PhD Amran Al-Ashouri: Doubling down for the energy transition

Climate change has Amran Al-Ashouri concerned. As a physicist, he knows how urgently and quickly measures need to be taken to limit the global temperature increase to between 1.5 and 2 degrees Celsius. In his private life, the 29-year-old scientist is accordingly a member of the association “climactivity”, which aims to educate as many people as possible about important matters in climate protection.

At Helmholtz-Zentrum Berlin, he is researching ways to significantly improve the efficiency of solar cells, which are extremely important for Germany’s planned energy transition. And he has been extraordinarily successful: in Steve Albrecht’s group, his doctoral thesis played a central role in the development of a tandem solar cell that brought the world record in this field to Berlin in 2020 for an efficiency of more than 29 percent. On 28 April 2022, Amran Al-Ashouri will be awarded the Helmholtz Doctoral Prize for mission-oriented research for this work.

The physicist describes his now award-winning doctoral thesis matter-of-factly and very modestly: “Most of it went wrong”. That makes him no different from the many thousands of others who find themselves becoming frustrated more than once during their doctorate, only to draw from their failures the decisive conclusions that ultimately lead to their success. Although, it is still rather rare for this to involve a world record that improves the odds in the fight against climate change.

Conventional silicon solar cells are reaching their limits in this fight; they have barely more than about 24 percent efficiency in them. Tandem solar cells, at least in theory, ought to be considerably more efficient. A tandem cell is one that combines a silicon solar cell, which generates electricity primarily from the infrared light from the sun, with a completely different type of cell made from a perovskite absorber that uses the higher-energy light in the visible spectrum.

The only problem is that, in practice, there are considerable losses when the two solar cell types are combined. Guest researcher Artiom Magomedov from Lithuania postulated that these losses could be prevented by adding an ultra-thin intermediate layer just one molecule thick. In addition, he reasoned that the ultra-thin layer should be self-organising because each of its molecules contains a blueprint for the monolayer. However, the success hoped for did not materialise at first; the efficiency of these tandem solar cells was initially worse than that of the top-performing conventional cells.

The solar cells only improved after Amran Al-Ashouri specifically investigated how the chemical blueprint of the individual molecules influences the formation of the ultra-thin layer. “The simpler the molecules are structured, the better the monolayer forms,” the physicist says, summarising the result of long, methodical experiments. Now, with the appropriate type of molecules, the monolayer forms within seconds, even on very irregular substrates.

The crucial factor for their efficiency was the speed at which the particles generated by the separation of electrical charges travel out from the perovskite solar cell. The negatively charged electrons do so in a few billionths of a second, while the simultaneously created, positively charged “holes” initially took hundreds to thousands of times longer. However, with the new molecule design, the holes are now only ten to fifty times slower than the electrons, and have the added benefit of keeping the solar cell much more stable. Also, with conventional materials, around 99 percent of all holes would be lost by producing heat and no electrical current. The losses in the new monolayer, in contrast, drop to only one hundredth. “This enabled us to increase the efficiency by two to three percentage points and thus break the world record,” Amran Al-Ashouri explains.

On top of all this, the developments at HZB can be quickly scaled up from the tiny laboratory scale to the industrial scale needed to become a used technology. This means the tandem cells could be market-ready in just a few years. It is no surprise, therefore, that Amran Al-Ashouri is now set to receive the Helmholtz Young Investigator Award.