01-07-2025
Solar energy systems: What is the science behind clean energy generation?
— Arunangshu Das
India's imports of solar photovoltaic (PV) cells from China jumped 141 per cent, seemingly driven by the increase in domestic solar PV module manufacturing capacity. But what are solar photovoltaic (PV) cells and how are they used in solar panels?
There are fundamentally two ways to generate electricity. The first is based on electromagnetic induction developed by Michael Faraday in 1821 and became commercially viable by 1890. It remains the backbone of global electricity production today. The second method uses photovoltaic (PV) cells, which are made from semiconductors like elemental silicon. It was first observed by Alexander Becquerel in 1839 as the photovoltaic effect.
But it wasn't until 1954 that researchers at Bell Labs (Chapin, Fuller, and Pearson) created the first practical solar cell using doped silicon. This progress was built on two key breakthroughs: Albert Einstein's explanation of the photoelectric effect, for which he won the Nobel Prize, and Polish scientist Jan Czochralski's development of single-crystal silicon, which is the standard material in solar cell manufacturing today.
Unlike PVs, which feed tradeable, regulated and taxed electricity into the grid, technologies like solar heating, water heating and even solar cooling are primarily standalone systems. For instance, solar cooling operates through an absorption refrigeration mechanism that can achieve indoor temperatures as low as 19°C even when outdoor temperatures reach 40°C. These technologies are similar to PV panels installed in remote areas far away from grids and are mainly used for battery charging and basic lighting.
Worldwide solar insolation – the amount of solar radiation received – across different regions varies tremendously. Although solar energy is abundant, it is diffuse and spread thinly across large areas. That is why different technologies (like parabolic troughs, Fresnel lenses, and other concentrators) are used to focus sunlight for various purposes, including heating, cooking, desalination, or even generating electricity.
PV cells are made from semiconductors like elemental silicon. Unlike metallic conductors like copper – known as Ohmic conductors, whose resistance to the flow of current increases with temperatures – silicon behaves differently. It is a poor conductor at room temperature, but its conductivity increases as the temperature rises, making it a non-Ohmic material.
According to quantum theory, electrical conduction requires the presence of electrons in a higher energy quantum state called conduction band, where they flow much like water in seas. In contrast, electrons in the lower-energy valence band are localised and cannot contribute to electric current.
For transition from lower-energy valence band to higher-energy conduction band, energy must be supplied. This energy can come from high thermal motions of atoms grossly manifested as the temperature of the system or some other energy input like light.
Light as a form of energy is observed either as a wave or particle behaving like discrete packets of energy called photons depending on the nature of the experiment. When photons strike electrons in the valence band, they can transfer their energy to those electrons, allowing them to jump to the conduction band.
But this transition happens if certain conditions, first explained by Einstein in his photoelectric effect theory, are met – the energy of the photon must be equal to the difference of energy between the two bands, also called band gap, which is measured in electron volts. Photons with higher energy will transfer the excess energy as heat leading to loss of electrons.
Apart from the energy criterion, there is also a symmetry criterion which is less relevant in this case. These two conditions immediately render approximately 50.4 per cent of the total solar spectrum unusable for electricity generation from PV cells made of crystalline silicon – 20.2 per cent of photons have lower energy, while 30.2 per cent have higher energy that is wasted as heat.
Other materials – such as gallium arsenide, cadmium telluride, and copper indium selenide – can capture different portion of the solar spectrum. However, issues like scarce natural abundance, handling difficulty, and environmental toxicity limit their widespread use.
In silicon-based PV cells, small amounts of phosphorus and boron are deliberately added to create regions with an excess of electrons and others with a deficit – known as 'holes'. This difference in charge between the two regions creates what's called a p-n junction. The junction of two such regions generates a driving force (electric potential) when sunlight strikes the material creating a cell just like a battery.
