Solar energy is hampered by the efficiency and cost-effectiveness of the photovoltaic cells. File Photo: IOL

CAPE TOWN – Environmental concerns and the rising cost of power generation have forced the world to look at sustainable and environmentally friendly energy resources. Solar energy is one of the alternatives to fossil fuels and currently contributes about 103 gigawatt to the energy needs of the world. 

Unfortunately solar energy is hampered by the efficiency and cost-effectiveness of the photovoltaic cells. This has led to numerous innovative efforts to make solar power increasingly competitive with traditional energy sources.

About 90% of solar panels in use today consist of mono- and polycrystalline silicon (c-Si) cells, which have dominated the solar market for decades. Polycrystaline silicon has cut the manufacturing costs to some extent, but unfortunately sacrificed some of the efficiency. 

The second-generation thin-film solar cells use innovative alternative materials such as copper indium gallium selenide (CIGS) and cadmium telluride (DcTe). Although they are simpler and cheaper to produce they also tend to sacrifice some efficiency. Currently these ultra thin layer materials did not have a major impact on the market, although thin-film photovoltaic innovations consisting of thin coatings of semiconductor material onto an underlying material did result in an easier incorporation of photovoltaics into buildings.

Over the years researchers have grappled with the problem of efficiency and cost-effectiveness of solar cells. The average efficiency rate of traditional solar cells is approximately 15%, meaning that 85% of sunlight is not converted into electricity. Researchers thus constantly experimented with new technologies to increase the efficiency and conversion rate of the solar cells.

However, the relative recent introduction of third generation solar cells containing perovskites may revolutionise the solar energy world. Perovskites are compound materials with a special crystal structure formed through chemistry. They are composed of calcium titanium oxide (CaTiO3) and have a high absorption coefficient that enables ultra-thin films around 500 nanometers (1 nanometer is 1 billionth of a meter). 

Interestingly enough, the first perovskite was discovered in the Ural Mountains in western Russia already in 1839 and was named after Count Lev Perovski, a Russian mineralogist. But it was only in 2006 that Tsutomu Miyasaka of Toin University in Japan discovered that some perovskites are semiconductors and could be used as the basis of a new type of solar cell.

In 2009 Japanese researchers were the first to incorporate perovskite in their solar cells, but the solar cells had low efficiencies and lacked stability. In 2012 Henry Snaith and his fellow researchers from the University of Oxford in the UK found a way to make perovskite solar cells with a light to electricity efficiency of just over 10%, significantly bringing down the cost per watt. 

Physicists are constantly enhancing the performance of solar cells to levels never before reached. Over the last two years the efficiency has increased to 20% or more, while costing much less than the silicon cells that are predominantly used today.

Dr Yanfa Yan from the University of Toledo in Spain, together with the US National Renewable Energy Lab and the University of Colorado made significant breakthroughs in the chemical formula and processing of tandem (two or more layers) perovskite solar cells. Prof Yan is convinced that full-sized tandem perovskite solar cells would soon be available in the consumer market. 

Using two or more layers of different materials allow for the harvesting of photons from different bands. The higher band gap material on the surface absorbs high-energy photons, while the lower band gap material beneath absorbs the lower-energy photons. These multi-junction cells render much higher efficiencies.

According to their research paper published in 2019 in the academic journal Science Dr Yan and his team fine-tuned a mix of lead and tin through the use of a chemical compound called guanidinium thiocyanate to markedly improve the structural and optoelectronic properties of the lead-tin mixed perovskite films.  This optimisation allowed them to reach an efficiency of 23%, which is much higher than the average efficiency of about 15% of silicon solar panels. In addition to the higher efficiency, the perovskite panels also cost less than half of their silicon counterparts.

The ultra-high efficiency tandem Perovskite solar cell outperforms any traditional polycrystalline or thin-film solar cell panel since it is able to absorb the full visible solar spectrum. It can thus easily be incorporated into new buildings without disturbing the aesthetic appearance. The new wave of solar power will most likely be incorporated in houses, office buildings, vehicles and even clothing. Soon the now famous rooftop solar panels will be old technology. 

But the quest for higher efficiency is continuing. Oxford PV in the UK is currently working on developing solar cells that could achieve conversion efficiencies of 37%. 

At the University of Toronto in Canada a team of scientists demonstrated a new type of light-sensitive nanoparticle called “colloidal quantum dots” that are about 8% more efficient at converting sunlight into electricity.

Researchers at the Imperial College University in London discovered another promising new material, called “gallium arsenide” that could make solar photovoltaic systems almost three times more efficient than any traditional solar cell. The solar cells are more efficient since they can be chemically altered to optimise the capturing of sunlight. The model also tracks the sun and use “light-pipes” to guide the sunlight into the system. 

The Massachusetts Institute of Technology (MIT) in the US announced a few months ago that they developed a new technology that could double the efficiency of solar cells. This new technology captures and utilises the waste thermal energy that is emitted by solar panels and would lead to a further decrease in the cost of solar energy.

But solar cells are not limited to rooftops of buildings anymore. Wearable solar devices are nothing new. Solar-powered watches, for instance, have been on the market for several years. However, what is new is the incorporation of tiny solar cells into textiles. At Nottingham Trent University in the UK researchers developed a way to integrate solar cells into clothing by embedding micro-solar cells into yarn that could then be woven into textiles. This innovation makes it possible for curtains to act as electricity generators.

Perovskite is so versatile that it can even be formulated as ink, which can be printed on any possible object using inkjet technology or even through the use of a paintbrush. Any printed object or painted wall can now become a source of electricity.

Solar panels are being reinvented through the use of ultra thin material with a high absorption rate. The new material has a tensile strength to meet architectural and engineering standards and improves the energy output tremendously. Due to the versatility of perovskite cell technology they will increasingly be incorporated in new windows, walls, roofs and building designs instead of the traditional panels on the roof of the building.

Although the contribution to the world’s current energy demand of about 16 Terawatts is still relatively small, perovskite solar cells may in future help to solve the world’s growing energy need. The most affordable and efficient way to harness the renewable energy of the sun is one step closer to reality and will soon replace silicon panels. However, the lifetime of the material is still unknown and researchers are working in all earnest to further improve the efficiency and stability.

With perovskite technology, solar energy is becoming more viable and affordable and could protect our planet for our children and future generations. The costs of solar energy has fallen with about 77% from 2010 to 2018, with the result that solar technology reached grid parity in many parts of the world. Due to the extraordinary increases (above the inflation rate) in the cost of electricity in South Africa, solar energy is a becoming a cost-effective alternative to consumers.

The conventional model of big, centrally distributed electricity (as in the case of Eskom) is globally being replaced by modular and evenly distributed consumer-driven power generation. Perhaps this may be part of the solution for the struggling, inefficient and expensive Eskom.

Professor Louis C H Fourie is a futurist and technology Strategist. [email protected]

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