by Scott Stonemeyer
If we collected 100% of the sunlight from the area of an ordinary school classroom over one day and stored that energy, we could power an entire home for nearly twenty days! While there has been a lot of success in developing solar cells, there remains a big question: How do we efficiently collect 100% of the energy from sunlight?
An emerging technology that could efficiently collect energy from sunlight is perovskite-based solar cells. Perovskites are a class of materials with similar crystal structures that are remarkable absorbers of solar light. Researchers have only been utilizing perovskites in solar cells since 2009, but already they have shown a staggering improvement in efficiency (that is, what amount of sunlight can be turned into energy) from an initial 3.8% to their current record of 25.5%! In 11 years, perovskite-based solar cells already rival the most efficient silicon-based solar cells (27.6%), which had a 55-year head start.
In a recent study by scientists at the Molecular Foundry, researchers investigated a specific type of ultra-thin perovskites, known as 2D perovskites (only a handful of atoms thick, but many, many atoms wide), and revealed how the internal structure could further enhance solar cell efficiency. There are specific 2D perovskites with structures that generate internal, intrinsic (naturally occurring) electric fields. Liang Tan, one of the primary Molecular Foundry scientists on the team, emphasizes that “this is important because these intrinsic electric fields improve the optoelectronic properties of these materials, like the long luminescence lifetimes,” referring to how long electrons remain in their excited state. This work sheds light on some key factors of the perovskite structure that can be modified to change its properties and enhance solar cell efficiency.
The team, comprised of staff scientists Liang Tan and David Prendergast with postdoc Jisook Hong, studied these 2D perovskite systems through first-principles density functional theory (DFT) simulations. These DFT simulations essentially generate a computer model of the perovskite material. You can think of this like a computer-generated floor plan you might use to remodel a home. The scale, spacing, and placement of everything is consistent in the model to see how any changes to the layout would look, except instead of a home, these DFT simulations build the perovskite material with the scale, spacing, and placement of the atoms laid out. This allows the researchers to study how the material properties change when they change the model, and how those properties differ in various regions of the perovskite.
The Molecular Foundry team studied how the number of layers in the 2D perovskites and the internal alignment of ions affect the solar cell efficiency. They found that when ions in the 2D layers were ferroelectrically aligned (the charges are oriented in the same direction), this created finite dipole moments, or tiny electric fields, in the layers of material. By changing the number of layers in the simulations (from 2-4), the researchers could effectively change how strong these tiny electric fields were in the material. If the intrinsic electric fields were too weak or too strong, then the solar cell efficiency suffered.
But why does this affect solar cell efficiency? The basic premise of a solar cell is that incoming sunlight excites electrons in the material, which can then flow to create electricity. These electrons are negatively charged, and when excited to higher energy levels, they leave behind a positively charged ‘hole’ where the electron used to live. Similar to the opposite poles of a magnet, this negatively charged electron and positively charged hole are very strongly attracted to each other. However, to efficiently produce electricity, the electron and hole must remain separated so the electron can continue to flow. The intrinsic electric field produced by the ferroelectrically aligned ions helps keep the electron and hole separated by directing the electrons towards the edges of the material and keeping the holes towards the center of the material.
These results could prove very important for the future design of solar cells. Researchers now know that creating a ferroelectric alignment of ions in the perovskite layers and adjusting the number of layers in the material plays a large role in the efficiency of the electron and hole separation. Therefore, when future perovskite materials are designed, scientists will know that the material should be designed to have properly aligned ions and a specific number of layers if they are hoping to achieve the highest solar cell efficiencies.
Read the paper:
Jisook Hong, David Prendergast, and Liang Z. Tan. Nano Lett. 2020. DOI: 10.1021/acs.nanolett.0c03468