Metal-organic framework thin films with diverse redox-active/inactive components for enhanced optical modulation and coloration efficiency

•The redox and coloration mechanisms of different MOFs are reported.•Co2+ doping into Cu-TCA for substitution of Cu2+ was firstly reported.•The Co/Cu-TCA film exhibits high optical modulation and coloration efficiency. Electrochromism is attractive due to the increasing demand for intelligent devices such as smart windows and display apparatuses. Metal-organic frameworks (MOFs) are found to be promising electrochromic materials thanks to their tailorable structures and chemical compositions via combining various metal centers and organic ligands. However, the uncovered redox mechanism, the insulating and inactive redox nature for majority of MOFs have hampered their practical application as efficient electrochromic materials. Herein, four MOF thin films (Cu-TCA, Zn-TCA, HKUST-1 and Zn-MOF-74) constructing from redox-active/inactive metal centers and organic ligands were prepared, and their electrochromic performance in a Li-ion-conducting electrolyte was present. We demonstrate that redox-active metal centers and organic ligands in MOFs contribute to the reversible and sustainable optical modulation. A further investigation was carried out to dope redox-active metal center Co2+ into the Cu-TCA thin film, serving as the second metal center. Unambiguous evidence shows that Co2+ doping can enlarge the specific surface area and pore volume of Cu-TCA to facilitate the transport and intercalation/deintercalation of ions and electrons, resulting in the enhanced transmittance modulation (70% at 705 nm) and coloration efficiency (337 cm2·C−1). Our work provides an important approach and guidance in designing efficient MOFs for electrochromic application.