Researchers from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences have designed an optimized electron transport layer for inverted perovskite solar cells that increases both energy conversion efficiency and operational stability. The work, led by Prof. Pan Xu of the Institute of Solid State Physics, focuses on controlling the behavior of the fullerene derivative PCBM at the perovskite interface.
Perovskite solar cells have now achieved energy conversion efficiency of almost 27 percent, making them among the leading candidates for the next generation of photovoltaic technologies. The Hefei team had previously reported a method to homogenize the cation distribution within the perovskite absorber, improving the performance of that layer. In parallel with the absorber optimization, they note that semiconducting charge transport layers are essential for efficient charge separation and extraction in complete devices.
PCBM, or -phenyl-C61-butyric acid methyl ester, is widely used as an electron transport material in inverted perovskite architectures, but tends to form dimers when exposed to heat and light. This dimerization reduces charge carrier mobility, lowers device efficiency, and accelerates performance degradation, which is a barrier to practical implementation. To understand and mitigate this effect, the researchers examined how PCBM molecules stack on different perovskite surface ends and identified heterogeneity in molecular orientation as a key factor promoting dimer formation.
Building on this analysis, the team designed a PCBM precursor additive, 2,3,5,6-tetrafluoro-4-iodobenzoic acid (FIBA), to tune molecular packing at the interface. FIBA interacts with PCBM at the perovskite surface and guides the molecules into a more ordered stacking arrangement, homogenizing their orientation. This alignment changes the local topology so that the configuration required for the cycloaddition reaction that produces PCBM dimers is suppressed, thus inhibiting dimer formation at the transport layer.
Molecular dynamics simulations helped clarify how the additive alters the stacking and orientation of PCBM at the microscopic level, linking molecular-scale organization to changes in the behavior of macroscopic devices. The researchers then integrated the optimized PCBM layer into inverted perovskite solar cells and systematically evaluated the photovoltaic performance of different device sizes.
Using this approach, the group reported an energy conversion efficiency of 26.6 percent for devices with a small footprint and an active area of about 0.1 square centimeters. Single-cell devices with an area of 1 square centimeter achieved an efficiency of 25.3 percent, while modules with a large area of 762 square centimeters achieved an efficiency of 21.3 percent. These results indicate that the interfacial strategy can be applied from laboratory-scale cells to larger modules.
The modified transport layer also improved the stability of the device under combined environmental stress. Optimized cells retained more than 85 percent of their initial efficiency after 2,000 hours of continuous use under simultaneous exposure to heat, humidity and light. The authors conclude that guiding PCBM stacking and suppressing dimerization provides a practical route to simultaneously improve efficiency and stability in inverted perovskite solar cells and may be applicable to other perovskite-based device structures.
Research report:Suppression of PCBM dimer formation in inverted perovskite solar cells
