Scientists at the Indian Institute of Technology, Madras, have crossed a significant milestone in the evolution of the ‘solar perovskite cell’ (SPC) — namely, making it both stable and reducing the downsides that come with stability.

In material science, perovskite refers to a certain arrangement of molecules. Materials whose structure resembles that of calcium titanate (CaTiO3) are called perovskites. In other words, perovskite is a material that has two positively charged ions (cations) and one negatively charged ion (anion). The general formula for such material is ABX3.

The solar industry loves perovskites because they milk more electricity out of sunlight than do the conventional crystalline silicon photovoltaic cells. With the regular PVs it is hard to convert more than 22 per cent of sunlight into electricity; it is generally believed that perovskites can do much better. However, SPCs go phut soon. While a regular crystalline silicon PV cell can go on for over 25 years, perovskites last about a year.

The combination of stability and energy efficiency has been elusive despite long years of effort by scientists the world over. Now, a team led by Prof Sudakar Chandran of IIT-Madras, who heads the institute’s Multifunctional Materials Laboratory, has achieved significant success.

To appreciate their work, it is first necessary to delve a little deeper into how PV cells work.

Electricity is a flow of electrons. As per the classical understanding of the atomic structure, electrons keep orbiting the nucleus of the atom. They have different energy levels. Even electrons in the same orbit could have varying energy levels. The energy levels are ‘grouped’ into different ‘energy bands’. The energy band of the electrons that are farthest from the nucleus — valence electrons — is called the ‘valence band’.

To get a flow of electrons going, you need to ‘nudge’ the electrons in the valence band to leave the atom. The electron should be supplied with energy from outside, to raise its energy level to the higher ‘conduction band’. The electron will then move to the next atom. As electrons jump from atom to atom, their vacant places are immediately taken by other electrons, just as the space vacated by a droplet of water in a river is immediately taken by another water droplet.

The difference between the valence band and conduction band is called ‘band gap’ and represents the minimum energy required to nudge electrons in the outermost orbits to leave the atom.

In a PV cell, the job of ejecting electrons is done by the photons of sunlight. PV materials that have smaller band gaps convert more solar energy into electricity.

Chandran and his team took interest in a structure called ‘double perovskite’, which is essentially a three-dimensional extension of a single perovskite. The formula of a double perovskite is A2B2X6.

Typically, solar cells have band gaps less than 2 eV (or electron-volt, the energy unit of subatomic particles). Good solar cells have band gaps of 1.5 eV. But the ‘double perovskites’ have much higher band gaps, which is a thumbs-down, as it calls for more sunlight to get a certain amount of electricity.

Chandran’s team engineered the bandgap of a double perovskite alloy exploiting bandgap bowing effect using cesium, silver, sodium and bismuth as cations and chlorine as the anion. This had a much smaller band gap (around 2.6 eV).

Of course, this is not good enough, given they have to get to 1.5 eV, or at least under 2 eV. However, Chandran sees the team’s work as being “on the right track”. Now it requires only further tinkering with other materials (such as tin or indium) to crack the band gap problem.

Chandran hesitates to describe the work as a “breakthrough”, but says it is a significant milestone crossing. It is promising because further work in this area, which is happening, could give a juicy solar perovskite cell that can make solar energy really cheap.

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