Hybrid organic–inorganic perovskites have attracted considerable interest due to their impressive performance in solar energy applications. Many experiments show that a slight excess of PbI₂ significantly enhances the properties of the most studied CH₃NH₃PbI₃ compound. We use real-time time-dependent density functional theory and nonadiabatic molecular dynamics to demonstrate that the effect arises due to decreased electron–phonon interactions responsible for nonradiative charge recombination. The fast organic CH₃NH₃⁺ (MA) cations, present on surfaces of stochiometric and MAI-rich perovskites, are particularly mobile and introduce high-frequency phonons and strong electric fields that couple to the charge carriers and create large nonadiabatic coupling. Excess PbI₂ decreases MA surface coverage, reduces the nonadiabatic coupling by up to an order of magnitude, and extends the charge carrier lifetime. Generally, charges in perovskites are long-lived because the nonadiabatic coupling is very small, less than 1 meV, and quantum coherence formed during charge recombination is short, less than 10 fs. Our results rationalize why decreasing the concentration of the organic cations on perovskite surfaces can suppress nonradiative charge carrier recombination and improve material performance.

中文翻译：

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从头算起非绝热分子动力学合理化的富含PbI _2的钙钛矿中较长的载流子寿命

混合的有机－无机钙钛矿因其在太阳能应用中的出色表现而引起了人们的极大兴趣。许多实验表明，稍微过量的PbI _2可以显着增强研究最多的CH ₃ NH ₃ PbI ₃化合物的性能。我们使用实时时变密度泛函理论和非绝热分子动力学来证明这种效应是由于负责非辐射电荷重组的电子-声子相互作用降低而产生的。快速有机CH ₃ NH ₃ ⁺存在于化学计量和MAI丰富的钙钛矿表面上的（MA）阳离子特别易移动，会引入高频声子和强电场，这些电荷耦合到电荷载流子上并产生大的非绝热耦合。过量的PbI _2会降低MA表面的覆盖率，将非绝热耦合降低一个数量级，并延长电荷载流子的寿命。通常，钙钛矿中的电荷是长寿命的，因为非绝热偶合非常小，小于1 meV，并且在电荷复合期间形成的量子相干很短，小于10 fs。我们的结果合理地解释了为什么降低钙钛矿表面上有机阳离子的浓度可以抑制非辐射载流子复合并改善材料性能。