College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University , Beijing 100875, P. R. China.
School of Physics, Complex & Adaptive Systems Lab, University College Dublin , Dublin 4, Ireland.
J Am Chem Soc. 2016 Mar 23;138(11):3884-90. doi: 10.1021/jacs.6b00645. Epub 2016 Mar 10.
Advancing organohalide perovskite solar cells requires understanding of carrier dynamics. Electron-hole recombination is a particularly important process because it constitutes a major pathway of energy and current losses. Grain boundaries (GBs) are common in methylammonium lead iodine CH3NH3PbI3 (MAPbI3) perovskite polycrystalline films. First-principles calculations have suggested that GBs have little effect on the recombination; however, experiments defy this prediction. Using nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory, we show that GBs notably accelerate the electron-hole recombination in MAPbI3. First, GBs enhance the electron-phonon NA coupling by localizing and contributing to the electron and hole wave functions and by creating additional phonon modes that couple to the electronic degrees of freedom. Second, GBs decrease the MAPbI3 bandgap, reducing the number of vibrational quanta needed to accommodate the electronic energy loss. Third, the phonon-induced loss of electronic coherence remains largely unchanged and not accelerated, as one may expect from increased electron-phonon coupling. Further, replacing iodines by chlorines at GBs reduces the electron-hole recombination. By pushing the highest occupied molecular orbital (HOMO) density away from the boundary, chlorines restore the NA coupling close to the value observed in pristine MAPbI3. By introducing higher-frequency phonons and increasing fluctuation of the electronic gap, chlorines shorten electronic coherence. Both factors compete successfully with the reduced bandgap relative to pristine MAPbI3 and favor long excited-state lifetimes. The simulations show excellent agreement with experiment and characterize how GBs and chlorine dopants affect electron-hole recombination in perovskite solar cells. The simulations suggest a route to increased photon-to-electron conversion efficiencies through rational GB passivation.
推进有机卤化物钙钛矿太阳能电池需要理解载流子动力学。电子-空穴复合是一个特别重要的过程，因为它是能量和电流损失的主要途径。晶界（GBs）在甲脒碘化铅 CH3NH3PbI3（MAPbI3）钙钛矿多晶薄膜中很常见。第一性原理计算表明，GBs 对复合的影响很小；然而，实验却违背了这一预测。本研究采用非绝热（NA）分子动力学与含时密度泛函理论相结合的方法，表明 GBs 可显著加速 MAPbI3 中的电子-空穴复合。首先，GBs 通过局域化和贡献电子和空穴波函数，以及创建与电子自由度耦合的附加声子模式，增强电子-声子 NA 耦合。其次，GBs 降低了 MAPbI3 的能带隙，减少了容纳电子能量损失所需的振动量子数。第三，由于电子-声子耦合增加，声子诱导的电子相干损失并没有加速，这与人们的预期相反。此外，在 GBs 处用氯取代碘会降低电子-空穴复合。通过将最高占据分子轨道（HOMO）密度推离边界，氯将 NA 耦合恢复到原始 MAPbI3 中观察到的值附近。通过引入更高频率的声子并增加电子间隙的波动，氯缩短了电子相干。这两个因素都成功地与相对于原始 MAPbI3 减小的能带隙竞争，并有利于长激发态寿命。模拟结果与实验吻合得很好，并说明了 GBs 和氯掺杂如何影响钙钛矿太阳能电池中的电子-空穴复合。模拟结果表明，通过合理的 GB 钝化可以提高光子到电子的转换效率。