If an external load is added to this cell, electrons will flow from the negative charge rich region through the load to the positive charge region completing the circuit. These can go on infinitely if light is available. Even from the usable 49.6 per cent of the solar spectrum, additional losses occur. PV cells can heat up to 30–40°C above ambient temperature, and radiate heat which accounts for about 7 per cent energy loss. Another 10 per cent loss comes from the imbalance in charge mobility – known as the saturation effect – which weakens the electric potential over time.
These lead to the final theoretical efficiency of 33.7 per cent for single-junction silicon solar cells called Shockley-Queisser limit. Other inefficiencies can still stem from real world situations like differential illumination of cells and variations during production leading to different open circuit potentials between the cells.
Considering real world losses due to further downstream processes like DC to AC conversion and maintaining maximum peak power (MPP), the average efficiency of crystalline PV cells stands at 25 per cent under best laboratory conditions and best commercial cells achieve 20 per cent efficiency. For comparison, photosynthesis captures only 3-6 per cent of the total available solar radiation energy.
Since natural silicon is quite reflective, an anti-reflection transparent coat of tin oxide or silicon nitride is applied that gives the cells and modules the blue colour. Compared to the completely renewable, ambient temperature assembly of proteins in biological photosystems, PV systems require significant energy input. The production of PV cells begins with the purification of elemental silicon to 99 per cent by the Czochralski process – it involves melting silicon and slowly crystallizing it into single-crystal ingots.
Slicing the purified ingots into wafers causes about 20 per cent material loss as silicon dust. Thus, the high cost of single crystal technology has called for the development of alternative methods like ribbon technology that largely bypass sawing of ingot. Amorphous silicon-based cells also have lower cost and their inherent crystal defects are mended by alloying with hydrogen.
Multijunction amorphous cells have been designed to capture a larger solar spectrum and achieve theoretical efficiency up to 42 per cent, yet under practical conditions, 24 per cent efficiency have been realised. PV technologies are currently categorised into three generations: first-generation thick crystalline wafers (~200 µm), second-generation thin wafers (1-10µm) and third generation multijunction tandem cells and quantum dots, which can generate more charge separation per photons thereby some of them can exceed Shockley-Queisser efficiency limit.
The price of PV electricity, measured in dollars per watt of direct current (DC) peak power, fell from $4–5 in 2010 to $2.8 in 2023 (and $1.27 for utility-scale systems), aligning with the US Department of Energy's SunShot programme target of $1 per watt for installed systems.
Broken down system costs in categories: 38 per cent is for the modules, 8 per cent for power electronics (mostly inverter), 22 per cent for wiring and mounting and 33 per cent for hardware balance systems, which include labour, permit, overhead costs and profit. With single crystal PV cells reaching their theoretical maximum output, the greatest scope of cost reduction lies in hardware balancing categories.
Annual efficiency loss is around 0.5 per cent, with most modules remaining effective for 20–25 years of operation. Contrary to common belief, while tropical and desert regions receive more sunlight, PV modules operate more efficiently in cold, clear conditions due to lower thermal losses.
Thus, the dominance of PV as a renewable energy source in low- and middle-income countries – many of which are in tropical or equatorial regions – remains challenged by climatic and infrastructural constraints. In addition, rising air pollution can reduce solar insolation by ~2–11 per cent, while soiling contributes a further 3–4 per cent annual loss in output.
Regular cleaning of panels is a hazardous task as under sunlight cells are electrically active and can also be water intensive. In heavily populated areas PVs can trap large amounts of heat triggering urban heat island phenomenon. While other solar technologies can partially complement PVs, its role in complete carbon neutral energy generation is an ongoing scientific debate.
What are solar photovoltaic (PV) cells and how are they used in solar panels?
Why is regular cleaning of photovoltaic (PV) panels considered both hazardous and resource-intensive?
What is the urban heat island effect, and how might photovoltaic (PV) installations contribute to it in densely populated areas?
In what ways do infrastructural and climatic constraints limit the effectiveness of photovoltaic (PV) systems in tropical and low-income regions?
(Dr. Arunangshu Das is the Principal Project Scientist at the Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi.)
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