The use of solar energy is expanding worldwide but the efficiency of silicon solar cells has made very little progress in the last few decades. Could perovskite solar cell be the answer to high-efficiency solar power?
Conventional solar cells today are made of silicon with power conversion efficiencies that have been stagnating in the low-20% range for about 15 years. This might strike you as bizarre, since glancing at data from the National Renewable Energy Laboratory (NREL) reveals theoretical efficiencies that approach 50%.
Reported timeline of solar cell energy conversion efficiencies since 1976 from the National Renewable Energy Laboratory
Solar efficiency refers to what portion of sunlight energy is converted to electricity via the use of photovoltaic technology.
So why are we still using what might appear to be dinosaur technology when it comes to relying on low-efficiency silicon solar cells? As usual, a gap is present between application (in the context of the real world) and theory (think idealistic sterile laboratory conditions). For instance, Sharp Corporation announced in April 2013 they had achieved the world's highest solar cell conversion efficiency (44.4%) with their concentrator triple-junction compound solar cell.
These high-efficiency solar cells are made with proprietary technology that utilizes three photo-absorption layers made of several elements - including indium gallium arsenide, indium and gallium. A lens-based concentrator system focuses sunlight on the cells to generate power.
A 44.4% seems like a huge improvement over the typical silicon cells used in the market today but one solar cell measures a mere 0.165 cm2. That's where the majority of the problem lies: keeping the efficiency of a solar power cell while increasing its surface area is an engineering project of epic problems. According to the former Chair of the UK Office for Renewable Energy, Bernie Bulkin, scaling Copper Indium Gallium Selenide (CIGS) solar cells to a usable size, for instance, would cost just about a billion dollars.
So how do we maintain the high-efficiency of a photovoltaic solar cell while enlarging its surface area and using it an outside environment subject to weather conditions - thunder, lightening, or rain and whatever else those three Macbeth witches proclaimed?
The answer to producing high efficiency, scalable photovoltaic solar cells may lie in the development of perovskite solar cell technology.
Perovskite, which are not Russian delicacies contrary to what their name might suggest, are cubo-octahedral crystals - cubes with their corners cut off - and have six octagonal and eight triangular faces. They are a broad class of materials in which carbon and hydrogen (and other organic molecules) form a three-dimensional crystal lattice by binding with a metal (such as lead) and a halogen (such as chlorine).
When it was first introduced, perovskite solar cell technology had an embarrassingly low solar efficiency. In 2009, perovskite solar cells composed of lead, iodide and methylammonium converted just about 3.8% of sunlight into usable electric power. But the technology has been tenacious: perovskite solar efficiency had soared to about 20% by the year 2014. The pace at which these perovskites are improving is outstanding - partially due to the fact that thousands of chemical compositions are possible when assembling the three-dimensional crystal lattice recipe for a perovskite solar cell.
Perovskite occurs naturally as a mineral - calcium titanium oxide - but other combinations of elements can adopt the exact same shape. Thus scientists can experiment with different chemical compositions to see what elements end up absorbing which frequency of light most efficiently.
There are various advantages that perovskites present over traditional silicon solar cells. While silicon dioxide (SiO2) is plentiful in the form of beach sand, separating the oxygen molecules attached to the silicon requires a gargantuan amount of energy. Melting silicon dioxide in an electrode arc furnace at high temperatures ranging between 1500 to 2000 degrees Celsius is required, which paradoxically releases more carbon dioxide emission into the atmosphere and also creates a fundamental limit on the production cost of silicon solar cells.
Furthermore, perovskite solar cells can be manufactured at much l0wer costs. In fact, Dr. Henry Snaith of Oxford's University and co-founder of Oxford Photovoltaics, predicted in 2012 that using perovskites would eventually cut the cost of solar energy by three quarters. This would threaten the commercial advantage that fossil fuels have had over the power supply market for decades.
Another complication of silicon photovoltaic cells is that they are heavy and rigid; they work most effectively when positioned flat in large, heavy panels as seen on large-scale installations such as rooftops and solar farms. These heavyset panels contribute to the high costs of assembling silicon photovoltaic arrays and modules.
By mixing up batches of liquid solutions, manufacturers can deposit light-weight thin films of perovskites on any type of shape and without the use of an electrode arc furnace. This eliminates high production costs on top of the production of toxic greenhouse gases.
