Seed Crystal Strategy Yields 23.51% Efficient Perovskite

A research team has developed a new method to stabilize a key perovskite material without chemical additives, achieving a solar cell efficiency of 23.51%. The work, led by scientists from Saudi Arabia and Greece, uses pre-formed seed crystals to guide the growth of the desired photoactive crystal phase.

According to the researchers, formamidinium lead iodide (FAPbI₃) is a leading candidate for efficient perovskite solar cells but is notoriously unstable under ambient conditions. Conventional stabilization methods involve adding elements like cesium or rubidium, which can widen the material's bandgap and limit its ultimate photovoltaic performance. Corresponding author Essa A. Alharbi told pv magazine their approach avoids this compromise. "Rather than using conventional α-phase stabilizers..." he said.

The Seeded-Growth Fabrication Process

The experimental work involved fabricating two types of solar cells with an n-i-p architecture: control devices and seed-assisted devices. For the seed-assisted cells, researchers mixed pre-formed α-FAPbI₃ seed crystals into the lead iodide precursor solution. A second deposition step used formamidinium iodide and methylammonium chloride, followed by annealing. The control devices were made identically but without the seed crystals.

Alharbi explained the mechanism. "The pre-existing α-FAPbI₃ seeds lower the nucleation barrier and direct the growth of the desired photoactive α-phase while suppressing the formation of the photoinactive δ-phase." This process resulted in films with larger grains, fewer defects, and lower surface roughness. The outcome was a dramatic jump in performance. The seed-assisted devices reached a power conversion efficiency of 23.51%, compared to just 15.5% for the control devices. They also maintained 99% of their initial performance after 3,000 hours of continuous operation under ambient conditions and one-sun illumination without any encapsulation.

Insights from Multiscale Simulations

The study combined experimental work with extensive computer simulations to understand why the seed strategy works. The team used density functional theory calculations to compare the energy of different crystal phases growing on a seed. Molecular dynamics simulations tracked how a 10 nm seed dissolved in the precursor solution. Further simulations examined nucleation pathways and the optical-electrical effects on cell performance.

The simulations revealed that dissolving seed crystals leave behind structural patterns that favor the growth of the efficient α-phase over the inactive δ-phase. The researchers concluded that seeded growth fundamentally changes the crystallization pathway. This leads to fewer defects, significantly reduced non-radiative recombination, negligible hysteresis, and more balanced charge transport within the cell.

Path to Commercial Scale and Broader Applications

The research team now aims to scale the technology from small lab devices to large-area solar modules. They plan to optimize seed concentration, size, and processing conditions for uniform crystallization over big substrates. Alharbi stated the method is well-suited for manufacturing. "The seeded-growth strategy is fully compatible with sequential deposition, making it well suited for large-area manufacturing and commercial-scale perovskite photovoltaics."

The approach may also benefit other devices beyond solar cells. The researchers noted its potential to improve perovskite-based light-emitting diodes, where suppressing defect-assisted recombination is equally critical for high performance. The international collaboration included researchers from King Abdulaziz City for Science and Technology, Princess Nourah bint Abdulrahman University, Taibah University, the Foundation for Research and Technology, Hellas, the Hellenic Mediterranean University, the University of Ioannina, and University College London. Their findings were published in the journal Materials Horizons.