cell surface [112].

From a physical perspective, the increase in carrier generation rate under concentration enhances the quasi-Fermi level separation, which directly translates into higher open-circuit voltage. However, this benefit is accompanied by an increase in recombination processes, particularly radiative and Auger recombination, which scale with carrier density. Therefore, the ultimate efficiency gain is determined by the balance between increased generation and enhanced recombination losses. In high-quality III–V semiconductor systems, where defect densities are minimized, radiative recombination dominates allowing operation closer to the thermodynamic efficiency limits. Moreover, under high-illumination conditions, the performance of photovoltaic devices becomes increasingly governed by recombination mechanisms and carrier transport limitations highlighting the importance of minimizing non-radiative pathways and optimizing material quality to approach ideal diode behavior [113].

The theoretical framework for understanding these effects is rooted in the extension of the Shockley–Queisser limit to multijunction systems under concentration. It has been demonstrated that concentration effectively reduces entropy-related losses and allows for higher achievable efficiencies, particularly when combined with optimal bandgap engineering across multiple subcells. In this context, CPV systems are typically based on high-efficiency multijunction solar cells and advanced optical designs. These systems enable concentration ratios exceeding 1000×, significantly increasing the photon flux density incident on the absorber. While concentration ratios exceeding 1000× are commonly explored in terrestrial CPV systems, such approaches are generally not suitable for space applications due to constraints related to tracking, system complexity, thermal management, and mass. However, such high levels of concentration introduce challenges related to optical alignment, thermal management, and long-term environmental stability. Under ideal conditions, efficiencies exceeding 50% are theoretically possible for MJ cells operating under high concentration ratios [114,115]. Advanced optical concepts such as microlenses and metasurface-based optics (metalenses) are being widely investigated for photovoltaic applications. However, their implementation in space systems remains at an early research stage and is not yet widely adopted in operational technologies.

5.2. Materials and Architectures of III–V Multijunction Solar Cells

Ultra-high efficiency MJ solar cells are predominantly based on III–V compound semiconductors due to their direct bandgaps, high absorption coefficients and the possibility of precise bandgap engineering. The most established architecture consists of lattice-matched GaInP/GaAs/Ge triple-junction cells, which have been further extended into four-, five-, and six-junction devices through the incorporation of additional subcells with tailored bandgaps [116,117]. While these architectures represent the current standard in multijunction solar cell design, further improvements in efficiency and spectral utilization require more advanced approaches that go beyond the limitations of lattice-matched systems.

Three main design strategies can be distinguished in multijunction solar cell architectures: lattice-matched, metamorphic, and inverted metamorphic (IMM) approaches. These strategies differ fundamentally in the way lattice mismatch is managed and how bandgap engineering is implemented across the device structure [118,119]. The fundamental differences between these architectures are illustrated schematically in Figure 5. The comparison highlights the key structural and materials-related trade-offs associated with each approach, particularly in terms of lattice matching, defect formation, and bandgap engineering flexibility.

As shown above, lattice-matched architectures offer superior crystalline quality due to the absence of lattice mismatch, but at the cost of limited bandgap tunability. In contrast, metamorphic designs enable greater flexibility in bandgap engineering through the use of graded buffer layers, although this introduces strain and increases the risk of defect formation. Inverted metamorphic (IMM) structures overcome these limitations by relocating the metamorphic junction to the bottom of the stack and removing the growth substrate, thereby reducing defect propagation into the active layers. This approach enables the integration of lattice-mismatched materials while maintaining high device performance, making IMM architectures particularly attractive for next-generation high-efficiency and lightweight space photovoltaic systems.

Recent developments have demonstrated six-junction devices with carefully engineered bandgaps spanning a wide portion of the solar spectrum, significantly reducing thermalization losses. These architectures rely heavily on precise control of epitaxial growth processes such as metal–organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), where interface quality and abruptness are critical for minimizing recombination losses [120,121]. The integration of novel materials, including dilute nitrides and antimonides, has further expanded the accessible bandgap range, enabling improved spectral utilization. These material systems are particularly relevant for applications requiring optimization under specific spectral conditions, such as AM0 illumination in space environments [122,123].

5.3. Efficiency Limits and Record Devices

The efficiency of MJ solar cells under concentration has reached unprecedented levels, surpassing 47% under laboratory conditions. These record values are achieved through the combination of optimized bandgap sequences, high material quality and advanced optical concentration systems. The fundamental advantage of MJ architectures lies in their ability to divide the solar spectrum into multiple energy ranges, each absorbed by a subcell with a corresponding bandgap, thereby minimizing thermalization and transmission losses. Under concentrated illumination, the efficiency limit increases significantly compared to single-junction devices. Theoretical analyses indicate that efficiencies exceeding 50% are achievable for systems with an optimal number of subcells and ideal bandgap alignment. However, practical limitations such as series resistance, imperfect current matching and non-radiative recombination reduce the achievable performance. State-of-the-art devices utilize advanced concepts such as tunnel junctions with extremely low resistive losses, anti-reflection coatings optimized for concentrated light, and highly efficient heat dissipation systems. The interplay between optical, electrical and thermal design is crucial for achieving record efficiencies. Additionally, CPV systems often operate at concentration ratios exceeding 500 suns, which necessitates precise tracking systems and robust thermal management. However, such concentration levels are not representative of space photovoltaic systems, where strict limitations on mass, tracking accuracy and system complexity typically preclude the use of high-concentration optics. The continuous improvement of MJ solar cells is closely linked to advances in epitaxial growth, defect control and interface engineering, which directly influence recombination mechanisms and carrier transport properties [115]. Nevertheless, the physical principles of concentrated photovoltaics remain relevant for understanding the behavior and efficiency limits of high-performance multijunction devices.

