The surfaces of perovskite solar cells (PSCs) are significant in determining the devices' efficiencies and stabilities. Here, we first uncover that the 4-tert-butylpyridine (tBP), as an essential additive in hole transport layers (HTLs), could recrystallize the amorphous and defective perovskite surface layers and passivate the defective sites on grain surfaces. The reconstruction induces a larger surface work function and mitigates the interface energy level misalignment between perovskite and HTLs, enlarging the photovoltage of the device. Then, we engineer the chemical bonding strength and develop a more effective HTL additive 4-tert-butylpiperidine (tBPp), which possesses a stronger interaction with perovskite surface defective sites than tBP. With the enhanced adsorption, the tBPp-reconstructed perovskite surface exhibits lower densities of defects and better stability under the stimuli of heat, light and humidity. As a result, the optimized tBPp PSC reaches a champion efficiency of 24.2% with much better operation stability. Tracked at the maximum power point under a continuous bias, the unsealed devices in a N 2 atmosphere can nearly maintain their initial efficiency after continuous light exposure for over 1200 h. Our findings provide an underlying understanding of the HTL additives, which markedly affect the efficiency and stability of n-i-p PSCs.

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The perovskite prototype is one of the most promising solar cell materials. However, perovskite suffers from a phase transition leading to thermodynamic instability, which tightly influences the solar cell operation performance. Thus, modulating transition dynamics would extend its lifetime, which needs an in-depth understanding of the potential energy surface (PES) and the phase transition kinetics at the atomic level. In this work, taking CsPbI 3 as an example of a perovskite prototype, we map out the PES and resolve the three lowest energy barrier paths of γ-CsPbI 3 degradation by using a stochastic surface walking method integrated with high-dimensional neural-network potential. Path I is γ-CsPbI 3 to hexagonal δ′-CsPbI 3 , a five-step transition with (110) Pv to (001) hex with the energy barrier 0.25 eV/f.u.; Path II is γ-CsPbI 3 to cmcm-CsPbI 3 , a two-step transition with an over all energy barrier 0.22 eV/f.u. and (001) Pv //(110) cmcm + [010] Pv // [001] cmcm ; Path III is γ-CsPbI 3 to δ-CsPbI 3 , a one-step transition without forming an inherent interface, with the highest energy barrier 0.34 eV/f.u. Interestingly, We find that with the substitution of the A-site and/or B-site by other atoms, such as Bi and Te, the γ-CsPbI 3 to δ-CsPbI 3 transition could be extensively hindered. In this work, by resolving the potential energy surface, we not only reveal the degradation mechanism at the atomic level but also find a way to design perovskites with high and long-term stability.

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