Terahertz Analysis of CHNHPbI Perovskites Associated with Graphene and Silver Nanowire Electrodes.

In order to investigate the thermal and chemical (in)stabilities of MAPbI 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 MAPbI, 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 MAPbI 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 PbI octahedra and our results indicate that graphene forms a stable interface with MAPbI and is also effective in suppression of the undesirable phase transition. Meanwhile, for MAPbI coupled with the AgNW bottom electrode, the THz conductivity was found to be as low as that of the MAPbI single layer, attributed to the chemical reaction between Ag atoms and iodide ions. The THz conductivity is greatly increased when an ultrathin AlO interlayer is introduced to cover the AgNW network via the atomic layer deposition (ALD) method. ALD of AlO 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.