Perovskites are a particular class of crystals that can be based on different elements and are promising materials for solar cells. They are sensitive to a wider spectrum of light wavelengths than silicon-based materials, and are less expensive to manufacture. In the last decade, their efficiency in converting light into electricity has caught up with that of the best silicon solar cells.
But researchers are still trying to establish how stable perovskites are in real world applications, under vast temperatures variations. Now, a group of researchers has demonstrated that, by adding a special polymer during the crystallization process of a lead-based perovskite film, the resulting solar cell maintains a nearly stable conversion efficiency when temperature varies between -60 °C and +80 °C at a rate of 20 °C per minute.
The study, published in Science1, has been coordinated by Antonio Abate, an expert in perovskite solar cells at the University of Naples Federico II and Helmholtz-Zentrum Berlin für Materialien und Energie, and Meng Li, also at the Helmoltz-Zentrum.
“Since the perovskite has a crystalline structure, it has a certain degree of rigidity”, Abate says. “Temperature variations tend to create fractures in the film, undermining its mechanical stability and decreasing the efficiency. The polymer we chose made the crystalline film more plastic”, he adds.
The cell used in the experiments comprises five different layers. The perovskite film, containing lead, is in the middle of the stack, between two semiconductors that extract the charged particles produced in the film when it is exposed to light. The two outermost layers are made by conductors which collect the charges accumulated in the inner layers and generate a current. Fractures in the film are caused by the fact that temperature variations cause the different layers to expand and compress.
“We selected a polymer with an unusually ordered structure, composed of chains of regularly spaced dipoles”, says Giuseppe Nasti, a researcher at the University of Naples and a co-author of the study. “When the polymer is added in the liquid solutions containing the precursors of the perovskite film, it acts as a lubricant between the grains that are the building blocks of the crystal”, he adds.
Scanning the surface of the perovskite film with an electron microscope, the scientists observed that the film with the added polymer had wider grains, with fewer spaces between them compared to a control film with no polymer. When observing the film during subsequent cycles of temperature variations, they noted that the film with the polymer retained this more compact structure, whereas the defects in the one without polymer worsened.
The polymer, made of a long chain based on hydrogen, sulfur, and carbon atoms, helped even at room temperature. At 25 °C, the cells with the polymer reached an efficiency of 24.6%, whereas the other ones stopped at 22.3%.
The scientists also simulated the exposure of the cells to one year of solar light at room temperature, and saw that those with the polymer retained around 96% of their initial conversion efficiency, versus 88% of the other one.
Abate and his collaborators are now trying to use the same polymer with perovskites based on tin, that unlike lead is not toxic. These materials are the subject of Abate’s ERC grant that he is using between Naples and Berlin.