5.4. Band Alignment and Tunnel Junctions in Multijunction Devices

In MJ solar cells, the interfaces between subcells play a critical role in determining overall device performance. Proper alignment of conduction and valence bands is essential for efficient carrier transport across heterointerfaces. Misalignment can lead to the formation of energy barriers that impede carrier flow and increase recombination losses [69,124].

Tunnel junctions are employed to electrically connect individual subcells in series while maintaining optical transparency. These junctions rely on heavy doping to create a narrow depletion region enabling quantum mechanical tunneling of carriers. The performance of tunnel junctions is governed by parameters such as doping concentration, band alignment, and defect density, which significantly influence carrier transport and overall device efficiency. The key parameters affecting tunnel junction operation are summarized in

Table 4. Key parameters influencing tunnel junction performance in multijunction solar cells and their impact on carrier transport and device efficiency.

Parameter	Physical Role	Impact on Device Performance
Doping concentration	Determines depletion width and tunneling probability	Higher doping enables efficient tunneling but may increase defect density
Band alignment	Controls barrier height and carrier transport	Misalignment leads to energy barriers and increased recombination
Defect density	Introduces recombination centers	Reduces carrier lifetime and junction efficiency
Current density	Affects carrier transport regime	High current may induce degradation and thermal effects

.

As shown in Table 4, tunnel junction performance arises from a complex interplay between material quality, doping levels, and band alignment, all of which require precise control during epitaxial growth. Consequently, high-quality tunnel junctions exhibit low electrical resistance and high peak tunneling current, which are essential for maintaining high fill factors, particularly under concentrated illumination conditions. However, under high current densities, phenomena such as thermal degradation and defect generation may significantly deteriorate junction performance. Band offset engineering is particularly important in metamorphic and IMM structures, where lattice mismatch introduces additional complexity. Careful design of interface layers and grading profiles is required to minimize dislocation propagation and maintain efficient carrier transport [125,126].

These limitations become particularly significant in CPV systems, where elevated current densities and operating temperatures impose additional constraints on device performance and long-term stability. One of the primary challenges associated with optical concentration is the significant increase in operating temperature.

5.5. Thermal Management and Degradation Under Concentration

One of the primary challenges associated with optical concentration is the significant increase in operating temperature. Elevated temperatures negatively affect device performance by reducing open-circuit voltage, increasing recombination rates, and accelerating material degradation processes. Thermal management strategies are therefore critical in CPV systems. These include passive cooling using heat sinks, active cooling systems, and the use of materials with high thermal conductivity. In addition, device design must account for thermal expansion mismatches between layers, which can induce mechanical stress and lead to defect formation. Degradation mechanisms under concentration include increased defect generation, contact degradation, and changes in material composition due to diffusion processes. In space applications, these effects are further exacerbated by radiation-induced damage, which introduces additional defect states that act as recombination centers.

5.6. Relevance for Space Applications

Although optical concentration is rarely employed in space photovoltaic systems due to constraints related to tracking, mass, and system complexity, the underlying physics and material systems developed for CPV have significantly influenced the design of high-efficiency space solar cells. Multijunction III–V solar cells remain the dominant technology for space applications, where maximizing power-to-weight ratio is critical. Under AM0 illumination conditions, the spectral distribution differs from terrestrial spectra, requiring careful optimization of bandgaps and layer thicknesses to achieve current matching across subcells. Furthermore, space environments introduce additional challenges such as radiation damage, which affects carrier lifetimes and increases recombination. The insights gained from high-concentration operation—particularly regarding recombination mechanisms, thermal effects, and material robustness—are directly applicable to improving the performance and durability of space solar cells. Future developments may involve hybrid approaches that combine multijunction architectures with advanced light management techniques, as well as the integration of novel materials such as GaN and wide-bandgap oxides for enhanced radiation resistance and thermal stability. Despite these limitations, several concepts involving optical concentration in space have been proposed and experimentally explored. These include deployable concentrator systems using lightweight Fresnel optics, as well as reflective concentrator designs intended to reduce the required photovoltaic area while maintaining high power output. In addition, low-to-moderate concentration approaches (typically below ~10×) have been considered for space applications as a compromise between efficiency enhancement and system complexity, particularly in missions where mass and area constraints are critical. These studies demonstrate that, while high-concentration CPV systems are generally impractical in space, controlled optical concentration may still play a role in specialized or future mission architectures [127,128].

