Stability of perovskite solar cells reaches next milestone

In the control perovskite layer (left), the microcrystals are not perfectly ordered and voids can form. In the new variant (right), the dipolar polymer cushions the tiny crystals and thus reduces the thermomechanical stress.

In the control perovskite layer (left), the microcrystals are not perfectly ordered and voids can form. In the new variant (right), the dipolar polymer cushions the tiny crystals and thus reduces the thermomechanical stress. © G. Li/HZB

Under the scanning electron microscope (SEM), clear voids can be seen at the grain boundaries of the control perovskite film (left). These defects can lead to losses and reduce the efficiency. With b-pV2F (right) the voids are reduced.

Under the scanning electron microscope (SEM), clear voids can be seen at the grain boundaries of the control perovskite film (left). These defects can lead to losses and reduce the efficiency. With b-pV2F (right) the voids are reduced. © G. Li/HZB

Perovskite semiconductors promise highly efficient and low-cost solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. Adding a dipolar polymer compound to the precursor perovskite solution helps to counteract this. This has now been shown in a study published in the journal Science by an international team led by Antonio Abate, HZB. The solar cells produced in this way achieve efficiencies of well above 24 %, which hardly drop under rapid temperature fluctuations between -60 and +80 Celsius over one hundred cycles. That corresponds to about one year of outdoor use.

The material class of halide perovskites is seen as a great hope for even more solar power at even lower costs. The materials are very cheap, can be processed into thin films with minimal energy input and achieve already efficiencies that are significantly higher than those of conventional silicon solar cells.

The Goal: 20 Years Outdoor Stability

However, solar modules are expected to provide stable output for at least 20 years in outdoor conditions while exposed to large temperature fluctuations. Silicon PV manages this easily, whereas the semi-organic perovskites lose performance rather fast. "Sunlight can heat up the inside of a PV cell to 80 Celsius; in the dark, the cell then cools down immediately to the outside temperature. This triggers large mechanical stresses in the thin layer of perovskite microcrystals, creating defects and even local phase transitions, so that the thin film loses its quality," explains Prof. Antonio Abate, who heads a large group at HZB.

Chemical Variations examined

Together with his team and a number of international partners, he has investigated a chemical variation that significantly improves the stability of the perovskite thin film in different solar cell architectures, among them the p-i-n architecture, which normally is a little less efficient than the more often used n-i-p architecture.

A "Soft Shell" against Stress

"We optimized the device structure and process parameters, building upon previous results, and finally could achieve a decisive improvement with b-poly(1,1-difluoroethylene) or b-pV2F for short," says Guixiang Li, who is doing his PhD supervised by Prof. Abate. b-pV2F molecules resemble a zigzag chain occupied by alternating dipoles. "This polymer seems to wrap around the individual perovskite microcrystals in the thin film like a soft shell, creating a kind of cushion against thermomechanical stress," Abate explains.

Record Efficiency for p-i-n Architecture 24,6%

In fact, scanning electron microscope images show that in the cells with b-pV2F, the tiny granules nestle a little closer. "In addition, the dipole chain of b-pV2F improves the transport of charge carriers and thus increases the efficiency of the cell," says Abate. Indeed they produced cells on a laboratory scale with efficiencies of up to 24.6%, which is a record for the p-i-n architecture.

One Year Outdoor Use

The newly produced solar cells were subjected over a hundred cycles between +80 Celsius and -60 Celsius and 1000 hours of continuous 1-sun equivalent illumination. That corresponds to about one year of outdoor use. "Even under these extreme stresses, they still achieved 96 % efficiency in the end," Abate emphasises. That is already in the right order of magnitude. If it is now feasible to reduce the losses a little further, perovskite solar modules could still produce most of their original output after 20 years - this goal is now coming within reach.

arö

  • Copy link

You might also be interested in

  • Green hydrogen: MXenes shows talent as catalyst for oxygen evolution
    Science Highlight
    09.09.2024
    Green hydrogen: MXenes shows talent as catalyst for oxygen evolution
    The MXene class of materials has many talents. An international team led by HZB chemist Michelle Browne has now demonstrated that MXenes, properly functionalised, are excellent catalysts for the oxygen evolution reaction in electrolytic water splitting. They are more stable and efficient than the best metal oxide catalysts currently available. The team is now extensively characterising these MXene catalysts for water splitting at the Berlin X-ray source BESSY II and Soleil Synchrotron in France.
  • Langbeinites show talents as 3D quantum spin liquids
    Science Highlight
    23.08.2024
    Langbeinites show talents as 3D quantum spin liquids
    A 3D quantum spin liquid has been discovered in the vicinity of a member of the langbeinite family. The material's specific crystalline structure and the resulting magnetic interactions induce an unusual behaviour that can be traced back to an island of liquidity. An international team has made this discovery with experiments at the ISIS neutron source and theoretical modelling on a nickel-langbeinite sample.
  • Green hydrogen: ‘Artificial leaf’ becomes better under pressure
    Science Highlight
    31.07.2024
    Green hydrogen: ‘Artificial leaf’ becomes better under pressure
    Hydrogen can be produced via the electrolytic splitting of water. One option here is the use of photoelectrodes that convert sunlight into voltage for electrolysis in so called photoelectrochemical cells (PEC cells). A research team at HZB has now shown that the efficiency of PEC cells can be significantly increased under pressure.