EPSRC-FNR Collaborative Proposal: Radiative Efficiency in Advanced Sulfide Chalcopyrites for Solar Cells (REACh)

The renewable and carbon free energy system of the future will rely heavily on electricity generated by solar cells. Solar modules have decreased in price so much in the last few years, that now the cost related to the rest of the system has becomes the main expense. These cost are related to the area of the solar cell. Therefore it remains essential to increase the efficiency of solar cells, because then we need less area to produce the same amount of electricity.

Conventional solar cells have been refined over several decades and their efficiencies are now approaching theoretical limits. A new way forward is provided by tandem solar cells, where we stack two different solar cells on top of each other, so that each can make better use of the solar spectrum and the efficiency becomes higher. We are working on a new material, sulfide chalcopyrite, which can be used in a thin film (using only small amounts of raw materials), is stable and has already shown promising efficiencies. We want to use this material as the top cell, combining it with well-known solar cell materials like silicon to produce a tandem cell. These thin films are not single crystals but consist of tiny crystals - on the micrometre scale - known as "grains", which are butt up against one another at "grain boundaries". These grain boundaries can be problematic because they differ from the perfect crystal, and are hence places where we can lose the electrons generated by the incoming sunlight at a rapid rate. Also, grain boundaries can block the current moving through the solar cell. Usually, there are many different grain boundaries in a thin film, some are detrimental for the solar cell and some are benign.

In this project we want to understand the role of grain boundaries in sulfide chalcopyrite solar cells. We will study luminescence, which is the light that is emitted by a solar cell material when it is excited by a laser or an electron beam, thereby generating electrons. We use this light to check the quality of the thin films. A good solar cell material will also emit a lot of luminescence, because the electrons that lead to light emission are also those that would carry the current in the solar cell - and if they are able to emit light they are not lost at defects like grain boundaries. For the best films we will use an electron microscope, which allows us to study the luminescence with a very high spatial resolution. Thus, in the electron microscope, we can examine each individual grain boundary and see how much it affects the luminescence. We can then check with the electron microscope what is special about those grain boundaries that do not lead to loss of electrons. This information will help us adapt the growth process of our thin films to make them into better solar cells, by avoiding growth of the more damaging types of grain boundary.