Perovskite solar cells hit 21.1% efficiency and record reproduciblity

The research was carried out by Michael Grätzel’s lab at EPFL, who set out to address the problem of thermal stability, which can significantly limit the efficiency of a solar cell in the long term.

Because of their enormous potential for the future of PV, perovskite solar cells are under immense research and thermal stability has been addressed with the addition of cesium. But this poses two problems:

• First, the resulting inorganic cesium lead trihalide perovskite (CsPbBr₃) does not offer an ideal band gap for PV. This means that it requires too much energy to excite bound electrons enough to free them so that they can flow as electrical current.
• The second problem of using cesium is that, at room temperature, it forms crystals that do not respond to light. In fact, the cesium mixture is only stable at temperatures above +300°C.

Consequently, the pure perovskite compounds fall prey to thermal or structural instabilities. However, the perovskite phase of CsPbI3 offers a more suitable band gap, and as such is being thoroughly investigated. One approach, called the 'antisolvent method', uses mixed cations and halides.

In the EPFL study, led by postdoc Michael Saliba, the researchers used a mixture of a triple cation (Cs/MA/FA) for the first time. The new films are more thermally stable and less affected by fluctuating surrounding variables such as temperature, solvent vapours or the heating protocol used for the device. But more importantly, they show stabilised power-conversion efficiencies of 21.1% and outputs at 18% under operational conditions, even after 250 hours.

Michael Saliba commented: “This is an absolute breakthrough. These properties are crucial for commercialising perovskite PV, especially since reproducibility and stability are the main requirements for cost-effective large-scale manufacturing of perovskite solar cells.”

The work is published in Energy and Environmental Science.

This study involved a collaboration of EPFL’s Laboratory of Photonics and Interfaces, Laboratory of Photomolecular Science, Group for Molecular Engineering of Functional Materials and the Panasonic Corporation.

The project was funded by the European Union's Seventh Framework Programme for research, technological development and demonstration, the SNSF-NanoTera (SYNERGY) and the Swiss Federal Office of Energy (SYNERGY), CCEM-CH, and the King Abdulaziz City for Science and Technology (KACST).