Flexible perovskite solar cell device before (top) and after (bottom) corrosion of the silver electrode. Image: Energy Materials and Surface Sciences Unit, OIST.
Flexible perovskite solar cell device before (top) and after (bottom) corrosion of the silver electrode. Image: Energy Materials and Surface Sciences Unit, OIST.

Perovskite solar cells are a rising star in photovoltaics. They absorb light across almost all visible wavelengths, they have exceptional power conversion efficiencies exceeding 20% in the lab, and they are relatively easy to fabricate. So why are perovskite solar cells not yet found on rooftops?

One major problem is that perovskite solar cells either employ gold electrodes, which are expensive, or silver electrodes, which have a short lifespan. In a new study published in Advanced Materials Interfaces, researchers in the Energy Materials and Surface Sciences Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan reveal the reason for the short lifespan of silver electrodes.

Currently, the most common electrode material in perovskite solar cells is gold, which is extremely expensive. A low-cost alternative to gold is silver, around 65 times cheaper. To keep the cost even lower, the researchers want to use solution processing to fabricate the layers of the solar cell, rather than expensive vacuum-based techniques.

The problem with using silver electrodes and the solution-based method is that the silver becomes corroded within days of fabrication. This corrosion makes the electrode turn yellow, and reduces the efficiency of the solar cell. The OIST team, headed by Yabing Qi, has now uncovered the cause of this degradation and proposed an explanation.

Perovskite solar cells are composed of a sandwich of layers that work together to transform light into electricity. Light is absorbed by the perovskite material and stimulates excited electrons, generating so-called electron-hole pairs. In simple terms: when the electrons are excited, they jump to a higher energy level and leave holes behind.

The excited electrons and holes are transported in opposite directions by the adjacent layers of the solar cell. These layers comprise an electron-transport titanium dioxide layer, a spiro-MeOTAD hole-transport layer (HTL), a glass layer coated with a transparent conductive material, and a silver electrode. The whole mechanism generates electric current, but each layer of the solar cell needs to be functioning correctly in order to work efficiently.

“If one layer fails, the whole solar cell will suffer,” explains Luis Ono, a staff scientist and group leader in Qi’s unit. In this study, the team analyzed the composition of the corroded silver electrode and identified the formation of silver iodide as the cause of the corrosion; the observed color change is due to oxidation of the silver to silver iodide. They also found that exposure to air accelerates the corrosion.

The team proposed a mechanism for this damage: silver iodide forms because gas molecules from the ambient air reach the perovskite material and degrade it to form iodine-containing compounds. These iodine-containing compounds diffuse to the silver electrode and corrode it. The migration of both air molecules and the iodine-containing compounds occurs through small pinholes present in the spiro-MeOTAD HTL layer. These pinholes are produced by solution processing and were identified some months ago by Zafer Hawash, a PhD student in the same laboratory.

The OIST team believes that understanding the corrosion mechanism is the first step to increasing the electrode lifetime. Since preventing the formation of pinholes in the spiro-MeOTAD HTL layer is one way to do this, the team is now working on ways to produce pinhole-free solar cells using solution processing. They have already fabricated pinhole-free HTL using vacuum-based methods.

“Perovskite-based solar cells show potential for commercial use as the next generation photovoltaic technology. Our goal is to design and fabricate large-area and low-cost photovoltaic modules with extended lifetime by employing appropriate HTLs and encapsulation materials,” explains Qi.

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