Modern Solar Cell Engineering

A modern solar cell converts more than a quarter of the sunlight that hits it into electricity. The cell is a few hundred micrometres thick, made mostly of silicon, and costs a few cents per watt to manufacture. The story of how the photovoltaic cell went from a laboratory curiosity in the 1950s to the cheapest source of new electricity in most of the world is, in retrospect, one of the most remarkable success stories of late-twentieth- and early-twenty-first-century engineering.

The photovoltaic effect

The photovoltaic effect — the generation of a voltage when a material is exposed to light — was discovered by the French physicist Edmond Becquerel in 1839, using a silver-chloride electrode in an acidic solution illuminated by sunlight. The effect was studied by several groups in the late nineteenth and early twentieth centuries, including Heinrich Hertz, who observed the photoelectric effect in 1887, and Albert Einstein, who explained it in 1905 with the quantum theory of light, for which he received the 1921 Nobel Prize in Physics. The first solid-state photovoltaic cell, using a junction between p-type and n-type silicon, was patented by Russell Ohl at Bell Labs in 1940. The first practical silicon solar cell, with an efficiency of about 6 percent, was demonstrated at Bell Labs in 1954 by Gerald Pearson, Calvin Fuller, and Daryl Chapin.[1, 2, 9]

The first major application of solar cells was in space. The Vanguard 1 satellite, launched by the US in 1958, was the first satellite to be powered by solar cells; it operated for more than six years until its transmitter failed, far longer than the chemical batteries on earlier satellites. The success of solar cells in space was a major driver of the early technology development, and the US space programme was the dominant customer for solar cells until the 1970s. The terrestrial market was, in the early years, negligible: a solar module in 1970 cost about $80 per watt (in inflation-adjusted dollars), and the levelised cost of electricity was more than $1 per kWh, more than 50 times the cost of conventional electricity.[1, 2, 9]

The cost reduction that made solar competitive began in the 1970s, driven by the oil crisis and by the development of new manufacturing techniques. The first major cost reduction came from replacing the expensive single-crystal silicon wafers with cheaper multicrystalline silicon. The second came from increasing the size of the wafers (from 50 mm diameter in 1970 to 200 mm in 2000 to 300 mm in 2020) and the size of the modules. The third came from the development of new cell architectures (PERC, PERT, TOPCon) that pushed the efficiency from 15 percent in 1990 to more than 25 percent in 2024. The result is a module cost, in 2024, of about $0.15 per watt, more than 100 times cheaper than in 1970. The levelised cost of solar electricity, in sunny regions, is now about $0.03 per kWh, cheaper than any other source of new electricity.[2, 3, 10]

The basic physics of a solar cell

A solar cell is, in essence, a large-area semiconductor diode. The cell is made of a semiconductor (typically silicon) with a p-n junction near the front surface. The p-type region is doped with acceptor atoms (typically boron) to create an excess of holes; the n-type region is doped with donor atoms (typically phosphorus) to create an excess of electrons. The junction between the two regions creates a depletion region, with a built-in electric field that separates photogenerated electron-hole pairs. When light is absorbed in the semiconductor, photons with energy greater than the band gap generate electron-hole pairs; the built-in field sweeps the electrons to the n-type contact and the holes to the p-type contact, and the resulting current can be drawn through an external circuit.[3, 4, 11]

The voltage of the cell is set by the band gap of the semiconductor, and is about 0.7 V for silicon. The current is set by the light intensity and the absorption of the cell, and is typically 40–45 mA/cm² under standard test conditions (1 kW/m² of AM1.5 sunlight, 25°C). The power output is the product of voltage and current, and is typically 20–25 mW/cm², giving an efficiency of 20–25 percent. The maximum theoretical efficiency of a single-junction solar cell under standard sunlight is 33.7 percent, the Shockley-Queisser limit. The limit is set by the fact that photons with energy below the band gap are not absorbed, and photons with energy above the band gap lose the excess energy as heat. Real cells are below the limit because of recombination (electrons and holes recombine before being collected), reflection (some light is reflected from the front surface), and resistance (the series resistance of the contacts and the shunt resistance of the cell).[3, 4, 12]

