Finding the right solvent
Mohite’s team looked at two important properties of the solvent, the dielectric constant and Gutmann donor number. The dielectric constant is the ratio of the electric permeability of the material to its free space. This determines how well a solvent can dissolve a compound. The donor number is the electron-donating capability of the solvent’s molecules.
By studying these properties for perovskites, Mohite and his research team found that there were just four solvents that could dissolve perovskites and still process them without disrupting the lower layer, the press release said. The researchers tested the solar cells to light for over 2,000 hours but did not see even one percent degradation. The cell layers are just one micron thick.
Using their discovery, one could manufacture perovskites solar cells at a capacity of 30 meters per minute, Mohite claims. Interestingly, the applications are not limited to solar energy alone. The method also opens up areas like green hydrogen, non-grid solar for cars, and building integrated photovoltaics, the press release added.
The research findings were published today in the journal Science.
Realizing solution-processed heterostructures is a long-enduring challenge in halide perovskites because of solvent incompatibilities that disrupt the underlying layer. By leveraging the solvent dielectric constant and Gutmann donor number, we could grow phase-pure two-dimensional (2D) halide perovskite stacks of the desired composition, thickness, and bandgap onto 3D perovskites without dissolving the underlying substrate. Characterization reveals a 3D–2D transition region of 20 nanometers mainly determined by the roughness of the bottom 3D layer. Thickness dependence of the 2D perovskite layer reveals the anticipated trends for n-i-p and p-i-n architectures, which is consistent with band alignment and carrier transport limits for 2D perovskites. We measured a photovoltaic efficiency of 24.5%, with exceptional stability of T99 (time required to preserve 99% of initial photovoltaic efficiency) of >2000 hours, implying that the 3D/2D bilayer inherits the intrinsic durability of 2D perovskite without compromising efficiency.