Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells

Abstract

Hybrid organic-inorganic perovskites emerged as a new generation of absorber materials for high-efficiency low-cost solar cells in 2009. Very recently, fully inorganic perovskite quantum dots also led to promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins. Currently, the record efficiency is obtained with CsPbI, whose crystallographical characterization is still limited. Here, we show through high-resolution in situ synchrotron XRD measurements that CsPbI can be undercooled below its transition temperature and temporarily maintained in its perovskite structure down to room temperature, stabilizing a metastable perovskite polytype (black γ-phase) crucial for photovoltaic applications. Our analysis of the structural phase transitions reveals a highly anisotropic evolution of the individual lattice parameters versus temperature. Structural, vibrational, and electronic properties of all the experimentally observed black phases are further inspected based on several theoretical approaches. Whereas the black γ-phase is shown to behave harmonically around equilibrium, for the tetragonal phase, density functional theory reveals the same anharmonic behavior, with a Brillouin zone-centered double-well instability, as for the cubic phase. Using total energy and vibrational entropy calculations, we highlight the competition between all the low-temperature phases of CsPbI (γ, δ, β) and show that avoiding the order-disorder entropy term arising from double-well instabilities is key to preventing the formation of the yellow perovskitoid phase. A symmetry-based tight-binding model, validated by self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties, with evidence of Rashba effect for both cubic and tetragonal phases when using the symmetry-breaking structures obtained through frozen phonon calculations.

The Role of Dimethylammonium Iodide in CsPbI3 Perovskite Fabrication: Additive or Dopant?

“…1.68 eV), which could be assigned to b-CsPbI 3 perovskite.T he XRD patterns of Cs-1.0DMAI and Cs-1.5DMAI samples also exhibited the diffraction peaks at 14.28 8 and 28.68 8,w hich is at ypical diffraction of b-CsPbI 3 perovskite. [10,29] These results suggested that the DMAI content can significantly affect the crystal phase of the final CsPbI 3 perovskite.T he scanning electron microscopy (SEM) was used to explore the morphologies of these different CsPbI 3 thin films.The DMAI content induced different crystallization processes,which lead to the different grain sizes and morphologies of Cs-xDMAI thin films.T he Cs-0.5DMAI thin film exhibits compact thin film with small grain (ca. 100 nm);t he crystal grain size of the Cs-0.7DMAI thin film markedly increases to circa 200-400 nm.…”

All‐Inorganic CsPbX3 Perovskite Solar Cells: Progress and Prospects

“…CsPbX 3 has four types of crystal polymorph (shown in Figure 1a): [23] the cubic structure (a-, Pm3m); the tetragonal structure (b-, P4/mbm),t he orthorhombic structure (g-, Pbnm); and an on-perovskite structure (d-, Pnma). Thus, for example the RT stable phase of CsPbI 3 is the nonperovskite d-y ellow phase that is relatively inactive as aphotovoltaic material, while what is known as the black (a-) phase is only stable at RT under special conditions.T here are also two other black phases (b and g)t hat are considerably less common.…”

Soft Template‐Controlled Growth of High‐Quality CsPbI3 Films for Efficient and Stable Solar Cells

“…Nevertheless, the all‐inorganic CsPbI 3 perovskite without any volatile organic component faces serious challenges in terms of low‐phase stability. As this problem was solved, the reversibility of the phase transition was found to be a unique advantage of high‐temperature‐processed inorganic perovskite films . By simply reheating the film at a temperature above the phase‐transition temperature, the undesired yellow‐phase CsPbI 3 will immediately turn back to black‐phase CsPbI 3 , which offers a new route to enhance the long‐term stability of all‐inorganic PSCs .…”