Compared with ferroelectric oxides, organic ferroelectric materials are lightweight, flexible and easy to process. They are ideal for applications in the next-generation portable electronics. Here, we demonstrate the room-temperature growth of imidazolium perchlorate (C3N2H5ClO4) ferroelectric films on various substrates, including Pt-coated Si, quartz and, more importantly, on flexible and transparent polyethylene terephthalate (PET). The films have a preferred (0 −1 −1) or (1 0 −1) orientation. The former shows a piezoelectric response comparable with the response of the Pb(Zr0.2Ti0.8)O3 film. This is attributed to the smaller elastic constant of the film, which makes it less susceptible to substrate clamping. When grown on PET, the film is transparent and can be bent to radii of a few millimetres without affecting its ferroelectric properties. Our discovery may significantly promote the application of molecular ferroelectrics in flexible and transparent electronics. Flexible organic ferroelectric films exhibiting a high piezoelectric response have been fabricated by researchers in China and Singapore. Organic ferroelectric materials possess many advantages over ferroelectric oxides, including being lightweight, flexible and easy to process as well as usually being transparent. These properties make them ideal for use in flexible and portable electronic devices, but this requires growing thin films of them on substrates. Now, Guoliang Yuan and co-workers have achieved room-temperature growth of high-quality thin films of the ferroelectric imidazolium perchlorate (C3N2H5ClO4) on various substrates. Importantly, they succeeded in growing thin films on polyethylene terephthalate (PET) substrates and show that the system can be bent to radii of a few millimetres without affecting its ferroelectric properties. This demonstration raises the possibility of using molecular ferroelectrics in flexible and transparent electronics. The transparent C3N2H5ClO4 ferroelectric film on ITO coated PET substrate can be bent to 3.1 mm radius without affecting its ferroelectric properties. Furthermore, its local piezoelectric response is comparable to that of Pb(Zr0.2Ti0.8)O3 film.