Effects of CH3NH3 polymer formation on performance of organic-inorganic hybrid perovskite solar cell

Abstract

CH₃NH₃PbI₃ is one of the most promising candidates for high-performance hybrid organic-inorganic perovskite solar cells. The CH₃NH₃PbI₃ 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 cm²·V^–1·s^–1 to 100 cm²·V^–1·s^–1. Experimentally, the temperature dependence of mobility is described as T^–1.3 to T^–1.6 due to the acoustic phonon scattering. To be sure, the rotating CH₃NH<inline-formula><tex-math id="Z-20210812103905">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.png"/></alternatives></inline-formula> cations are disadvantageous to the carrier transport and performance for CH₃NH₃PbI₃ solar cells. The effect of the rotating CH₃NH<inline-formula><tex-math id="Z-20210812103911">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.png"/></alternatives></inline-formula> cations on high-performance CH₃NH₃PbI₃ solar cells remains an open question. The Gaussian 09 software has been utilized to optimize the geometrical structures of CH₃NH₃ dimer, trimer, tetramer, and pentamer in isolated state at the MP2 level with using the cc-PVTZ basis set. For CH₃NH₃ polymer, the mean distance between two centroids of neighboring CH₃NH₃ decreasing with the number of CH₃NH₃ is slightly smaller than the lattice constant 6.28 Å of tetragonal CH₃NH₃PbI₃, which is advantageous to structural stability and higher structural order of inorganic [PbI3]^– framework. It signifies that the long range order of electrically neutral CH₃NH₃ is easily formed for room-temperature CH₃NH₃PbI₃. The total dipole moment linearly increases with the number of CH₃NH₃ for CH₃NH₃ polymer, and attains a large value 19.7 Debye for CH₃NH₃ pentamer, which may be the origin of strong polarization in CH₃NH₃PbI₃ heterojunction. The molecular orbitals of five unpaired electrons for CH₃NH₃ pentamer are distributed around NH₃-sides of five different CH₃NH₃ pentamers respectively, and these orbital energies are in a range from –4.4 eV to –3.2 eV. The unpaired electrons in CH₃NH₃ polymer have an electrostatic attraction on the CH₃-side of neighboring CH₃NH₃, which is the key cause of forming the ordered CH₃NH₃ polymer. Hence it can be inferred that the orbital energies of unpaired electrons are getting closer when the longer range order of CH₃NH₃ are formed in room-temperature CH₃NH₃PbI₃ through the interfacial electron injection. The vector field map of electrostatic potential (ESP) shows that CH₃NH<inline-formula><tex-math id="Z-20210812103926">\begin{document}${}_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 NH₃-side has a stronger electrophilic character than CH₃-side, however, CH₃NH₃ monomer and polymer have weak electrophilic and nucleophilic character. Thus, the forming of CH₃NH₃ polymer at the CH₃NH₃PbI₃ 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 CH₃NH₃ 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 CH₃NH₃PbI₃ material through the heterojunction, the CH₃NH₃ polymer is easily formed, and the unpaired electrons in polymer are transferred between two neighboring CH₃NH₃ through hopping mechanism. ii) The decoupling between organic CH₃NH₃ and inorganic [PbI3]^– 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 CH₃NH₃ polymer at the CH₃NH₃PbI₃ heterojunction can enhance the order of inorganic [PbI3]^– framework. Our researches may help to further understand the origin of high power conversion efficiency (PCE) for hybrid organic-inorganic perovskite solar cells.