Perovskite solar cells use high-quality perovskite, usually consisting of an organic-inorganic lead compound or tin-halide material as their light-harvesting layer.

The high cost of these materials is a significant barrier to the widespread adoption of perovskite solar cells, but a recent development by engineers at the École Polytechnique Fédérale de Lausanne (EPFC) has yielded an alternative material for these cells that would reduce their price by 80 percent while maintaining efficiency above 20 percent.

Hole-Transporting Materials

Traditionally, solar cells consist of silicon as the semi-conducting absorbent layer. When a light source hits the cells, photons are absorbed and create electron-hole pairs, which is when the electron transitions from the valence band to the conductive band.

Once separation occurs and electrons are within the cell, the next step is to create an imbalance to move the electrons into one direction and create an electric field. In silicon cells, the movement of electrons is through the p-n junction, where the p junction contains a higher concentration of electrons and the n-junction is the “hole” where there are less electrons, creating the electric field at the junction.

However, silicon cells and internal hole-transporting materials are expensive and require intensive treatment. This is the advantage of perovskite cells: they use simpler wet chemistry processing methods that don’t require the use of special clean room facilities, as silicon processing does.

The two prevalent types of hole-transporting materials for perovskite-based cells are a spiro-type and tetrakis compounds, but both are costly to synthesize. However, without hole-transporting materials, solar cell efficiency declines significantly.

Making Perovskite Solar Cells Cheaper

3D image of FDT molecules on perovskite crystal film. (Image courtesy of Sven M. Hein and EPFL.)

The new hole-transporting material consists of dissymmetric flourene-dithiophene (FDT). Compared to spiro-type and tetrakis compounds, FDT is easily modifiable, providing an array of possible hole-transporting materials.

Tests showed FDT’s efficiency as 20.2 percent, which is higher than the other two more expensive alternatives.

"The best performing perovskite solar cells use hole transporting materials, which are difficult to make and purify, and are prohibitively expensive, costing over €300 per gram, preventing market penetration," said lead researcher Mohammad Nazeeruddin.

"By comparison, FDT is easy to synthesize and purify, and its cost is estimated to be a fifth of that for existing materials - while matching, and even surpassing their performance," he added.

For more information, see the published research in Nature Energy.