The engineering of a high-efficiency silicon cell involves minimising all three loss mechanisms. Recombination is minimised by using high-purity silicon (with minority-carrier lifetimes of milliseconds), by passivating the surfaces (with thin layers of silicon dioxide, silicon nitride, or aluminium oxide), and by using a PERC (Passivated Emitter and Rear Cell) architecture that puts the rear contact behind a passivating layer. Reflection is minimised by an anti-reflection coating (typically a 75-nm layer of silicon nitride), by texturing the front surface (to make the light bounce multiple times), and by using bifacial cells (that absorb light from the back as well as the front). Resistance is minimised by using screen-printed silver contacts on the front and aluminium on the back, with a fine grid of fingers to minimise the distance the current has to travel. The best commercial cells, in 2024, have efficiencies of about 25 percent; the best lab cells are above 26.8 percent.[3, 4, 13]

Silicon: the workhorse

Crystalline silicon is, in many ways, an ideal solar-cell material. It is the second most abundant element in the Earth’s crust (after oxygen); it is non-toxic; it has a band gap (1.12 eV) that is well-matched to the solar spectrum; and the technology for purifying, crystallising, slicing, and processing it is mature and well understood. The downside is that silicon is an indirect-bandgap semiconductor, which means that the absorption of a photon requires the simultaneous absorption or emission of a phonon (a quantum of lattice vibration) to conserve momentum. The absorption coefficient is therefore much lower than for a direct-bandgap semiconductor, and the cell must be at least 100 micrometres thick to absorb most of the incident light.[3, 4, 14]

The dominant silicon-cell architectures, in 2024, are PERC (Passivated Emitter and Rear Cell), TOPCon (Tunnel Oxide Passivating Contact), and HJT (Silicon Heterojunction). PERC, which has been the dominant architecture since the early 2010s, adds a passivating layer on the back of the cell, allowing more of the longer-wavelength light to be absorbed and reducing recombination. TOPCon, which is rapidly taking market share from PERC, adds a thin tunnel oxide and a heavily-doped polysilicon layer on the back, providing even better passivation and a higher efficiency (about 25 percent vs about 23.5 percent for PERC). HJT, which uses amorphous silicon to passivate the crystalline-silicon surface, has the highest efficiency (above 26 percent) and the best bifacial performance, but is more expensive to manufacture.[3, 4, 15]

The silicon wafer itself is the most expensive part of the cell. The wafer is sliced from a single-crystal ingot (grown by the Czochralski process) or a multicrystalline block (cast in a crucible), with wire saws that use a thin steel wire and a slurry of silicon carbide abrasive. The kerf loss (the silicon lost as sawdust) is about 50 percent, and the wafer is then etched to remove the surface damage. The trend, since 2015, has been to use larger wafers (M10, 182 mm × 182 mm, and M12, 210 mm × 210 mm) to reduce the cost per watt, and to use thinner wafers (now down to 130 micrometres, with 100 micrometres in development) to reduce the silicon consumption. The combination of larger and thinner wafers has reduced the polysilicon consumption per watt by more than 50 percent since 2015.[3, 4, 16]

The limits: Shockley-Queisser

The Shockley-Queisser limit, calculated in 1961 by William Shockley and Hans-Joachim Queisser, is the maximum efficiency of a single-junction solar cell under standard sunlight. The limit is 33.7 percent, and is set by the fundamental physics of light absorption and carrier collection. Photons with energy below the band gap cannot be absorbed, and so are lost; photons with energy above the band gap lose the excess energy as heat, and so are partially wasted. The optimal band gap is about 1.34 eV (silicon is close, at 1.12 eV, but not optimal), and the optimal efficiency is 33.7 percent.[4, 5, 17]

