Effects of CH<sub>3</sub>NH<sub>3</sub> polymer formation on performance of organic-inorganic hybrid perovskite solar cell
Authors: Zhang Ao , Zhang Chun-Xiu , Zhang Chun-Mei , Tian Yi-Min , Yan Jun , Meng Tao ,   Department of Science, Beijing Institute of Graphic Communication, Beijing 102600, China,   Institute of Applied Micro-Nano Materials, School of Science, Beijing Jiaotong University, Beijing 100044, China
Published: June 2021

CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> is one of the most promising candidates for high-performance hybrid organic-inorganic perovskite solar cells. The CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> single crystal and polycrystalline thin film exhibit the unique features of long carrier lifetimes and diffusion lengths, however, their carrier mobilities are in fact rather modest in a range from 1 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> to 100 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. Experimentally, the temperature dependence of mobility is described as T<sup>–1.3</sup> to T<sup>–1.6</sup> due to the acoustic phonon scattering. To be sure, the rotating CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103905">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.png"/></alternatives></inline-formula> cations are disadvantageous to the carrier transport and performance for CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells. The effect of the rotating CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103911">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.png"/></alternatives></inline-formula> cations on high-performance CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells remains an open question. The Gaussian 09 software has been utilized to optimize the geometrical structures of CH<sub>3</sub>NH<sub>3</sub> dimer, trimer, tetramer, and pentamer in isolated state at the MP2 level with using the cc-PVTZ basis set. For CH<sub>3</sub>NH<sub>3</sub> polymer, the mean distance between two centroids of neighboring CH<sub>3</sub>NH<sub>3</sub> decreasing with the number of CH<sub>3</sub>NH<sub>3</sub> is slightly smaller than the lattice constant 6.28 Å of tetragonal CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, which is advantageous to structural stability and higher structural order of inorganic [PbI3]<sup>–</sup> framework. It signifies that the long range order of electrically neutral CH<sub>3</sub>NH<sub>3</sub> is easily formed for room-temperature CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. The total dipole moment linearly increases with the number of CH<sub>3</sub>NH<sub>3</sub> for CH<sub>3</sub>NH<sub>3</sub> polymer, and attains a large value 19.7 Debye for CH<sub>3</sub>NH<sub>3</sub> pentamer, which may be the origin of strong polarization in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction. The molecular orbitals of five unpaired electrons for CH<sub>3</sub>NH<sub>3</sub> pentamer are distributed around NH<sub>3</sub>-sides of five different CH<sub>3</sub>NH<sub>3</sub> pentamers respectively, and these orbital energies are in a range from –4.4 eV to –3.2 eV. The unpaired electrons in CH<sub>3</sub>NH<sub>3</sub> polymer have an electrostatic attraction on the CH<sub>3</sub>-side of neighboring CH<sub>3</sub>NH<sub>3</sub>, which is the key cause of forming the ordered CH<sub>3</sub>NH<sub>3</sub> polymer. Hence it can be inferred that the orbital energies of unpaired electrons are getting closer when the longer range order of CH<sub>3</sub>NH<sub>3</sub> are formed in room-temperature CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> through the interfacial electron injection. The vector field map of electrostatic potential (ESP) shows that CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103926">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103926.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103926.png"/></alternatives></inline-formula> has strong electrophilic character, and the NH<sub>3</sub>-side has a stronger electrophilic character than CH<sub>3</sub>-side, however, CH<sub>3</sub>NH<sub>3</sub> monomer and polymer have weak electrophilic and nucleophilic character. Thus, the forming of CH<sub>3</sub>NH<sub>3</sub> polymer at the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction leads the organic and inorganic portions to be decoupled, which can effectively reduce the anharmonic phonon modes. Under an applied electric field, the unpaired electrons in CH<sub>3</sub>NH<sub>3</sub> pentamer can transfer along the C-N axis through the hopping mechanism. According to these results, we can draw three useful conclusions below. i) The electrons under an applied electric field are easily injected into the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> material through the heterojunction, the CH<sub>3</sub>NH<sub>3</sub> polymer is easily formed, and the unpaired electrons in polymer are transferred between two neighboring CH<sub>3</sub>NH<sub>3</sub> through hopping mechanism. ii) The decoupling between organic CH<sub>3</sub>NH<sub>3</sub> and inorganic [PbI3]<sup>–</sup> framework can effectively reduce the anharmonic phonon modes, which can lead the carrier scattering decrease and the efficiency of carrier separation and transport to improve; iii) The ordered CH<sub>3</sub>NH<sub>3</sub> polymer at the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction can enhance the order of inorganic [PbI3]<sup>–</sup> framework. Our researches may help to further understand the origin of high power conversion efficiency (PCE) for hybrid organic-inorganic perovskite solar cells.