# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 3 Results and Discussion
### Photodiode Characterization  
Although spin-coating is a well-established technology in the fabrication of OPDs [32], it presents few major disadvantages. In a first instance, the use of spin-coating as a deposition technique prevents a large-scale uniform deposition of thin films, especially on flexible or uneven substrates such as plastics or paper. Furthermore, spin-coating is intrinsically a full-sample deposition method, and the patterning is only possible through classical photolithography, a relatively complex process which includes the use of solvents, but not always compatible with plastic and paper electronics. The usage of easy-to-pattern deposition techniques is then necessary for the fabrication of the OPDs into an integrated printed  
Figure 5: Experimentally measured (green solid line) and simulated (red solid line) magnitude (**a**); and phase (**b**) of differential S-parameter of the dipole antenna.  
system, rendering inkjet printing [23] and spray-coating [33] as primary choices for the fabrication of integrated printed systems.  
The typical material stack of an OPD includes a low work function conductor to be used as a cathode, the photoactive material and a high work function conductor to be used as an anode, and at least one of the two contacts must be semi-transparent to allow the light to penetrate into the photoactive material. Albeit the realization of semi-transparent solution-processable high work function electrodes has been demonstrated in the past with the use of conductive polymers, such as PEDOT:PSS [23; 26; 34], the obtainment of low work function materials is hindered by their high reactivity to oxygen. For instance, printed silver, which is characterized by a nominal work function of 4.26 eV, as a consequence of oxygen chemisorption, can show an increase in this parameter of up to 1 eV, leading to poor reproducibility and sub-optimal devices [23; 35]. One of the most reliable solutions to this problem is the use of surface modifiers, such as polyethylenimine (PEI), to induce permanent dipoles, consequently reducing and stabilizing the work function of the material [22]. The PEI thin layer (around 10 nm) is typically deposited through spin-coating or dip coating, which leads to the deposition onto the whole substrate and could compromise the functionality of the other devices integrated there. Conversely, the obtainment of a spray-deposition method through a stencil guarantees the realization of a precisely patterned PEI layer, which represents the basis for the production of the stacked photodetector in a complex system.  
Figure 6 shows the Current Density vs. Voltage (JV) characteristics of the photodiode in dark conditions and under AM1.5 illumination. The blue curve is relative to an OPD where the Ag electrode has not been modified throughout the PEI deposition, while the red curve is relative to an OPD with PEI modification, whose structure is depicted in Figure 3. The comparison of the JV characteristics clearly shows a substantial difference in the dark current (more than two orders of magnitude), associated with an overall instability in time, which is reflected in the noisy and irregular shape of the characteristic curve. On the other hand, the characteristic curve of the photodiode with the PEI layer is well aligned to photodetectors obtained with similar material stacks [24; 25; 26; 36]. Finally, Figure 7 shows the EQE under a bias of \(-\)4 V of such a device, which results in being essentially flat in the wavelength range between 400 nm and 600 nm, with a maximum value of 70%. The responsivity of this sensor is equivalent to other photodetectors in the literature [26; 37].  
Figure 6: Dark (dashed lines) and illuminated (solid lines) JV characteristics, in linear (**a**) and logarithmic (**b**) scale, of the fully-sprayed organic photodiode with and without PEI. Current density as low as \(5\times 10^{-4}\) mA/cm\({}^{2}\) have been reliably achieved.