Challenges in Perovskite Solar Cells

The exploitation of fossil fuels keeps rising to satisfy the global demands­, since the beginning of the industrial revolution. In the recent survey from BP energy, the use of fossil fuels in 2019 is estimated to about 120,000 TWh. Although the use of fossil fuels causes a great threat to the environment, their role in the economic growth of a country is inevitable. In recent years, the government around the world has realized the engraving situation induced by the use of fossil fuel and began to introduce the policies on renewable sources of energy [1]. Owing to these new policies, the use of a renewable source of energy such as solar, wind and hydroelectric power grew to new heights. In order to promote the wide use of these technologies, cost effective technologies like Perovskite solar cells were introduced. Between 2013-2020, the efficiency of Perovskite solar cells (PSC) reached to 26.1% and continues to increase, in a far cheaper processing cost than the silicon solar cells [2]. In this aspect, the PSC provides a great opportunity for research and development to make a renewable source of energy cheaper and widely employed. 

Figure 1. An ink-jet printed Perovskite solar module on a flexible matrix material. Image courtesy: Sollartek.

What is Perovskite?

Perovskites are minerals that discovered in the Ural Mountains of Russia, named after the scientist Lev Perovski in 1839. The mineral is composed of calcium, titanium and oxygen CaTiO₃, which is formulated with general structure ABX₃. Depending on the atomic structure, composition, and the properties of the Perovskite crystals are explored for superconductivity, spintronics, thermoelectrics and photovoltaics applications. In order to form a Perovskite structure, the elements involved should satisfy the Ruddlesden rules about the atomic radii. Perovskites MAPbI₃, owing to their ambipolar behavior and excellent absorption of visible light, they are deeply explored in solar photovoltaic industries [3].

What makes them unique?

When it exposed to the solar radiation, the photon induces the formation of free electrons-hole pairs on the Perovskite semiconducting layer which are transported to the positive and negative terminals to power the device. Optimizing this Perovskite layer to generate a larger number of electron hole pairs can increase the solar cell efficiency to the theoretical limit. Dye sensitized solar cells [4] are succeeded by PSC owing to their drawback of leaking of dyes and refilling problems. In this way, PSCs are the first solid-state solar cells which attained growth within a short time. The attractive features of the PSCs are their open circuit voltage, large optical absorbance, cell efficiency and scalability [5]. These properties of PSC make them superior over its competitors like Cadmium telluride, Copper-Indium-Gallium-Selenide(CIGS), amorphous silicon solar cells. On commercialization, the potential of PSC will overtake the traditional silicon solar cell technology.

What are the drawbacks?

Though the PSC has a lot of potential and advantages, the problem of the crystal structure stability under moisture and prolonged exposure to UV light degrades their performance rapidly. Now the problem of stability to moisture is greatly addressed through various methods like sealing and packing the solar cell. The problem of light soaking in PSC is a long standing issue, where the use of electron transport layers [6] like TiO₂, SnO₂ and Al₂O₃ are modified to suit the charge transfer mechanism in the Perovskite layer. Both the light induced degradation and the environmental moisture stability were duly addressed and now the lifetime of the solar cells is accelerated to over 10000 hours. Other major issues like mechanical durability, thermal degradation, current-voltage behaviors are being studied detailed by the researchers. Methyl ammonium Lead Iodide (MAPbI₃) is the widely used organo-metallic perovskite for solar cells. Here the amount of lead used is estimated as 2 mg/Watt. The use of lead in electronic devices is greatly restricted in many countries due to their ill effects on human life. Lead is a heavy toxic metal substance, which when mixed in water bodies causes contaminations.  Many other electronic devices using Lead are discontinued, like lead acid batteries.

Scalability to meet zero emissions by 2050

PSC overcome the disadvantage of the traditional silicon solar cell by simple fabrication and easy packing process. Such capabilities can make them the best source of emission free energy to meet the goal of emission free world by 2050. The established crystalline silicon solar module based industry is pointed to as mature and ready to scale. But silicon-based solar cells faces challenges to meet these energy needs. In order to meet the demand of 20% global energy, the photovoltaic industry needs to install about 300 to 500 GW annually in the next 30 years. However, the current installed manufacturing capacity of Silicon solar cell is limited to 100 GW annually. In this aspect, implementing manufacturing plants for PSC, the goal of 500 GW annually can be easily satisfied at a lower price. Depending on the business model, Perovskite modules can be manufactured in facilities that cost 50% less than other solar factories and use less materials. The supply chain to support PSC manufacturing is also small, allowing for factories to be sited close to end markets.

Commercialized Perovskite Photovoltaic Technology

In 2015, Dyesol, an Australian based solar cell manufacturer recorded a major breakthrough in enhancing the stability and assembling a Perovskite photovoltaic module having an efficiency of 10% and an operating lifetime of 1000 hours. Oxford PV Ltd. in association with the University of Oxford has planned their Perovskite solar cell technology. Their technique to print Perovskites directly onto glass has led to a semi-transparent coating ideal for BIPV applications and, once integrated into the glazing units of a building, the technology is capable of providing a significant percentage of the building’s electrical energy requirements directly from sunlight. Recently, a researcher from Iowa State University has created a flexible PSC with efficiency of 11.8% with heavy stability to any environmental conditions. They have used the layer by layer vapor deposition technique to deposit an inorganic Perovskite layer made of CsPbI_3-xBr_x. Using this technique, any scale of flexible solar cells can be fabricated rapidly. Commercialization Tests on this solar cell is nearing the final stage to be introduced in market [7].

 

References:

1. P. Stevenson and R. Hyde, Elem. Energy Ecofys Imp. Coll., 50 (2014).
2. N. Islavath et al., J. Energy Chem., 26, 584 (2017).
3. G. Ding et al., Adv. Electron. Mater., 6, 1900978, (2020).
4. J. Jerrard, Fire Rescue Magazine. 2008.
5. J. S. Manser et al., Acc. Chem. Res., 49, 330 (2016).
6. S. Dubey et al., ACS Nano, 11, 11206 (2017).
7. H. Gaonkar et al., ACS Appl. Energy Mater., 3, 3497 (2020).

Blog Written by

Dr. K. Vaithinathan

Department of Materials science and engineering 

City University of Hong Kong, Hong Kong

Editors

Dr. A. S. Ganeshraja

Dr. S. Chandrasekar

Dr. K. Rajkumar

Reviewers

Dr. Y. Sasikumar

Dr. S. Thirumurugan