In recent times the focus on renewable energy has increased exponentially. Energy consumption is surging and solar energy’s contribution is expected to grow from 19% to 29% by 2050. Renewable energy is estimated to make up over 50% of generation by 2035. The solar and wind energy generation put together is expected to reach 30-50% of the total share generated.  Growth in Solar Energy will be fueled by improved performance resulting in reduced cost of the generated solar power. We have already witnessed that in many countries the levelized cost of electricity has breached the limits of coal-based power generation and this trend is going to continue. This will make renewables cheaper than coal and gas in most regions before 2030.

To meet these future goals, scientists and engineers are working tirelessly to improve the performance of renewable energy systems and are working to reduce the cost of solar power generation by innovating new designs and material combinations in photovoltaic cells. Presently, the multi-junction solar cells hold the records for best cell efficiency of 39.2%; however, the use of multijunction devices are limited to space technologies due to complex fabrication processes. Best efficiency of crystalline silicon based solar cells are ~ 26.7%, whereas for thin film technology it is ~23.4%.Many of the above solar cell technologies are mature, and some of them are already in production. However, the production process for these cells are complex and energy intensive.

Researchers were working on a couple of solar photovoltaic cell technology which were inexpensive and could be easily fabricated to make solar cells. Such different  solar cell technologies have been grouped as Emerging PV technologies. Organic solar cells, dye-sensitized solar cells, quantum dot solar cells. Perovskite Solar cells (PSC’s) are few examples. Out of these, PSC is one such technology which shows very promising results and has capabilities to meet the efficiency of c-Si solar cells.

Perovskite describes a large category of compounds having crystal structures like calcium titanium oxide (CaTiO3), which was first discovered in the Ural Mountain, Russia. The materials being used for newly developed Perovskite solar cells (PSC) have their structure in the form of ABX structure where A and B are cations and X is the bonding anion. This material can absorb sunlight with a hundred times thinner active layer than our traditional silicon solar cells. This three-part structure makes it a mix and match case where we can have multiple compounds by substituting any of the constituents. Rate of increase in the performance of the PSC’s has left all other solar cells technologies far behind. In a decade of its inception, PSC’s have already crossed the efficiency levels of established thin film solar cells technologies like cadmium telluride (CdTe) or copper-indium-gallium-selenide (CIGS) and reached the level of crystalline silicon (c-Si) solar cells.

The best reported efficiency for the single junction PSC’s has been reported at 25.2% by the A team led by Prof. Michael Saliba at KRICT2 whereas the theoretical efficiency for these PSC’s are about 31% .  Interesting results have been reported by researchers when they used the Perovskite in tandem with other solar cell technologies like c-Si or CIGS. CIGS-perovskite tandem cells have shown efficiencies up to 21.5%. Further, when combined with c-Si technology the Perovskite- c-Si tandem cells have shown efficiencies up to 28% and have outperformed both silicon single junction and perovskite solar cells. These efficiency numbers are close to the efficiency of 32.8% for gallium-indium-phosphide/gallium arsenide (GaInP/GaAs) tandem solar cells which have a more complex processes and a higher manufacturing cost. The results show that the Perovskite/silicon tandem cells’ efficiencies can be pushed toward 30%4. The below table shows the comparison of non-concentrator based solar cells.

Table 1: Comparison of different Solar cell technologies

The best part of the PSC’s is that the active layers are solution-processable using inexpensive processes like spin coating, spray coating printing and many more, at low processing temperature. This makes it suitable for commonly used substrates including flexible plastics.  This flexibility in choosing substrates and processing technologies makes the device suitable for unconventional applications as well as for new applications like IOT.

Initial PSC devices have poor stability and there is a sharp drop in observed performance due to hydro-decomposition as well as thermal decomposition. Researchers are working on various methods to improve the stability of PSC devices like modifying the chemical constituents or structure of the perovskite, modifying the charge transport layers, interface optimization, encapsulation etc. M. Saliba and M. Gratzel ET al. reported that PSC devices were able to retain 95% of its initial power after 500Hrs sun-soaking at 85°C. However, more work is needed to make them as reliable as current c-Si based PV modules.

Scaling up of PSC devices are another challenge which is mainly caused due to non-uniform coating of chemicals in the solar cells. The researchers are working on various methods to have a pin hole free uniform perovskite layer on a larger area. The work includes interface and chemical molecular engineering.  There are already a couple of organizations who have started putting up production set-ups for making large area PSC’s. However, this will take some more time to have a reliable and practically large area.

At present, the Perovskite seems to be at the same junction where the polymer solar cells were in year 2011-12, where the issues related to scaling up and stability were the main concerns. However, the efficiency number with PSC’s are much better and so the researchers have little extra space to play around for solving these issues. The collective work on these challenges will lead to faster adaptation of this in initial production and will pave the path for cost effective Tandem Solar cells.

Once resolved, Perovskite can be the next revolution in the Photovoltaic industry. By using abundant low-cost material and simple processing technologies, it will lead the energy security and fulfilling demand at a cost much lower than the existing coal based or even nuclear power plants.

Reference:

1. McKinsey Energy Insights’ Global Energy Perspective, January 2019
2. https://www.nrel.gov/pv/cell-efficiency.html
3. http://news.mit.edu/2019/perovskites-microstructure-solar-cells-0207
4. https://www.cei.washington.edu/education/science-of-solar/perovskite-solar-cell
5. RSC Adv., 2018, 8 , 10489; Large area perovskite solar cells- a review of recent progress and issues