Recycling Perovskite Solar Cells

Recycling Perovskite Solar Cells

By Judy Tsao, Member-at-Large for the GCI

Solar energy is arguably the most abundant and environmentally friendly source of energy that we have access to. In fact, crystalline silicon solar cells have been employed in parts of the world at a comparable cost to the price of electricity derived from fossil fuels.1 The large-scale employment of solar cells, however, remains challenging as the efficiency of existing solar cells still needs to be improved significantly.

An important recent breakthrough the field of solar cells is the use of perovskite solar cells (PSC), which includes a perovskite-structured compound as the light-harvesting layer in the device (Figure 1). Perovskite is a name given to describe the specific 3-D arrangement of atoms in such materials. Even though the first PSC was reported only in 2009, its power conversion efficiency (PCE) has already been reported to exceed 20%, a milestone in the development of any new solar cells which typically takes decades of optimization to achieve.2

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Figure 1. Thin-film perovskite solar cell manufactured by vapour deposition (photo credit: Boshu Zhang, Wong Choon, Lim Glenn & Mingzhen Liu)

PSC has several advantages compared with traditional solar cells, including low weight, flexibility, and low cost.3 There are, however, several challenges that must be overcome before PSC can be brought to the market. The most common PSC to date includes CH3NH3PbI3 and related materials, which contain soluble lead (II) salts that are toxic and strictly regulated.

Interestingly, there has been a consensus in the literature that the lead content in the perovskite layer is not actually the main issue in the environmental impact of PSC production.4 Part of the reason for this conclusion is simply that the thickness of perovskite layer required would amount to less than 1000 mg of lead in one square meter of material. This value is only modest compared to lead pollution from other human sources such as lead paints or lead batteries.5

The main environmental concerns regarding PSCs appear to lie in the use of gold and high temperature processes during the manufacturing of the devices.6 It has thus been suggested that, in order to reduce the environmental impact of PSCs, recycling of raw materials is very important. In a recent study by Kadro et al., 7 a facile protocol for the recycling of perovskite solar cell was developed. The entire procedure takes place at room temperature and takes less than 10 minutes (Figure 2).

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Figure 2. Schematic process for recycling PSC components [7].

As it turns out, components of a fully assembled PSC can be extracted by sequentially placing the device in different solvents. Step 1 of the procedure uses chlorobenzene to remove the gold layer, while step two uses ethanol to dissolve CH3NH3I. This then leaves PbI2 to be the only component remaining on the device, which can be removed by just a few drops of N,N-dimethylformamide. It is also worth noting that the recycled materials can be fabricated into a complete PSC again without significant drop in performance.

Even though the discovery of PSC has only been made less than a decade ago, its potential in applications in photovoltaics has been underlined by numerous studies. It is especially gratifying to see that the environmental impacts of such devices are already under active research before PSCs are introduced to the market. While these studies have demonstrated that PSCs have low environmental impacts when properly recycled, there are other challenges still facing researchers in this field. In particular, the short lifetime of such devices needs to be improved to match that of traditional silicon-based solar cells. Nevertheless, the facile method of recycling PSCs without compromising the performance will certainly make them even more competitive than traditional solar cells.

References:

  1. Branker, K. et al. Renewable Sustainable Energy Rev. 2011, 15, 4470.
  2. Yang, W. S. et al. Science, 2015, 348, 1234.
  3. Snaith, H. J. Phys. Chem. Lett. 2013, 4, 3623.
  4. Serrano-Lujan, L. et al. Energy Mater. 2015, 5, 150119.
  5. Dabini, D. Phys., 2015, 6, 3546.
  6. Espinosa, N. et al. Adv. Energy Mater. 2015, 5, 1.
  7. Kadro, J. M. et al. Energy, Environ. Sci. 2016, 9, 3172.

 

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A Green Iodine Clock Reaction

By Judy Tsao, Member-at-Large for the GCI

The iodine clock is a popular chemistry demonstration among high school teachers and science demonstrators. In this activity, two clear and colourless solutions are mixed together, which leads to no initial observable change. However, after a short period of time the mixture abruptly turns deep blue.

Clock Reaction

This variation of the iodine clock reaction has the same dramatic colour change as the traditional demo!

In addition to the dramatic colour change, this activity also serves to illustrate important concepts in reaction kinetics. The time it takes for the colour change to take place can be altered by varying the concentration and temperature of the solutions. A number of similar procedures can be found online, yet the overall reaction always consists of the same three steps: 1) In the first step, iodine is reduced to iodide, 2) iodide then gets oxidized back to iodine, and finally 3) the iodine reacts with the starch contained in the mixture to give the deep blue colour. The first step is faster than the second step, thus no change is observed until all the reducing agents in the solution are consumed and step 1 cannot occur anymore, allowing step 2 to proceed to step 3.

While the reagents required for the iodine clock reaction can be easily found in a chemistry research lab, they are not necessarily the most user-friendly when it comes to doing this demonstration in a public setting. Typically, one solution containing sodium bisulfite, potassium iodide, and soluble starch and a second solution containing 20% hydrogen peroxide and concentrated sulfuric acid are employed in the demonstration. I began doing chemistry demonstrations at local high schools and quickly realized that this experiment would be challenging as it is not feasible for me to use some of these reagents; both 20% hydrogen peroxide and concentrated sulfuric acid are corrosive and must be treated before disposal.

Clock reaction ingredients

All the reagents needed for this modified procedure of the iodine clock demonstration.

Luckily, I stumbled upon a very well written paper published in the Journal of Chemical Education where the authors managed to achieve the same demonstration using only consumer products. With the authors’ modifications, no concentrated acid or peroxide is needed. I was able to purchase all the required reagents at Shoppers Drug Mart and the demonstration can be safely repeated by students at home. The first solution in the new procedure is made up of vitamin C powder dissolved in water and a few drops of tincture of iodine, while the second contains 3% hydrogen peroxide and laundry starch. The same results are achieved – a dramatic colour change that gets students excited about chemistry. The difference now is that I could comfortably carry all the required chemicals on public transit and could rinse all of the waste from this demonstration safely down the sink.

The transportation of reagents and the safe disposal of chemicals are often two of the biggest challenges facing chemistry demonstrators. Putting green chemistry principles into practice by eliminating the use of toxic chemicals and hazardous waste is an excellent way to help mitigate these challenges. Additionally, it helps students understand the importance of green chemistry at an early stage in their learning career.

References:

“The Vitamin C Clock Reaction”, S. W. Wright and P. Reedy, J. Chem. Ed. 200279, p 41.