In space environments, photovoltaic devices are subjected to pronounced thermal cycling driven by periodic transitions between direct solar illumination and eclipse. In low Earth orbit (LEO), solar arrays typically experience temperature variations ranging from approximately −150 °C to +120 °C, depending on spacecraft configuration, surface properties, and thermal control strategies. These transitions occur over relatively short timescales (typically 45–90 min per orbital cycle), leading to repeated thermal loading and unloading of materials [129,130,131]. Such extreme and rapid temperature fluctuations differ significantly from terrestrial conditions, where solar cells usually operate within a narrower temperature range (typically 25–75 °C) and under comparatively stable thermal conditions. As a result, space photovoltaic systems are exposed not only to higher peak temperatures but also to significantly larger thermal gradients and cycling frequencies.

From a device physics perspective, temperature strongly influences photovoltaic performance. Increasing temperature leads to enhanced carrier recombination and a reduction in open-circuit voltage, resulting in a decrease in overall efficiency. For terrestrial silicon-based solar cells, the temperature coefficient typically ranges from −0.30 to −0.44%/K [132], whereas III–V multijunction solar cells—commonly used in space—exhibit lower temperature sensitivity, typically in the range of −0.10 to −0.20%/K. Consequently, despite exposure to higher temperature extremes, III–V devices can maintain relatively stable performance compared to conventional terrestrial technologies. Beyond instantaneous efficiency effects, thermal cycling introduces additional degradation mechanisms at the material and device levels. Repeated expansion and contraction due to mismatches in coefficients of thermal expansion between different layers can lead to the formation of mechanical stress, microcracks, delamination, and degradation of interfaces. These effects are particularly critical in multilayer structures such as multijunction solar cells and in emerging thin-film and flexible photovoltaic systems, where mechanical robustness is inherently reduced. Moreover, thermal cycling can accelerate defect evolution, diffusion processes, and degradation of passivation layers, thereby indirectly affecting carrier lifetime and recombination dynamics over long mission durations. As a result, thermal management in space photovoltaics must address not only heat dissipation but also long-term structural and functional stability under cyclic thermal stress.

In addition to efficiency considerations, long-term stability and durability are critical for space photovoltaic systems. Devices must withstand combined effects of radiation, thermal cycling, vacuum conditions and mechanical stress over extended mission durations. Degradation processes include defect accumulation, interface deterioration and material fatigue, which can progressively reduce carrier lifetime and device performance. Therefore, material selection and device design must prioritize radiation tolerance, thermal stability and mechanical robustness to ensure reliable long-term operation [31,133].

Radiation exposure in space leads to the formation of point defects and defect clusters within semiconductor materials, which act as recombination centers and reduce carrier lifetime. This results in degradation of key parameters such as short-circuit current and open-circuit voltage. The extent of degradation depends on particle type, energy and fluence, with displacement damage caused by protons and electrons being particularly significant. Advanced materials such as III–V multijunction solar cells exhibit improved radiation resistance. Nonetheless, performance degradation remains a critical factor in long-term mission design [134,135].

In practical space systems, photovoltaic devices operate as part of a broader electrical subsystem that includes power conditioning and control circuits. These typically involve maximum power point tracking (MPPT) units, voltage regulation systems and energy storage interfaces. Such circuits are essential for optimizing power extraction under varying illumination and temperature conditions, as well as for ensuring stable operation of onboard systems. Therefore, the overall performance of space photovoltaics depends not only on the solar cells themselves but also on the efficiency and reliability of the associated power electronics [136,137,138].

6. Ultra-Lightweight and Flexible Space Solar Cells

The development of ultra-lightweight and flexible solar cells represents one of the most important technological directions in modern space photovoltaics. In contrast to terrestrial applications, where efficiency is often the primary performance metric, space systems are strongly constrained by mass and volume. As a result, the specific power (W/kg) of a photovoltaic system becomes a critical parameter directly influencing launch costs and system-level design. Reducing the mass of solar cells while maintaining high efficiency is therefore a key objective in the development of next-generation space power systems [139]. Thin-film III–V solar cells fabricated using ELO technology have emerged as one of the most promising approaches to achieving this goal. These devices combine the high efficiency and radiation resistance of III–V multijunction architectures with drastically reduced mass and enhanced mechanical flexibility. As a result, they are increasingly considered as a viable alternative to conventional wafer-based space solar cells [140,141].

6.1. Epitaxial Lift-Off (ELO): Principle and Process

Epitaxial lift-off is a technique that enables the separation of epitaxially grown semiconductor layers from their original substrate. The process typically involves the insertion of a sacrificial layer, most commonly AlAs, between the substrate (e.g., GaAs) and the active III–V device structure. This layer can be selectively removed using wet chemical etching, typically in hydrofluoric acid, allowing the thin-film device to be detached without damaging the functional layers [142,143]. The key steps of ELO process, including sacrificial layer removal and thin-film separation, are illustrated schematically in Figure 6.

The concept of ELO was first demonstrated by Yablonovitch et al., who showed that epitaxial GaAs layers could be selectively lifted off with high precision [144]. Earlier work by Konagai et al. also demonstrated the feasibility of producing thin-film GaAs solar cells via layer separation techniques [145]. Since then, the method has evolved significantly and is now compatible with complex multijunction structures. A key advantage of ELO is the possibility of substrate reuse. Since III–V substrates such as GaAs are expensive, their recovery and reuse can significantly reduce fabrication costs. At the same time, the transferred thin-film device can be integrated onto lightweight and flexible carriers such as polyimide, metal foils, or composite substrates. This enables the fabrication of photovoltaic devices with extremely low mass per unit area [146,147,148].

