Stable inorganic thin film photovoltaic devices for a sustainable Internet of Things – Siphon

The Internet of Things (IoT) is the next technological revolution. 12.2 billion IoT devices were already connected in 2021 and the proliferation rate forecast is very high. Most of these devices are wireless, operate indoors and are powered through a primary battery, whose lifetime is about 8-25 months for such applications. In few years, hundreds of millions of IoT batteries may have to be replaced every day, which may doom the sustainability of the IoT ecosystem. Recent developments in low-power electronics and low-energy wireless communication protocols have considerably lowered the energy and power demand for IoT devices and opened new perspectives for powering them through artificial light energy harvesting. Indoor photovoltaic harvesters have a high Power Conversion Efficiency (PCE) potential because of the narrow-band photon light spectra of the Light Emitting Diode (LED) sources. Efficiencies at laboratory scale are yet dramatically lower than expected, because the quality of the material is crucial at low irradiance. Organic or hybrid organic-inorganic solar cells show the highest PCE for such indoor light applications but suffer from stability issues. Inorganic III-V or amorphous silicon devices have been developed but have low PCE. Among Indium-free chalcogenide absorbers, CuGaSe2 (CGS) is the most promising candidate but the synthesis of homogeneous and single phase CGS thin films at a temperature compatible with a lightweight and flexible substrate, mandatory for IoT applications, is difficult. We recently demonstrated that metal halide post-deposition treatments can be used to drastically decrease the synthesis temperature of CGS films as well as produce single phase thin film with large grains. This new and unique approach consisting in using metal halides as transport agent has therefore all the ingredients to break the technological locks that, to date, restrain the use of stable and industrially compatible CGS for indoor PV applications.