Femtosecond events dictate efficiency of future solar cells

The research, at the Universities of Cambridge and Milan, studied how quickly electrons created as sunlight hits the solar cell material need to reach the cell’s electrode to be converted into flowing electric current before their energy starts to decline. The teams, led by Prof Sir Richard Friend at St John’s College, Cambridge, and Prof Guilio Cerullo at the Polytechnic University of Milan, found that this time is of the order of 10 femtoseconds — a femtosecond is 10^-15 seconds, or a millionth of a billionth of a second.

If the cells can work that fast, they could achieve an efficiency of 30 per cent or possibly greater — in rough terms, the greatest efficiency that solar cells could conceivably achieve. Today’s best silicon-based solar cells typically operate at efficiencies closer to 20 per cent. But perovskite cells are much thinner than silicon cells, which gives the team hope that this speed could be achieved.

Friend’s team in Cambridge used two-dimensional spectroscopy,  a technique developed in Milan, to carry out the study. They used two lasers to simulate sunlight on a sample of lead iodide perovskite, a candidate material for solar cells, with a third laser acting as a probe to measure how much of the sunlight was being absorbed. When a free electron is first created by absorption of a photon of light by the perovskite, it is moving very fast and is referred to as “hot”. But soon after, it starts to collide with other electrons and loses energy. This process changes the amount of light absorbed by the cell — a change detected by the probe laser. The researchers found that electron collisions begin between 10 and 100 femtoseconds after light is absorbed by the perovskite.

In order to reach the maximum efficiency, the electron needs to reach an electrode and be extracted from the cell while it is still hot. This does not happen in conventional silicon solar cells, which are about a millimetre thick. Perovskite cells, however, are only a thousandth of this thickness.

"The timescale that we calculated is now the time limit that we have to operate within if we want to create super-efficient, hot carrier solar devices,” explained Johannes Richter of Friend’s Cambridge group, lead author of a paper on the research in Nature Communications.

“We would need to get electrons out before this tiny amount of time elapses. We are talking about doing this extremely quickly, but it's not impossible that it could happen. Perovskite cells are very thin and this gives us hope, because the distance that the electrons have to cover is therefore very short." Moreover, the team believes that nanostructures could be created within the cells that could further reduce the distance the electrons need to travel.

Perovskites are seen as having great potential for solar cells because they can be made using simple, low-cost solution-based processes, making them cheaper to make than conventional silicon cells. Moreover, being so thin, they are flexible and could potentially be incorporated into fabric.