6.2. Thin-Film III–V Solar Cells: Structure and Properties

Thin-film III–V solar cells typically retain the same fundamental device architecture as their bulk counterparts, including multijunction stacks such as GaInP/GaAs/InGaAs or more complex inverted metamorphic structures. However, the total thickness of the active layers is reduced to a few micrometres, which significantly lowers the overall mass without compromising optical absorption. This is possible due to the intrinsic properties of III–V semiconductors. Their direct bandgaps result in high absorption coefficients, allowing efficient photon absorption within very thin layers. As a consequence, thickness reduction does not lead to significant optical losses, provided that proper light management strategies, such as anti-reflection coatings and back reflectors, are implemented [149,150]. Another important feature of thin-film III–V devices is their compatibility with IMM architectures. In such structures, the inverted growth sequence naturally facilitates substrate removal after epitaxy, making them particularly well suited for ELO processing. This synergy between IMM design and thin-film processing is one of the key reasons why many of the highest-performance lightweight solar cells are based on inverted architectures [151,152].

One of the most important consequences of ELO processing is the ability to produce mechanically flexible solar cells. Once transferred onto a thin carrier, the device can bend without significant degradation of electrical performance. Typical minimum bending radii reported for flexible III–V solar cells fabricated via ELO range from approximately 5 to 20 mm, depending on the device thickness and supporting substrate. For emerging thin-film and perovskite-based devices, even smaller bending radii (down to a few millimeters) have been demonstrated. This opens the possibility of designing deployable, foldable, or conformal photovoltaic systems, which are highly attractive for modern spacecraft architectures. Flexible solar cells can be stowed compactly during launch and deployed in orbit, enabling large-area power generation systems with minimal volume. In addition, conformal solar cells can be integrated directly onto curved or irregular surfaces, increasing design flexibility at the system level. These features are particularly relevant for small satellites, constellations, and next-generation space missions where compactness and adaptability are essential [153,154].

6.3. Performance and Specific Power

The combination of high conversion efficiency and low mass enables exceptionally high specific power values in thin-film III–V solar cells, making them highly attractive for space applications. Recent studies [32,155] have demonstrated inverted metamorphic multijunction (IMM) devices achieving efficiencies exceeding 30% under AM0 illumination, while reaching specific power values of at least 3 W/g, representing a significant improvement over conventional GaAs/Ge-based multijunction solar cells. Beyond performance metrics, thin-film architectures enable a shift toward system-level optimization. Instead of focusing solely on conversion efficiency, parameters such as power-to-mass ratio, deployability, and integration flexibility become equally important in the design of advanced space photovoltaic systems [32,155]. A summary of the key performance advantages and technological challenges associated with thin-film III–V solar cells is presented in

Table 5. Key advantages and challenges of thin-film III–V solar cells and their implications for space applications.

Category	Key Aspect	Impact on Space Applications
Efficiency	>30% (AM0, multijunction)	High energy conversion capability
Specific power	>3 W/g; up to ~500 W/kg	Significant mass reduction and improved payload efficiency
Mechanical flexibility	Thin-film, flexible substrates	Enables deployable and conformal systems
Integration	Compatibility with advanced architectures (e.g., IMM)	Enhanced design flexibility
Thermal behavior	Reduced thermal mass	Faster heat dissipation but increased sensitivity to overheating
Mechanical stability	Risk of cracking or delamination	Requires careful handling and encapsulation
Surface effects	Increased surface-to-volume ratio	Higher recombination losses without proper passivation
Scalability	Challenges in large-area fabrication	Limits industrial-scale implementation

.

As shown in Table 5, thin-film III–V solar cells offer a unique combination of high efficiency, low mass, and mechanical flexibility. However, these advantages are accompanied by technological challenges related to mechanical integrity, surface recombination, and scalability of fabrication processes. The ELO process, while enabling lightweight device architectures, must be carefully controlled to avoid mechanical damage during layer transfer. The handling and integration of ultrathin semiconductor layers remain non-trivial, particularly for large-area devices. In addition, the reduced thickness increases the relative importance of surface and interface recombination, necessitating advanced passivation strategies to maintain high carrier lifetimes and minimize recombination losses.

Another critical challenge is related to scalability. Although ELO-based approaches have been successfully demonstrated at laboratory scale, extending these processes to large-area manufacturing while maintaining yield and uniformity remains an active area of research. Overall, thin-film III–V solar cells based on epitaxial lift-off represent a highly promising direction for next-generation space photovoltaic systems. By combining high efficiency, low mass, and mechanical flexibility, they offer a unique set of properties that cannot be achieved with conventional wafer-based technologies. Their compatibility with advanced multijunction architectures, particularly inverted metamorphic designs, further enhances their potential. Nevertheless, continued progress in process scalability, mechanical robustness, and defect control will be essential to fully realize their advantages at the system level.

