Perovskite Solar Cells Show Promise as Researchers Mitigate Challenges with Production

Solar panels have continued to increase in efficiency and are becoming cost-competitive against non-renewable sources of power. While silicon-based solar systems have been the most accessible so far, perovskite-based cells may steal the spotlight in coming years.

Silicon solar panels have undergone extensive research and development with solar cells costing less than $1 per watt at efficiencies of up to 20%. However, unless silicon panels can continue to rise in efficiency and lower in cost then non-renewable sources such as coal, oil, and gas will remain the dominant option for electricity generation.

While renewable sources of energy are presented as a cleaner alternative to fossil fuels, there are some drawbacks to solar panels that may not be as environmentally friendly as people may think.

Creating silicon wafers for solar cells requires a large amount of energy as the silicon needs to be heated to 1000 degrees centigrade and kept at this temperature for up to an entire month as the seed crystal is drawn from the molten material. Such high-temperature requirements directly result in high electrical power requirements and, since most power on the planet comes from non-renewable sources, this translates to CO₂ production. So the production stage, itself, is an energy-hungry venture. Silicon panels also contain chemicals that are potentially hazardous to the environment (the dopants used to change the electric charge of the silicon), and so when solar panels go through their end-of-life cycle, they require proper handling or they risk releasing pollutants.

Getting around environmental, efficiency, and cost problems associated with silicon solar panels has been a gradual process for the past 40 years. Now, however, researchers have been pursuing alternatives to silicon-based cells. One of the major alternatives is based on the perovskite crystal.

The Perovskite Panel

Perovskite solar cells, unlike silicon cells, have a very different chemical structure. These cells include a perovskite-structured compound which has a structure identical to the crystal structure of calcium titanium oxide. Often, these types of cells are based on organic-inorganic lead or tin halide-based materials as they layer that receives incoming photons. Importantly, perovskite panels often have a broad absorption spectrum and can often absorb most (if not all) visible light. 

Back in 2009, these solar cells had efficiencies of 3.8% but as of 2017 they have gone beyond common silicon cells and are currently around 22.7% efficient. In the interest of increasing this efficiency, research teams from both the University of Potsdam and the HZB (Helmholtz-Zentrum Berlin) have been working to better understand where inefficiencies occur. Their research found that the interface between the perovskite crystal and the layer of the solar cell that allows the transportation of electrons can be defective, causing the loss of important charge carriers. This highlights one of the major drawbacks associated with perovskite, which are presently being addressed by researchers across the globe.

Cost, Longevity, and Materials Sciences: Perovskite's Challenges and Solutions

Perovskite cells, for all their benefits, have some major problems that have also plagued silicon. Among these problems are efficiency, hazardous materials, and making perovskite cells a viable option for mass production. The good news is that researchers around the globe are presenting possible solutions for these issues.

Reducing Hazardous Materials

Like silicon, perovskite cells are often made with toxic and even potentially carcinogenic materials that could make them very harmful to humans. One chemical in particular that causes problems is PbI (lead iodide) which is a breakdown product as the cell degrades over time. PbI is very toxic and will cause acute and chronic consequences similar to lead poisoning. To keep these dangerous chemicals sealed in the cell would require further materials which would drive up their cost.

However, researchers may have solved the lead toxicity issue where a research team from KAIST in South Korea has proposed a new material, CS2AU2I6, which does not use lead. While the main materials in these cells are low cost and easy to manufacture, the electrodes used are often materials such as gold which have a very high market price, making cost a stubborn challenge to overcome.

Lead Iodide is a very toxic chemical. Image courtesy W. Oelen [CC BY-SA 3.0]

Increasing Efficiencies with New Manufacturing Methods

Producing perovskite solar cells has proven difficult in terms of manufacturing. In hopes of mitigating this issue, researchers are looking for more viable production methods for perovskite cells.

As discussed above, one of the major causes of inefficiency in perovskite is the interface between the perovskite and the transport layer. This is currently being actively addressed by a new method of applying the transport layer.

In June, NYU's Tandon School of Engineering announced a new concept for perovskite cell production developed by researchers from their department, headed up by Professor André D. Taylor, in conjunction with Peking University, the University of Electronic Science and Technology of China, Yale University, and Johns Hopkins University. The researchers sprayed a layer of PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester) onto perovskite cells to act as the electron transport layer or ETL. 

In a solar cell design known as the p-i-n structure, the electrons in the negatively-charged ETL are excited when light hits its surface and are ushered along the light-trapping "intrinsic" layer (the perovskite crystal) through to a positively-charged hole transport layer (HTL). One of the issues with this p-i-n structure so far has been that the ETL is difficult to consistently place atop the perovskite crystal. 

The NYU design for the airbrushed perovskite solar cell. Image courtesy of NYU.

In essence, the researchers accomplished higher conductivity for the electron transport layer atop the perovskite crystal by making it more uniform with their airbrush method. This change reportedly returned a 30% gain in power conversion efficiency, from 13% to 17%, and resulted in fewer defects in the ETL (which, again, can cause costly issues when mass producing cells).

Increasing Longevity

Another problem currently being investigated is that perovskite cells have a very short lifespan. At present, the National Renewable Energy Laboratory produces cells that last 5,000 hours. To put that into perspective, the cell will last 208 days whereas traditional panels have a lifespan of 20 years (keep in mind that solar panels can likely last longer much longer than this).

The reason for such a short lifespan is the fact that perovskite panels are highly susceptible to moisture and are easily broken down. Unless these panels can be made to last much longer, perovskite cells may be completely uneconomical as panels would require changing once every six months. Having said that, the first cells lasted a few hours and researchers are working to find methods to improve the lifespan.

In April, doctoral candidate Armi Tiihonen defended her dissertation at Aalto University in Finland on this subject. Along with fellow researchers, Tiihonen has been working to develop perovskite cells with longer lifespans and that are also easier to monitor for signs of wear. As dye-sensitized perovskite cells age, they turn from a dark color to yellow, indicating wear. According to Tiihonen, reducing the factors that cause degradation in perovskite cells, such as water and UV light, was important but not as changing the electrolytes used in these dye-sensitized cells. By changing the electrolyte composition from iodine to cobalt, fellow researcher Kati Miettunen reported a "tenfold increase in the lifetime of solar cells."

Combining Strengths: Perovskite Inks and Perovskite and Silicon Hybrids

Perovskite panels could be the key to future solar technology but, even though they have high efficiencies, are cheap to produce, and can be applied in thin film form, they still need to solve their toxicity issues and their incredibly short lifespan to be taken seriously as a renewable alternative.

If perovskite cells cannot become accessible on their own steam, however, there may be other viable paths forward. 

Researchers have managed to develop perovskite cells in form of ink that can be painted to surfaces to create a thin film solar cell. One idea for implementation is to coat existing silicon solar cells in this ink an effort to increase the overall efficiency of the two panels (since thin films are often transparent).

Perovskite ink that can be painted on silicon solar cells for additional efficiency. Image courtesy NREL

This method would basically improve the efficiency of silicon without needing to alter the original panels much.

But coating silicon with perovskite may not be the only option in joining the two materials together. In June, researchers from EPFL (École polytechnique fédérale de Lausanne) in France introduced the concept of coating the silicon crystal, itself, with perovskite. This silicon-perovskite hyrbid was reportedly able to achieve 25.2% efficiency. 

Whether perovskite solar panels replace silicon or whether the two materials are combined, the future of solar is looking sunnier all the time.