Terahertz Analysis of CH₃NH₃PbI₃ Perovskites Associated with Graphene and Silver Nanowire Electrodes

In order to investigate the thermal and chemical (in)stabilities of MAPbI₃ incorporated with graphene and silver nanowire (AgNW) electrodes, we employed the terahertz (THz) time-domain spectroscopy, which has a unique ability to deliver the information of electrical properties and the intermolecular bonding and crystalline nature of materials. In in situ THz spectroscopy of MAPbI₃, we observed a slight blue-shift in frequency of the 2 THz phonon mode as temperatures increase across the tetragonal-cubic structural phase transition. For MAPbI₃ with the graphene top electrode, no noticeable frequency shift is observed until the temperature reaches the maximum operating temperature of solar cells (85 °C). Phonon frequency shift is sensitive to the strain-induced tilt of PbI₆ octahedra and our results indicate that graphene forms a stable interface with MAPbI₃ and is also effective in suppression of the undesirable phase transition. Meanwhile, for MAPbI₃ coupled with the AgNW bottom electrode, the THz conductivity was found to be as low as that of the MAPbI₃ single layer, attributed to the chemical reaction between Ag atoms and iodide ions. The THz conductivity is greatly increased when an ultrathin Al₂O₃ interlayer is introduced to cover the AgNW network via the atomic layer deposition (ALD) method. ALD of Al₂O₃ on the AgNW surfaces at low temperature guarantees a conformal coating, which strongly affects the ohmic contacts between the NWs. Our results demonstrate the advantage of THz spectroscopy for the comprehensive analysis of thermal and chemical stabilities of perovskites associated with the electrode materials.