有机阳离子对卤素钙钛矿太阳能电池性能的影响

Effects of organic cations on performance of halide perovskite solar cell

采用第一性原理计算了CH_3NH_3PbI_3中有机部分CH_3NH_3^+和CH_3NH_3的静电特性.结果表明:CH_3NH_3^+具有强的亲电特性,CH_3NH_3的CH_3^-端具有弱亲电性,而NH_3^-端具有弱亲核性.发现在CH_3NH_3PbI_3中CH_3NH_3^+之间强静电排斥作用在相变中起着重要的作用,且在室温条件下CH_3NH_3^+在无机笼中具备活性和无序的特性,使得TiO_2/CH_3NH_3PbI_3异质结中n型TiO_2的电子通过界面扩散到CH_3NH_3PbI_3材料,并与CH_3NH_3^+结合形成CH_3NH_3,CH_3NH_3的静电特性导致在内建电场作用下更容易取向,取向的CH_3NH_3周围形成的静电场会变得更弱和更加均匀.这对无机框架上载流子的产生和传输更加有利,这样的异质结比传统的pn结具有更大优势.这是CH_3NH_3PbI_3太阳能电池高的光电转换效率的重要原因.

The halide perovskite solar cells employing CH3 NH3 PbX3（X = Cl-, Br-, I-） and CH3 NH3 PbI（3-x）Clx as light absorbers each have shown a rapid rise in power conversion efficiency（PCE） from 3.8% to 22.1% in recent years. The excellent photovoltaic performance is attributed to good optical and electrical properties such as appropriate bandgap,large absorption coefficient, high carrier mobility, long carrier lifetime and long carrier diffusion length. However, the physical mechanism of high PCE for halide perovskite solar cells is still unclear. The Gaussian 09 software is utilized to optimize the geometries of isolated CH3 NH3＋ and CH3 NH3 at a B3 LYP/6-311＋-G（d, p） level, and the Multiwfn software is used to visualize the electrostatic potentials（ESPs） of CH3 NH3＋ and CH3 NH3. Based on the ESPs of CH3 NH3＋and CH3 NH3, it is found that the CH3 NH3＋ has a strong electrophilic character, however, the NH3-side and CH3-side of CH3 NH3 have weak nucleophilic and electrophilic character, respectively. So the electrostatic characteristics of CH3 NH3＋ and CH3 NH3 are significantly different. The strong electrostatic repulsive interaction between two neighboring CH3 NH3＋ radicals plays an important role in structural phase transition of CH3 NH3 PbI3 material. At room temperature,the CH3 NH3＋ in the inorganic cage is activated and disordered, and has a strong electrophilic character. Due to these characteristics of CH3 NH3＋, the interfacial electrons at TiO2/CH3 NH3 PbI3 heterojunction are combined with CH3 NH3＋ to form CH3 NH3 in the inorganic [PbI3-framework. The CH3 NH3 at the heterojunction under the built-in electric field is more easily oriented than CH3 NH3＋. Two initial geometrical configurations for CH3 NH3＋：CH3 NH3 and CH3 NH3：CH3 NH3 dimers are optimized by using Gaussian 09 at an MP2/Aug-cc-PVTZ level. On the basis of the electrostatic characteristic of CH3 NH3＋：CH3 NH3 dimer, the interfacial electrons at TiO2/CH3 NH3 PbI3 heterojunction are easily injected into the CH3 NH3 PbI3 material, which leads to the strong polarization of CH3 NH3 PbI3 material at the heterojunction. From the ESP of optimized CH3 NH3：CH3 NH3 dimer, it is found that the weak electrostatic field of the inorganic framework, parallel to C-N axis, is induced by the CH3 NH3 orientational order, which is made for improving the photogenerated electron-hole pair separation and carrier transport. The TiO2/CH3 NH3 PbI3 heterojunction has more advantage than traditional p-n junction because of no consumption of carrier for CH3 NH3 PbI3 material in the process of forming built-in electric field.The physical mechanism is the origin of high PCE for CH3 NH3 PbI3 solar cells. According to the experimental results and first-principle calculations, we can draw an important conclusion that the electrostatic characteristics of organic CH3 NH3＋ cations in the inorganic [PbI3]-framework result in the high performances of halide perovskite solar cells.