In a first, a researcher from Pune’s Indian Institute of Science Education and Research (IISER) has successfully produced a stable, high-efficiency, all-inorganic perovskite nanocrystal solar cells. The new material has 10.77% efficiency to convert sunlight to electricity. The results were published on October 7 in the journal Science. Traditional research has been around a hybrid organic-inorganic halide perovskite material. Though the hybrid material has high efficiency of over 22%, the organic component in it is volatile and becomes completely unstable under ambient conditions within a short span of time. This renders the material unsuitable for commercial photovoltaic applications.
Problems in bulk form
So Abhishek Swarnkar, a research scholar from the Department of Chemistry at IISER and lead author of the paper, and others from the National Renewable Energy Laboratory, Colorado, U.S., replaced methyl ammonium, the organic component, with cesium to produce the material of cesium lead iodide. “Though the completely inorganic material is stable, there are problems. In bulk form [bigger size crystal], 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 Mr. Swarnkar.
One way of making the bulk material capable of absorbing the entire range of visible sunlight (400-700 nm) is to heat it to 300 degree C so that is attains a desirable crystal structure (cubic phase). But when the material cools down to room temperature, where photovoltaics normally operate, it once again regains its undesired crystal structure (orthorhombic) 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,” he says. 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.
‘More energy required’
“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,” Mr. Swarnkar says.