  Communication range in passive Radio-Frequency Identification (RFID)
front-end devices is a critical barrier in the real-world implementation of
this low-cost technology. Purely passive RFID tags power up by harvesting the
limited RF energy transmitted by the interrogator, and communicate by
backscattering the incident signal. This mode of communication keeps
manufacturing costs below a few cents per tag, but the limited power available
at the tag undermines long-range deployment. In this paper, we present an
approach to use Photovoltaics (PV) to augment the available energy at the tag
to improve read range and sensing capabilities. We provide this extra-energy to
the RFID integrated circuit (IC) using minimum additional electronics yet
enabling persistent sensor-data acquisition. Current and emerging thin-film PV
technologies have significant potential for being very low-cost, hence
eliminating the barrier for implementation and making of PV-RFID wireless
sensors. We reduce the long-range PV-RFID idea to practice by creating
functional prototypes of i) a wireless building environment sensor to monitor
temperature, and ii) an embedded tracker to find lost golf balls. The read
range of PV-RFID is enhanced 8 times compared to conventional passive devices.
In addition, the PV-RFID tags persistently transmit large volumes of sensor
data (>0.14 million measurements per day) without using batteries. For
communication range and energy persistence, we observe good agreement between
calculated estimates and experimental results. We have also identified avenues
for future research to develop low-cost PV-RFID devices for wireless sensing in
the midst of the other competitive wireless technologies such as Bluetooth,
Zigbee, Long Range (LoRa) backscatter etc.
