This shows the structure of ß-CuSCN and a cross-sectional scanning electron microscope image of a complete CuSCN-based perovskite solar cell. Image: M. Ibrahim Dar/EPFL.
This shows the structure of ß-CuSCN and a cross-sectional scanning electron microscope image of a complete CuSCN-based perovskite solar cell. Image: M. Ibrahim Dar/EPFL.

Perovskite solar cells (PSCs) can offer high light-conversion efficiencies with low manufacturing costs. But to be commercially viable, perovskite films must also be durable and not degrade under sunlight over time.

Scientists at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have now found a way to improve the operational stability of PSCs. They have developed versions that retain more than 95% of their initial 20% conversion efficiency under full sunlight illumination at 60°C for more than 1000 hours. The breakthrough, which marks the highest stability ever achieved for PSCs, is published in a paper in Science.

In conventional silicon solar cells, efficiencies have plateaued at around 25%, while problems with their high cost of manufacturing, heavy weight and rigidity have remained largely unresolved. In contrast, despite being a much more recent technology, PSCs have already achieved more than 22% efficiency.

Given the vast chemical versatility and low processing costs of perovskite materials, PSCs hold the promise for creating cheap, lightweight and highly efficient solar cells. But until now, only highly expensive, organic hole-transporting materials (HTMs), which selectively transport positive charges in a solar cell, have been able to achieve power-conversion efficiencies of over 20%. And by virtue of their ingredients, these hole-transporting materials adversely affect the long-term operational stability of the PSC.

Scientists are therefore actively investigating cheap and stable hole transporters with high efficiencies to allow the large-scale deployment of perovskite solar cells. Among various inorganic HTMs, cuprous thiocyanate (CuSCN) stands out as a stable, efficient and cheap candidate ($0.5/g versus $500/g for a commonly used organic HTM known as spiro-OMeTAD). But previous attempts at using CuSCN as a hole transporter in perovskite solar cells have had limited success. This is due to problems associated with depositing a high-quality CuSCN layer on top of a perovskite film and the chemical instability of the CuSCN layer when integrated into a PSC.

Now, researchers in Michael Grätzel's lab at EPFL, led by postdocs Neha Arora and Ibrahim Dar, have introduced two new concepts that overcome the major shortcomings of CuSCN-based PSCs. First, they developed a simple dynamic solution-based method for depositing highly conformal, 60nm-thick CuSCN layers to produce PSCs with stabilized power-conversion efficiencies exceeding 20%. This is comparable to the efficiencies of the best performing, state-of-the-art spiro-OMeTAD-based PSCs.

Second, the scientists introduced a thin spacer layer of reduced graphene oxide between layers of CuSCN and gold. This innovation allowed the PSCs to achieve excellent operational stability: they retained over 95% of their initial efficiency while operating at maximum power for 1000 hours under full-sun illumination at 60°C, surpassing the stability of organic HTM-based PSCs. It also shows that the instability of previous CuSCN-based PSCs originated from the degradation of the CuSCN/gold contact during operation.

"This is a major breakthrough in perovskite solar-cell research and will pave the way for large-scale commercial deployment of this very promising new photovoltaic technology," says Grätzel.

"It will benefit the numerous scientists in the field that have been intensively searching for a material that could replace the currently used, prohibitively expensive organic hole-transporters," adds Dar.

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.