有机阳离子对卤素钙钛矿太阳能电池性能的影响

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.

Effects of organic cations on performance of halide perovskite solar cell

The halide perovskite solar cells employing CH₃NH₃PbX₃ (X=Cl^-, Br^-, I^-) and CH₃NH₃PbI_3-xCl_x 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 CH₃NH₃⁺ and CH₃NH₃ at a B3 LYP/6-311++G(d, p) level, and the Multiwfn software is used to visualize the electrostatic potentials (ESPs) of CH₃NH₃⁺ and CH₃NH₃. Based on the ESPs of CH₃NH₃⁺ and CH₃NH₃, it is found that the CH₃NH₃⁺ has a strong electrophilic character, however, the NH₃^- side and CH₃^- side of CH₃NH₃ have weak nucleophilic and electrophilic character, respectively. So the electrostatic characteristics of CH₃NH₃⁺ and CH₃NH₃ are significantly different. The strong electrostatic repulsive interaction between two neighboring CH₃NH₃⁺ radicals plays an important role in structural phase transition of CH₃NH₃PbI₃ material. At room temperature, the CH₃NH₃⁺ in the inorganic cage is activated and disordered, and has a strong electrophilic character. Due to these characteristics of CH₃NH₃⁺, the interfacial electrons at TiO₂/CH₃NH₃PbI₃ heterojunction are combined with CH₃NH₃⁺ to form CH₃NH₃ in the inorganicPbI₃]^- framework. The CH₃NH₃ at the heterojunction under the built-in electric field is more easily oriented than CH₃NH₃⁺. Two initial geometrical configurations for CH₃NH₃⁺:CH₃NH₃ and CH₃NH₃:CH₃NH₃ dimers are optimized by using Gaussian 09 at an MP2/Aug-cc-PVTZ level. On the basis of the electrostatic characteristic of CH₃NH₃⁺:CH₃NH₃ dimer, the interfacial electrons at TiO₂/CH₃NH₃PbI₃ heterojunction are easily injected into the CH₃NH₃PbI₃ material, which leads to the strong polarization of CH₃NH₃PbI₃ material at the heterojunction. From the ESP of optimized CH₃NH₃:CH₃NH₃ dimer, it is found that the weak electrostatic field of the inorganic framework, parallel to C-N axis, is induced by the CH₃NH₃ orientational order, which is made for improving the photogenerated electron-hole pair separation and carrier transport. The TiO₂/CH₃NH₃PbI₃ heterojunction has more advantage than traditional p-n junction because of no consumption of carrier for CH₃NH₃PbI₃ material in the process of forming built-in electric field. The physical mechanism is the origin of high PCE for CH₃NH₃PbI₃ solar cells. According to the experimental results and first-principle calculations, we can draw an important conclusion that the electrostatic characteristics of organic CH₃NH₃⁺ cations in the inorganicPbI₃]^- framework result in the high performances of halide perovskite solar cells.