Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals

Abstract

The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI)(MAPbBr) to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of δ-phase. These findings benefit the understanding of α-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.

Recent Progress in Single‐Crystalline Perovskite Research Including Crystal Preparation, Property Evaluation, and Applications

“…Over the last 2 years, an increasing number of researchers have begun studying single‐crystalline perovskites, including crystal preparations,21, 27, 28, 43, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 property studies,21, 27, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 and applications 56, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91. For example, an extremely long carrier lifetime (up to 20 µs) and carrier diffusion length (175 µm) have been found,21 which has received a great deal of attention.…”

A New Perspective on the Role of A‐Site Cations in Perovskite Solar Cells

“…One way to circumvent this problem is to tune the size of A-site cation (or halides), and many researchers tried to increase the number of carbon chain or amine group of MA+. [26,28] As the low energy spectra near the Fermi level (E F ) contain only Pb 2+ (conduction) and I − (valence) states without Asite cation, the role of A-site cations has been overlooked with a limited understanding as a structural tuner with its ionic radius.…”

Anti-Solvent Crystallization Strategies for Highly Efficient Perovskite Solar Cells

“…This was a motivation for researchers to adopt the approach for various mixed perovskites and enhance further the efficiencies. First, the~20% efficiencies were confirmed by others [43]. Then, the Grätzel group was able to enhance the efficiency up to 20.8% (with a record open-circuit potential of 1.18 V) by simply optimizing the PbI 2 :FAI molar ratio at 1.05 [44].…”