# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 1 Introduction  
The increasing interest in the Internet of things (IoT) has opened a new paradigm in the interaction between objects and human beings. In the IoT, sensors and actuators are embedded in physical objects and interconnected through wired and wireless networks [1]. One of the existing alternatives to connect objects is the radiofrequency identification (RFID) technology. RFID tags with sensing capabilities can be attached to objects, allowing not only their identification at different locations but also obtaining information about their quality or the environment where they are placed. In the case of products, it is especially interesting to know its status through the supply chains, leading to improvements in the inventory management as well as reductions in working capital and logistics costs [2]. This situation has led to a new paradigm in packaging [3]. Different environmental parameters are meaningful to control, such as relative humidity, temperature or exposure to certain chemicals [4; 5; 6]. European studies estimates that 40% to 50% of all food produced in Europe is wasted, where 10% to 15% of this wastage happens during supply and delivery operations to consumers [7]. Furthermore, the food industry disposed about $25 billion of spoiled goods every year. In addition to this, within every home, annually $300 worth of bruised fruit, bad meat, and other perishable goods are thrown away [7]. This situation has severe consequences in the global supply of adequate food stocks and contributes to escalating food costs. In particular, the exposure to light and its dose is a very significant environmental factor influencing the qualityand lifetime of perishables [8]. Apart from that, light controlling can be used as a security measure, detecting if a package has been opened or badly manipulated. Some strategies have been followed, such as the inclusion of a tape that can be used to seal the package. This tape consists of two parallel conductive lines connected by conductive bridges that has to be broken to open the package, thus allowing the detection of any intrusion into the package [9]. In this regard, printed electronics (PE) results are especially attractive to develop these kinds of solutions. Printing has the capability of manufacturing over large areas with ultra-cost-effectiveness. Other relevant features of PE are the capabilities of achieving lightweight, thin, and environmental-friendly devices [10]. Different techniques can be used to fabricate RFID tags with PE, from screen-printing to inkjet printing or gravure; furthermore, the tag can be fabricated directly on the desired substrate, such as paper or plastic substrate. Not only the antenna and the connections can be manufactured by PE [11; 12], but also the sensor capabilities can be developed with these techniques [13; 14; 15]. Many sensors useful for packaging monitoring have been already developed [16]. It is crucial to minimize the number of discrete components that are integrated into these types of devices in order to reduce its cost per unit. To do that, the inclusion of printed sensors and antennas is mandatory. In this work, we present a novel ultra-high frequency (UHF) RFID tag developed on a flexible substrate including a photodetector. Some silicon solutions have been already developed in this direction [17; 18], but the development of a flexible and printed one has not been described until now. This tag has been fully fabricated by printing techniques, except for the silicon RFID chip, showing the strategy followed to develop a final working device making profit of different manufacturing techniques. In particular, the antenna and layout have been inkjet printed while the photodiode structure has been realized by spray deposition. Indeed, the major challenge of this work is to successfully fabricate the sprayed photodetector on a flexible substrate, using as the bottom electrode an inkjetted silver line. We present the difficulties of the fabrication integration and how we overcome them.