# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 2 Materials and Methods
### Fabrication Process  
The antenna, interconnections and bottom electrode have been defined with DMP-2831(tm) Dimatix printer (Fujifilm Dimatix Inc., Santa Clara, CA, USA). The substrate temperature has been fixed at 60 \({}^{\circ}\)C while printing. All of these elements have been manufactured with only one printed layer. A drop space of 20 \(\upmu\)m has been settled in the printer for 40 \(\upmu\)m-landed diameter drops followed by a curing step at 125 \({}^{\circ}\)C for 30 min. The error caused by the accuracy of the printing process is one extra drop, a half drop on each side of the layout, as detailed in [21]. Thus, the error will be 40 \(\upmu\)m, which is insignificant compared to the printing dimensions of the prototype. Higher temperatures are not recommended because the glass transition temperature of this PEN film is 155 \({}^{\circ}\)C.  
An inkjet printed line of Ag ink has been used as a cathode for the OPD. The measured work function of the Ag line, however, is too high (\(\sim\)4.4 eV) for the realization of functioning photodiodes and has to be lowered throughout the use of a PEI thin film, as reported in the past in literature [22; 23]. Nevertheless, since the spin-coating of a PEI layer might affect the functionality of the whole printed systems, it has been necessary to realize it on a definite area via spray-deposition throughout a stencil, using the parameters developed and validated in [24]. Subsequent to the deposition of the PEI film, a 700 nm thick P3 HT:PCBM film as an active layer has been sprayed, followed by a 50 nm thick hole injection layer and a 300 nm thick PEDOT:PSS anode. The fabrication parameters of these layers are presented in full detail elsewhere [25; 26]. Figure 3 depicts the cross sectional view of the OPD, including its electrodes.  
The spray-deposition has been carried out through self-made spray-deposition setup. An atomizing gun (Krautzberger GmbH, Eltville am Rhein, Germany) is attached to a platform with adjustable height and fixed \(x\)-\(y\) position and it is controlled by an electromechanical 3/2-way valve. The valve is activated by an electronic timer to guarantee repeatable and accurate deposition times, and the nitrogen pressure is set by a pressure gauge. The substrate temperature is adjusted via a programmable hot-plate. PEI, P3 HT:PCBM and PEDOT:PSS were spray deposited at a nozzle-to-sample distance of 15 cm, with a substrate temperature of 60 \({}^{\circ}\)C, 120 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, respectively, with an atomization pressure of 0.5 bar, 1 bar and 1 bar, respectively. These parameters were chosen as optimal in terms of roughness and substrate coverage, based on the results of previous works [24; 25].  
The last step has been the assembly of the chip and the external components to the flexible tag. For this purpose, a three-step process has been followed. First, interconnects between RFID chip and silver pads have been made by using the conductive resin H20E. A double layer of dry adhesive has been placed on the bottom part of the chip to fix it to the substrate. Finally, the conductive resin has been cured by heating in an oven at 80 \({}^{\circ}\)C for 3 h. This heating step also improves the dry film adhesion, better gluing the chip to the substrate.