Abhishek Swarnkar (left) made the all-inorganic perovskite stable at ambient temperature by reducing the size of the crystals to nanometre range.

In a first, Abhishek Swarnkar, a research scholar from the Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune, has successfully produced a stable, high-efficiency, all-inorganic perovskite nanocrystal solar cells. The new material has 10.77 per cent efficiency to convert sunlight to electricity. The results were published on October 7 in the journal Science.

Swarnkar carried out research for six months (November 2015 to April 2016) as an intern at the National Renewable Energy Laboratory, Colorado, U.S. In 2014, he was selected for the Bhaskara Advanced Solar Energy Fellowship from the Government of India’s Department of Science and Technology (DST) and Indo-US Science and Technology Forum (IUSSTF).

Traditional research has centred around a hybrid organic-inorganic halide perovskite material. Though the hybrid material has high efficiency of over 22 per cent, the organic component in it is volatile and becomes completely unstable at ambient conditions within a short span of time. This renders the material unsuitable for commercial photovoltaic applications.

So Swarnkar replaced methyl ammonium, which is the organic component, with cesium to produce an all-inorganic perovskite material of cesium lead iodide.

The nanometer-sized perovskite crystals absorb visible sunlight (400-700 nm) at ambient temperature.“Though the completely inorganic material is stable, there are other problems. In bulk form (bigger size crystals), the cesium lead iodide perovskite absorbs sunlight light only up to about 400 nm. So it does not have much application as a photovoltaic material,” says Swarnkar.

One way of making the bulk material capable of absorbing the entire range of visible sunlight (400-700 nm) is by heating it to 300 degree C so that is attains a desirable crystal structure (cubic phase). But the problem is when the material cools down to ambient temperature, where photovoltaics normally operate, it once again regains its undesired crystal structure (orthorhombic phase) and loses the ability to absorb sunlight beyond 400 nm.

“We found that by reducing the size of the crystals to nanometre range, the material at ambient temperature is able to absorb visible sunlight till 700 nm. This is because the material retains the desirable crystal structure (cubic phase) even at room temperature,” he says. The nanocrystals were found to be stable from -196 degree C to about 200 degree C.

By reducing the size of material to nanometer range, the surface to volume ratio increases tremendously.  As a result, high surface energy comes into play and makes the high-temperature cubic phase crystal structure stable even at room temperature.

The researchers assembled the nanocrystals as a thin film. The thin film was used for making both solar cells and red LEDs. Solar cells made using the nanocrystal thin film has 10.77 per cent efficiency to convert sunlight to electricity and produce a high voltage of 1.23 volts.

“Generally, more electrical energy is required to get low energy emission in LEDs. But less electrical energy (voltage) was sufficient to produce red light in LEDs made using our method,” Swarnkar says.

Earlier at IISER, Swarnkar had worked on caesium lead bromide perovskite and published a paper in November 2015 in the journal Angewandte Chemie. “I had demonstrated that the optical properties of the material is good for display technology such as LED,” he says. “So at NREL I proposed to work on caesium lead iodide and formed a team of eight people.”

Published in The Hindu on October 10, 2016