Towards efficient, lead-free solar cells
Photovoltaic cells, also known as solar cells, convert light into electricity. The most common type, silicon solar cells, has been in production since the 1950s. However, manufacturing this type is difficult and energy-intensive. A newer version was supposed to solve these problems. Available since 2009, perovskite solar cells are named after the mineral. Technically, the mineral has a certain crystalline structure, which many materials can exhibit, so within the solar cell industry the name perovskite applies to that structure rather than the mineral. Perovskite cells are electrically efficient while also being easy and cheap to make. Having numerous clear advantages over established solar cell technologies, perovskite cells quickly became the most promising new type of new solar cell worldwide. Unfortunately, these cells also contain toxic lead, which is being phased out of electronics manufacture. The solar cell researchers are now searching for alternative perovskites.
Investigating double perovskites
The EU-funded PhotSol project, undertaken with the support of the Marie Skłodowska-Curie Actions programme, investigated such alternatives. The team examined lead-free perovskites, including one called double perovskite. However, the electrical efficiency of these materials is still low. Therefore, PhotSol researchers tried to improve their efficiency by identifying and removing bottlenecks, which affect performance in extremely complicated ways called loss mechanisms. Bottlenecks are often caused by defects in the material. Defects can include crystal malformations or impurities. Surfaces and interfaces can also be sources of defects. “Once we have identified what kind of defect limits the performance,” explains Wolfgang Tress, senior project researcher, “we can specifically target this defect in the device fabrication. For instance, if interfaces are the major culprit, as is often the case in solar cells, passivation layers or replacement of interfacial layers might reduce the defect density.”
Setbacks, and a new direction
Although the team successfully documented the bottlenecks of one type of double-perovskite solar cell, this material did not show sufficient potential for use as an efficient solar cell. Furthermore, the team’s work was difficult to reproduce, a problem that often plagues perovskite researchers. So the consortium has abandoned this direction and will be examining similar but newer materials. Nevertheless, the study yielded useful characterisation protocols. “During the characterisation we made an unprecedented discovery,” adds Tress. “For the first time, we managed to measure the electroluminescence spectrum.” This can be obtained by operating the solar cell as a light-emitting diode, first applying a current and then detecting the emission. Although the signal was very weak, as expected, the spectrum was surprisingly different to the photoluminescence. In lead perovskites, the electroluminescence and photoluminescence spectra are normally identical. The reason for the difference is unknown, but the team will enthusiastically investigate. PhotSol researchers will continue developing new photovoltaic technologies, focusing on understanding the physics of the devices to improve their design. To help with this, Tress has received an ERC Starting Grant. Ultimately, the work will contribute to the development of truly efficient and lead-free solar cells. These, once available and widely installed, will lessen the world’s reliance on fossil fuels, significantly reducing greenhouse gas emissions.
Keywords
PhotSol, solar cells, perovskite, lead-free, photovoltaic, double perovskite
