The Promise of Perovskite

Drawbacks of Silicon Photovoltaics

For decades, silicon has been the mainstay of the photovoltaic solar panel, the device that sucks in sunlight and spits out electricity as sustainable energy. Unfortunately, making highly pure crystalline polysilicon for these panels comes at a high price in terms of the energy consumed in the manufacturing process. As such, the energy payback time for a silicon solar panel is rather long, ultimately limiting the rapid scalability of this technology.

Fortunately, there is another class of photovoltaic materials that have begun to emerge from research laboratories in recent years – the perovskites. The archetypal member of the mineral class is calcium titanium oxide (calcium titanate), first discovered in Russia’s Ural Mountains by Gustav Rose in 1839 and named for the mineralogist Lev Perovski. The term perovskite is used to classify any mineral with the same cubic crystal structure.

Do Perovskites’ Green Credentials Live up to the Hype?

Research on the light-converting possibilities of perovskites has led to a range of new photovoltaic materials that require little energy in fabrication and have high power-conversion efficiencies. Jian Gong and Fengqi You of Northwestern University in Evanston, IL, USA, and Seth Darling of Argonne National Laboratory, Lemont, and the University of Chicago, IL, USA, have performed a life-cycle assessment of two different types of solution-processed perovskite solar modules. They hoped to shed light on the environmental performance of this promising class of photovoltaic materials and to validate the materials’ green credentials.

They used two modules as models to test the ecological impact of novel photovoltaic materials. The first module is equipped with fluorine doped tin oxide (FTO) glass, a gold cathode and a mesoporous titania scaffold. The second has indium tin oxide (ITO) glass, a silver cathode and a zinc oxide thin film. Their assessment of these two modules takes into account all the energy costs of materials and component fabrication and assembly, and offsets these against the likely energy gains of the use of such modules to generate electricity from sunlight. The team also compared CO₂ emissions for the technologies. The lifecycle assessment demonstrates that in comparison with existing commercial technologies, the energy payback time (EPBT) is the shortest for perovskite modules.

The team also found, however, that carbon emissions are not entirely offset by such a module. They suggest that perovskite panels should be the focus of photovoltaic research, but having proven themselves effective, research should now be directed towards improving system performance ratio and device lifetime as well as reducing the need to use rare or toxic metals.

The Sun’s Rising Importance for Sustainable Energy

The researchers point out that currently solar energy represents only a small contribution to global electricity generation, but the number of photovoltaic systems in operation has been increasing over the last decade. In the long term, solar energy is expected to become an increasingly important component of a sustainable energy mix, as fossil fuels become a less and less attractive option for power generation given their emissions and slowly dwindling reserves.

Although perovskites were originally investigated as dye sensitizers for conventional solar panels, the discovery of their standalone potential sparked an entirely new field of investigation. “The efficiency of perovskite solar cells has surged from 3.8 % to 20.1 % in just five years, and will likely continue to climb toward 25 % in the future”, the researchers report. As such, these materials look set to become a significant component of the whole solar offering. “With continuous record-breaking power conversion efficiencies reported in the past few years, perovskite photovoltaics must now be considered a potential serious challenger to other technologies for electricity generation”, the team concludes.

Mike McGehee of Stanford University, CA, USA, is also pioneering the use of perovskites in photovoltaic devices. “It is great to see an analysis of how much energy is needed to make perovskite solar cells and to see that the results are very encouraging,” he told ChemViews Magazine. “We have to keep in mind that we still do not know precisely how the cells would be manufactured, so the paper only provides rough estimates. Nonetheless the results are exciting. The results show that it will be critically important to improve the lifetime.”