For solar panels, wringing every drop of energy from as many photons as possible is imperative. This goal has sent chemistry, materials science and electronic engineering researchers on a quest to boost the energy-absorption efficiency of photovoltaic devices, but existing techniques are now running up against limits set by the laws of physics.
Existing solar cells all work in the same fundamental way: they absorb light, which excites electrons and causes them to flow in a certain direction. This flow of electrons is electric current. But to establish a consistent direction of their movement, or polarity, solar cells need to be made of two materials. Once an excited electron crosses over the interface from the material that absorbs the light to the material that will conduct the current, it can’t cross back, giving it a direction.
Finding a material that exhibits the bulk photovoltaic effect for visible light would greatly simplify solar cell construction. Moreover, it would be a way around an inefficiency intrinsic to interfacial solar cells, known as the Shockley-Queisser limit, where some of the energy from photons is lost as electrons wait to make the jump from one material to the other.
As no known materials exhibited the bulk photovoltaic effect for visible light, the research team turned to its materials science expertise to devise how a new one might be fashioned and its properties measured.
Starting more than five years ago, the team began theoretical work, plotting the properties of hypothetical new compounds that would have a mix of these traits. Each compound began with a “parent” material that would impart the final material with the polar aspect of the bulk photovoltaic effect. To the parent, a material that would lower the compound’s bandgap would be added in different percentages. These two materials would be ground into fine powders, mixed together and then heated in an oven until they reacted together. The resulting crystal would ideally have the structure of the parent but with elements from the second material in key locations, enabling it to absorb visible light.
This structure is something known as a perovskite crystal. Most light absorbing materials have a symmetrical crystal structure, meaning their atoms are arranged in repeating patterns up, down, left, right, front and back. This quality makes those materials non-polar; all directions “look” the same from the perspective of an electron, so there is no overall direction for them to flow.
A perovskite crystal has the same cubic lattice of metal atoms, but inside of each cube is an octahedron of oxygen atoms, and inside each octahedron is another kind of metal atom. The relationship between these two metallic elements can make them move off center, giving directionality to the structure and making it polar.
After several failed attempts to physically produce the specific perovskite crystals they had theorized, the researchers had success with a combination of potassium niobate, the parent, polar material, and barium nickel niobate, which contributes to the final product’s bandgap.
The researchers used X-ray crystallography and Raman scattering spectroscopy to ensure they had produced the crystal structure and symmetry they intended. They also investigated its switchable polarity and bandgap, showing that they could indeed produce a bulk photovoltaic effect with visible light, opening the possibility of breaking the Shockley-Queisser limit.
Moreover, the ability to tune the final product’s bandgap via the percentage of barium nickel niobate adds another potential advantage over interfacial solar cells.
Another way to get around the inefficiency imposed by the Shockley-Queisser limit in interfacial solar cells is to effectively stack several solar cells with different bandgaps on top of one another. These multi-junction solar cells have a top layer with a high bandgap, which catches the most valuable photons and lets the less valuable ones pass through. Successive layers have lower and lower bandgaps, getting the most energy out of each photon, but adding to the overall complexity and cost of the solar cell.
This story is reprinted from material from the University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.