# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 3 Results and Discussion
### Tag Functioning  
Finally, the whole tag has been exposed to different levels of illumination and the readout of the sensor has been acquired 30 times consecutively. Table 1 represents the obtained ADC counts for each particular conditions as mean value with its standard deviation. In all cases, reference voltages and feedback resistor values of the SFE have been optimized in order to cover the maximum dynamic range of the ADC, 0-1023 counts. Those particular configurations are indicated in Table 1. Two main configurations have been tested to reflect two different real world conditions. First, the exposure to a full sun (AM1.5) has been considered, showing that the photodiode is responsive and its current can be easily detected by the RFID chip. This kind of illumination can be predominant into natural environments; however, it does not cover every typical range of application. In fact, a more realistic and cost-effective approach to detect light permeability of packages could involve the exposure of the parcel to a low-intensity light integrated into the shipping chain. In this second case, a monochromatic light (wavelength 600 nm to match the intensity peak of the EQE of the photodiode) has been shone on the photodetector, with different power densities in the range of some tens of \(\mu\)W (20-60 \(\mu\)W). A proper adjustment of the feedback resistor led to a clear detection of the light and a net discernment between the dark and the illuminated conditions, leading to stable measurements with a low dispersion. Indeed, the tag is capable of not only detecting exposure to light, but also obtaining the absolute luminance level of this exposure.  
Noticeably, all of these measurements were performed in a solar-cell operation regime for the OPD, namely, without any external bias applied, in order to guarantee the readability of the chip in passive mode and without any need for energy accumulators. Figure 8 represents the obtained photocurrent with respect to the optical power density. The sensing resoluti  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Device/Configuration** & **Optical Power Density (mW/cm\({}^{2}\))** & **ADC Count** & **Photocurrent (\(\mu\)A)** \\ \hline Sun simulator & & & \\ \(R_{\text{ap}}=185\) k\(\Omega\) & 0 & 0 \(\pm\) 0 & Below 1.5 \\ V\({}_{\text{refl}}=0\) mV & & 100 & 810 \(\pm\) 1 & 3.23 \\ \hline Monochromator (600 nm) & 0.02 & 0 \(\pm\) 0 & Below 0.8 \\ \(R_{\text{p}}=3875\) k\(\Omega\) & 0.04 & 256 \(\pm\) 8 & 0.12 \\ V\({}_{\text{refl}}=0\) mV & 0.08 & 884 \(\pm\) 10 & 0.19 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sensor performance in the RFID tag.  
Figure 7: **External Quantum Efficiency** (EQE) of the OPD fabricated with the PEI interlayer. In the visible range, peak EQE of 70% and responsivity up to 0.34 A/W are achieved, comparable to similar OPDs with evaporated cathode.  
247 \(\mu\)W/cm\({}^{2}\) in high optical power density mode (\(R_{ph}\) = 185 k\(\Omega\)) and circa 1.3 \(\mu\)W/cm\({}^{2}\) in low power density mode (\(R_{ph}\) = 3875 k\(\Omega\)).  
Finally, the tag was introduced in the climatic chamber varying the temperature from 30 \({}^{\circ}\)C to 20 \({}^{\circ}\)C in 10 min at Relative Humidity (RH) room conditions. The temperature measurements extracted from the tag were in agreement with the chamber sensor within a 0.5 \({}^{\circ}\)C difference, which is covered by their respective uncertainties.