The role of perovskites in tandem photovoltaics
The use of perovskites has become a defining trend in double and triple junction solar technologies. These synthetic materials can be engineered with a wide band gap, around 1.7–1.8 eV, enabling efficient absorption of high-energy light while allowing lower-energy photons to pass through.
In a tandem configuration, this translates into a perovskite-based top cell capturing photons in the orange, yellow, green, blue, and ultraviolet spectrum, paired with a silicon bottom cell that absorbs near-infrared light due to its narrower band gap.
Stability challenges in three-dimensional structures
While this combination is theoretically ideal, practical limitations remain. The best band gap performance has traditionally been achieved with three-dimensional perovskites. However, these materials are highly sensitive to moisture, leading to gradual degradation over time.
Why two-dimensional perovskites are more resilient
Two-dimensional perovskites offer greater resistance thanks to their structure, which incorporates long hydrophobic organic chains acting as spacers that repel water molecules.
However, this advantage comes at a cost. Unlike fully inorganic semiconductors such as silicon, which form highly ordered and symmetric crystal structures, 2D perovskites combine organic and inorganic components, resulting in a softer lattice that is more prone to distortions. This lack of symmetry hinders charge transport and reduces overall efficiency.
A multilayer breakthrough with near-perfect symmetry
The research team, led by Aditya Mohite at Rice University, developed a multilayer 2D perovskite that overcomes these distortions and approaches near-perfect crystal symmetry.
“As far as we know, this is the first demonstration of such symmetry in a multilayer 2D perovskite system at room temperature,” Mohite explained.
To achieve this, the researchers drew inspiration from the 3D perovskite structure FAPbI3, engineering an improved 2D version. By applying precise crystallization protocols and custom-designed organic molecules, they obtained well-defined tetragonal space groups and reduced the interlayer spacing to just 4 Å.
Improved exciton transport and light interaction
This structural refinement significantly enhances exciton transport properties, both in diffusion length and mobility.
“All the absorbed light generates excitons that can travel through the material for more than two micrometers without energy loss,” Mohite noted. “This is a remarkable result, as very few materials can achieve this.”
The approach also enabled the creation of thicker multilayer structures, altering how the material interacts with light. This is critical, as single-layer 2D perovskites typically exhibit a band gap that limits absorption to violet light, around 2.2–2.5 eV.
Band gap tuning for broader solar absorption
As additional layers are stacked, the band gap decreases. At three layers, it reaches 1.7–1.8 eV, allowing the material to capture a broader portion of the solar spectrum, making it highly suitable for tandem solar applications.
Testing new two-dimensional perovskites
The researchers tested the material in self-powered photodetectors, devices that convert light into electrical signals. These prototypes demonstrated higher sensitivity and faster response compared to devices based on other 2D perovskites.
“One of the main challenges in tandem photovoltaic devices is finding wide band gap materials,” said Faiz Mandani, co-author of the study. “The two-dimensional perovskites we are developing offer greater stability. This specific material also has a nearly ideal band gap for integration with silicon or other semiconductors.”
The study, titled Exciton diffusion beyond 2 μm enabled by maximum symmetry in two-dimensional perovskites, was published in Nature Synthesis.