Effects of Rb Incorporation and Water Degradation on the Stability of the Cubic Formamidinium Lead Iodide Perovskite Surface: A First-Principles Study

Abstract

Despite the major improvements in device efficiency, perovskite solar cells (PSCs) are limited by their poor stability due to the degradation toward moisture. Recently studies have demonstrated that the perovskite materials can be stabilized by adding more inorganic elements such as Cs and Rb. However, the specific mechanism is not yet fully established. In this paper, first-principles calculations were employed to investigate the effects of Rb incorporation and water degradation on cubic formamidinium lead iodide (FAPbI3) perovskite. The termination dependence of the structural stability of the representative (100), (010), and (001) surfaces was characterized by surface energies. The PbI2-terminated (001) surface was found to be the most stable surface structure. The Rb dopants were found to be energetically favorable, existing on the surface more than staying in the inner atomic layers. Furthermore, the degradation mechanism was investigated by water adsorption on the Rb-incorporated FAPbI3 (001) surface. According to our calculations, the water molecules tend to interact with the surface on the Pb-top sites with the largest adsorption energy. The orientation of the surface FA cations exerted a crucial influence on the adsorption energy as well as the surface energy. The moisture-induced decrease in optical absorption in the visible region could be observed in absorption spectra. It indicates that the adsorbed water molecules subsequently reduce the conversion efficiency. More importantly, we find that Rb incorporation at the surface region with the largest adsorption energy will destroy the surface structure leading to the obvious drop in the absorption spectra. These findings further illustrate the initial degradation process of Rb-incorporated FAPbI3 under the humid condition, which should be helpful to improve the stability of Rb-doped FAPbI3 perovskites in the presence of moisture.

Structural, electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation

“…The band gaps calculated from DFT-PBE and DFT-vdW are decreased by about 1.0 eV aer inclusion of the SOC effects. The HSE06 calculation including SOC effects has a band gap that is about 0.82 eV lower, decreasing from 1.9937 eV to 20 6.42, 24 -1.368, 19 1.40, 20 1.75, 21 1.45, 24 1.43, 37 1.43, 38 0.224, 19 0.66, 21 0.82, 24 1.47, 38 Experimental 6.3503, 11 6.3620, 37 6.3558, 39 -1.53, 11 1.489, 9 1.45, 39 - 1.1732 eV. The good consistency of the band-gap energy with experiments in the DFT calculations is caused by the strong relativistic effect of the Pb atoms offsetting the underestimation of the band gap by typical DFT calculations.…”

“…23 Guo et al studied the effects of Rb incorporation and water degradation of the FAPbI 3 surface. 24 In this work, by using the rst-principles total energy calculation method, we systematically investigated the structure, electronic properties, charge effective mass, k-dependent band splitting and optical properties of cubic a-FAPbI 3 . Different calculation methods were evaluated, specically, standard density functional theory (DFT), screened hybrid DFT, and the GW approach, both with and without incorporation of spin-orbit coupling (SOC) effects.…”

Recent theoretical progress in the development of perovskite photovoltaic materials

“…Ab initio nonadiabatic molecular dynamics performed by Long et al revealed that small amounts of water adsorbed on the MAIterminated (001) surface of tetragonal MAPbI 3 can perturb perovskite surface, make localized photoexcited electron close to the surface and more importantly avoid deep electron traps, which results in the decrease of electron-hole overlap and the increase of the excited state lifetime [128] . Some theoretical efforts focus on the effect of water molecules on the stability of other perovskite materials, like MAGeI 3 and formamidinium lead iodide (FAPbI 3 ) [129,130] . These works also found easy diffusion of water into the inner of perovskite, structural destructions, formation of hydrated intermediates and the moisture-induced decrease in optical absorption observed in MAPbI 3 .…”

Effect of Sr substitution on the property and stability of CH3NH3SnI3perovskite: A first‐principles investigation

“…Many experimental and theoretical studies on the adsorptions of oxygen and water molecules on the surfaces of MAPbI 3 and their roles played in the degradation of the perovskites have been reported . However, till now, there are no reports for adsorptions of reactive molecules, such as oxygen and water, on the surfaces of Sn‐ and Sr‐mixed perovskites.…”

“…Their results indicated that both water and oxygen molecules can be strongly attracted by both MAI‐ and PbI 2 ‐terminated surfaces, and the interplay of water and oxygen molecules on the surfaces are more easily to degrade the perovskites. Guo et al studied the water degradation of cubic formamidinium lead iodide (FAPbI 3 ) perovskite by first‐principles calculations. They found that water molecules tend to interact with the surface at the top of Pb site, and the adsorbed water molecules subsequently reduce the conversion efficiency.…”