Unraveling UV Stability in Metal Halide Perovskites: From Degradation Mechanisms to Molecular Passivation

TL;DR

The paper addresses UV-induced instability in MAPbI3 perovskites and introduces real-time TDDFT-NOB to capture ultrafast hot-carrier cooling and lattice distortions under UV excitation. It identifies two distortion pathways—orbital-occupation effects (Pb–p and I–p antibonding) and direct cooling-induced distortion (DCID)—with hole dynamics dominating early stages, and finds MA+ rotation accelerates under UV. A multidentate BDO passivant dissipates energy via additional phonon channels, suppresses PbI2-like distortions, and enhances UV stability, with GIWAXS and device aging experiments validating the improvements. The work provides mechanistic insight into UV degradation and a design principle for passivation strategies to improve the UV resilience of perovskite devices.

Abstract

Understanding the mechanisms of UV-induced degradation is crucial for enhancing the UV stability of perovskite solar cells. The UV-driven structural dynamics of CH3NH3PbI3 (MAPbI3) are investigated using real-time TDDFT simulations, revealing that under the electron and hole excitation, the distortion of the inorganic framework (PbI) is primarily driven by the electron occupation of Pb-p and I-p antibonding states, whereas in the hole case, it is mainly governed by the direct cooling induced distortion. We also find that UV accelerates the rotation of MA+ molecules. Further, a BDO molecule is introduced as a passivant, which suppresses structural distortions and provides multi-phonon channels to dissipate carrier cooling energy. Experimental results confirm the UV-protective role of BDO, with suppressed PbI2 formation and improved device stability. These results clarify the mechanism of the UV-induced degradation in the MAPbI3 perovskite and further elucidate how passivation molecules enhance UV stability.

Figures (9)

• Figure 1: (a) Electron excitation: Time evolution of eigenenergies in the conduction band (gray lines); the blue line denotes the average energy of the excited electron (the averaged energy of carrier is computed with Equ.S4). (b) Hole excitation: Time evolution of eigenenergies in the valence band (gray lines); the red line denotes the average energy of the excited hole. The right panels show the corresponding projected density of states (PDOS) calculated based on time t=0 structure. We find that the states in the energy range that we are interested are dominated by PbI inorganic frame. The contribution of MA is out of this range. The PDOS plots also highlight the state ranges corresponding to different orbital characters. In panel (a), the energy zero point corresponds to the energy of the CBM at t=0, while in panel (b), the energy zero point corresponds to the energy of the VBM at t=0.
• Figure 2: Time evolution of $L$ and $\theta$ (deviations to ideal PbI$_x$ polyhedral) for layer 1 and layer 2.
• Figure 3: Time evolution of $\beta$ (in-plane Pb-I-Pb angles) for layer 1, layer 2, and the interlayer (out-of-plane Pb--I--Pb angle).
• Figure 4: The evolution of $\delta$.
• Figure 5: The autocorrelation function of MA$^+$ orientation as a function of time. To ensure sufficient sampling for averaging, $\tau$ is set to half of the total simulation time, i.e., 750 fs.