Solar energy, the third-largest renewable energy source after hydropower and wind, has emerged as a clean, sustainable, and powerful alternative to fossil fuels. The sunlight striking the Earth is more than 10,000 times the world’s total energy use, and technologies to harvest as much solar energy as possible are surging rapidly. Since the first commercial silicon (Si) solar panels created by Bell Laboratories in 1954, the most common technologies today use different forms of Si-based solar cells and convert up to 20% of the sunlight to electricity.
According to IEA’s market analysis, the generation of solar photovoltaics (PV) — the process of converting sunlight into electricity — has reached 720 TWh in 2019 from 585 TWh in 2018 and is expected to grow up to 1,940 TWh by 2025. The current maximum global capacity of solar energy is 592 GW, contributing 2.2% to global electricity generation.
What are the current and upcoming innovative materials?
A typical solar cell consists of semiconducting materials such as p- and n-type silicon with a layered p-n junction connected to an external circuit. Sunlight illumination on the panels causes electron ejection from silicon. The ejected electrons under an internal electric field create a flow through the p-n junction and the external circuit, resulting in a current (electricity). With a swiftly growing market and the development of creative applications, R&D on innovative solar energy materials is at its peak to achieve maximum solar-to-electricity efficiency at low cost. Three types of highly investigated semiconducting materials of today are crystalline Si, thin films, and the next-generation perovskite solar cells (PSCs).
Crystalline silicon (c-Si) is the most used semiconducting material in solar panels, occupying more than 90% of the global PV market, although the efficiency is significantly under the theoretical limit (~30%). Solar cells made of alternative low-cost and high-efficiency materials are emerging.
The National Renewable Energy Laboratory (NREL) is driving the development of high-efficiency crystalline PVs, which includes III-V multijunction materials (with target efficiency of >30%) and hybrid tandem III-V/Si solar cells. Their six-junction III-V solar cells have reached an efficiency of 47.1% under concentrated light. Moreover, Si-based bifacial technology can harvest solar energy from both sides of the panel, with 11% more efficiency compared to standard panels.
Second-generation thin-film solar cells are appearing as one of the most promising PV technologies due their narrow design (350 times smaller light-absorbing layers compared to standard Si-panels), light weight, flexibility, and ease of installation. Typically, four types of materials are used in their construction: cadmium-telluride (CdTe), amorphous silicon, copper-indium-gallium-selenide (CIGS), and gallium-arsenide (GaAs). While CdTe has a toxicity concern due to the cadmium, the CIGS solar cells are turning out to be the more promising high-efficiency and economic options for both residential and commercial installations, with efficiency up to 21%.
Ascent Solar is one of the top players in the manufacturing of high-performance CIGS modules, with their superlight and extreme CIGS technology being used in space, aerospace, government, and public sectors.
Perovskite Solar Cells
Among the next-generation solar cells, hybrid metal halide perovskite solar cells (PSCs) have garnered a great amount of attention due to their low price, thinner design, low-temperature processing, and excellent light absorption properties (good performance under low and diffuse light). PSCs can be flexible, lightweight, and semitransparent. Notably, perovskite thin films can also be printed, leading to scalable high-throughput manufacturing, and a recent roll-to-roll printed PSC has reached 12.2% efficiency, the highest among printed PSCs.
Notably, combined perovskite and Si-PV materials have shown a record efficiency of up to 28% under laboratory conditions, as demonstrated by Oxford PV. While stability and durability have remained a major concern, a recent low-cost polymer-glass stack encapsulation system has enabled PSCs to withstand standard operating conditions. Although PSCs are still not commercialized, they hold significant economic and efficiency advantages to drive the future of the solar energy market.
What are the breakthrough integrative solar cells technologies?
Apart from innovative materials, creative methods of harvesting maximum solar energy are also emerging. For example, Swiss start-up Insolight is using integrated lenses as optical boosters in the panels’ protective glass to concentrate light beams by 200 times while reaching an efficiency of 30%.
Another recent development is the designing of prototypes of thermoradiative PV devices, or reverse solar panels, that can generate electricity at night by utilizing the heat irradiated from the panels to the optically coupled deep space, which serves as a heat sink.
Interestingly, along with innovative materials, integrative applications other than standard rooftop installations are also rising and are currently in their infancy. For instance, solar distillation can harvest solar energy while utilizing the dissipated heat from panels to purify water, if there is an integrated membrane distillation attachment.
Another transformative technology of the future could be solar paints, which include solar paint hydrogen (generates energy from photovoltaic water splitting), quantum dots (photovoltaic paint), and perovskite-based paints.
Furthermore, transparent solar windows are highly innovative applications, and Ubiquitous Energy has achieved a solar-to-electricity conversion efficiency of 10% with their transparent materials. A demonstration from Michigan State University, a pioneer in this technology, can be seen in this video:
With the rapid development of low-cost, high-performance semiconducting materials, space-saving thin films, and easily installable technologies, the solar energy market is expected to boom in the next five years. Despite the setback caused by the pandemic, the anticipated cost reduction of 15% to 35% by 2024 for solar installations is encouraging and could make this renewable energy more affordable.
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