The Shockley-Queisser limit can be exceeded by stacking multiple cells with different band gaps, each absorbing a different part of the solar spectrum. The most efficient cells in the world, with efficiencies above 47 percent, are multijunction III-V cells — typically a stack of three or four subcells made of gallium arsenide, indium gallium phosphide, and germanium, grown by molecular beam epitaxy or metalorganic vapour phase epitaxy. These cells are used in space (where their high efficiency and radiation hardness justify the cost) and in concentrator photovoltaics (where mirrors or lenses focus the sunlight by a factor of 100–1000, reducing the area of expensive cell needed). The cost, however, is more than 100 times that of silicon, and the terrestrial market is essentially zero.[4, 5, 18]

The other ways to exceed the Shockley-Queisser limit include concentrator photovoltaics (which use lenses or mirrors to focus the sunlight), intermediate-band solar cells (which use an intermediate energy level to absorb below-bandgap photons), hot-carrier solar cells (which extract the energy of the photogenerated carriers before they thermalise), and multiple-exciton generation (in which a single high-energy photon generates multiple electron-hole pairs in a quantum dot). All of these are still in research, and none has yet reached commercial production.[4, 5, 18]

Perovskites: the new hope

Perovskite solar cells are the most promising new solar-cell technology. The active material is a hybrid organic-inorganic lead halide with the perovskite crystal structure (ABX3, where A is methylammonium or formamidinium, B is lead, and X is iodide or bromide). The first perovskite cell, with an efficiency of 3.8 percent, was reported by Tsutomu Miyasaka and colleagues in 2009. The efficiency has since risen at a pace faster than any other solar-cell technology in history, reaching 26.1 percent in 2024, and is still rising. The perovskite can be deposited by simple solution processes (spin-coating, slot-die coating, inkjet printing) on a variety of substrates, including flexible ones. The combination of a high band gap (1.55–2.3 eV, tunable by composition) and a high absorption coefficient makes the perovskite an ideal top cell for a tandem cell.[5, 6, 19]

The main challenge for perovskites is stability. The early perovskite cells degraded in days or weeks under solar illumination, due to a combination of moisture sensitivity, thermal degradation, and photo-induced ion migration. The best modern cells, with improved encapsulation and composition engineering, have demonstrated stability of more than 1,000 hours under accelerated testing, and the best encapsulated modules have shown less than 5 percent degradation after 1,000 hours of damp heat and 1,000 hours of illumination. The stability is improving rapidly, and the first commercial perovskite-silicon tandem modules are expected in 2025–2027, with efficiencies above 30 percent.[5, 6, 20]

The other challenge is the lead content. The most efficient perovskites contain lead, which is toxic. Lead-free perovskites (using tin instead of lead) have been developed, but their efficiencies are much lower (about 14 percent vs 26 percent for lead-based), and they are less stable. The toxicity concern is, in practice, much smaller than for other energy technologies: the lead content of a perovskite-silicon tandem module is about 0.5 g per square metre, and the lead is encapsulated in a glass-and-polymer sandwich. The total lead content of a 1 MW solar farm (perhaps 5,000 m² of modules) would be about 2.5 kg, less than the lead content of a single car battery. The industry is, nevertheless, working on lead-free alternatives as a hedge against regulatory risk.[5, 6, 20]

Tandem cells and the next decade

The most promising near-term path to higher efficiency is the tandem cell, in which a perovskite top cell is stacked on a silicon bottom cell. The perovskite absorbs the high-energy (blue) part of the solar spectrum, and the silicon absorbs the lower-energy (red) part. The combination is more efficient than either cell alone, because the energy that would have been wasted as heat in the silicon is instead used efficiently in the perovskite. The theoretical efficiency of a two-junction tandem with a 1.7 eV perovskite on a 1.1 eV silicon cell is about 42 percent, well above the 33.7 percent Shockley-Queisser limit. The best experimental results, in 2024, are above 33.9 percent (LONGi and NREL, with various partners), and the trajectory is still steep.[6, 7, 21]

