The race is on to achieve an efficiency of 30 percent in perovskite-silicon tandem solar cells – and the Helmholtz Centre Berlin (HBZ) has just pulled out in front. The HBZ team, which pulled expertise together from different research groups, has just recorded 29.8 percent efficiency certified by the Fraunhofer Institute for Solar Energy Systems (ISE) CalLab, surpassing the previous record held by the UK company Oxford PV, which announced 29.5 percent in December last year.

The potential for improving conventional silicon solar panels (which typically achieve an efficiency of between 18 to 22 percent) has almost been exhausted. All bets are now on the new class of flexible, crystalline “metal halide perovskite” cells to replace the older technology. "An efficiency of 30 percent is like a psychological threshold for this fascinating new technology which could revolutionise the photovoltaic industry in the near future," explains Steve Albrecht, a team member from the HBZ HySPRINT lab in a press release.

In January 2020, HBZ reached 29.15 percent efficiency. To achieve the extra performance, the researchers focused on two areas: the texturing of the silicon and perovskite elements, arranged in different geometries, and the infrared reflective material on the back side of the cell. "Even the nanotexturing on one side improves the light absorption and enables a higher photocurrent compared to a flat reference," says doctoral student Johannes Sutter.

The team also used a dielectric reflector at the back of the cell, designed to channel infrared light back into the silicon absorber, which resulted in a higher photocurrent. The computer simulations used to develop the cell suggest that it could be made even more effective by nano-structuring the absorber layers on both sides, thereby surpassing the magic 30 percent mark.