Overview
Solar energy was once limited to advanced spacecraft and novelty devices, but it is now a major component of the global energy mix. Over the past decade, solar power has transitioned from a niche resource to a significant energy source worldwide.
Earth receives about 173,000 TW of solar radiation, roughly ten times the average global electricity demand.[1] This indicates that solar energy has the potential to meet global energy needs.
In the first half of 2023, the share of electricity generation from solar in the United States rose from 4.95% in 2022 to 5.77%.[2] Although fossil fuels (mainly natural gas and coal) still accounted for 60.4% of U.S. electricity generation in 2022,[3] the growing impact of solar and rapid technological progress in the field are notable.
Types of Solar Cells
Solar cells, also called photovoltaic (PV) cells, are commonly grouped into three categories: crystalline, thin-film, and emerging technologies. Each category has trade-offs in efficiency, cost, and lifespan.
Crystalline
Most residential rooftop solar panels are made from high-purity monocrystalline silicon. These cells have achieved efficiencies above 26% and lifetimes exceeding 30 years in recent years.[4] Typical residential panels currently have efficiencies around 22%.
Multicrystalline silicon is less expensive than monocrystalline silicon but offers lower efficiency and shorter lifetime. Lower efficiency requires more panels and larger installation area.
Compared with conventional silicon cells, multijunction cells based on gallium arsenide (GaAs) can achieve higher efficiencies. These cells use multiple stacked layers, each made from different materials such as gallium indium phosphide (InGaP), indium gallium arsenide (InGaAs), and germanium (Ge), to absorb different wavelengths. Although multijunction cells offer very high efficiency potential, high manufacturing costs and immature development limit their commercial viability and practical deployment.
Thin-Film
The dominant thin-film PV product in the global market is cadmium telluride (CdTe) modules, which have been deployed in millions of installations worldwide with peak capacity exceeding 30 GW, mainly at utility-scale sites in the United States.
In these thin-film modules, the cadmium content per square meter is lower than that of an AAA nickel-cadmium (Ni-Cd) battery. Cadmium in the modules is bound to tellurium, and tellurium is insoluble in water and stable at high temperatures up to 1200°C. These factors reduce the toxicity risk associated with CdTe thin-film modules.
Tellurium is rare in the Earth's crust, with abundance around 0.001 ppm. Like other scarce elements such as platinum, tellurium scarcity can significantly affect CdTe module costs. Recycling can mitigate this supply issue.
CdTe modules can reach efficiencies up to 18.6%, and laboratory cell efficiencies can exceed 22%.[5] Replacing long-used copper doping with arsenic doping has substantially improved module lifetime, bringing it closer to that of crystalline silicon modules.
Emerging Technologies
Emerging PV technologies using ultrathin films (less than 1 micrometer) and direct-deposition techniques aim to reduce manufacturing costs and provide high-quality semiconductor materials. These approaches could compete with established materials such as silicon, CdTe, and GaAs.[6]
Notable thin-film research candidates include copper zinc tin sulfide (Cu2ZnSnS4 or CZTS), zinc phosphide (Zn3P2), and single-walled carbon nanotubes (SWCNT). In laboratory settings, copper indium gallium selenide (CIGS) solar cells have demonstrated peak efficiencies of around 22.4%. Replicating such efficiencies at commercial scale remains challenging.[7]
Halide lead perovskite thin-film cells are a prominent emerging technology. Perovskites are materials with a crystal structure typically described by the formula ABX3. Perovskite materials have achieved rapid efficiency gains: a commercial-scale silicon-based perovskite tandem cell produced by Oxford PV recorded a 28.6% efficiency and is slated for production.[8]
Perovskite cells reached efficiencies comparable to CdTe thin films within a few years. Early perovskite devices suffered from very short lifetimes measured in months. Progress since then has extended operational lifetimes toward 25 years or more. Perovskite cells offer high conversion efficiency (over 25% in advanced demonstrations) while requiring lower processing temperatures and potentially lower manufacturing costs.
Building-Integrated Solar
Some solar cells are designed to capture only part of the solar spectrum while allowing visible light to pass through. These transparent cells, such as dye-sensitized solar cells (DSC) invented in 1991 in Switzerland, have seen recent research advances that improved efficiency and may enable market entry in the near future.
Other approaches embed inorganic nanoparticles into polycarbonate interlayers in glass. These nanoparticles shift specific parts of the spectrum toward the glass edges while allowing most of the spectrum to pass through. Light concentrated at the glass edges is then collected by PV cells. Research is also exploring the application of perovskite thin films to transparent solar windows and building fa?ades.
Material Requirements for Solar
Expanding solar deployment increases demand for key materials such as silicon, silver, copper, and aluminum. The U.S. Department of Energy notes that about 12% of metallurgical-grade silicon (MGS) is processed into polysilicon for solar panels.[10]
China is a major producer in this sector: in 2020 roughly 70% of global MGS and 77% of polysilicon supply were produced in China.[11] Converting silicon to polysilicon requires extremely high temperatures. In China, these processes have been largely powered by coal. Regions with abundant coal resources and low electricity costs have become significant producers, with one region accounting for around 45% of global polysilicon output.[12]
Silver used in solar panel production accounts for about 10% of global silver consumption. Major silver mining countries include Mexico, China, Peru, Chile, Australia, Russia, and Poland. Mining activities can cause heavy metal pollution and community displacement.
Copper and aluminum mining also present land-use and environmental challenges. The U.S. Geological Survey reports that Chile produces about 27% of the world's copper, followed by Peru (10%), China (8%), and the Democratic Republic of the Congo (8%). The International Energy Agency estimates that if global renewable deployment reaches levels consistent with net-zero scenarios by 2050, solar projects could roughly double copper demand compared with current levels.[13]
Conclusion
Will solar become our primary energy source? Prices are falling and efficiencies are improving, and multiple technological pathways are available. Determining which technologies will scale and how to integrate large amounts of solar into power grids remain open questions.
The transition of solar from a specialized energy source to a mainstream provider highlights its potential to meet or exceed significant portions of energy demand. While crystalline silicon currently dominates the market, advances in thin-film and emerging technologies such as CdTe and perovskites pave the way for higher-efficiency and more integrated solar applications. Challenges remain, including environmental impacts from material extraction and production bottlenecks, but the sector continues to grow and innovate.
With balanced technical progress and sustainable practices, solar energy development can contribute to a cleaner and more abundant energy future. Its growth in the U.S. electricity mix illustrates its expanding role, and it has the potential to become a global component of sustainable energy systems.
