Over the last few decades, perovskite solar cell (PSC) technology has leaped forward. The power conversion efficiency (PCE) of perovskite materials in solar cells has reached over 27%, causing organizations to put more research into the subject.
Today, much of this research centers on how formamidinium iodide (FAI) can improve the PCE of these solar cells. Understanding the uses of formamidinium iodide in solar cells can help to explain why it’s one of the most popular perovskite solar cell materials.
FAI supplies the formamidinium cation that occupies the A-site in the ABX₃ perovskite crystal lattice, where A = FA⁺, B = Pb²⁺, and X = I⁻. This cation is critical, as it leads to formamidinium lead iodide (FAPbI₃), which is a hybrid perovskite material. FA⁺ sits in the 12-fold coordinated cavity formed by corner-sharing PbI₆ octahedra.
Thanks to its larger ionic radius and dual-ammonia group, FA⁺ typically outperforms methylammonium (MA⁺), which was the previous choice for A-site cation. These characteristics create a more stable structure, which generally means better long-term device performance, especially under stress from heat or humidity.
To combine FAI and PbI₂, FAI must be dissolved in a polar solvent, such as DMF or DMSO. This solution can then be combined with PbI₂ and deposited as a thin film via spin-coating, blade-coating or another solution-processing method. Alternatively, one component can be deposited first, after which the second component is deposited on top. Whether the combined precursor solution or the sequential deposition method is used, the film is then heated through thermal annealing, which allows the FAI and PbI₂ molecules to form the photoactive α-phase.
It’s vital that the FAI has a high purity level, as trace impurities can:
FAPbI₃ has become the preferred absorber material for high-efficiency perovskite solar cells for several reasons. First, FAPbI₃ has a bandgap of approximately 1.45 eV, which sits closer to the Shockley-Queisser theoretical optimum than methylammonium lead iodide (MAPbI₃). This means that FAPbI₃ can absorb a wider range of sunlight. Additionally, with a bandgap of around 1.5-1.6 eV, MAPbI₃ can’t deliver as high a short-circuit current density as FAPbI₃ can. As a result, FAPbI₃ can deliver a PCE of 27.6%, superior to the PCE of MAPbI₃.
FAPbI₃ is also considered to be more stable than MAPbI₃, since it maintains stability at a higher temperature.
The main challenge of using FAI-based perovskites is phase stability. FAPbI₃ exists in two phases — black α-phase (Cubic) and yellow δ-phase (Hexagonal). The black α-phase is the photoactive phase, but is thermodynamically metastable at room temperature. This means that, given enough time or the right triggers, it will transition to the more stable but photoinactive yellow δ-phase.
Triggers that could cause this change include:
To stabilize the perovskite in its black α-phase, researchers can use mixed-cation formulations that incorporate Cs⁺, MA⁺ or Rb⁺ into the A-site. They can also use mixed-halide compositions and MACl additives that template α-phase crystallization.
FAI is a key component across many practical applications in perovskite photovoltaics, such as:
FAI is also used in research applications, such as phase transition studies, defect chemistry work and composition engineering investigations.
Whatever the application is, FAI purity is vital, with a consistently high purity helping to ensure reproducible device performance and avoid batch-to-batch variation.
At Iofina, we know how important it is to have high-purity formamidinium iodide that you can rely on. With over four decades of experience, we’re a vertically integrated producer of iodine and iodine derivatives, including formamidinium iodide. With an ISO 9001:2015 and ChemStewards certified facility, we operate with a strong emphasis on research and development, meaning our products are well-suited for these applications.
To find out more about how we can support your research or manufacturing operations, contact us today.