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Photon-recycling for high-efficiency energy harvesting in GaAs photovoltaic devices on silicon

Periodic Reporting for period 2 - RECHARGE (Photon-recycling for high-efficiency energy harvesting in GaAs photovoltaic devices on silicon)

Reporting period: 2019-06-05 to 2020-06-04

Billions of wireless sensors are expected to be installed over the coming decade, with almost half to be located inside buildings. Due to the limited lifetime of batteries, this vision will not be realised without energy harvesting solutions providing power autonomy to sensors, a topic specifically prioritised in the 2016-2017 Work programme of Horizon 2020. Currently, the use of batteries to power these devices places significant constraints on their power consumption, where the range and frequency of data transmission are curtailed to achieve sufficient battery life, and the range of applications is also limited to the ones that allow battery replacement. Additional operation and maintenance costs are also incurred by providing replacement batteries.
Indoor photovoltaics has the potential to solve these hardware issues, providing greater reliability and operational lifetimes in wireless sensor networks. Persistently powering individual nodes by harvesting ambient light using small ∼cm2 photovoltaic cells is becoming possible for more and more wireless technologies and devices. In this project, we characterize the performance of various indoor photovoltaic cells under ambient light sources. Given the interest in commercializing different photovoltaic cells in this growing market, we have also built techno-economic models to compare different technologies and tackled the outstanding research questions that must be answered by the indoor photovoltaic community to enable self-powered, indoor-located IoT nodes.
If successful, this project will enable the deployment of wireless sensors at scale to generate the Big Data required to optimize operations and increase efficiency across many industries primarily located indoors such as energy-efficient and smart buildings, logistics and inventory sensing and tracking, health monitoring, or robotic systems. In order to widely deploy wireless sensors for indoor applications, we are investigating the manufacture of devices that are low-cost, self-powered, easily deployable at scale and could remain operational for many years.
The project has made excellent progress, testing novel GaAs and GaAs-on-Si, perovskite and CdTe indoor photovoltaic harvesters. In addition, the project has demonstrated a battery-free wireless temperature sensor; one of the first perovskite powered IoT devices. We fabricated wide-bandgap indoor photovoltaic (IPV) cells, better matched to absorb indoor light sources. We varied the Iodide-Bromide composition to produce a 1.84 eV band-gap device, close to the optimum for IPV cells. We measured high efficiency (18.5%) and high photo-voltage (0.95 V) under ambient illumination. We subsequently demonstrated a low-cost self-powered environmental sensor by using three wide-bandgap perovskite IPV cells to power an RFID Ultra-High-Frequency (UHF) integrated chip with an onboard temperature sensor, connected to a laser-cut copper antenna. Compared to a standard backscatter RFID tag sensor without an external power source, we demonstrated a maximum 7.2x boost in communication range, achieved by reducing the amount of RF power harvested that is used to power the tag’s IC, using the external power supplied by the perovskite IPV cells instead, such that the maximum RF signal is backscattered.
As a confirmation of the efficacy of our wireless temperature sensor, we measured the temperature in our laboratory over a period of ~8 hours. Our tag sensor setup resulted in 23625 measurements over a period of 29454 seconds or a measurement every 1.24 seconds. These high frequency measurements are possible because our design is not dependent on waiting for a supercapacitor or battery to charge up after every measurement; instead current is drawn from the IPV module to boost the backscatter signal as required. Furthermore, the RFID reader manufacturer’s claim maximum throughputs exceeding 1000 tags per second, whereas practical throughput is around 60 tags per second (depending on the total population of tags in the environment and the reader parameters). We find the IPV-backscatter device is able to communicate every few seconds, allowing the collection of 1000’s of data points in an hour. This is a two order of magnitude increase in data collection rate compared to other indoor-light harvesting sensors and can undoubtedly provide the high-resolution data required to train machine learning and artificial intelligence algorithms for many applications.

The results of this project were the disseminated through multiple high impact journal papers in Advanced Functional Materials, Joule (x2), Advanced Materials Technologies, Applied Energy, Optics Express, IEEE Sensors and the IEEE Internet of Things Journal . Additionally the work was presented at 3 international conferences including the Materials Research Society's 2019 Fall conference and the 2019 IEEE Photovoltaics Specialists Conference.
There are multiple potential long-term impacts from this project. The potential for integrating small solar cells into the wireless sensors needed to power the fast-growing internet of things (IoT) ecosystem, many of which are located indoors, is large. This market could represent a unique opportunity for thin film PV technologies, and perovskites in particular, to reduce the risk inherent to ramping up commercial scale production. The market for indoor PV cells, such as those used to power watches and calculators, was worth just US $140 million in 2017. But price reductions in solar are beginning to line up with decreasing energy requirements for technologies such as wireless sensors, RFID tags, and Bluetooth beacons. We predict billions of these sensors will be installed in the coming years, despite their current reliance on battery power, leading to sacrificed performance for increased battery life, and additional operations and maintenance costs associated with replacing these batteries.
Integrating PV cells into the devices could solve a lot of these problems, leading to a boom in the indoor PV market, surpassing $1 billion annually in 2024. Around half of the sensors are expected to be placed indoors, with little or no access to sunlight, meaning the PV cells would have to rely artificial light, typically at intensities three orders of magnitude lower than sunlight. Our techno-economic analyses so far, suggest that the poor low light performance of silicon would not make it a good candidate for indoor PV applications, opening the door for various thin film technologies. Emerging technologies including GaAs, organic PV and perovskites have exhibited the kind of low light performance needed for indoor PV, and their well-documented stability issues would be less of a problem in an indoor setting. The type of sensor being powered may have a lifetime much shorter than the 20 years plus which has become the industry standard. For perovskites in particular, we theorize that the indoor PV market could provide an opportunity to mitigate many of the risks associated with commercial introduction: Our market analysis makes it clear that the rapid growth of the indoor IoT market could provide an ideal jumping-off point for perovskite products, allowing a new PV company to establish customers, revenue, and credibility before establishing larger-scale solar panel manufacturing facilities.