Catalytic metal-induced crystallization of sol–gel metal oxides for high-efficiency flexible perovskite solar cells

Abstract

Sol–gel metal oxide films are an important type of functional materials for energy technologies and beyond owing to their versatile properties and ease of processing, but their applications are limited by the typically required high sintering temperatures. This study reports a novel phenomenon where the sintering temperatures of sol–gel metal oxide films were substantially lowered when a metal phase was embedded within the sol–gel precursor films. Morphological and compositional analyses revealed that the reduction in sintering temperatures was enabled by a mechanism similar to the metal-induced crystallization (MIC) process, with a distinction that catalysis of sol–gel reactions occurred alongside the induction of crystallization. We observed this catalytic MIC (c-MIC) mechanism in a variety of sol–gel material systems including Ni_xO with embedded Au or Ag; TiO_x with embedded Ni, Au, or Pt; and SnO_x with embedded Ni. Based upon this concept, we demonstrated highly efficient flexible and rigid organic/inorganic hybrid perovskite solar cells (PSCs) using a low-temperature-sintered Ni_xO film embedded with Au nanoislands as the hole-transporting layer (HTL). The c-MIC mechanism reduced the sintering temperature of the Au-embedded Ni_xO HTL by 100 °C, making the Ni_xO process compatible with plastic substrates while allowing the Ni_xO HTL to retain more NiOOH surface groups, which enhanced hole collection and improved the quality of the perovskite layer. Thanks to the advantages of the c-MIC Ni_xO film, the flexible and rigid PSCs achieved high power conversion efficiencies of up to 15.9 and 19.0%, respectively, which were sustained for >1200 h at 65 °C and 65% relative humidity. Our findings provide a practical route for low-temperature fabrication of high-quality oxide functional films by solution-based sol–gel processes, which will be valuable to a wide variety of applications in addition to thin film solar cells.