To advance the green energy transition, solar panels have experienced exponential growth in installations each year. However, these silicon-based devices are quite thick, using a lot of resources. This extraction and waste processing thus leads to solar panels having greenhouse gas emissions equivalent to 5 t of CO₂ per GWh.

Therefore, a lot of research is currently focused on making solar panels with perovskite, which is a material that can achieve similar efficiencies of above 25% using far less material. According to studies such as those conducted by Celik et al. and Vidal et al., they have a smaller ecofootprint than their silicon-based counterparts. Since they struggle with long-term performance issues, quite some research is still needed before they are fully marketable. Thus, thousands of researchers work on improving perovskite solar panels to contribute to the development of this more environmentally-friendly alternative.

Though, let us remove our green-tinted glasses, shall we?

Despite its advantages, perovskite faces many issues. For example, the best-performing perovskite is lead-based, which is highly toxic to humans and the environment. Even though efforts are made to encapsulate it, eventual leaks are inevitable.

Furthermore, when making small solar panels in the lab, the most popular technique is spin-coating. Here, the perovskite is prepared in a solution which is then pipetted onto the unfinished solar panel. Then, the panel is spun rapidly to remove all but a thin layer of solid perovskite. This ensures the thinness of the panels but also means that a lot of the material is quite literally thrown away (see picture below), causing a lot of waste.

The solutions themselves used for making the perovskites also present a problem: The main solvent, dimethyl formamide, is classified in the worst category for green solvents by Pfizer, GSK, and Sanofi. The other solvent used, dimethyl sulfoxide, is listed in the middle category by all three companies, though this still means a substitution is advised.

What are the alternatives?

Some research groups look into replacing lead with a different metal, the most popular being tin. However, the resulting efficiencies are far lower than those for lead-based perovskites. Therefore, some groups research using a mixture of lead and tin to at least reduce the toxicity. This has somewhat more promising results, as efficiencies of above 22% have been reached.

For making perovskite solar panels on a large scale, a manufacturing method other than spin-coating is needed. Instead of spinning the panels, a popular idea is to essentially print the layers onto the panels. This, too, needs further refinement, though: Only specific components are compatible with an ink format. Additionally, the way the perovskite layer is formed changes, meaning that the methods used for the record-breaking perovskite solar panels may not be suitable for large-scale fabrication.

Some research has also gone into reducing the toxicity of the solvents. Even if a substitution still is advised for most of the solvents used, Doolin et al. developed a mixture of solvents without any components with major issues. Using this modified solution, they reached practically identical efficiencies as when using dimethyl formamide. However, their reached efficiencies lie well below the standard for high-performing perovskite solar panels, potentially indicating an incompatibility with the manufacturing methods for commercial devices.

What does this all mean? Should we stop producing silicon solar panels until greener alternatives are ready for the market?

Based on all the aspects that could be improved upon in current solar panels, one could be very tempted to wait until we get things right. Indeed: If governments continue pouring their resources into silicon solar panels, we might face a (smaller but still relevant) path dependence, as we currently face with the fossil fuel industry. Even if the silicon solar panels must be taken out once their lifetime of 20-25 years runs out, it may be easier to replace them with other silicon ones as the factories are equipped to produce those. If the trend of rising production of silicon solar panels is continued, it will become more and more difficult to change to greener options.

However, it may take some time before truly green alternatives are available for the market. Large-scale production of perovskite solar panels is barely beginning now, with only a few factories producing them. This means holding back on silicon solar panel production would inadvertently benefit the fossil fuel companies as they will supply the energy to meet demands. In other words, waiting for a greener solution would play right into the hands of those who seek to delay climate action.

So… what now?

It is clear that halting solar panel production is not an option. Instead, methods must be found to not only replace silicon with perovskite but also improve the perovskite production itself. Even if not identical, the general methods for making different kinds of solar panels are quite similar. This means that changing the infrastructure will not be nearly as big as moving away from fossil fuels.

Perhaps, before perovskite fully steps into the role that silicon plays, it can be used as an additional tool in the energy transition. When printing perovskite, due to its thinness, it can be produced as a flexible film (see picture below). This means it can be used in applications where the rigidity of the silicon solar panels would otherwise inhibit the application of photovoltaics.

Furthermore, it is possible to combine silicon and perovskite layers within one solar panel to absorb a wider range of sunlight wavelengths. This leads to an overall higher efficiency, which would lower the overall ecofootprint per generated energy unit. As this technology is foreseen to have a considerable market share within the next two years, it presents an additional interim solution until greener options are more accessible.

After all, it does not make sense to push for the greenest technology if it is not affordable and cannot cover people’s needs. To make environmentally friendly solar panels, they cannot only be green: they have to be sustainable. As we strive for sustainable development, we must, for now, use the somewhat dirtier silicon solar panels to replace the outdated fossil fuel technologies. In the meantime, research and development efforts should be focused on making truly green solar panels a sustainable option in the near future.