7. Tandem Solar Cells Beyond Silicon: Emerging Perovskite-Based Architectures

Tandem solar cells are widely regarded as one of the most effective routes for surpassing the efficiency limits of single-junction photovoltaics. Their operating principle is based on combining absorbers with different bandgaps so that high-energy photons are harvested in the top cell, while lower-energy photons are transmitted to deeper-lying subcells. In this way, thermalization and transmission losses are reduced, and the detailed-balance limit of a single p–n junction can be exceeded [156,157]. In recent years, tandem photovoltaics have evolved from a largely theoretical concept into a technologically mature and highly competitive field, with record efficiencies in perovskite-based tandems already exceeding those of conventional crystalline silicon cells. At the same time, current tandem research is no longer focused only on absolute efficiency. Stability, scalability, light management, interconnection design, and manufacturability have become equally important [3,4]. These issues are particularly relevant for space photovoltaics, where high specific power, low mass, and tolerance to radiation and thermal cycling are critical [5,6]. Altogether, this makes perovskite-based tandems one of the most dynamic and strategically important areas in next-generation solar-cell research [158,159,160].

The main reason for the rapid rise of perovskite tandems is the exceptional flexibility of metal-halide perovskite absorbers. Their bandgap can be tuned over a broad range by modifying halide composition and cation chemistry, which makes them highly suitable as top cells in tandem designs [161]. Wide-bandgap perovskites in the range of about 1.68–1.80 eV are especially attractive for tandem integration with lower-bandgap bottom cells because they can efficiently harvest the visible part of the spectrum while transmitting near-infrared photons. In addition, perovskites exhibit high absorption coefficients, long carrier diffusion lengths, and compatibility with relatively low-temperature processing routes. These features allow the fabrication of optically efficient absorber layers with sub-micrometre thickness, which is highly attractive from the perspective of lightweight photovoltaics. For this reason, perovskites are now being investigated in tandem with silicon, narrow-bandgap perovskites, CIGS, and, more exploratorily, with III–V materials. Among these pairings, perovskite/silicon tandems are by far the most mature, while direct perovskite/III–V tandems remain at a much earlier stage of development [162,163,164].

7.1. Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandems are currently the dominant branch of perovskite-based tandem photovoltaics. Their attractiveness comes from a very practical combination of scientific and industrial advantages: perovskites provide bandgap tunability and strong visible-light absorption, while silicon contributes mechanical robustness, mature manufacturing infrastructure, and excellent long-wavelength response [165]. In most architectures, the perovskite top cell has a bandgap close to 1.7 eV, whereas the silicon bottom cell remains near 1.12 eV. This pairing is close to ideal from the standpoint of spectral splitting, which explains the rapid progress of monolithic two-terminal perovskite/silicon tandem solar cells [166]. Early landmark progress was demonstrated by K. A. Bush et al. [167], who reported a 23.6–efficient monolithic perovskite/silicon tandem with improved stability, establishing the viability of this architecture beyond proof-of-concept devices. Since then, the field has progressed rapidly through improvements in interface passivation, recombination layers, textured substrates, and optical coupling. A major milestone was reported by J. Xu et al. [168], where advanced interface passivation enabled perovskite/silicon tandem efficiencies exceeding 31%, highlighting the critical role of interfacial engineering in suppressing recombination losses. Another important advance was reported by E. Aydin et al. [169], who achieved efficient and stable perovskite/silicon tandem solar cells through contact displacement by MgFₓ, emphasizing the importance of interfacial control and transport-layer design. More recent records summarized in the latest efficiency tables show that certified perovskite/silicon tandem efficiencies have already reached 34.85% [170].

The rapid progress of perovskite/silicon tandems has been driven not only by the absorber itself but also by progress in device architecture. In monolithic two-terminal devices, the subcells are connected in series through a recombination junction or interconnecting layer, and the photocurrent is limited by the weaker of the two junctions. This makes current matching one of the most important design rules [170,171]. To achieve high efficiency, parameters such as the perovskite bandgap, absorber thickness, and parasitic absorption in transport layers must be carefully optimized in a coupled manner. At the same time, the use of textured silicon bottom cells improves light trapping and enhances photocurrent generation but significantly complicates the deposition of high-quality perovskite layers [172,173].

Recent studies have also shown that high performance can be maintained on textured or industry-relevant silicon platforms such as Silicon Heterojunction (SHJ) and Tunnel Oxide Passivated Contact (TOPCon), which is important for future manufacturing. In practice, this means that the field has moved beyond simple laboratory cells toward architectures that are much closer to scalable technology. From the perspective of a review article, this is one of the strongest arguments for giving perovskite tandems a central role in a discussion of future space photovoltaics [174,175,176].

