Solar photovoltaics (PV) utilise solar cells to convert light (photons) into electricity (voltage). Bell Labs showcased the first practical silicon solar cell in April 1954, using these cells to power a small toy Ferris wheel and a solar-powered radio transmitter. The demonstration achieved a modest 6 per cent efficiency in converting solar radiation into electricity. Photovoltaics have come a long way since then and are now used for space satellites, as well as household items such as calculators and watches.
Currently, in addition to its earlier applications, massive volumes of solar cells are being used to produce solar modules for power generation. Due to rapid advancements in solar PV technology, power generation from solar cells has become one of the cheapest ways of generating electricity in many parts of the world. As a result, solar power has emerged as one of the fastest-growing segments in the energy industry. Improvements in materials, processes and designs have resulted in the development of solar cells with high efficiencies, reaching up to 40 per cent. These upgrades are crucial for achieving higher solar power production and greater cost-effectiveness.
Types of PV technologies
The most widely used solar cells today are based on crystalline silicon, where silicon atoms are connected to form crystal lattices. Crystalline silicon cells can be categorised as either polycrystalline, which is made from several silicon fragments melted together, or monocrystalline, which is made from single crystals of silicon. The monocrystalline cell has a higher efficiency as compared to polysilicon because it is composed of a single crystal with more room for electrons to move. The majority of the advancements in the solar power segment are focused on silicon-based solar cells, which currently dominate the global solar PV market.
Passivated emitter and rear cell (PERC) is a modified version of a silicon solar cell and has become a globally utilised PV technology. PERC solar cells have a reflective passivation layer on the back surface field (BSF). This layer reflects unused light across the solar cell, allowing more sunlight to be captured for increased efficiency. PERC can be either n-type or p-type depending on the number of electrons. Another PV technology, passivated emitter rear totally diffused cells, has a diffused back surface, unlike conventional cells with aluminium-alloy BSF. In tunnel oxide passivated contact solar cells, a layer of tunnel oxide and polysilicon is added to the rear side to increase voltage handling capabilities and energy generation. Another common PV technology is thin-film solar cells, which are made from thin layers of semiconductor materials such as cadmium telluride or copper indium gallium diselenide. These thin-film solar cells are made by the deposition of semiconductor materials onto a supporting material, such as glass, plastic or metal. The perovskite solar cell is a novel thin-film cell that consists of materials that are printed, coated or vacuum-deposited onto a supporting layer.
A technology that is gaining traction across the globe is bifacial. Bifacial modules can generate electricity from both the front and back of the modules. This is made possible by harnessing the sunlight reflected from the ground onto their rear surfaces, resulting in increased energy production. Consequently, they produce more solar power than monofacial modules. Bifacial solar modules are commonly based on silicon cells, and progress in the bifacial thin-film cells segment is limited. Meanwhile, heterojunction solar cells consist of a crystalline silicon cell sandwiched between two layers of amorphous thin-film cell, thereby increasing the efficiency of solar cells.
Other solar PV technologies include the III-V solar cell, organic solar cell, concentration PV, quantum dots and multijunction PV. The III-V solar cell is named after the elements that were used to make them – elements from Group III and Group V of the periodic table. These cells are quite expensive to manufacture but are highly efficient and are often used in satellites and unmanned aerial vehicles. Meanwhile, organic solar cells are composed of carbon-rich compounds, while concentration PV works by concentrating sunlight using a mirror or lens onto a solar cell.
Quantum dot solar cells are made of tiny nanometer-sized semiconductor particles deposited on a substrate using methods such as spin-coating, spraying, or roll-to-roll printers. Additionally, multiple layers of semiconductors are stacked to create multijunction cells, which demonstrate higher efficiency compared to single-junction cells that utilise only one semiconductor.
Innovations and advancements
As the global demand for solar cells and modules continues to rise, there is a strong emphasis on improving their efficiency and cost-effectiveness. Thus, ongoing research efforts are focused on reducing the materials used in manufacturing, decreasing their thickness and finding alternative options for expensive materials, while simultaneously improving efficiency. For instance, since metallisation pastes with silver are quite expensive, manufacturers are working to reduce the consumption of silver, and even replace it entirely through alternatives such as copper. Further, larger cell formats are being used to increase module power.
The cell is typically the most expensive component of a module, and other non-cell materials also contribute significantly to the overall module price. Glass constitutes the largest portion of a module, and manufacturers are focusing on reducing the thickness of the glass to achieve two primary goals: lowering material costs and reducing the weight of the module. Similarly, copper wires that dominate the cell interconnection technology are also expected to have reduced thickness in the coming years.
Polysilicon is the most expensive material in a crystalline silicon solar cell, and thus, impacts the overall price of solar modules. There is extensive ongoing research to make wafers with significantly less polysilicon by improving the yields of crystallisation and wafering processes. Thinner wafers and improved material recycling rates are currently contributing to cost savings for solar cells.
Outlook for India
There has been a significant emphasis on improving domestic manufacturing capabilities in India, as the country aims to install over 200 GW of new solar power capacity. The aim is to reduce the dependence on imported solar cells and modules for these upcoming installations. Thus, basic customs duty of 25 per cent on solar cell imports and 40 per cent on solar module imports have been implemented from April 1, 2022 onwards to make imports more expensive than local products.
An attractive production-linked incentive scheme has been launched to incentivise the domestic manufacturing of solar cells and modules and to create vertically integrated supply chains for high-efficiency solar products. Under this initiative, two tranches of successful capacity and amount allocations have already been conducted. As a result, a total of 48,337 MW of capacity has been sanctioned, with a cumulative support of over Rs 185 billion by the government.
While these initiatives are encouraging steps to promote local manufacturing, the focus should be on creating efficient and quality products that can last the entire 25 years lifetime of solar projects. Furthermore, these products should incorporate the latest technologies to prevent obsolete cells and modules from entering the market. With the solar PV technology landscape evolving rapidly, manufacturers must continually innovate and align with global trends in order to compete on the world stage.