Photovoltaics to Power the Internet of Things
It is hard for any new photovoltaic (PV) technology to beat silicon in bulk power generation, but there are many new applications where the unique properties of novel PV can have particular benefits. As the “Internet of Things” (IoT) develops, there is an increasing need for power harvesting to drive autonomous sensors and communications devices integrated into everyday products, where connecting to mains electricity or providing primary batteries is prohibitively expensive or inconvenient. The requirements for PV in this application are wide and varied – efficiency is still important, but the relevant efficiency is under indoor illumination levels and typical indoor spectra, where silicon cells typically perform badly. Other features such as robustness, flexibility, ease of integration with storage and electronics, and aesthetic appearance are often also of crucial importance. This project will focus on understanding the physics determining the performance of solution-processed photovoltaics under indoor lighting conditions. The project will study both organic (polymer:fullerene, polymer:small-molecule, and polymer:polymer) PV devices, which are generally considered to work well at low illumination levels, and perovskite devices. Expertise in perovskite device fabrication and analysis will be provided by Dr Sam Stranks. A key initial challenge will be to set up accurate measurements of cell performance under relevant conditions, which are far from the 1-sun AM1.5G conditions usually investigated. The science questions to be answered include: (i) What recombination processes limit the efficiency at low intensity, and how do they differ from the processes dominating under solar intensities? (ii) What features of the density of states in organic bulk heterojunctions and perovskites determine the intensity dependence of the open-circuit voltage and maximum-power-point voltage? and (iii) How can processing and materials choice be tuned to further reduce these intensity dependences? In addition to carefully calibrated current-voltage measurements, transient photovoltage (large signal and small signal with background illumination) will be a key measurement technique. Having established scientific understanding of the issues, engineering of device architectures will be carried out to match the various application-dependent requirements for indoor PV operation. The project is well-aligned with the goals of Cambridge’s Energy@Cam strategic research initiative, which has identified a Grand Challenge on Materials for Energy-Efficient ICT for particular focus. This Challenge forms the basis for the Cambridge spoke of the Sir Henry Royce Institute, which will provide access to state-of-the-art equipment and will facilitate interactions with potential manufacturers (including Eight19 Ltd) and users of PV for IoT applications.