Photovoltaics for indoor energy harvesting

The Internet of Things revolution requires a low-cost, stable, and highly efficient power source to allow autonomous operation of smart objects and wireless sensors even at very low light levels. Indoor photovoltaics (PV) has the potential to fulfil these requirements, providing independence from the main grid, portability, and improved sustainability for low-consumption devices. Whereas polycrystalline silicon dominates the outdoor solar cell market, amorphous silicon is commercially more suited for products used inside buildings, delivering higher efficiencies under indoor illumination (with its extremely lower intensities and narrower spectra compared to sunlight). In very recent years, there has been a remarkable rise in the research and development of new generation photovoltaic solar cells, i.e., those based on organic, dye-sensitized and perovskite absorbers, focused on indoor applications with efficiencies rising well above those possible under the sun reaching and even surpassing the 30 % power conversion efficiency threshold. This review provides a systematic overview of indoor PV devices, highlighting the main progress achieved and the strategies to design highly efficient cells as well as the issues to be resolved for this field to continue to prosper. We also analyse the differences in device design for solar cells meant for operation in the outdoors vs indoors. Markets and applications to be tapped by indoor photovoltaics for light harvesting are huge, ranging from building-integrated elements to consumer products, biomedical devices, wireless sensors and communication technologies.