KAUST and University of Toronto team develops perovksite-silicon tandem cell with 25.7% efficiency

Scientists from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and University of Toronto, have developed a perovskite-silicon tandem solar cell which they claim showed excellent operational stability under accelerated tests.

The device was made by combining solution-processed, micrometer-thick perovskite top cells with fully textured silicon heterojunction bottom cells.

The researchers claim the approach enabled a threefold expansion of the width of the cell depletion region ' the area near the p-n junction where free electrons diffuse across the junction and combine with holes to form negative ions. The width of the depletion region depends on the amount of impurities added to the semiconductor and the number of charge carriers in the p-type and n-type semiconductors is proportional to the amount of impurity atoms added. A wider cell depletion region in the Saudi cell is said to have enhanced its carrier collection properties.

A conformal passivation strategy for rough surfaces was adopted in the manufacture of the cell and a self-limiting passivant was anchored on the wide-band-gap perovskite surface. Controlling perovskite morphology and film thickness was central to enhancing charge drift and diffusion, according to the researchers.

The KAUST group used the perovskite film to completely cover the micrometer-sized pyramids which typically arise in conventional manufacturing processes for planar perovskites. The pyramid structures lead to low shunt resistance, reducing the current flowing through a solar cell. 'This [manufacturing] process enabled us to achieve uniform perovskite coverage of the pyramids and eliminated the need for additional flattening processes,' the scientists stated.

The cell achieved a certified conversion efficiency of 25.7% and exhibited negligible performance loss after 400-hour thermal-stability tests at 85 degrees Celsius and after the same period under maximum power point tracking at 40 degrees Celsius.

'The fully textured bottom cells minimized reflection losses and efficient light trapping was achieved for the bottom cells, crucial to satisfying current-matching conditions,' stated the device's developers.

The cell was also said to display low hysteresis ' the retardation of an effect when forces acting upon a body are changed ' and high reproducibility.