7.2. Stability, Reliability and Space Relevance

Despite their excellent efficiency, perovskite tandems still face a central challenge that is much less severe in conventional III–V multijunction cells: long-term stability. For terrestrial deployment, the most important degradation factors include moisture, oxygen, temperature, ion migration, and bias-induced interfacial changes [177,178]. In space, the environment is different but not necessarily easier. Thermal cycling, ultraviolet irradiation, vacuum exposure, electron and proton bombardment, and atomic oxygen in low Earth orbit all need to be considered. This is why the relevance of perovskite tandems for space applications cannot be judged only by power conversion efficiency. Their operational durability under combined stresses is equally important [179,180]. Recent reviews dedicated specifically to space applications show that perovskite devices offer several attractive features, including low mass, very high power-to-weight potential, and in some cases surprisingly good radiation tolerance. At the same time, these reviews also make it clear that encapsulation, compositional stability, and interfacial robustness remain major bottlenecks for real space deployment. In other words, perovskite tandems are extremely promising for space, but they are not yet a drop-in replacement for mature III–V space cells. Their importance lies in the fact that they may offer a radically different balance of efficiency, mass, and cost once reliability is sufficiently improved [181].

Another reason why perovskite tandems are especially interesting for space systems is their compatibility with thin, lightweight, and potentially flexible device formats. This is not a minor advantage. In space photovoltaics, cell efficiency is always important, but specific power can be even more decisive at the system level. Perovskite top cells require only very thin absorber layers and can in principle be processed on lightweight supporting structures. Flexible perovskite/silicon tandems and lightweight perovskite-based tandems are therefore being actively pursued as routes toward high power-to-mass performance. Even when silicon remains part of the stack, the perovskite layer contributes very little to mass while substantially boosting efficiency. This is one of the reasons why perovskite tandems are now discussed not only as terrestrial high-efficiency devices but also as candidates for low-Earth-orbit satellite power systems [182,183].

7.3. All-Perovskite and III–V/Perovskite Tandem Concepts

A second major direction beyond perovskite/silicon tandem architectures is the development of all-perovskite tandem solar cells, in which both subcells are based on perovskite absorbers with different bandgaps. These devices are particularly attractive because they exploit the same material family, potentially simplifying fabrication and enabling even lower mass compared to perovskite/silicon tandems [184,185]. In such architectures, a wide-bandgap (≈1.7–1.9 eV) perovskite top cell is typically combined with a narrow-bandgap (≈1.1–1.3 eV) Sn–Pb-based perovskite bottom cell, allowing efficient utilization of the solar spectrum [184]. However, despite rapid progress, the development of all-perovskite tandems remains strongly limited by the stability of narrow-bandgap absorbers, particularly due to Sn²⁺ oxidation and increased non-radiative recombination losses [186]. Even so, recent advances have demonstrated significant improvements in efficiency and device design, including better control of recombination layers, interfacial engineering, and defect passivation [187]. These developments highlight the rapid evolution of perovskite-based multijunction strategies that are no longer constrained by silicon integration. In contrast, direct III–V/perovskite tandem architectures remain at a much earlier stage of development. In this case, the literature is still dominated by theoretical analyses and conceptual device designs rather than fully optimized experimental devices [188]. However, this approach is highly attractive due to the potential combination of the excellent radiation hardness and high efficiency of III–V semiconductors with the tunable bandgap and low-temperature processing of perovskites.

Recent studies suggest that such hybrid tandems could enable very high efficiencies, particularly in space applications, where the trade-off between performance, mass, and radiation resistance is critical [189,190]. Although still largely exploratory, this direction significantly expands the design space of tandem photovoltaics beyond the currently dominant perovskite/silicon approach.

A comparison of the key characteristics of emerging tandem concepts is summarized in

Table 6. Comparison of emerging tandem solar cell architectures beyond perovskite/silicon systems.

Architecture	Maturity Level	Key Advantages	Main Challenges
Perovskite/Si	High	Proven high efficiency, compatibility with industry	Limited flexibility due to Si substrate
All-perovskite	Medium	Ultra-lightweight, tunable bandgaps, simplified materials system	Stability of Sn-based absorbers, recombination losses
III–V/perovskite	Low	Highest efficiency potential, radiation resistance	Early-stage development, cost, integration complexity

.

As shown in Table 6, each tandem concept offers a different balance between efficiency, technological maturity, and system-level applicability. While perovskite/silicon tandems currently represent the most advanced and scalable solution, all-perovskite tandems offer unique advantages in terms of weight and material tunability. At the same time, III–V/perovskite tandems remain a promising but still largely conceptual direction, with the potential to combine the best properties of both material systems. From a forward-looking perspective, it is important to recognize that future photovoltaic technologies may not converge on a single architecture but rather evolve toward application-specific solutions depending on requirements such as efficiency, cost, weight, and environmental stability.

Taken together, the current literature indicates that tandem photovoltaics have entered a new stage of development, in which the key question is no longer whether tandem concepts can outperform single-junction devices, but which material combinations offer the best pathway toward practical, scalable, and application-driven photovoltaic systems.

8. Comparative Analysis of Photovoltaic Materials for Space Applications

Mission-Dependent Requirements

The selection of photovoltaic materials for space applications is strongly dependent on mission-specific conditions, including orbital environment, radiation exposure, mission duration, and system-level constraints such as mass and reliability. Unlike terrestrial systems, where cost-efficiency often dominates, space photovoltaics require a careful balance between efficiency, durability, and resistance to extreme conditions.

These requirements vary significantly between LEO, GEO and deep-space missions. Therefore, a comparative overview of mission-dependent constraints and corresponding technological solutions is provided in

Table 7. Comparison of different materials by application and mission type.