The engineering challenges for tandem cells are formidable. The two subcells must be optically and electrically matched, with the current through the two subcells equal at the operating point. The perovskite top cell must be deposited on the silicon bottom cell without damaging it, and the two must be connected electrically (either in series, with a tunnel junction, or in parallel, with a transparent conducting layer). The encapsulation must protect both cells from moisture and from light-induced degradation. The manufacturing process must be scalable to gigawatt volumes. The first commercial tandem modules are expected from LONGi, Trina, and JinkoSolar in 2025–2027, with efficiencies above 30 percent and costs comparable to or lower than single-junction silicon modules.[6, 7, 22]

The other promising direction is bifacial cells, which absorb light from both the front and the back. Bifacial cells, combined with single-axis tracking, can produce 10–20 percent more energy per year than a fixed-tilt monofacial system of the same nameplate capacity. The technology is now standard in utility-scale solar, and the share of bifacial modules in new installations is now above 80 percent. The combination of bifacial cells, single-axis tracking, and high-efficiency perovskite-silicon tandems is the most likely path to a solar electricity cost of $0.01 per kWh by 2035. The cost target, if achieved, would make solar the dominant source of new electricity worldwide, and would complete the transition that began with the first practical cell at Bell Labs in 1954.[6, 7, 23]

Comparison: leading solar-cell technologies (2024)

Technology	Cell architecture	Lab efficiency (2024)	Commercial efficiency	Cost ($/W)	Best suited to
Crystalline silicon (PERC)	p-type Si, Al-BSF rear	24.5%	~23.5%	~$0.15	Utility-scale, residential
Crystalline silicon (TOPCon)	n-type Si, poly-Si rear	26.8%	~25.0%	~$0.15	Utility-scale (replacing PERC)
Crystalline silicon (HJT)	a-Si/c-Si heterojunction	27.0%	~25.5%	~$0.20	Premium, bifacial
Perovskite single junction	CH3NH3PbI3	26.1%	~20% (pilot)	TBD	Single junction (in development)
Perovskite-silicon tandem	Perovskite on Si	33.9%	~30% (pilot)	TBD	Premium (commercialising 2025–2027)
Multijunction III-V (concentrator)	GaInP/GaAs/Ge stack	47.6%	~40%	>$5	Space, CPV
CdTe thin film	CdTe/CdS	22.1%	~19%	~$0.20	Utility-scale (First Solar)
CIGS thin film	Cu(In,Ga)Se2	23.6%	~18%	~$0.30	Building-integrated

Key numbers

• ~33.7%: Shockley-Queisser limit for a single-junction cell.
• ~26.8%: best lab efficiency for a silicon cell (2024).
• ~33.9%: best lab efficiency for a perovskite-silicon tandem (2024).
• ~47.6%: best lab efficiency for a multijunction III-V cell (2024).
• ~$30/MWh: levelised cost of utility-scale solar in the US (2024).
• ~$0.15/W: cost of a commercial silicon solar module (2024).
• ~600 GW: global annual production of solar PV modules (2024).
• ~2 TW: global installed capacity of solar PV (2024).
• 1954: year of the first practical silicon solar cell at Bell Labs.

