The search for green energy has steadily been gaining traction in the past years. A research group from Oxford University, lead by physicist Henry Snaith, has recently discovered that adding cesium as an absorption material in solar cells of the calcium titanate mineral perovskite can improve the performance of the photovoltaics – the method by which semiconducting materials convert solar energy into a direct electrical current. The goal of much of today’s solar energy research is to solve the problem of inefficiency with respect to the limited solar spectrum absorption range of the cells, which prevent solar cells from utilizing the full potential of solar energy. While much research is being conducted to find new materials and innovative ways to increase the efficiency of the cells, efforts are also directed toward refining the current standards: silicon and perovskite cells. Snaith’s group found that combining the two together by replacing some of the ions in the material’s structure with cesium ions could increase the efficiency by 25 per cent when compared to current solar panels. Although this technology has yet to be marketed, and the scientists are still figuring out what sort of life expectancy we should expect from these cells, this tandem – a combination of the benefits of silicon and perovskite – could very well help overcome the obstacles provided by the current narrow solar spectrum range.
Affordable perovskite solar cells are quickly gaining ground, but they are sensitive to environmental factors, such as moisture, air, heat or – yes, strangely enough – too much sunlight, which reduces the panels’ efficiency and lifespan. The inclusion of cesium provides some stability for the solar cell against these forces of nature. “This is really a breakthrough for the field,” says Michael Graetzel, a chemist at the Swiss Federal Institute of Technology in Lausanne, although the life expectancy of these perovskite solar cells is still uncertain.
Six years ago, a group of Japanese researchers developed the first perovskite solar cell, which converted 3.8 per cent of sunlight energy into electricity. Since then, perovskites have come a long way. Only a month ago, South Korean researchers at the Materials Research Society meeting attained an efficiency of 21.7 per cent for perovskite cells, giving silicon cells, which convert 25 per cent of solar energy into electricity, a run for their money. Presently, no other photovoltaic technology has developed at such a quick rate. If the predictions of Moore’s Law on the exponential growth of technology – essentially doubling every two years – stays true in the years to come, who knows how efficient solar cells will be in, say, ten years.
Silicon cells and perovskite cells each absorb sunlight for a specific range of wavelengths. This is because the amount of extra energy required to vibrate an electron and allow it to travel across the material is different for both cells – this is known as the band gap. Perovskites are characterized by a band gap of 1.5 electronvolt (eV), absorbing blue photons; conversely, silicon cells absorb larger wavelengths, such as red photons, due to their lower band gap (1.1 eV). When both are combined in a tandem cell, the solar panel captures more of the solar spectrum, allowing more energy to be harvested. Snaith’s team is attempting to broaden this range even further by substituting some of the iodine with bromine. This increases the band gap of perovskites so that it may absorb more blue light; however, by increasing the band gap the cell also becomes more susceptible to light and heat.
Snaith estimates that these tandem cells should sooner or later surpass 30 per cent efficiency. Gallium arsenide cells are, at the moment, the only solar cells that have exceeded 30 per cent efficiency, but they will likely never be commercialized, as they are quite expensive to produce. Cesium-altered perovskite, on the other hand, are built of relatively cheap materials, such as lead and iodine, in a layered crystalline structure with a simple organic compound – either methyl ammonium or formamidinium. The fact that no high-temperature apparatus nor clean room facility is required to design these cells makes them even more attractive.
Naturally, as solar panels become more energy efficient, they will be an ever better alternative to utility companies for average consumers and, all in all, a wiser investment. For manufacturers, the barrier posed by the cost of materials and assembly should become smaller, allowing mass production to become more feasible. The moment at which a solar energy source can generate power at a levelized cost of electricity which is no higher than the price of power from an electrical grid, called grid parity, has already been reached in several Western European countries and Latin American countries, such as Chile and Argentina. A number of states in the U.S. are on the verge of doing so as well. This could be the birth of a new age of energy, one that no longer eats at the unsustainable resources of the earth and preserves the integrity of the environment for future generations.