Mission Type /Parameter	LEO	GEO	Deep Space
Radiation Environment	Moderate	High	Very high (extreme)
Mission Duration	5–7 years	~15–20 years	>15–20 years
Expectations from the Cell Technology	Low production cost High availability Proven standard Low weight Acceptable degradation (not a priority)	Resistance to environmental conditions and durability of the solution are key Low tolerance to degradation Cost-effectiveness is a lower priority	Maximizing energy efficiency, resilience, and stability Cost-effectiveness is the lowest priority
Technological Solution	Most commonly mono-/poly Si (silicon cells) Single-junction III–V (GaAs) for highly demanding LEO missions	High-class multi-junction cells, most often Triple-junction III–V	Multi-junction cells with the highest efficiency, e.g., High-end Multi-junction

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As shown in Table 7, the technological requirements evolve from cost-driven solutions in LEO missions toward performance- and reliability-driven approaches in deep-space applications. This transition directly influences material selection, shifting the focus from conventional silicon-based technologies to advanced III–V multijunction architectures. In this context, silicon-based photovoltaic technologies remain an important reference point for understanding the evolution of space solar cells.

Silicon as a material and silicon-based photovoltaic cells themselves are still widely used in satellites and space missions, but this segment will experience long-term declines. Silicon cells are popular among manufacturers of small satellites and installations, in projects focused on cost-effectiveness relative to power yield. They are often the first-choice option due to the availability, maturity, and testability of the technology, and significantly less complex production than advanced cells such as GaAs or CIGS. However, silicon PV cells, despite their many advantages and effectiveness in numerous applications, have significant limitations in the extreme conditions of space. Numerous observations from completed missions indicate very high exposure and sensitivity of silicon cells to radiation degradation—cosmic radiation (protons and electrons) significantly damages their crystalline structure, leading to a permanent decline in performance over time. However, due to their long-standing market dominance, numerous research projects are underway in the field of silicon PV cells to increase their radiation resistance using radiation hardening technology, which potentially enables the automatic removal of radiation damage at temperatures typical of outer space. Another direction of silicon cell development is the production of ultra-thin cells, which use polymer layers instead of protective glass, resulting in a lightweight and flexible cell [191].

In contrast to silicon, III–V semiconductor technologies—particularly gallium arsenide (GaAs)—represent the current benchmark for high-performance space photovoltaics. GaAs dominates the market as a material for space applications, accounting for nearly 50% of its market value. As a material, it exhibits unparalleled performance in extreme conditions, and GaAs cells, known for their exceptional radiation resistance and energy conversion speed, remain the best choice for high-altitude and deep-space missions. It is the industry standard for space applications due to its ability to convert a broad spectrum of sunlight into electricity with the highest efficiency. GaAs cells are so-called III–V (three–five) cells—advanced, multi-junction solar cells made of semiconductors composed of elements from groups III and V of the periodic table, achieving the highest efficiency in the world. They are primarily used in the space industry and for power concentration due to their high cost and complexity of production, in contrast to third-generation cells, which include dye-sensitized (DSSC) and organic technologies, which are characterized by lower cost and lower efficiency. These cells, often in a multi-junction configuration (e.g., with the addition of aluminium indium phosphide), achieve efficiencies exceeding 30% and demonstrate exceptional resistance to degradation caused by cosmic radiation. Their main barrier is the very high production cost, which limits their use to specialized applications where efficiency and reliability are paramount. The multi-junction GaAs PV cell market is estimated at $1.8 billion by 2025. Further growth to $6.9 billion, measured at a CAGR of 25% per year between 2025 and 2033, is driven by the growing demand for highly efficient solar energy conversion, low cell weight, and high resistance to the very demanding atmospheric conditions found in space.

Four-junction GaAs PV cells are also available on the market, characterized by very high efficiency (a very wide operating temperature range and negligible efficiency losses with increasing temperature), optimal use of the solar spectrum (the multi-junction structure allows for the absorption of various wavelengths of light, allowing for maximum energy conversion), and reliability in demanding space conditions. However, due to their extremely high production costs, they are mainly used in projects of strategic importance, such as artificial satellites (ensuring constant power in orbit), space probes and stations (planetary missions where reliability is the highest priority), and Mars rovers (high pressure for low mass and limited (marginalized) installation space for the cells).

As an alternative to conventional III–V technologies, thin-film materials such as CIGS have gained increasing attention in recent years. CIGS is gaining increasing attention due to its light weight and flexibility, making it suitable for various satellite applications. Additionally, it has a high solar absorption capacity and high absorption coefficient. CIGS PV cells are currently being developed primarily in single- and double-junction configurations. The potential of multi-junction CIGS cells is being explored in laboratory settings, as the material itself allows for adjustment of the bandgap over a wide range (from 1.0 to 1.7 eV) by varying the gallium to indium ratio. However, single- and double-junction CIGS modules currently dominate the space market due to their proven stability and efficiency without the need for heavy protective glass.

The CIGS cell market was valued at USD 3.3 billion in 2024 and is projected to grow to USD 14.5 billion by 2033 (with a weighted average growth rate of 17.8%).