References

1. Wikipedia, “Photovoltaic effect.” (Standard reference on the photovoltaic effect and its history.) https://en.wikipedia.org/wiki/Photovoltaic_effect
2. Wikipedia, “Solar cell.” (Standard reference on solar cells, with a section on the history.) https://en.wikipedia.org/wiki/Solar_cell
3. Wikipedia, “Crystalline silicon.” (Standard reference on crystalline silicon as a solar-cell material.) https://en.wikipedia.org/wiki/Crystalline_silicon
4. Wikipedia, “Shockley - Queisser limit.” (Standard reference on the fundamental efficiency limit of a single-junction cell.) https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit
5. Wikipedia, “Multi-junction solar cell.” (Standard reference on multijunction cells, with the record efficiency data.) https://en.wikipedia.org/wiki/Multi-junction_solar_cell
6. Wikipedia, “Perovskite solar cell.” (Standard reference on perovskite solar cells, with efficiency history.) https://en.wikipedia.org/wiki/Perovskite_solar_cell
7. Wikipedia, “Tandem solar cell.” (Standard reference on tandem cells, with a section on perovskite-silicon tandems.) https://en.wikipedia.org/wiki/Tandem_solar_cell
8. Wikipedia, “Levelized cost of electricity.” (Standard reference on LCOE, with a section on solar cost trends.) https://en.wikipedia.org/wiki/Levelized_cost_of_electricity
9. Wikipedia, “Russell Ohl.” (Standard reference on Russell Ohl, who patented the first solid-state photovoltaic cell.) https://en.wikipedia.org/wiki/Russell_Ohl
10. Wikipedia, “Vanguard 1.” (Standard reference on Vanguard 1, the first solar-powered satellite.) https://en.wikipedia.org/wiki/Vanguard_1
11. Wikipedia, “P - n junction.” (Standard reference on the p-n junction, the heart of every solar cell.) https://en.wikipedia.org/wiki/P%E2%80%93n_junction
12. Wikipedia, “Edmond Becquerel.” (Standard reference on Edmond Becquerel, the discoverer of the photovoltaic effect.) https://en.wikipedia.org/wiki/Edmond_Becquerel
13. Wikipedia, “Passivated emitter and rear cell.” (Standard reference on PERC, the dominant silicon cell architecture.) https://en.wikipedia.org/wiki/Passivated_emitter_and_rear_cell
14. Wikipedia, “Czochralski process.” (Standard reference on the Czochralski process, used to grow silicon ingots for solar wafers.) https://en.wikipedia.org/wiki/Czochralski_process
15. Wikipedia, “Tunnel oxide passivating contact.” (Standard reference on TOPCon, the silicon cell architecture replacing PERC.) https://en.wikipedia.org/wiki/Tunnel_oxide_passivating_contact
16. Wikipedia, “Longi Green Energy.” (Standard reference on LONGi, the world’s largest solar wafer and module manufacturer.) https://en.wikipedia.org/wiki/Longi_Green_Energy
17. Wikipedia, “National Renewable Energy Laboratory.” (Standard reference on NREL, the leading US solar research lab, which maintains the solar cell efficiency record table.) https://en.wikipedia.org/wiki/National_Renewable_Energy_Laboratory
18. Wikipedia, “Concentrator photovoltaics.” (Standard reference on concentrator photovoltaics, the technology that uses lenses or mirrors to focus sunlight.) https://en.wikipedia.org/wiki/Concentrator_photovoltaics
19. Wikipedia, “Miyasaka Tsutomu.” (Standard reference on Tsutomu Miyasaka, who first demonstrated a perovskite solar cell in 2009.) https://en.wikipedia.org/wiki/Miyasaka_Tsutomu
20. Wikipedia, “Perovskite (structure).” (Standard reference on the perovskite crystal structure used in perovskite solar cells.) https://en.wikipedia.org/wiki/Perovskite_(structure)
21. Wikipedia, “Trina Solar.” (Standard reference on Trina Solar, one of the largest solar module manufacturers.) https://en.wikipedia.org/wiki/Trina_Solar
22. Wikipedia, “JinkoSolar.” (Standard reference on JinkoSolar, the world’s largest solar module manufacturer by shipments.) https://en.wikipedia.org/wiki/JinkoSolar
23. Wikipedia, “Bifacial solar cell.” (Standard reference on bifacial solar cells, which absorb light from both front and back.) https://en.wikipedia.org/wiki/Bifacial_solar_cell
24. Wikipedia, “First Solar.” (Standard reference on First Solar, the leading manufacturer of CdTe thin-film solar cells.) https://en.wikipedia.org/wiki/First_Solar