Reducing energy loss can greatly increase the yield of solar cells. Before the incoming sunlight can generate electricity, a large part of the energy is lost in heating up the solar cell. Now researchers from Technology Foundation STW, NanoNextNL, the University of Amsterdam and TU Delft have gained experimental insights into a quantum process in silicon nanocrystals by which excess light energy is transferred to adjacent nanocrystals, thereby avoiding energy losses to heat. The mechanism was only recently suggested. The researchers’ measurements show that the experimental behaviour is in line with the theoretical predictions. This opens the way to producing solar cells with a considerably higher yield than the 25% that is currently achieved in practice. The researchers published their findings on 18 March in an Advance Online Publication on the Nature Photonics website.
Band gap
A solar cell consists of semiconducting material. This material absorbs light particles (photons) due to the fact that electrons in the valence band absorb the energy from the photons and thereby end up in the conduction band. As a consequence of this, they leave a hole in the valence band. Electrons in the conduction band are able to move around freely. Electron-hole pairs can thus be extracted from the material, and in this way, electrical current can be generated. All of the energy that is in excess of the so-called band gap (the energy difference between the valence band and the conduction band) is lost in a few femtoseconds (millionths of a billionth of a second) to lattice vibrations in the material, thereby generating heat in the solar cell.
For silicon, the most frequently used semiconductor in solar cells, this means that only a maximum of around 30% of the energy from the solar spectrum can be converted into electricity. In practice, however, yields of just 25% are achieved; the rest of the incoming energy is lost.
Increasing the energy yield
This can be countered with the silicon nanocrystals in the material. As the silicon nanocrystals are the size of a few nanometres, quantum effects come into play: a process develops that produces many more charges than is the case for the absorption of photons alone. In the jargon, this process is known as Space-Separated Carrier Multiplication (SSCM). It causes the energy from the light that is in excess of the abovementioned band gap to spread very rapidly to other electron-hole pairs in adjacent nanocrystals before heating takes place. In this way, one high-energy photon is responsible for multiple electron-hole pairs in adjacent nanocrystals. In the ideal scenario, this process can increase the efficiency of silicon solar cells to levels approaching 50%. Given that the current battle to improve efficiency is being fought over fractions of percentages, this represents a substantial improvement.
Figure 1. Silicon nanocrystals (in the circles) in a sample of a silicon dioxide matrix in which the crystals have been embedded. The image was made using a scanning transistor electron microscope (STEM).
An incredible insight
While scientists already knew of the SSCM process, until recently, it had only been demonstrated indirectly, by measuring how many photons are emitted by the nanocrystals in the material; this is a measure of the energy that had previously been absorbed. Now the researchers have directly observed the process. To do so, they directed femtosecond laser pulses onto a sample containing nanocrystals and were able to directly observe the results using the same laser. Because the pulses are shorter than the time it takes for heating to occur, the researchers could use their pump-and-probe set-up to ‘observe’ the absorption process at first hand and thus see the moment at which the new energy carriers are created. This yielded an incredible insight: SSCM appears to take place in a direct way. That is to say, the absorption of a high-energy photon is directly followed by splitting in energy and space, leading to the forming of multiple electron-hole pairs in adjacent nanocrystals. This process is particularly attractive because it makes the lifetime of the incoming energy from the photons much longer than the initial absorption process, meaning that there is much more opportunity for energy transfer.
Figure 2. Artistic impression of the SSCM process. As high-energy photons hit various nanocrystals, a number of electron-hole pairs are produced in one go. Illustration by Wieteke de Boer.
Towards much greater efficiency
Silicon nanocrystals in solar cell material absorb and re-emit photons, partly at different wavelengths. This spectrum-reshaping effect produced by the nanocrystals is being comprehensively researched in order to increase the efficiency of solar cells in an indirect manner. The new insights into the SSCM process make it clear that the nanocrystals can also contribute directly to the more efficient conversion of sunlight. With this, the new insights represent a fundamental step forward in the development of knowledge in this area, and appear to clear the way for solar cells that are composed of silicon nanocrystals that can reach substantially higher efficiency levels than existing cells.