Among emerging materials, gallium nitride (GaN) has attracted significant attention as a potential next-generation platform for space photovoltaics. Its unique physicochemical properties offer clear advantages over conventional materials such as silicon, GaAs, and CIGS, particularly under extreme space conditions.

To further highlight the trade-offs between different photovoltaic technologies, a comparative overview of their key technical and economic parameters is presented in

Table 8. Comparison of key technical and cost parameters of the technology.

Technology	Technical Characteristics	Production Cost	Application	Summary
GaAs and III–V multi-junctions	High efficiency: typical field-of-life efficiency around 28–32% for standard GaAs, multi-junctions (triple-, quadruple-junction) can achieve >30–40% in space Radiation resistance higher than silicon, very slow degradation	Very high production costs, even in small quantities; standard GaAs cells for space: approximately $300–800/W, including testing, certification, and processing Epitaxial processes (MBE/MOCVD) and limited production scale significantly increase costs	Standard in GEO and deep space satellites due to its longevity, efficiency and radiation resistance	Best performance and durability in space, with the highest production cost per Wp
CIGS	Average efficiency: Commercial CIGS cells can exceed ~17–22% (in tests), with the potential for higher values in tandem technology Low thickness and weight thanks to thin-film construction and lightweight substrates enable very high power density (W/kg) Good radiation resistance, although dependent on configuration and protective layers	Theoretically, significantly lower than GaAs due to smaller material quantities and thin-film processes. However, there is no mass market for space-grade CIGS yet, so costs depend on small batches and R&D, not large volumes Estimated at $250–500/W for space-grade CIGS	Early Adoption for Small Satellites and CubeSat	The standard allows for lightweight, thin-film panels No broad market for certified space-grade
Silicon	Lowest efficiency in AM0 (space) conditions compared to GaAs: typically 12–18% Good durability in LEO/LEO-like conditions, but significant power degradation over time at high radiation doses Heavier panels (thicker wafers) mean higher overall cost	The lowest of the three cell-level technologies—space-grade is cheaper than GaAs, but often more expensive than standard Earth-based panels due to mandatory testing Estimated at $200–350/W at space-grade	Traditionally used on smaller satellites and LEO where price is critical and lifespan may be shorter (short, budget missions)	Cheapest base material Lowest efficiency and higher electrical degradation in space

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As summarized in Table 8, each photovoltaic technology presents a distinct balance between efficiency, cost, and environmental resilience. Silicon-based devices remain the most cost-effective solution but suffer from relatively low efficiency and significant radiation-induced degradation. In contrast, GaAs and other III–V multijunction technologies provide superior performance and durability, albeit at a substantially higher cost. Thin-film CIGS technologies offer an attractive compromise, combining reduced mass with moderate efficiency and good radiation tolerance, making them particularly suitable for small satellite platforms. Finally, emerging materials such as GaN represent a promising direction for future development, with the potential to simultaneously achieve high efficiency, low degradation, and reduced system mass.

Overall, the selection of photovoltaic technology for space applications is inherently mission-dependent, requiring a trade-off between performance, cost, and operational constraints.

9. Conclusions and Future Perspectives

The rapid evolution of photovoltaic technologies for space applications clearly demonstrates that future development will not be driven by a single material system or device architecture, but rather by a combination of approaches optimized for specific mission requirements. As discussed throughout this review, conventional III–V multijunction solar cells remain the benchmark for high-efficiency and radiation-resistant space photovoltaics. At the same time, emerging concepts such as inverted metamorphic structures, thin-film architectures, and perovskite-based tandems are redefining the design space by introducing new trade-offs between efficiency, mass, cost, and scalability.

A key trend in next-generation space photovoltaics is the shift from device-level optimization toward system-level performance metrics. While power conversion efficiency remains a critical parameter, factors such as specific power (W/kg), mechanical flexibility, deployability, and integration with complex spacecraft structures are becoming increasingly important. In this context, thin-film and lightweight photovoltaic technologies, including epitaxial lift-off III–V devices and perovskite-based tandems, offer significant advantages that cannot be achieved with conventional wafer-based solutions.

However, these emerging technologies also introduce new challenges that must be addressed before large-scale implementation in space systems becomes feasible. For III–V multijunction devices, further improvements in defect control, tunnel junction performance, and scalable epitaxial growth remain essential. For perovskite-based tandems, long-term operational stability under combined stress conditions—including radiation, thermal cycling, and vacuum exposure—represents the most critical limitation. In addition, issues related to encapsulation, interfacial robustness, and reproducibility must be resolved to ensure reliable performance over mission lifetimes.

Looking forward, it is likely that space photovoltaics will evolve toward a diversified technological landscape, where different architectures are selected based on application-specific constraints. High-efficiency III–V multijunction cells will continue to dominate missions requiring maximum reliability and performance, while lightweight and flexible tandem technologies may enable new classes of space systems, including small satellites, deployable structures, and space-based solar power platforms.

Ultimately, the future of space photovoltaics lies in achieving an optimal balance between efficiency, mass, durability, and cost. Continued progress in materials science, device engineering, and system integration will be essential to translate emerging photovoltaic concepts into practical technologies capable of meeting the increasingly demanding requirements of modern space missions.