So far perovskites are much more cost-efficient than silicon photovoltaic cells, don't contribute to global warming with the release of carbon dioxide and are much more flexible and manageable. So about their conversion efficiency?
Since combining materials leads to capturing a broader spectrum of sunlight (meaning that photons belong to a wide range of visible-light wavelengths), some researchers have been focusing on creating a "multijunction cell" - a combination of perovskites with silicon. For instance, methylammonium lead tri-halide perovskite and silicon solar cells could form together a highly efficient pair: the perovskite solar cell on the top layer is designed to capture short wavelength photons, while a silicon solar cell at the bottom focuses on capturing longer wavelength photons.
In 2015, a research team at Stanford University published research in the journal Energy & Environmental Science, showing that stacking perovskites onto a silicon cell could boost the solar system's overall efficiency.
By combining a silicon cell with a 11.4% efficiency with 12.7%-efficiency perovskites, the Stanford University research team, led by Michael McGehee, created a 17%-efficiency hybrid solar cell. However Michael McGehee did pessimistically add that they had "ways to go to show that perovskite solar cells are stable enough to last 25 years."
In April 2016, a new breakthrough was made by the Hong Kong Polytechnic University: the world's highest power conversion efficiency of 25.5% was reached with the development of perovskite-silicon solar cells.
Led by Professor Charles Chee Surya and Clarea Au Endowed Professor in Energy of the Department of Electronic and Information Engineering, the research's goals have been focused on developing innovative ways to improve the conversion efficiency of solar energy. This could lower utility costs of $0.5 per watt - the average cost of electricity generated by silicon solar cells - to $0.35 per watt.
The Hong Kong Polytechnic University's research was able to achieve this high-efficiency with new innovative ways - including the assembling of a perovskite layer made of molybdenum trioxide, gold and molybdenum trioxide, each designed with an optimized thickness. The transparency of these materials optimize the passage of light through the perovskite triple layer in order to be absorbed into the silicon part at the bottom of the solar cell. The silicon portion of the photovoltaic cell was designed by silicon efficiency expert Professor Shen Hui of Sun Yat-sen University and Shun De SYSU Institute for Solar Energy.
Also adding to the research's ability to boost the efficiency of energy conversion is the addition of a low-temperature annealing process that minimizes the impact of defects in the perovskite material, as well as the development a haze film designed to trap more light into each solar cell.
Co-founder of Oxford Photovoltaics Dr. Henry Snaith announced in March 2015 that perovskite solar technology would be on the market by 2017.
Oxford Photovoltaics has large dreams when it comes to perovskites. Their plans include taking advantage of the material's transparency and coating office buildings with perovskite solar cells. According to estimates, this architectural idea, not unlike Polysar's plans to turn buildings into power plants, would allow a 35-story building in London to generate about 60% of the electricity that it uses.
The fact that perovskite solar cells are now competing efficiency-wise with silicon cells is remarkable. Reaching a 20% efficiency (let alone 25%) is a milestone in solar power technology, and one that has taken other types of solar cells decades to accomplish. While silicon solar cells are now considered to be a mature technology, the progress of perovskites continues to bloom. In seven years, the efficiency of perovskite solar cells has increased five-fold, and has doubled in just the past two years. Many predict that their efficiency will keep reaching new heights in years to come.
Problems still need to be addressed with perovskites, such as their durability in weather conditions. Silicon PV cells are extremely resistant, which is not the case for their perovskite cousins; these remain susceptible to water, air and light. In fact, their fast degradation explains why the NREL energy chart above labels perovskites as "not stabilized".
Furthermore, the question of how to produce perovskite solar cells in large quantities in a way that competes with silicon technology is still a question mark. But even small supplies of perovskite solar cells could help bring solar power in developing areas that have yet to be connected to any electrical grid.
With its exponential increase of energy conversion efficiency, its low production costs, environmentally friendly engineering methods and its "shape-shifting" flexible abilities, the potential of perovskite solar cells is promising and bright. So who knows - the clunky, rigid silicon photovoltaic cells stuck in their heavy frames sprawled out in vast endless fields may just soon be a thing of the past.
Photo Credits: under CC license
Read more on UnderstandSolar.com. This post was written by one of its writers, Dr. Kat
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