Researchers have managed to quantify the astoundingly high speeds at which solar cells of the future would have to work in order to stretch what are currently seen as natural limits on their energy conversion efficacy.
The study, which examined photovoltaic devices based on a type of materials known as perovskites, indicates that these could attain unparalleled levels of super-efficiency. But to achieve that, they will have to convert sunlight into electrons and then extract these as electrical charge within only quadrillionths of a second – a few “femtoseconds”, to confer them their scientific name.
Moving electrons at this ultrafast rate would enable the manufacture of “hot carrier” cells. These are solar cells which can produce electricity more efficiently by utilizing the extra kinetic energy that electrons have for a short-lived moment just after they are formed, while they are traveling at high speed.
The quantity of electrical energy that can be taken from a hot carrier cell, compared with the quantity of light absorbed, could potentially match or even break an energy efficiency rate of 30%. In general terms, this is the highest energy efficiency that solar cells can possibly realize – although basic silicon cells usually have efficiencies closer to 20% in practice.
In spite of the minute fractions of time involved, the authors of the recent paper say that it is possible that perovskites could eventually push this efficiency boundary.
The research paper, published in the journal Nature Communications, was authored by academics in Italy and the UK. The British team involved researchers in the Cavendish Laboratory’s Optoelectronics research group of Professor Sir Richard Friend, a Fellow of St John’s College, Cambridge. The Italian team is based at the Politecnico di Milano in the group of Professor Guilio Cerullo.
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. 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.
Johannes Richter, a PhD student in the Optoelectronics group and the lead author of the paper,
Perovskites are a group of materials which could before long swap silicon as the material of choice for a number of photovoltaic devices. Although perovskite solar cells have only been built within the last couple of years, they are already nearly as energy-efficient as silicon.
Relatively because they are significantly thinner, they are a lot cheaper to produce. While silicon cells are around a millimeter thick, perovskite equivalents have a thickness of approximately one micrometer, which is about 100 times thinner than a human hair. They are also highly flexible, meaning that besides being used to power machines and buildings, perovskite cells could ultimately be integrated into things like tents, or even clothing.
In the latest study, the researchers wanted to know for how long the electrons created by these cells retain their maximum possible levels of energy. When sunlight touches the cell, light particles (or photons), are changed into electrons. These can be drawn out via an electrode to harvest electrical charge.
For a short moment after they are produced, the electrons are moving very rapidly. However, they then begin to collide, and lose energy. Electrons which preserve their speed, before collision, are referred to as “hot” and their extra kinetic energy means that they have the potential to create more charge.
Imagine if you had a pool table and each ball was moving at the same speed. After a certain amount of time, they are going to hit each other, which causes them to slow down and change direction. We wanted to know how long we have to extract the electrons before this happens.
The Cambridge team exploited of a technique formulated by their colleagues in Milan known as two-dimensional spectroscopy. This involves pumping light from two lasers on to samples of lead iodide perovskite cell so as to mimic sunlight, and then employing a third “probe” laser to measure how much light is being absorbed.
After the electrons have collided and slowed down, and are beginning to occupy space in the cell, the quantity of light being absorbed varies. The time it took for this to occur in the study effectively allowed the team to define how much time is available to extract electrons while they are still “hot”.
The research found that electron collision events began to occur between 10 and 100 femtoseconds after light was first absorbed by the cell. To maximize energy efficiency, the electrons would therefore need to reach the electrode within 10 quadrillionths of a second.
The researchers are however hopeful that this might be possible. As well as exploiting the intrinsic thinness of perovskite, they believe that nanostructures could be developed within the cells to decrease further the distance that the electrons need to travel.
That approach is just an idea for now, but it is the sort of thing that we would require in order to overcome the very small timescales that we have measured.
The research paper, ‘Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy’, has been published in Nature Communications.