Enhancing charge carrier extraction and energy band alignment in perovskite solar cells using interfacial passivation with polar organic molecules

The wide utilization of perovskite material as an absorber layer in solar cells necessitates favorable alignment with the perovskite's conduction band, governed by FTO/TiO₂ (SnO₂). Instead of an ideal electric-selective contact, further improvement of the hole-selective contact is crucial to enhance hole extraction and minimize carrier recombination at the interface between perovskite and spiro-OMeTAD. In this study, we employed series of polar organic molecules [2,4-dimethyl-6,8-bis(4-(methylthio)phenyl)pyrrolo [1,2-a]pyrimidine-7-carbonitrile (PCNS), 2,4-dimethyl-6,8-diphenylpyrrolo [1,2-a]pyrimidine-7-carbonitrile (PCN), and 2,4-dimethyl-6-8-(pyren-1-yl)pyrrolo [1,2-a]pyrimidine-7-carbonitrole (PCNP)] with cyano groups as the interfacial passivate layer to facilitate energy band matching between the perovskite and the hole transport layer as confirmed by the energy band bending at the perovskite surface. Consequently, we achieved effective charge carrier extraction and suitable bandgap alignment. A detailed comparative analysis of the photophysical and electrical properties among the three molecules elucidated the origin of higher open circuit voltage (1.18 ​V) and improved fill factor (83.15 %) in the solar cell device based on PCN molecule (24.22 %).