Ambient stable, hydrophobic, electrically conductive porphyrin hole-extracting materials for printable perovskite solar cells

Abstract

Fundamental properties of organic molecules such as symmetry and conjugation have a major impact on their functional properties and applications. In this study, we designed and synthesized hydrophobic, electrically conductive porphyrin derivatives using N-octyl phenothiazine as the donors and a porphyrin as the π-spacer in a D–π–D configuration. These materials are candidates for hole transport layers (HTL) within optoelectronic devices, and specifically perovskite solar cells (PSC). Detailed optical, electrical and electrochemical characterization of the porphyrin molecules were used to assess their properties, revealing good conductivity, clear electronic transitions and HOMO energy levels well aligned with the valence band of methylammonium lead iodide, the archetypal absorber in PSCs. We fabricated all-solution processed perovskite devices through screen-printing of the various layers and, adopting a carbon nanoparticle–graphene composite cathode, achieved a high photocurrent density of >19.5 mA cm⁻² and power conversion efficiency of >11% for our porphyrin derivatives. In addition, by the introduction of hydrophobic octyl groups on the porphyrin substituents, we could achieve excellent water stability of our devices without the need for encapsulation, confirming the promise of these materials for stable HTLs. Such hydrophobic porphyrin systems will have broad academic and industrial interest for use in photovoltaics, light emitting diodes, photodetectors and other optoelectronic devices.