## 1 Introduction  
Flexible organic electronics are considered a valuable technology for the development of innovative devices that can be complementary to those based on standard electronic processes. Indeed, overturning the standard paradigm of electronics, this peculiar technology emphasizes peculiar properties, such as flexibility and transparency, and the possibility of cost-effective fabrication over large areas. All these features are of great importance for the future exploitation of flexible organic electronic devices in real products for electronic, optoelectronic and bioengineering applications: optical transparency is fundamental in different biochemical applications [1; 2; 3], and extreme flexibility is needed for the development of imperceptible, wearable and even tattoo-like devices [4; 5; 6; 7; 8; 9; 10; 11]. In this scenario, the possibility of up-scaling device fabrication to an industrial size is not secondary, in order to ensure future market exploitation, as well as low-voltage operation for actual portability and user safety. Among flexible organic devices, Organic Field-Effect Transistors (OFETs) are surely of great interest, as they can employed both as basic building blocks in electronic circuits [12; 13; 14] and as sensors [15; 16; 17]. In the bioelectronic scenario, peculiar optical and mechanical properties must be combined with low-voltage operation of the devices, in order to ensure portability and user safety. Although significant steps forward have been reported by the recent literature, the actual combination of low-voltage operation, transparency, flexibility and large-area fabrication processes is substantially missing. Indeed, several examples of transparent OFETs may be found in the literature [18; 19; 20; 21; 22] with moderately or even high operating voltages and fabricated over rigid substates, as well as low-voltage and transparent devices which are not flexible [23; 24]. When flexibility and transparency are combined, moderately high voltages are reached or small-area fabrication processes are involved [25; 26; 27; 28; 29; 30]. This is mainly related to the fact that low-voltage operation is generally reach by ensuring large gate insulator capacitance; this can be obtained by using high-k dielectrics [31] or reducing the gate insulator thickness. High-k materials, such as inorganic dielectrics, can be transparent and reasonably flexible, but normally requires high process temperatures, and are thus not suitable for the employment of most of the plastic substrates. Organic dielectrics can be processed at room temperature and by liquid phase, but low-voltage operation is allowed by depositing them in very thin films, as their dielectric constant is relatively low. Unfortunately, obtaining such thin films (generally in the range of a few tens of nanometers) is not trivial with large-area processes, and only a couple of examples have been reported so far in the literature. As a matter of fact, only a few examples combining low voltage operation, transparency and flexibility are available to date [32; 33], but small-area techniques, such as spin-coating, still cover a central role in the fabrication process, e.g., for the deposition of a gate insulator layer thin enough to ensure low-voltage operation. In this paper, transparent, all-organic, low-voltage OFETs over a fully flexible substrate will be reported. The fabrication process here presented employs only large-area techniques, namely ink-jet printing and Chemical Vapor Deposition (CVD), that can be easily up-scaled to an industrial size. As CVD allows the uniform deposition of very thin dielectric layers at room temperature, such a fabrication process can skip the limitations of solution-processed dielectrics, and can be adapted to different kind of substrates, from flexible--but relatively thick--plastics to ultra-conformable, tattoo-like (thickness \(<1\) um) ones.