The Rise of Battery-Free Electronics

This post is about a very interesting concept I've come across, namely, indoor solar cells. I give an alternative take on how this technology can compete with batteries, but more importantly, providing focus to this overlooked way of recycling energy.

 

As the internet of things are expanding, the demand for batteries has also increased. Costly man-hours are needed to provide and maintain charged batteries for all these small devices. Imagine if the micro-electronics could instead harvest the energy needed from indoor light sources. This would not only be financially beneficial, but it could also potentially lower the use of non-rechargeable batteries.

The concept of photovoltaics isn't new. We've all heard of green energy harvested by solar panels. Solar cells are typically made of crystalline Si and works by exploiting the photovoltaic effect converting sunlight directly into electricity. In simple terms, this is done by using the incident light to create mobile charged particles within a semiconductor from which an electrical current is produced. Photovoltaic cells exploit the band gap between the valance band and the conduction band for semiconductors. The width of this band gap determines how much energy a photon must have to be absorbed and promote an electron from the valance band to the conduction band. If the photon has excess energy, this energy will be lost as heat [1]. The solar irradiance makes materials with a band gap of 1.1 eV - 1.4 eV optimal for harvesting energy outdoors, however, indoor light sources typically emit light in the visible range of the electromagnetic spectrum (380 nm - 700 nm), making a band gap of 1.9 eV - 2.0 eV the optimal band gap for indoor photovoltaics cells, IPVs [2]. These differences in band gap underlines the importance of specialized indoor solar cells to ensure an optimal harvest of energy.

Materials Used for Indoor Photovoltaic Systems

Researchers are looking into both organic and inorganic materials for the IPVs. One of the materials that have received increased attention in recent years is perovskite photovoltaics, PPV, as they have a low-cost combined with energy conversion efficiencies as high as >25 %, exceeding the conversion efficiency of crystalline silicon (22.3 %). Furthermore, the energy conversion efficiency has been reached at band gaps of 1.6 eV, which is less than the optimal band gap for IPVs. It is believed PPVs can be modified by changing the chemical composition, reaching the optimal band gaps and a theoretical cell efficiency of more than 50 % [3].

Perovskites have the general formular ABX₃, where the A-site is typically occupied by methylammonium, MA, formamidinium, FA, rubidium or cesium, while lead typically occupies the B-site and X is occupied by a halide. These hybrid organic-inorganic perovskites allow for numerous different combinations as the A-site can be occupied by multiple different cations and the X-site might be occupied by a mix of halides [4]. It has been found that varying the ratio of I and Br in lead halide perovskites allows for bandgaps between 1.5 eV and 2.2 eV [3]. Mathews et al. have investigated (Rb_0.01Cs_0.05)(MA_xFA_1-x)_0.94Pb(Br_xI_1-x)₃ with x = 0.17 and x = 0.5. The bandgaps were determined to be 1.63 eV and 1.84 eV respectively. They simulated the light of an office space without solar insolation, by obtaining an illumination of 0.16 mW cm^-2 using a compact-fluorescent lamp. The cell with a bandgap of 1.63 eV showed an efficiency of 21.4 %, while the cell having a band gap of 1.84 eV yielded an efficiency of 18.5 %. When tested under solar illumination, the cells showed an efficiency of 16.5 % for x = 0.17 and 11.9 % for x = 0.5, underlining the importance of the bandgap matching the type of illumination [3].

Indoor Photovoltaic Systems for Micro Electronics

One can take different approaches to power IoT nodes from ambient energy. The harvest-store-use architecture, where a rechargeable battery is connected to the IoT node, allows for storage of energy whenever lights are on. Another design, the harvest-use architecture, is a battery-less system. This means the IoT node is not able to store the harvested energy, and the node may have low reliability and availability due to fluctuating ambient energy. The harvest-use architecture is a more environmentally friendly solution than the harvest-store-use architecture, however, it is only suitable for IoTs which allow for intermittent operation [2]. This type of architecture could indeed clear the way for battery-less systems, however, these battery-less systems will likely stay on IoT level, but indoor PV systems could potentially work as a supplement to obtain energy for other electronics used indoors. Even though IPVs do not make way for a battery-free world, they most certainly provide a tremendous opportunity to reclaim energy used to light up rooms which is not to be overlooked.