Scientists from the Cavendish Laboratory at the University of Cambridge (UK) and the Politecnico di Milano (Italy) have quantified the ultrafast speed at which perovskite solar cells need to process sunlight to stretch the natural energy conversion efficiency limits and ultimately achieve paradigm-shifting levels of super-efficiency.
The solar cells would have to convert sunlight to electrons in a few femtoseconds. “When a solar cell absorbs light, it creates an electron, which is initially moving very fast — we call this electron ‘hot´,” says Cambridge PhD student Johannes Richter from Cavendish Laboratory's Optoelectronics research group. There is an impediment, though, to extracting this extra energy: This ultrafast moving electron is surrounded by many resting electrons it can collide with. “Imagine a pool table,” says Richter. “You hit one ball and it moves, but as soon as it collides with the surrounding balls, it will lose its energy.” In this new study, Richter and his fellow researchers measured for the first time, how fast these electrons collide in perovskite solar cells. From that, they were able to deduce the time limit for extracting that extra kinetic energy — thus, for creating highly efficient hot-carrier perovskite solar cells.
Designing a super-efficient solar cell
“Solar cells are assumed to have an ultimate efficiency limit of around 30%,” Richter says. “The ultimate goal is to break the 30% efficiency limit by utilizing new design approaches — this is called a 'super-efficient' solar cell.”
The efficiency limit of 30% is based on the assumption that the electrons lose all of their speed before they are extracted, which is, in fact, the case in most solar cells. “However, if we were able to make use of the electron's speed — physicists call it 'kinetic energy´ — we could reach efficiencies well above 30%,” says Richter. “Such super-efficient devices are called 'hot-carrier solar cells.' ”
Key applications for super-efficient solar cells
Richter, who was also the lead author of the paper “Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy,” published in the journal Nature Communications, says super-efficient solar cells are favored anywhere where one requires a large electricity output from a small surface area, for example, in space for powering satellites or the International Space Station (ISS).
Finding nanostructuring approaches for perovskite solar cells
“By knowing how fast electrons collide, we can now determine the distance electrons can travel before collision,” Richter says about the potential impact of the new findings on the design of future perovskite solar cells. “In order to extract the electrons within a few tenths of nanoseconds, we now know that nanostructuring approaches will be required.”
Measuring hot-carrier processes with 2D spectroscopy
Richter says since the electron collision times were so fast — only 10 quadrillionths of a second — the team needed a special experimental tool. They collaborated with the research group of Professor Giulio Cerullo at the physics department at Politecnico di Milano. “[They] use a special technique called 2D spectroscopy, where they use three ultrashort laser pulses to measure these processes.”
Dr Felix Deschler from the Cavendish Laboratory, who supervised the study, and his team are now defining precise design criteria for hot-carrier perovskite solar cells in order to determine potential techniques for manufacturing such ultimate solar cells.
Written by Sandra Henderson, Research Editor, Solar Novus Today